soil degradation - ugentaverdood/cursus land degradation...soil degradation ann verdoodt academic...

Soil Degradation

Ann Verdoodt

Academic Year 2011-2012

Compilation of course notes by Prof. Donald Gabriëls

and new materials by Prof. Ann Verdoodt

Baringo Rangelands, Kenya

Photo by Stephen Mureithi

Soil Degradation | Introduction i

Table of Contents

1. Introduction ................................................................................................................. 1

1.1. What is land degradation....................................................................................... 1

1.1.1. Definition of land .......................................................................................... 1

1.1.2. Ecosystem functions and services ................................................................. 1

1.1.3. Definitions of land degradation ..................................................................... 2

1.1.4. Causes of land degradation ........................................................................... 3

1.1.5. Extent of land degradation ........................................................................... 4

1.2. What are the main land degradation types and processes? .................................... 4

1.2.1. Soil degradation ............................................................................................ 5

1.2.2. Vegetation degradation ............................................................................... 11

1.2.3. Water resource degradation ........................................................................ 12

1.2.4. Soil and water pollution .............................................................................. 13

1.3. Why should we combat land degradation? ........................................................... 13

1.4. How to assess land/soil degradation? ................................................................... 15

1.5. How to remediate land/soil degradation .............................................................. 15

2. Soil degradation assessment ...................................................................................... 17

2.1. Conceptual frameworks ....................................................................................... 17

2.1.1. DPSIR ......................................................................................................... 17

2.1.2. Ecosystem services framework.................................................................... 19

2.1.3. Sustainable livelihoods approach ................................................................ 21

2.2. Soil quality Indicators ......................................................................................... 24

2.2.1. Definition .................................................................................................... 24

2.2.2. Characteristics of good indicators ............................................................... 25

2.2.3. Selection of indicators ................................................................................. 27

2.2.4. Interpretation of indicators ......................................................................... 29

3. Soil structural degradation ........................................................................................ 32

3.1. Process of aggregate formation ............................................................................ 33

3.1.1. Physico-chemical processes ......................................................................... 33

3.1.2. Activities of soil organisms ......................................................................... 35

3.2. Factors affecting aggregation............................................................................... 36

3.2.1. Impact of raindrops ..................................................................................... 36

3.2.2. Flooding ...................................................................................................... 36

3.2.3. Drying and wetting ..................................................................................... 37

3.2.4. Freezing and thawing ................................................................................. 37

3.2.5. Biotic factors ............................................................................................... 37

3.2.6. Influence of Tillage ..................................................................................... 37

3.2.7. Soil amendments ........................................................................................ 38

3.3. Assessment of soil structure ................................................................................ 38

3.3.1. Laboratory methods .................................................................................... 39

3.3.2. Other indices............................................................................................... 41

3.4. Impact of soil structural degradation ................................................................... 42

4. Soil compaction .......................................................................................................... 44

Soil Degradation | Introduction ii

4.1. Soil compaction process ....................................................................................... 44

4.1.1. Soil strength ............................................................................................... 44

4.1.2. Factors affecting soil strength ..................................................................... 45

4.1.3. Propagation of external stress with depth................................................... 45

4.2. Susceptibility to compaction ................................................................................ 46

4.3. Resilience to compaction ...................................................................................... 48

4.4. Causes of soil compaction .................................................................................... 49

4.5. Consequences of soil compaction .......................................................................... 50

4.5.1. Influence on soil physical properties ........................................................... 50

4.5.2. Influence on soil chemical properties .......................................................... 51

4.5.3. Influence on soil biological properties ......................................................... 51

4.5.4. Influence on soil functions .......................................................................... 52

4.6. Assessing soil compaction .................................................................................... 52

4.6.1. Field observations ....................................................................................... 52

4.6.2. Soil profile descriptions ............................................................................... 52

4.6.3. Direct laboratory methods .......................................................................... 53

4.6.4. Indirect methods ......................................................................................... 54

4.7. Prevention and remediation ................................................................................ 56

4.7.1. Preventing soil compaction ......................................................................... 56

4.7.2. Remediating soil compaction....................................................................... 56

5. Soil surface sealing and crusting ................................................................................ 58

5.1. Sealing versus crusting ........................................................................................ 58

5.2. Types of crusts ..................................................................................................... 58

5.3. Surface sealing and crusting process ................................................................... 59

5.3.1. Morphology of structural crusts .................................................................. 59

5.3.2. Morphology of depositional crusts ............................................................... 60

5.3.3. Formation of soil crusts............................................................................... 60

5.4. Factors affecting slaking and deflocculation ........................................................ 62

5.4.1. Rainfall factor ............................................................................................. 62

5.4.2. Weather ...................................................................................................... 62

5.4.3. Soil Properties ............................................................................................ 62

5.4.4. Antecedent soil moisture and wetting rate.................................................. 64

5.4.5. Microrelief................................................................................................... 65

5.5. Characterisation of soil crusts ............................................................................. 65

5.5.1. Direct methods ............................................................................................ 66

5.5.2. Indirect methods ......................................................................................... 67

5.6. Sealing and crusting in different climates ........................................................... 68

5.7. Impact on crops ................................................................................................... 70

5.8. Crust Management .............................................................................................. 72

5.8.1. Preventive measures ................................................................................... 72

5.8.2. Curative measures ...................................................................................... 73

6. Soil erosion ................................................................................................................. 77

6.1. Soil erosion processes........................................................................................... 77

6.1.1. Soil erosion by water ................................................................................... 77

6.1.2. Soil erosion by wind .................................................................................... 79

Soil Degradation | Introduction iii

6.1.3. Disturbance, translocation or tillage erosion............................................... 80

6.2. On-site and off-site effects of accelerated soil erosion .......................................... 81

6.2.1. On-site damage ........................................................................................... 81

6.2.2. Off-site damage ........................................................................................... 81

6.3. Assessing soil erosion risk and losses .................................................................. 82

6.3.1. Field observations ....................................................................................... 82

6.3.2. Field experiments ....................................................................................... 82

6.3.3. Laboratory measurements .......................................................................... 85

6.3.4. Modelling soil losses by water erosion ......................................................... 85

6.3.5. Modelling soil losses by wind erosion .......................................................... 87

6.4. Soil loss tolerance ................................................................................................ 88

7. Salinisation ................................................................................................................ 91

7.1. Causes of soil salinisation .................................................................................... 91

7.1.1. Soluble salts ................................................................................................ 91

7.1.2. Evapotranspiration ..................................................................................... 92

7.1.3. Drainage ..................................................................................................... 93

7.1.4. Irrigation water quality .............................................................................. 93

7.2. Sources of soluble salts ........................................................................................ 93

7.2.1. Weathering ................................................................................................. 93

7.2.2. Secondary deposits ...................................................................................... 94

7.2.3. Irrigation water .......................................................................................... 94

7.3. Measuring salinity and sodicity ........................................................................... 95

7.3.1. Total dissolved solids (TDS) ........................................................................ 95

7.3.2. Electrical conductivity (EC) ........................................................................ 96

7.3.3. Exchangeable sodium percentage ............................................................. 102

7.3.4. Sodium adsorption ratio ............................................................................ 102

7.4. Effects of salinity and sodicity on physical soil degradation .............................. 104

7.5. Classification of salt affected soils ..................................................................... 106

7.5.1. Saline soils ................................................................................................ 107

7.5.2. Sodic soils ................................................................................................. 107

7.5.3. Saline-sodic soils ....................................................................................... 108

7.6. Impact on plant growth ..................................................................................... 109

7.6.1. How salts affect plant growth ................................................................... 109

7.6.2. Tolerance to salinity and sodicity .............................................................. 112

7.7. Reclamation of saline and sodic soils ................................................................. 116

7.7.1. Reclamation of saline soils ........................................................................ 116

7.7.2. Reclamation of sodic soils.......................................................................... 118

7.7.3. Reclamation of saline-sodic soils ............................................................... 118

7.8. Controlling salt-build up in soils ........................................................................ 118

7.8.1. Salt balance .............................................................................................. 119

7.8.2. Salinity hazard ......................................................................................... 121

7.8.3. Preliminary surveys and monitoring schemes .......................................... 123

7.8.4. Soil salinity modelling .............................................................................. 127

8. Decline in OM .......................................................................................................... 129

Soil Degradation | Introduction iv

8.1. Definition of soil organic matter ........................................................................ 129

8.2. Composition of soil organic matter..................................................................... 129

8.3. Influence on soil properties and the environment .............................................. 131

8.4. Management of soil organic matter ................................................................... 134

9. Decline in biodiversity .............................................................................................. 137

9.1. Definition........................................................................................................... 137

9.2. Assessment of biodiversity ................................................................................. 137

10. Aridity, drought and climate change ...................................................................... 140

10.1. Aridity .............................................................................................................. 140

10.1.1. Lang factor (1915) ................................................................................... 140

10.1.2. De Martonne Aridity Index (1926) .......................................................... 141

10.1.3. Emberger Aridity Index (1932) ............................................................... 142

10.1.4. Thornthwaite Classification.................................................................... 143

10.1.5. Gaussen-Bagnouls Classification ............................................................ 143

10.1.6. UNEP Aridity Index ............................................................................... 144

10.2. Rainfall distribution ......................................................................................... 147

10.2.1. Precipitation Concentration Index (PCI)................................................. 147

10.2.2. Modified Fournier Index (MFI) ............................................................... 147

10.3. Drought ............................................................................................................ 148

10.3.1. Different types of drought ....................................................................... 149

10.3.2. Drought and desiccation ......................................................................... 152

10.3.3. Early warning systems ........................................................................... 153

10.4. dryland zones ................................................................................................... 153

10.4.1. Delineation of dryland zones................................................................... 153

10.4.2. General characteristics of dryland zones ................................................ 154

10.4.3. Problems delineating dryland boundaries............................................... 155

10.4.4. Climatic variability and change in drylands ........................................... 156

11. Desertification........................................................................................................ 165

11.1. Evolution of the concept ................................................................................... 165

11.1.1. UNCOD (70s and 80s) ............................................................................ 166

11.1.2. UNEP (1991) .......................................................................................... 166

11.1.3. Earth summit (1992) .............................................................................. 167

11.1.4. Convention on desertification (1994) ...................................................... 167

11.2. Characteristics of dryland Degradation............................................................ 169

11.2.1. Soils in drylands ..................................................................................... 169

11.2.2. Susceptibility to degradation .................................................................. 169

11.2.3. Desertification processes ........................................................................ 170

11.2.4. Desertification within different land use systems ................................... 173

11.3. Causes of desertification .................................................................................. 174

11.3.1. Population growth and emigration ......................................................... 174

11.3.2. Over-cultivation ...................................................................................... 175

11.3.3. Overgrazing ............................................................................................ 177

11.3.4. Deforestation and fuel gathering ............................................................ 180

11.3.5. Unsound development projects ............................................................... 182

Soil Degradation | Introduction v

11.3.6. Urban and industrial activities ............................................................... 184

11.4. Extent of desertification ................................................................................... 184

11.4.1. Aerial extent ........................................................................................... 185

11.4.2. Economic impact ..................................................................................... 185

11.5. Combatting desertification ............................................................................... 187

12. Global land degradation assessments .................................................................... 189

12.1. GLASOD .......................................................................................................... 189

12.1.1. General Approach ................................................................................... 189

12.1.2. Soil degradation categories and types ..................................................... 190

12.1.3. Degree of soil degradation ...................................................................... 192

12.1.4. Relative extent of the degradation type .................................................. 195

12.1.5. The severity of soil degradation .............................................................. 195

12.1.6. Causes of soil degradation ...................................................................... 196

12.1.7. Rate of soil degradation .......................................................................... 197

12.1.8. Output .................................................................................................... 197

12.2. GLADA............................................................................................................. 200

12.2.1. Net primary productivity as land degradation indicator ......................... 200

12.2.2. General methodology .............................................................................. 201

12.2.3. Results .................................................................................................... 210

12.2.4. Limitations ............................................................................................. 213

12.3. Global dimension of desertification .................................................................. 214

1.2.1. USDA approach ........................................................................................ 214

1.2.2. UNEP – UNCCD approach ....................................................................... 220

12.3.2. Special patterns ...................................................................................... 222

12.3.3. Relative importance of different degradation types ................................ 223

13. Soil protection and conservation ............................................................................ 226

13.1. Principles of sustainable land management ..................................................... 226

13.1.1. Increasing land productivity ................................................................... 227

13.1.2. Improved livelihoods ............................................................................... 228

13.1.3. Improved ecosystems .............................................................................. 230

13.2. Technologies ..................................................................................................... 233

13.2.1. Integrated soil fertility management ...................................................... 233

13.2.2. Conservation agriculture ........................................................................ 235

13.2.3. Rainwater harvesting ............................................................................. 236

13.2.4. Smallholder irrigation management ....................................................... 238

13.2.5. Agroforestry ............................................................................................ 240

13.2.6. Pastoralism and rangeland management ............................................... 241

13.3. Approaches ....................................................................................................... 243

13.4. Decision support ............................................................................................... 244

Soil Degradation | Introduction 1

1. Introduction

1.1. WHAT IS LAND DEGRADATION

1.1.1. Definition of land

Land is commonly used to refer to a section of the earth‟s surface with all the physical,

chemical and biological features influencing the use of land. It thus comprises soils,

terrain, climate, hydrology, vegetation and fauna, as well as human activities (land

management).

1.1.2. Ecosystem functions and services

Each unit of land is part of an ecosystem (forest, rangeland, agro-ecosystem, wetland,

urban area, …). These ecosystems deliver ecosystem functions and services.

Ecosystem functions are natural processes contributing to the self-maintenance of

ecosystems (eg. photosynthesis, water cycle, nutrient cycle, …).

The benefits that humankind can extract from the functioning of ecosystems is referred

to as ecosystem services. Four categories of ecosystem services can be distinguished

(Table 1.1):

− Provisioning services refer to the products people obtain from the ecosystem

(these are sometimes also referred to as ecosystem goods).

− Regulating services are the benefits people obtain from the regulation of

ecosystem processes.

− Cultural services are the nonmaterial benefits people obtain from the ecosystem

(MA, 2003).

− Supporting services are those necessary for the production of all other ecosystem

services.

A good ecosystem functioning is necessary for the production of all other ecosystem

services. The impacts of supporting services on people are either indirect or occur over a

very long time.


Table 1.1. Overview of key ecosystem services

Supporting Provisioning Regulating Cultural

Soil formation and

retention

Nutrient cycling

Water cycling

Primary production

Production of

atmospheric O2

Provisioning of

habitat

Food

Fresh water

Fuel wood

Bio-chemicals

Fibers

Genetic resources

Ornamental resources

Climate regulation

Disease regulation

Water regulation

Water purification

and waste treatment

Pollination

Air quality

maintenance

Erosion control

Biological control

Storm protection

Spiritual & religious

Recreation &

ecotourism

Aesthetic

Inspirational

Educational

Sense of place

Cultural heritage

Cultural diversity

Knowledge systems

Social relations

1.1.3. Definitions of land degradation

Land degradation is a relatively new term (eighties). Its definition has been subjected to

shifts in emphasis and broadening over the last decades, and is still subjected to an

ongoing political and academic debate.

LADA (Land Degradation Assessment in Drylands, http://www.fao.org/nr/lada/) defines

land degradation as the long-term reduction in the capacity of the land to perform

ecosystem functions and services (including those of agro-ecosystems and urban

systems) that support society and development (LADA, 2005).

According to the United Nations Convention to Combat Degradation (UNCCD,

http://www.unccd.int/), land degradation is the reduction or loss, in arid, semi-arid, and

dry sub-humid areas, of the biological or economic productivity and complexity of rainfed

cropland, irrigated cropland, or range, pasture, forest, and woodlands resulting from

land uses or from a process or combination of processes, including processes arising from

human activities and habitation patterns. These processes include (i) soil erosion caused

by wind and/or water; (ii) deterioration of the physical, chemical, and biological or

economic properties of soil; and (iii) long-term loss of natural vegetation.

Consequently, the UNCCD defines land degradation as a persistent decline in the

ability of a dryland ecosystem to provide goods and services associated with primary

production (Safriel and Adeel, 2005).

Based on current uses of the term land degradation, Johnson and Lewis (2007) define it

as the substantial decrease, in either or both of an area‟s biological productivity or

usefulness to humans, due to human activities.

http://www.fao.org/nr/lada/

http://www.unccd.int/


A number of key issues can be highlighted from this overview of definitions. There is

general agreement that land degradation:

− decreases the capacity of managed land systems to meet user demands; and

− threatens the long-term biological and/or economic resilience and adaptive capacity

of the ecosystem in general.

Divergences in the definitions are related to:

− causes of land degradation; and

− its spatial extent.

1.1.4. Causes of land degradation

Land degradation is generally caused by inappropriate land use and management that

jeopardizes the soil‟s self-regulating capacity (and resistance and resilience). The

unsustainable land use and management practices themselves are often driven by socio-

economic and political forces (Lal et al., 1998).

The main causes of human-induced land degradation in agro-ecosystems are:

− Overexploitation of high potential land

Intensification of agricultural production requires increased use of herbicides,

pesticides, and fertilisers. Herbicides and pesticides may cause health hazards if

applied and disposed of inappropriately. Unregulated fertiliser input, often

subsidised, causes water pollution, biodiversity shifts and health threats.

− Misuse of marginal land

Extension of farming into marginal lands through the conversion of forests,

wetlands, and mountain slopes causes erosion, nutrient loss and rapid fertility

decline. It affects environmental services such as water supply and microclimate,

and often creates related hazards such as landslides and desertification. This leads

to land degradation through erosion, nutrient loss, soil contamination, soil

acidification and even desertification.

Further losses of agricultural land are induced by the ongoing urbanisation,

industrialisation and infrastructure development, resulting in soil sealing and soil and

water contamination.

In urban, peri-urban and industrial ecosystems, industrial activities and the expansion

of the built up land into natural or agricultural are the main causes of land degradation.

Land disposal of urban and industrial wastes is a major source of soil contamination and

pollution. Industrial pollution causes soil chemical and biological degradation with


drastic impacts on the current and future productivity (and on human health) (Lal et al.,

1998).

Land degradation processes such as for instance soil erosion, salinisation, and

acidification can also occur as a result of natural conditions. Consequently, when

assessing land degradation, a clear insight in the causal factors needs to be gained in

order to be able to suggest effective measure for its combat and remediation!

Examples of natural land degradation:

− Salinisation of sugarcane plantation in Zambia: salts can originate from (1) use of

bad quality irrigation water, (2) saline ground water (natural or human-induced), or

(3) released from rock during weathering; and

− In drylands, long periods of drought combined with unsustainable land management

trigger land degradation (desertification). In this case, land degradation is caused by

a negative interference of climate and land management.

1.1.5. Extent of land degradation

Land degradation is occurring in all parts of the world. Nevertheless, especially the

dryland ecosystems are very fragile and particularly susceptible to land degradation.

Desertification is land degradation in arid, semi-arid and dry sub-humid areas resulting

from various factors, including climatic variations and human activities (UNCCD,

UNEP, GEF). Desertification thus involves land degradation in environments under low

rainfall conditions.

1.2. WHAT ARE THE MAIN LAND DEGRADATION TYPES

AND PROCESSES?

Land degradation is caused by a variety of complex interrelated degradation processes.

These can be grouped into four major land degradation types, each of which can be

subdivided according to a specific sub-set of degradation processes:

− Soil degradation

− Vegetation degradation

− Water resource degradation

− Pollution (soil or water)


1.2.1. Soil degradation

Soil degradation occurs when there is a decline in the productive or functional capacity

of the soil as a result of adverse changes in its biological, chemical, physical and

hydrological properties.

Soil is in fact an extremely complex, variable and living medium that regulates our

environment and responds to the pressures imposed upon it. Ignored by the majority of

us, soil carries out a number of key environmental tasks that are essential to our

wellbeing.

1.2.1.1. Soil functions and services

Soil functions and uses can be divided into two main areas, ecological and socio-

economic (Table 1.2; Figure 1.1). The ecological or environmental functions comprise

biomass production; filtering, buffering and transformation; carbon sequestration; and

provision of a biological habitat and gene reserve. From a technical, industrial and socio-

economic point of view, the soil constitutes the physical supporting medium; a source of

raw materials and it protects our cultural heritage.

Table 1.2. Main soil functions and uses

Ecological Functions Technical, Industrial & Socio-Economic

Functions

Biomass

production

- Food production

- Renewable energy

Physical

medium

-Support for built

structures such as

housing, infrastructure,

factories, …

-Waste disposal

-Recreation activities

Filtering,

buffering &

transformation

-Cycling of major elements

(C, N, P, S, …)

-Regulate water flow

-Sorption reactions

-Microbial & biochemical

transformations

Source of raw

materials

-Quarrying & mining

activities

-Supply of water

Biological

habitat & gene

reserve

-Soil biomass (macro- &

microfauna, micro-organisms)

-Supporting biological habitat

for many organisms

-Gene reserve

Protecting &

preserving

cultural

heritage

-Protects archaeological &

palaeontological sites

-Contributes to the

appearance of the

landscape

Biomass production. Soil is the medium that enables us to grow our food, natural

fibre and timber. In Europe, for instance, the most productive agricultural soils are

found along the major river valleys, their estuaries and also on the glacial plains of


northern Europe where ice age winds have deposited a layer of fine rock-dust to form

fertile “loess” soil.

Filtering, buffering and transformation. Soil has a storing, filtering and

transforming capacity, and regulates atmospheric, hydrological and nutrient cycles.

Soil plays an essential role in the hydrological cycle. Excess water is redistributed

to surface or ground water. Soil can absorb much of the rain that falls on it but the

amount varies according to texture, structure and vegetative cover. Soil functions as a

natural filter for groundwater, and releases CO2, methane and other gases in the

atmosphere. The considerable storage and buffering capacity of soil is closely related to

the organic matter content.

Figure 1.1. Some functions (services) provided by soil

The soil also represents the largest terrestrial carbon pool on earth - often

regarded as a separate 7th soil function – and thus soil organic carbon holds a very

important role in global C cycle. Soil may act as a sink (CO2 and CH4) or source (CO2,

CH4 and N2O) in the carbon cycle depending on land use and management. Nearly all

models of global climate change predict a loss of carbon from soils as a result of global

warming. However, restoration of eroded and degraded soils and intensification of

agriculture can lead to C sequestration in soils.


Soil stores not only water, plant nutrients and gases, but can also immobilize or break

down a multitude of pollutants, for example from waste disposal. These filtering,

buffering and transformation capacities are however limited and vary according to

specific soil conditions. Contaminants may build up and subsequently be released in

different ways, in some cases exceeding regulatory thresholds. Anticipatory policies

based on monitoring and early warning systems are essential to prevent damage to the

environment and risks to public health.

Biological habitat and gene reserve. Sustaining biodiversity is an essential

ecological soil function as soil is the habitat for a huge amount and variety of living

organisms, and thus sustains a diverse gene pool. The total weight of organisms below

for example temperate grassland can exceed 5 t ha-1. A few grams of such soil contain

billions of bacteria, hundreds of kilometres of fungal hyphae, tens of thousands of

protozoa, thousands of nematodes, several hundred insects, arachnids and worms, and

hundreds of metres of plant roots.

In turn, the soil biological activity contributes to its properties and characteristics,

which are essential for its productive functions. Soil biota are involved in most of the key

soil functions, driving fundamental nutrient cycling processes, regulating plant

communities, degrading pollutants and helping to stabilise soil structure.

Physical medium. Soil provides the foundation upon which we construct our buildings,

roads and other infrastructures. One of the main problems in this context is the

exponential increase of urban and peri-urban areas, including transport facilities

between them.

Source of raw materials. Soil supplies clay, sand, gravel, minerals as well as energy

and water and as such forms the basis for technical, industrial and socio-economic

development. In Flanders, different types of sands (from pile sands, masonry and

concrete sands to very valuable quartz sand), loams and clays contribute to the

economically important house and road construction, bricks, synthetics and glass

sectors. The great challenge is to sustain the demands for surficial minerals and mining

sites as to guarantee the supply of raw materials.

Protecting and preserving cultural heritage. Soil protects the buried heritage of

archaeological and historic remains from damage and depletion. Much of the evidence of


our heritage remains buried within the soil, awaiting study by archaeologists. The

degree of preservation of such remains depends very much on the local soil conditions.

Waterlogged or very acid soil with low levels of oxygen has very little microbial activity

and provides an ideal environment for preserving organic remains.

1.2.1.2. Soil degradation

Soil degradation refers to the decline in a soil‟s inherent capacity to perform

environmental and socio-economic functions. Causes of degradation include

deforestation, overgrazing, agricultural practices, overexploitation of the vegetative

cover, and industrial activities. Figure 1.2 shows some human activities causing soil

degradation.

Figure 1.2. Human impacts on soil causing soil degradation

At a global scale it has been estimated that nearly 20 million km² or over 15% of the

land area has been degraded (GLASOD, 1991). The main causes are deforestation,

overgrazing and poor agricultural management. Some soil degradation processes are

natural phenomena but they are exacerbated by all kinds of unsustainable human uses.

Soil degradation is expected to continue and probably accelerate if appropriate measures

are not taken.

Sheet, rill and gully erosion, and the scouring and deposition of soil by wind are some of

the most visible symptoms of soil degradation; but other less visible forms are even more


widespread and sometimes more serious (e.g. depletion of nutrients, soil organic matter

decline).

Soil physical, biological, chemical and hydrological properties are degraded in different

ways:

Key processes that result in degradation of soil physical properties include:

− Surface crusting and compaction through the impact of raindrops, animal

hooves and farm machinery;

− Loss of topsoil structure through excessive tillage and loss of soil organic

matter;

− Sub-soil compaction due to the passage of heavy farm machinery and/or

ploughing to a constant depth.

Key processes that result in degradation of soil hydrological properties include

− Waterlogging involving a rise in the water table close to the soil surface due to

poor irrigation practices, or loss of deep rooted vegetation whose water needs

would have kept the water table low; and

− Aridification involving a decrease in soil moisture availability, typically due to

reduced rain water infiltration following deterioration in the soil‟s physical

structure.

Key processes that result in degradation of soil chemical properties include:

− Decline in the number and availability of soil nutrients (N,P,K, secondary and

trace elements) e.g. through leaching, gaseous losses, removal in harvested

products etc.

− Chemical imbalances and toxicities e.g. through application of inappropriate

types and quantities of fertiliser, pesticides etc.;

− Changes in soil pH (acidification or alkalinisation);

− Salinisation (build up of salts through poor irrigation practices in crop lands and

poor grazing practices in grasslands);

− Chemical pollution from over use of agro-chemicals, plastic mulches or poor

management of industrial and mining wastes.

Key processes that result in degradation of soil biological properties include:

− Reduction in the numbers or activity of beneficial soil organisms such as

bacteria, rhizobia, mycorrhiza, earth worms, termites etc;


− Increase in the numbers and activity of harmful soil organisms such as

nematodes, parasitic weeds etc.

Soil salinization is a particular type of land degradation that deserves specific attention.

Soil salinization often restricts options for cropping in a given land area as few plants

grow well on saline soils. It also degrades the quality of shallow ground water and

surface water resources, such as ponds.

Saline soils occur where the supply of salts, for example from rock weathering, capillary

rise, rainfall or flooding, exceed their removal by plant uptake, leaching and flooding.

Thus salinization on the soil surface occurs where the following conditions occur

together:

− the presence of soluble salts, such as sulphates of sodium, calcium, and magnesium

in the soil

− a high water table

− a high rate of evaporation

− low annual rainfall

Sodic soils contain a higher amount of sodium attached to clay particles. When in

contact with water, a sodic soil swells and disperses into tiny fragments. On drying

these tiny fragments block the soil pores which cause problems of crusting, hard-setting,

poor infiltration and water logging.

Excess salts hinder crop growth, not only by toxicity effects, but by reducing water

availability, regardless of the total amount of water actually in the root zone. Salts in

the soil increase the effort plant roots must make to take up water. High levels of salt in

the soil have a similar effect as drought: making water less available for uptake by plant

roots.

Key soil erosion processes can be grouped into broad categories, which are described

separately, however, anyone of them may occur in the same locality, either in

combination or at different times of year:

− Water erosion - is often quite widespread and can occur in all parts of drylands

where rainfall is intense (e.g. during a storm9) and surface runoff occurs. This

category includes processes such as splash, sheet, rill and gully erosion. Soils that

have lost organic matter and had their structural stability degraded through


excessive tillage, are more vulnerable to water erosion. Likewise surface and subsoil

compaction reduces the amount of rainfall that can infiltrate into the soil leading to

increased surface runoff and increased risk of water erosion.

− Wind erosion – is also widespread throughout drylands that are exposed to strong

winds. It includes both the removal and deposition of soil particles by wind action

and the abrasive effects of moving particles as they are transported. In areas with

extensive loose sandy material wind erosion can lead to the formation of mobile sand

dunes that cause considerable economic losses through engulfing adjacent farm land,

pastures, settlements, roads and other infrastructure.

− Gravitational erosion – tends to be more localised in regions with steep and rocky

slopes and mountain ranges. On sloping land when soil is saturated its weight may

be sufficient to exceed the forces holding the soil in place. Under such circumstances

the forces of gravity take over and mass movement may occur. This includes all

relatively large down-slope movement of soil and/or rock, e.g. landslides, slumps,

earth flows and debris avalanches. Landslides may be natural events, however, their

frequency and severity may greatly increase following destruction of the natural

vegetative cover by logging and/or clearing for cultivation.

− Freeze/thaw (frozen and melting) erosion - is restricted to high altitude areas and

areas with cold climates. It occurs when water in the topsoil initially freezes and

expands, and then melts, enabling loosened surface soil particles to be carried away

in melt water runoff. It is primarily a natural process rather than one which is

accelerated by particular human activities.

1.2.2. Vegetation degradation

Vegetative growth, especially in drylands, tends to be limited by a range of natural

factors, notably extreme temperatures, low and erratic rainfall, low soil water

availability, and shallow soils with low inherent fertility. In response, a number of

highly specialized vegetation types have evolved, adapted to the local climate,

topography and soils.

Vegetation degradation involves a combination of processes:

− Reduction in vegetative biomass – with fewer plants, at lower density, with

reduced vigour and growth producing less leaves, stems, flowers, fruits, seeds, etc.

(resulting in reduced yield of grassland, forest and woodland products);


− Reduction in vegetative ground cover – with expanding areas of bare ground

occurring in formerly vegetated areas;

− Reduction in the quality of the vegetative biomass – where, although the total

biomass may be about the same, plant species of high value (for fodder, timber,

fuelwood, food, medicines etc) have to a lesser or greater extent been replaced by

species of lower, or no value; or parts of the plants have been damaged or their

health affected through excessive removal of specific parts (for timber, fuelwood,

fodder, fruits, food, medicine etc.); and

− Reduction in species diversity and/or abundance (numbers/populations of

specific species). This can happen in natural plant communities or in the diversity of

local crop varieties and land-races.

In assessing vegetation degradation, we are concerned with adverse changes in the

quantity and quality of the plants that are found in grassland, forest and woodland

areas.

1.2.3. Water resource degradation

Water resource degradation includes:

− Increased fluctuations in quantity of surface water ‘stream’ flow leading to

increased storm peak flows and reduced dry season flow as a higher proportion of the

rain falling during storm events is lost rapidly as surface runoff rather than

infiltrating into the soil;

− Increased incidence of downstream flooding as upstream areas become degraded

and can no longer absorb the volume of rainfall received during storm events;

− Drying up of rivers, springs, lakes, ponds, boreholes, etc. more frequently and for

longer periods as water is lost in surface runoff rather than infiltrating to replenish

groundwater levels;

− Reduced groundwater recharge due to increased surface rainwater runoff;

− Lowering of the ground water table due to reduced recharge and increased

extraction;


− Increased sediment load in streams and rivers due to increased soil erosion in their

catchment areas;

− Reduced water storage capacity due to sedimentation of reservoirs;

− Pollution of surface and ground water resources from human and animal wastes,

agro-chemicals, industrial and mining wastes; and

− Increased salt content of surface and ground water resources due to excess salt

flushing from irrigated areas. see above soil salinity and salinisation.

1.2.4. Soil and water pollution

Agriculture or industry can lead to pollution of land resources:

− chemical imbalances and toxicities within the soil, as can occur with the

application of inappropriate types and quantities of fertiliser;

− the build-up of inorganic pollutants as a result of over use of agro-chemicals and

the deterioration in the topsoil of residues left following the use of plastic mulches;

− toxic chemicals emitted in the smoke from heavy industry settling on the soil

surface downwind of the factory;

− uncontrolled discharge of pollutants into water sources which then get onto

the land when the water is used for irrigation purposes, or flooding takes place; and

− erosion (by wind and/or water) and subsequent deposition, on land

downwind/ downstream, of the material from the spoil heaps and other wastes

associated with mining and quarry operations.

1.3. WHY SHOULD WE COMBAT LAND DEGRADATION?

Land degradation is not restricted to drylands, but affects many developed and

developing countries.


Land degradation has multiple and complex impacts on the global environment

through a range of direct and indirect processes affecting a wide array of ecosystem

functions and services.

It is a global issue because of its adverse impact on :

(1) agricultural productivity and sustainability; and

(2) ecology/environment

with consequences at local, regional and global scales: eutrophication of surface water,

contamination of groundwater, increase in greenhouse gases, availability of fresh water

supply and prime agricultural land (Lal et al., 1998). Consequently, land degradation

impacts occur on global development issues, especially food security and human health.

Since ecosystems provide the habitats for all living organisms, disruption to ecosystem

functions inevitably diminishes the diversity of above- and below-ground

biodiversity, as well as affecting aquatic life. Despite the fact that the potential impact

of deforestation on above-ground biodiversity is large and well documented, significant

gaps in knowledge exist on the linkages between other forms of land degradation and

biodiversity. Effects on below-ground biodiversity are likely to be the most severe.

The clearest and best-researched linkage is between land degradation and climate

change. Estimates of historical contributions of agriculture to atmospheric CO2, the

amounts and rates of carbon lost as a consequence of deforestation and conversion of

land to agriculture and other soil-vegetation-atmosphere carbon fluxes, all suggest that

land degradation has had a very significant impact, through raising atmospheric CO2

concentrations, on climate. Future impacts are certain.

Land degradation interrupts the regulating and provisioning services of ecosystems, in

particular nutrient cycling, the global carbon cycle and the hydrological cycle.

Sustainable land management critically depends upon the efficient functioning of these

cycles. For example, carbon pools in soil and above-ground vegetation, particularly

forests, are very large but easily disturbed. They are affected by unsustainable land

management practices and by the type of land degradation that is prevalent (e.g. water

erosion; deforestation; soil compaction).


1.4. HOW TO ASSESS LAND/SOIL DEGRADATION?

In the combat of soil degradation, four main steps can be distinguished:

(1) assessment of the current state of problems, causes and impacts;

(2) monitoring soil degradation;

(3) controlling soil degradation; and

(4) remediation measures.

Land degradation assessments aim at estimating the severity and intensity of the

ongoing land degradation processes, as well as at characterizing their spatial extent and

distribution. In these lecture notes, we will explore different tools for land degradation

assessments, to be applied at various spatial and temporal scales: use of expert and local

knowledge, field observations, laboratory measurements and field experiments,

simulation models and monitoring schemes.

1.5. HOW TO REMEDIATE LAND/SOIL DEGRADATION

Once the land degradation processes are understood and cause-effect relationships

established, appropriate methods of constraints/stress alleviation, soil restoration and

quality enhancement can be developed. This scientific information is needed by policy

makers and land use planners to identify policies/practices that reverse degradative

trends and set motion to soil restorative processes. The land users in their turn will only

be motivated to adopt these measures if they have a clear view on the short and long-

term impacts and consequences.

In these lecture notes, we‟ll handle some soil remediation and rehabilitation techniques.

Attention will also be paid to land degradation control measures that prevent or

mitigate land degradation in risk areas.

References

Johnson DL. and Lewis LA. 2007. Land Degradation: Creation and Destruction.

Lowman and Littlefield, USA.

Lal R. (Editor) 1998. Soil quality and soil erosion. Soil and Water Conservation Society,

SRC Press, USA.


Safriel U. and Adeel Z. 2005. Dryland systems. In: Hassan R, Scholes R, Ash N (eds)

Ecosystems and human well-being, current state and trends, vol 1. Island Press,

Washington, pp 625–658

Soil Degradation | Soil degradation assessment 17

2. Soil degradation assessment

2.1. CONCEPTUAL FRAMEWORKS

A sound land degradation assessment considers not only the type, extent and intensity

of the ongoing land degradation processes, but also focuses on an understanding of the

direct and indirect causes (or drivers) and the impacts on the environment and on local

peoples‟/stakeholders‟ lives and livelihoods.

LADA, in its Field Manual for Local Level Land Degradation Assessment in Drylands

(version 1, 2009), briefly introduces 3 different conceptual frameworks that can be used

to this aim. The DPSIR framework provides a structure for integrating the different

parts of the assessment. The Ecosystem Services and the Sustainable Livelihoods

frameworks help to think about land degradation (LD) and sustainable land

management (SLM) impacts in a structured and systematic way.

2.1.1. DPSIR

The Driving Force-Pressure-State-Impact-Response (DPSIR) framework (was also

adopted by the European Environmental Agency) is based on the assumption that

economic activities and society‟s behaviour affect environmental quality and takes into

account the different steps in the policymaking. The DPSIR framework shows a chain of

causes and effects from Driving forces (activities) to Pressures, to changes in the State of

the environment, to Impacts and Responses. DPSIR as such highlights the complex

connections between the causes of environmental problems, their impacts and society‟s

response to them (Figure 2.1).

Driving forces quantify human activities or other processes that have a positive or

negative influence on sustainable development. This exerts a pressure on the

environment and eventually changes the quality or quantity of natural resources (State)

with a possible impact on human health or environmental quality. The policymakers can

react on this with a response directed towards modifying the driving forces, pressures,

state or impact.


Figure 2.1. The DPSIR framework applied to soil

2.1.1.1. Driving force

A „driving force‟ is a need. Examples of primary driving forces for an individual are the

need for shelter, food and water, while examples of secondary driving forces are the need

for mobility, entertainment and culture. For an industrial sector a driving force could be

the need to be profitable and to produce at low costs, while for a nation a driving force

could be the need to keep unemployment levels low. In a macroeconomic context,

production or consumption processes are structured according to economic sectors (e.g.

agriculture, energy, industry, transport, households).

2.1.1.2. Pressure

Driving forces lead to human activities such as transportation or food production, i.e.

result in meeting a need. These human activities exert 'pressures' on the environment,

as a result of production or consumption processes, which can be divided into three main

types: (i) excessive use of environmental resources, (ii) changes in land use, and (iii)

emissions (of chemicals, waste, radiation, noise) to air, water and soil.


2.1.1.3. State

As a result of pressures, the „state‟ of the environment is affected; that is, the quality of

the various environmental compartments (air, water, soil, etc.) in relation to the

functions that these compartments fulfil. The „state of the environment‟ is thus the

combination of the physical, chemical and biological conditions.

2.1.1.4. Impact

The changes in the physical, chemical or biological state of the environment determine

the quality of ecosystems and the welfare of human beings. In other words changes in

the state may have environmental or economic „impacts‟ on the functioning of

ecosystems, their life supporting abilities, and ultimately on human health and on the

economic and social performance of society.

2.1.1.5. Response

A „response‟ by society or policy makers is the result of an undesired impact and can

affect any part of the chain between driving forces and impacts. An example of a

response related to driving forces is a policy to change mode of transportation, e.g from

private (cars) to public (trains), while an example of a response related to pressures is a

regulation concerning permissible SO2 levels in flue gases.

Understanding the full cycle of Driving Forces, Pressures, Status, Impacts and

Responses (DPSIR) within a highly complex environmental compartment like soil is a

challenging task for soil science. The development of effective soil protection strategies

requires a strong scientific knowledge of soil functioning, processes and properties.

Figure 2.2 illustrates how DPSIR can be used to link together the different data sets

collected by the land degradation assessment. It will often be logical to move from step 1

through to 5 when structuring the reporting.

2.1.2. Ecosystem services framework

The Ecosystem Services (ES) Framework focuses on the ecosystem services. This

framework encourages the assessment team to think broadly about the range and scale

of impacts of land degradation and sustainable land management. Some impacts are

easy to quantify, others not; some are felt locally and very differently according to the

socio-economic status of the land-user, others are felt nationally or globally.


Supporting services differ from provisioning, regulating, and cultural services in that

their impacts on people are often indirect or occur over a relatively long time period,

whereas changes in the other categories have relatively direct and short-term impacts

on people.

Figure 2.2. Linking and synthesizing the land degradation assessment using the DPSIR

framework

Land degradation is often a consequence of over-exploitation or poor management of one

or more provisioning service (e.g. food production, water extraction for irrigation etc.),

impacting negatively on key supporting and regulating services such as water

regulation or nutrient cycling. Negative impacts in these areas can then undermine the

provisioning services on which the land-users rely.

Trade-offs and potential conflicts between services or groups of services are common.

The most important challenges in decision-making about ecosystems can be around

understanding these trade-offs and using this understanding to improve the decision

making process. For example,

− increasing the flow of one service from a system, such as increasing the provision of

timber to meet demand, may decrease the stock or flow from other services, such as

carbon sequestration, water quality regulation or the provision of habitat.


− the short term increase in the flow of a provisioning service locally may have a

negative impact on services and flows that are less easy to quantify or that are

valued more at the global rather than local scale (e.g. C sequestration or

biodiversity).

− over-exploitation of a provisioning service may lead to demand exceeding the ability

of the system (via supporting/regulating services) to maintain key stocks (e.g.

nutrients, water, vegetation).

All of these impacts should be considered when trying to improve the management of

the system.

Cultural services are difficult to assess quantitatively. Qualitative information on some

of these is generated from community discussions and land user interviews. It is easy to

undervalue cultural services, however, and studies have shown that these are often

highly valued locally and they can be adversely affected by land degradation or

improved by SLM.

The ES framework is brought in more during the synthesis and analysis of findings of

the land degradation assessment. In most cases those responsible for the analysis and

assessment output will need to infer various impacts of land degradation on ecosystem

services rather than measuring them directly.

2.1.3. Sustainable livelihoods approach

The Sustainable Livelihoods framework or approach is used for understanding how

household livelihood systems interact with the natural, socio-economic and policy

environment. Impacts can be in both directions i.e. many pressures leading to land

degradation arise from the activities of land-users and LD/SLM causes impacts on land-

users‟ livelihoods. In this assessment the SL approach is used to help understand both:

− the drivers and pressures leading to LD/SLM and

− the impacts of LD/SLM on people.

A relatively simple example of this approach is shown in Figure 2.3.


The platform or core of a household‟s livelihood is its assets, classified into five classes

and denoted by a pentagon. Both the vulnerability context (on the left hand section of

Figure 2.3 and policies, institutions and processes affect the access people have to key

assets and what they can do with them. The livelihood strategies of different individuals

and categories of households are shaped by their asset base and by the vulnerability and

institutional context in which they live. When tracking back from LD/SLM pressures to

driving forces it is often in these two contextual areas (vulnerability and institutional)

that the driving forces are found.

Figure 2.3. The Basic Livelihoods Framework (Source: Ellis & Allison, 2004)

Five concepts are crucial for understanding the linkages within the framework (from left

to right in the diagram):

− the vulnerability context

− livelihood assets

− institutions

− livelihood strategies

− livelihood outcomes

2.1.3.1. Vulnerability context

Vulnerability context (or “risk exposure”) comprises cycles (e.g. seasonality), trends and

shocks that are beyond the household‟s control. An understanding of how people succeed

or fail in sustaining their livelihoods in the face of shocks, trends and seasonality can

help with the design of policies and interventions to assist peoples‟ existing coping and

adaptive strategies.


2.1.3.2. Livelihoods assets

Livelihood assets refer to the resource base of the community and of different categories

of households. The pentagon in Figure 2.3 represents the different types of assets

available to local people – human (H), natural (N), financial (F), physical (P) and social

(S). These assets are owned, controlled, claimed, or by some other means accessed by the

household. The physical capital or assets are key in determining livelihood activities and

the quality of land/natural resources management. Some assets that open up

opportunities for people are: credit, education, labour, secure land tenure, rights to use

natural resources e.g. harvesting fuelwood, road access to market. It is not just access to

assets that is important; the ability to use the assets productively and sustainably is

often determined by the vulnerability and institutional context.

2.1.3.3. Institutional context

Institutional context of people's livelihoods. This includes formal and informal policies

and regulations; social relations, markets and organisations. In many cases the main

obstacles to progress or opportunities for change are in this area and the SL approach

encourages us to ask important questions about these.

2.1.3.4. Livelihood strategies

Livelihood strategies. Most people are trying to follow some kind of strategy in terms of

their lives and household wellbeing and the SL approach encourages attempts to

characterize these, with a focus on what people are already doing or trying to do. In

dryland areas these strategies are often quite diversified, comprising a mix of crop

cultivation, livestock and off-farm work (often seasonal).

2.1.3.5. Outcomes

A livelihood is sustainable if people are able to maintain or improve their standard of

living related to well-being and income or other human development goals, reduce their

vulnerability to external shocks and trends, and ensure their activities are compatible

with maintaining the natural resource base – their land resources.


2.2. SOIL QUALITY INDICATORS

Soil quality is the ability of soil to provide ecosystem and society services through its

capacities to perform its functions and respond to external influence. Soil degradation

processes lead to a reduction in soil quality.

Soil functions are associated with certain biological, chemical and physical soil processes

such as the maintenance of plant available nutrients in the soil solution, the leaching of

pollutants through the soil to groundwater, erosion, .... In many cases, the rates of these

processes or specific properties may be difficult to measure directly, but we can readily

measure specific soil properties that are indicative of these rates.

2.2.1. Definition

Soil quality indicators can be defined as measurable soil properties that influence the

capacity of the soil to perform a specific function. Trends with time of these soil quality

indicators provide information on the severity and extent of soil degradation.

Physical indicators. These are principally concerned with the physical arrangement of

the solid particles and pores, and include texture, bulk density, porosity, aggregate

strength and stability, soil crusting, soil compaction and topsoil strength.

Chemical indicators. This list of potential soil attributes is very large and the final

selection will depend upon the function under consideration. Attributes include pH,

salinity, aeration status, organic matter content, cation exchange capacity, status of

plant nutrients, concentrations of potentially toxic elements and possibly the most

important attribute, the capacity of the soil to buffer against change.

Biological indicators. Biological attributes may be very dynamic and exceptionally

sensitive to changes in soil conditions, which is why there is often a preference for

biological attributes for short-term evaluations. Attributes that might be measured

include the populations of micro-, meso-, and macro-organisms, respiration rate or other

indicators of microbial activity, and more detailed characterisation of organic matter.

Visible indicators. It is often the observation of visible attributes that brings land

degradation to our attention and causes public awareness and alarm, but in many

respects, when there is visible evidence of the decline in soil quality, the process of


decline may have proceeded too far and the chance of restoring the quality may have

been lost. The visible attributes include evidence of erosion in the form of rills and

exposure of subsoil, surface ponding of water, surface run-off and poor plant growth.

A particular land use or management involves several soil functions; each function may

involve several processes; and each process may be associated with several biological,

chemical and physical indicator properties. Therefore the number of potential soil

quality indicators may be a dozen or more. Each of these measurements must be

interpreted with regard to implications for the soil functions under consideration.

For example, if the bulk density is measured as 1.5 Mg/m³, one must ask “is this too

high, too low or just right?” to facilitate the functions regulating water, providing

habitat, ...For some indicators high values may be more desirable (OM), for others lower

values are better (BD) and still for others (pH) there might be an optimum values, above

and below which soil function suffers.

2.2.2. Characteristics of good indicators

The type and number of indicators depends on the scale of the evaluation and the soil

functions of interest. In the literature some criteria, specifically developed for the

selection of soil quality indicators are reported:

(1) The indicator set needs to integrate chemical, physical and biological soil properties

relevant to the envisaged land use. Indicator groups must be sufficiently diverse to

represent the chemical, biological and physical properties and processes of the

complex soil system.

(2) There is an unambiguous relationship between an indicator and the soil function

being assessed. Significant indicators are meaningful to the problem under

consideration, i.e. they must provide relevant information with respect to the key

issue.

(3) They need to be sensitive to changes in management (early warning indicators).

Some soil quality indicator properties are much more susceptible to change by soil

management than are others (Table 2.1).

Properties such as soil texture, mineralogy, steepness of slope, and stoniness are

inherent characteristics of the soil and are not subject to change through land


management practices. These properties are important in determining the most

appropriate system to use.

At the other extreme are properties that may be subject to almost daily control so

that their effect on soil quality is immediate. Examples are the soil water content as

affected by irrigation and rainfall, and levels of plant-available nutrients that

change rapidly as chemical fertilisers are applied. Also soil density can be strongly

reduced by a single pass of tillage or a heavy vehicle. These properties are significant

to soil function, but their use in quality assessment is problematic since they can

change from day to day.

Table 2.1. Different types of soil quality indicators with respect to sensitivity to change

Ephemeral Intermediate Permanent

changes within days or routinely

managed

subject to management of

several years

inherent to profile or site

water content

field soil respiration

pH

mineral N

available K

available P

bulk density

aggregation

microbial biomass

basal respiration

specific respiration coefficient

organic matter content

soil depth

slope

climate

restrictive layers

texture

stoniness

mineralogy

Intermediate between these two extremes we find properties that are subject to

change only through long-term management efforts. Soil organic matter content,

along with microbial biomass and soil aggregation, are examples of this intermediate

class of soil quality indicators. These properties are important because of their major

influence on soil processes such as water and air movement, soil erosion and the

generation of biodiversity. But they can be developed only if we have an

understanding of the complex processes that generate them.

(4) They must be relevant and suited to the level of detail that is envisaged and take

into account the spatial and temporal variability of the soil quality. A frequently

ignored consideration in many attempts to evaluate soil quality is the scale at which

the evaluation is to be undertaken.

The optimum time and location for observing or sampling soil quality indicators

depends on the function for which the assessment is being made. The frequency of

the measurement also varies according to climate and land use.


(5) The methodology should be reliable, repeatable, and analytically sound. One should

select a fixed, well-defined method to determine the indicator value that is

maximally independent on external influencing factors. The methodological

approach to calculate the indicator has to be technically and scientifically sound,

based on international standards and international consensus about its validity and

its suitability for linkage to economic models, forecasting and information systems.

(6) They are preferably already available in existing databases and the proposed

methodology should be compatible with sampling and monitoring. For wide

application of the indicators the complexity as well as the effort and costs of data

gathering and calculation of the indicator values should be acceptable for decision

makers. This criterion is linked strongly with data availability. In order to be

operational, indicators should be easily measurable and quantifiable.

(7) The indicators need to be easily usable for non-scientists as well as scientists. The

indicators should be generally understandable in order to facilitate communication of

results provided by indicators to the public and political decision-makers.

In summary, good indicators are Specific, Measurable, Achievable, Relevant and Time-

bound – or SMART.

To evaluate soil quality these indicators can be assessed at one point in time or

monitored over time to establish trends.

2.2.3. Selection of indicators

Many different frameworks have been defined, and there is not yet a universally

accepted methodology. A recent concept is based on a functional approach. Based on the

envisaged land use(s), one defines which functions the soil needs to perform to be suited

for all these (this) land use(s). Then one identifies the different soil processes that are

important for the realisation of those different functions.

Identification of relevant soil processes to any soil function can be complicated due to the

spatial heterogeneity of soil and a lack of understanding of many of the processes,

particularly biological, required to sustain a particular function. Figure 2.4 illustrates

how soil process identification can be facilitated in a structured way by focussing on the

potential threats to which the soil function under consideration is exposed.


Finally, one defines which soil parameters play an important role in these soil processes

and as such can be used as indicators for soil quality (Fig. 2.4). Present day soil quality

assessments, in addition, also pay attention to the sampling strategies and

methodologies available for measuring these indicators.

Figure 2.4. Filtering and buffering function: identification of related soil threats and

affected soil processes

Collectively these indicators form a minimum dataset that can be used to determine how

well critical soil functions associated with each management goal are being performed.

Several minimum datasets for soil quality indicators have been proposed. This set is

realised through statistical analysis (ANOVA analysis, linear and multiple regression

analysis, and factor analysis) on a more elaborated set of indicators whereby those that

explain most of the variation in soil quality are retained. The indicators of the original

set proved earlier, based on a descriptive statistical analysis, that they are influenced by

buffering

capacity

run-off & erosion

m itigation

aeration water

supply

chem ical

transform ations

b io log ica l

breakdown

FILTER & B U FFER

decreased C

sequestra tion

decreased

storage

capacity

decreased

b io log ica l

activ ity

decrease in

chem ica l &

b io log ica l

transform ations

runoff causing

po llu tion o f

surface w ater

reduced

storage

capacity for

po llu tants

restricted

buffering

capacity

po llu tion

entering food

cha in and

ground w ater

decrease in

transform ations

R educed

transform ations

due to O 2

stress

R estricted

in filtra tion

increasing run-

off & erosion

A ffects

chem ica l &

b io log ica l

transform ations

O 2 o r H 2O

supply for so il

life d isturbed

R eduction in

b io log ica l

transform ations

DECLINE

O RG ANIC

M ATTER

ERO SIO N CO NTA -

M INATIO N

CO M PACTIO N D ISTURBED

W ATER

CYCLE

DECLINE

B IO DIVERSITY


changes in management. An example of such a minimum indicator set is given in Table

2.2.

Table 2.2. A proposed minimum dataset of physical, chemical and biological indicators

for soil quality

Indicator Rationale for selection

Physical

Texture

Topsoil/rooting depth

Infiltration

Bulk density

Water holding capacity

Aggregation

Retention and transport of water & chemicals

Estimate rooting volume for production & erosion

Runoff, leaching & erosion potential

Plant root penetration & porosity

Water retention, transport & erosion

Soil structure, erosion resistance & early indicator of

management effects

Chemical

Soil organic matter

pH

Electrical conductivity

Extractable N, P & K

Different forms of N

Defines soil fertility & structure, pesticide & water retention

Nutrient availability, pesticide adsorption & mobility

Defines crop growth, soil structure & water infiltration

Capacity to support crop growth

Leaching potential & mineralization/immobilisation

Suspected pollutants

Biological

Microbial biomass C & N

Potentially mineralisable N

Soil respiration

Plant quality & human and animal health

Microbial catalytic potential & repository for C & N

Soil productivity & N supplying potential

Biological activity, early warning management effects on

organic matter

2.2.4. Interpretation of indicators

Each indicator is then scored, often using ranges established by the soil‟s inherent

capability to set the boundaries and shape of the scoring function. Setting these

appropriate critical limits determines the accuracy and reliability of the soil quality

assessment.

Optimum values can be obtained from the soils of undisturbed ecosystems, where soil

functioning is at its maximum potential. Thresholds for each soil quality indicator can

be set based on the range of values measured in natural ecosystems or in best-managed

systems and on critical values for soil functioning reported in the literature.

Indicator scoring can then be accomplished in a variety of ways depending on the soil

function under consideration. Scoring can be performed using a fixed number of ordinal

values, ranging for instance from 1 to 5, with increasing performance; or one can use

different, continuous scoring functions:


linear scoring functions

Y x s t s (eq. 1)

Y 1 x s t s (eq. 2)

where, Y is the linear score, x the soil property value, s and t are the lower and upper

threshold values. Equation 1 is used for “More is better” scoring function, equation 2 for

“Less is better” and a combination of both for “Optimum” scoring function.

non-linear scoring functions

b x AY 1 1 e

where, x is the soil property value, A the baseline or value of the soil property where the

score equals 0.5 and b is the slope. Using the equation, three types of standardized

scoring functions were generated (1) „More is better‟, (2) „Less is better‟ and (3)

„Optimum‟.

Figure 2.5 gives an overview of different linear and non-linear scoring functions used to

evaluate soil quality for the biomass production function.

Figure 2.5. Linear and non-linear scoring functions to convert soil quality indicators in

index values to evaluate soil quality for the biomass production function.

For some management goals the same indicator may be included under different

functions and even score in different ways.


Amoeba (radar) diagrams prove useful in providing a clear overview of the different soil

quality indicator values recorded for each of the soil processes or functions under

consideration (Figure 2.6).

The complexity of co-evaluating the status of many chemical, physical, and biological

parameters however, has prompted researchers to integrate multiple indicators into a

soil quality index. In that case, the scoring functions need to convert (normalised) the

original parameter values, measured in many different units, into values ranging

between 0 and 1.

Figure 2.6 Radar diagram of soil quality evaluating the effect of different agricultural

management strategies

References

Ellis F., and Allison EH. 2004. Livelihood Diversification and Natural Resource

Access. Working Paper No.9, Livelihood Support Programme, FAO, Rome.

Soil Degradation | Soil structural degradation 32

3. Soil structural degradation

Soil physical properties profoundly influence how soils function in an ecosystem (Table

3.1) and how they can best be managed. Success or failure of both agricultural and

engineering projects often hinges on the physical properties of the soil used. The

occurrence and growth of many plant species closely relate to soil physical properties, as

is the movement over and through soils of water and its dissolved nutrients and

chemical pollutants (Brady and Weil, 2008). Thus, sustainable management of natural

resources requires optimization of soil structural characteristics.

Table 3.1. Soil Physical Properties and Processes That Affect Agricultural, Engineering,

and Environmental Soil Functions (Lal and Shukla, 2004)

Process Properties Soil functions

Biomass productivity (agricultural functions) 1. Compaction Bulk density, porosity, particle size

distribution, soil structure Root growth, water and nutrient uptake by plants

2. Erosion Structural stability, erodibility, particle size, infiltration and hydraulic conductivity, transportability, rillability

Root growth, water and nutrient uptake, aeration

3. Water movement Hydraulic conductivity, pore size distribution, tortuosity

Water availability to plants, chemical transport

4. Aeration Porosity, pore size distribution, soil structure, concentration gradient, diffusion coefficient

Root growth and development, soil and plant respiration

5. Heat transfer Thermal conductivity, soil moisture content

Root growth, water and nutrient uptake, microbial activity

Engineering functions 1. Sedimentation Particle size distribution, dispersibility

Filtration, water quality

2. Subsidence Soil strength, soil water content, porosity

Bearing capacity, trafficability

3. Water movement Hydraulic conductivity, porosity Seepage, waste disposal, drainage

4. Compaction Soil strength, compactability, texture Foundation strength Environmental functions 1. Absorption/adsorption Particle size distribution, surface area,

charge density Filtration, water quality regulation, waste disposal

2. Diffusion/aeration Total and aeration porosity, tortuosity, concentration gradient

Gaseous emission from soil to the atmosphere

The arrangement and placement of soil particles determines the response of soil to

exogenous stresses such as tillage, traffic, and raindrop impact. This arrangement of soil

particles, called “soil structure”, is a dynamic, complex property that varies in time and

space, and reflects the effect of numerous interacting factors (Lal and Shukla, 2004).


The first step to the development of soil structure is the process of aggregation, i.e. the

formation of soil aggregates or organo-mineral complexes from primary particles and

humic and other bonding substances (Figure 3.1).

Figure 3.1. Formation of soil aggregates (Brady and Weil, 2008)

3.1. PROCESS OF AGGREGATE FORMATION

Both biological and physico-chemical processes are involved in the formation of soil

aggregates. Physico-chemical processes of aggregate formation are associated mainly

with clays and, hence, tend to be of greater importance in fine-textured soils. In sandy

soils, aggregation is almost entirely dependent on biological processes.

3.1.1. Physico-chemical processes

The most important physico-chemical processes are flocculation, and swelling and

shrinking of clay masses.

Flocculation (mutual attraction of colloidal particles – e.g. clay and organic molecules)

occurs when charged colloidal particles collide with one another and adhere after the

collision due to favourable conditions in the electrical double layer. The reverse of this

process is called dispersion. The process of aggregation is therefore closely linked to


the behaviour of the diffuse double layer and its response to ionic composition of the

bulk solution.

3.1.1.1. Flocculation of clays and the role of adsorbed cations

Except in very sandy soils that are almost devoid of clay, aggregation begins with the

flocculation of clay particles into microscopic clumps of floccules. If 2 clay platelets come

close enough to each other, positively charged ions compressed in a layer between them

will attract the negative charges on both platelets, thus serving as bridges to hold the

platelets together. These processes lead to the formation of a small stack of parallel clay

platelets, termed a clay domain (Figure 3.2). Polyvalent cations also complex

hydrophobic humus molecules, allowing them to bind to clay surfaces, forming domains

that are more random in orientations. Clay/humus domains form bridges that bind to

each other and to fine silt particles, creating the smallest size groupings in the hierarchy

of soil aggregates. These domains, aided by the flocculating influence of polyvalent

cations and humus, provide much of the long-term stability for the smaller micro-

aggregates (Brady and Weil, 2008).

In certain highly weathered clayey soils, the cementing action of Fe-oxides and other

inorganic compounds produces very stable aggregates called pseudosand.

When monovalent cations, especially Na+ are prominent, as in some soils of arid and

semi-arid areas, the attractive forces are not able to overcome the natural repulsion of

one negatively charged clay platelet by another. Na+ ions cause an increase of the

electrical double layer for 2 reasons: (1) they have a large hydration shell of water and

(2) they have a lower charge than divalent cations. The clay platelets therefore cannot

approach closely enough the flocculate, so remain in dispersed condition that causes the

soil to become almost structureless (Brady and Weil, 2008) (Figure 3.2).

3.1.1.2. Volume changes in clayey materials

As a soil dries out and water is withdrawn, the platelets in clay domains move closer

together, causing the domains and, hence, the soil mass, to shrink in volume. As a soil

mass shrinks, cracks will open up along zones of weakness (Brady and Weil, 2008).

Plant roots also have a distinct drying effect as they take up soil moisture in their

immediate vicinity. Water uptake, especially by fibrous-rooted perennial grasses,

accentuates the physical aggregation processes associated with wetting and drying. This


effect is but one of many ways in which physical and biological soil processes interact

(Brady and Weil, 2008).

Freezing and thawing cycles have a similar effect, since the formation of ice crystals is a

drying process that also draws water out of clay domains. The swelling and shrinking

actions that accompany freeze-thaw and wet-dry cycles in soils create fissures and

pressures that alternately break apart large soil masses and compress soil particles into

defined structural peds (Brady and Weil, 2008).

Figure 3.2. Conceptual diagram illustrating how the type of cations present on the

exchange complex influences clay dispersion (adapted from Brady and Weil, 2008)

3.1.2. Activities of soil organisms

Among the biological processes of aggregation, the most prominent are (1) the burrowing

and moulding activities of soil animals, (2) the enmeshment of particles by sticky

networks of roots and fungal hyphae, and (3) the production of organic glues by

microorganisms, especially bacteria and fungi (Brady and Weil, 2008).

Aggregation is thus flocculation plus cementation with numerous agents that stabilize

and bind the floccules. Most common cementing agents include soil organic matter

(humus, microbial by-products, fungal hyphae), silicate clays, lime, and sesquioxides.

Structural stability is the ability of soil to retain its arrangement of solids and void

space when external forces are applied. The aggregate stability depends on the bonding

agents involved in cementing particles together.


3.2. FACTORS AFFECTING AGGREGATION

There are numerous factors that affect aggregation, including soil characteristics such

as texture, clay mineralogy, nature of exchangeable cations, quantity and quality of the

humus, as well as exogenous factors such as weather (drying and wetting, freezing and

thawing), biological soil processes (activity of soil fauna, root growth), land use and

management (Lal and Shukla, 2004).

These factors can either promote aggregate formation and stability, or induce soil

structural degradation. Physical soil degradation is mainly a result of soil structure

breakdown when soil aggregates are deformed by a force applied either externally from

the impact of rain or unsustainable land management practices, or internally from the

breaking out of trapped air when aggregates are flooded.

3.2.1. Impact of raindrops

Falling drops of water during heavy rains or sprinkler irrigation beat apart the

aggregates exposed at the soil surface. Mass, impact velocity and momentum (M = mv)

of the raindrops are the primary sources of energy that disrupt an aggregate. This is

expressed as the kinetic energy of impacting raindrops:

With m = mass of rain per unit area

v = impact velocity of rain drop

3.2.2. Flooding

The breakdown of soil aggregates when immersed in water into smaller sized micro-

aggregates is referred to as slaking. In some soils the dilution of salts by this water

stimulates the dispersion of clays.

Slaking, deflocculation, or dispersion of aggregates on rapid wetting or submersion in

water, is attributed to numerous factors including the effect of entrapped air,

predominance of Na+ on the exchange complex, and weak aggregate strength caused by

low level of soil organic matter content and weak ionic bonds.


3.2.3. Drying and wetting

Repeated cycles of drying and wetting play a major role in aggregation through

shrinking and swelling that lead to formation of aggregates. Swelling or rewetting leads

to reorientation of particles. Shrinking or drying leads to formation of cracks and

cementation. Rapid wetting breaks large clods into aggregates because of the effect of

entrapped air.

3.2.4. Freezing and thawing

Water expands on freezing and its impact on aggregation depends on the size,

distribution and duration of ice crystals. The in situ freezing of water in pores may lead

to a fracturing of the soil.

3.2.5. Biotic factors

Soil biota play an important role in aggregation and soil structure development (see

section 3.1.2).

3.2.6. Influence of Tillage

Effects of tillage on soil structure depend on the type, frequency, and timing of the

tillage operations.

Tillage can both promote and destroy aggregation. The antecedent soil moisture content

is an important parameter because it influences the dispersability of clay. If the soil is

not too wet or too dry, tillage can break large clods into natural aggregates, creating a

temporarily loose, porous condition conducive to the easy growth of young roots and the

emergence of tender seedlings. Tillage can also incorporate organic amendments into the

soil and kill weeds.

Over longer periods, however, tillage greatly hastens the oxidative loss of soil organic

matter from surface horizons, thus weakening soil aggregates. Tillage operations,

especially if carried out when the soil is wet, also tend to crush or smear soil aggregates,

resulting in loss of macroporosity, soil compaction and the creation of a puddled

condition (Figure 3.3).


Figure 3.3. Puddled soil (left) and well-granulated soil (right) (Source: USDA Natural

Resources Conservation Service in Brady and Weil, 2008)

3.2.7. Soil amendments

Addition of organic matter (compost, manure, sludge) has beneficial effects on soil

structure through formation of clay-organic complexes. Organic matter furthermore

provides the energy substrate for biological activity. During the aggregation process, soil

mineral particles (silts and fine sands) become coated and encrusted with bits of

decomposed plant residue and other organic materials. Complex organic polymers

resulting from decay chemically interact with particles of silicate clays and iron and

aluminium oxides to form bridges between individual soil particles, thereby binding

them together in water-stable aggregates.

The application of gypsum leads to improved aggregation of dispersed alkaline soils.

There exist also synthetic organic polymers (vinyl acetate) or soil conditioners or soil

stabilizers to improve soil structure. These synthetic polymers can be adsorbed by the

surface of clay particles and alter their relation to water and ions in the solution. The

polymer molecules can also link several clay particles through the formation of inter-

particle bonds that facilitate flocculation or stabilize an unstable arrangement.

3.3. ASSESSMENT OF SOIL STRUCTURE

Soil structure is a dynamic property and is difficult to characterize. It can furthermore

be characterized and assessed in different ways from different points of view:


− mechanistic approach: size, shape, arrangement and packing of particles into

aggregates/peds;

− functional approach: with focus on the soil-pore system, structure being defined

as the assemblage of aggregates and (intra-aggregate, inter-aggregate) voids; and

− engineering approach: related to functions of soil structure such as bearing

capacity, shear strength, slope stability, compressibility, permeability.

Figure 3.5 provides an overview of different techniques for the characterization of soil

aggregation.

Figure 3.5. Methods of assessment of aggregation (Lal and Shukla, 2004)

3.3.1. Laboratory methods

Laboratory methods of aggregate analysis can be broadly grouped into 3 categories: (1)

ease of dispersion, (2) assessment of aggregation and aggregate size distribution, and (3)

evaluation of aggregate strength.

3.3.1.1. Dispersion

A known quantity of air dry soil is poured into a beaker containing deionised or distilled

water. Quick wetting of aggregates leads to aggregate breakdown (slaking and

dispersion).

Assessment of soil aggregation

Field methods Laboratory methods

Raindrop impact Dry sieving Wet sieving

Equilibration

at different

humidity

Sample

pretreatment


Several indices have been developed to classify soils on the basis of their dispersion

characteristics:

Dispersion ratio

This index is a measure of the clay + silt fraction in dispersed rather than aggregated

condition:

With WDSilt = fraction of water-dispersible silt (%)

WDClay = fraction of water-dispersible clay (%)

Clay dispersion ratio

Likewise, the clay dispersion ratio can be calculated:

Clay flocculation index

The clay flocculation index is then given by:

The higher the CDR and DR the more the ability of the soil to disperse while the higher

the CFI the better aggregated the soil.

3.3.1.2. Aggregation and aggregate size distribution

These methods focus on the resistance soil solids offer to the mechanical abrasion

arising from the movement of particles relative to water or air.

Wet sieving is done to simulate erosion by water and stability to quick wetting. Dry

sieving is done to simulate aggregate resistance to wind erosion.

The dispersive energy per unit mass of soil is related to aggregate stability. Different

indices can be used to express the results of aggregate analysis by wet or dry sieving

(Table 3.2).


Table 3.2. Some commonly used indices to express results of aggregate analysis by wet or

dry sieving

3.3.1.3. Aggregate strength

Wet sieving uses a group of aggregates to determine aggregate stability. The simulated

raindrop technique to determine aggregate strength however tests aggregate stability on

individual aggregates and can therefore account for the heterogeneity of field aggregates

(Blanco and Lal, 2008).

In this method, an aggregate at predefined soil moisture content is subjected to

simulated raindrops of given size, weight and terminal velocity. The number of

raindrops needed to disintegrate the aggregate is recorded and converted to kinetic

energy.

3.3.2. Other indices

Pieri (1991) proposed the concept of critical level of soil organic matter concentration for

structural stability of tropical soils:

Based on the analysis of about 500 samples from semi-arid regions of West Africa, the

following limits of soil organic matter concentration for stabilization of soil structure

were proposed:


St ≤ 5% loss of soil structure and high susceptibility to erosion

5 < St < 7% unstable structure and risk of soil degradation

St> 9% stable soil structure

There are also other indices of soil structure, based on soil properties other than

aggregation (Table 3.3). Soil structure degradation can also be assessed indirectly by

monitoring changes in bulk density, permeability and infiltration capacity.

Table 3.3. Some indices of soil structure based on properties other than aggregates (Lal

and Shukla, 2004)

Soil property Index of soil structure

Porosity Total porosity

Pore size distribution

Aeration porosity Soil strength Penetration resistance

Modulus of rupture

Relative density Water retention Plant available water capacity

Least limiting water range

Water transmission Infiltration capacity

Profile hydraulic conductivity Soil drainage

Aeration Oxygen diffusion rate

Diffusion coefficient

3.4. IMPACT OF SOIL STRUCTURAL DEGRADATION

Structural degradation and decline in aggregation of structured soils lead to soil

dispersion, compaction, crusting, formation of pans, accelerated soil erosion, and

emission of CO2 (soil slaking and dispersion lead to exposure of C, otherwise tied or

locked within the aggregates, enhancing its microbial decomposition and oxidation) and

other greenhouse gases into the atmosphere (Figure 3.4).

These ramifications can have a drastic impact on plant growth and net primary

productivity, hydrologic cycle, water quality, elemental cycling, and emission of trace

gases. The interactive effects of soil processes, soil properties, plant growth, and

environment can adversely impact ecosystem functions (Lal & Shukla, 2004).

Structural resilience refers to the ability of soil structure to recover following a major

disruption in the aggregation process caused by changes in land use, cultivation or soil

management.


Structural resilience depends on numerous factors including soil organic matter content,

clay mineralogy, wettability characteristics, and biotic factors. Numerous soils exhibit

self-mulching properties (e.g. self-mulching Vertisols). In other soils, aggregation is

restored only when taken out of cultivation and put under a restorative fallow.

Figure 3.4. Impact of the decline in soil structure on soil physical quality

References

Blanco H. and Lal R. 2008. Principles of Soil Conservation and Management. Springer-

Verlag.

Brady NC., and Weil RR. 2008. The nature and properties of soils. 14th Edition. Pearson

Education, New Jersey.

Lal R., and Shukla MK. 2004. Principles of Soil Physics. Marcel Decker, New York.

Pieri C. 1991. Fertility of soils: A future for farming in the West African savannah.

Springer-Verlag.

Soil Degradation | Soil compaction 44

4. Soil compaction

When a soil is subjected to mechanical stress, its volume will decrease and its bulk

density will increase. Under saturated conditions, this increase in bulk density is only

possible through drainage of water. In that case, one refers to consolidation. In

unsaturated soils, this compression process is indicated as compaction.

Soil compaction is the compression of soil by external forces, leading to an increase in

bulk density and a decrease in the total pore volume. It thus comprises the

rearrangement of soil aggregates and/or particles in a denser way in comparison with

the arrangement of similar but not compacted soil.

Soil compressibility refers to the resistance of soil to a change in volume, whereas soil

compactibility refers to the resistance to changes in bulk density.

4.1. SOIL COMPACTION PROCESS

The response of the soil to mechanical stress strongly depends on the soil strength. Soil

strength is the resistance that has to be overcome to obtain a known soil deformation. It

refers to the capacity of a soil to resist, withstand, or endure an applied stress without

experiencing failure (rupture, fragmentation, flow), and thus reflects the capacity of

soils to bear loads. Consequently, soil strength is directly related to the likelihood of a

soil to compact.

4.1.1. Soil strength

In agriculture, soil strength has applications to root growth, seedling emergence,

aggregate stability, erodibility and erosion, compaction and compactibility, and draft

requirements for plowing. Positive effects are those related to soil trafficability and

bearing capacity, and resistance to compactive and erosive forces. Negative effects are

those due to high draft power requirement, poor root growth, low seedling emergence,

and poor crop stand.

Soil strength may be of 2 types: (1) resistance to volumetric compression or structural

strength, and (2) resistance to linear deformation or shear strength.


4.1.1.1. Soil structural strength

The soil structural strength determines the maximum level of vertical loads that can be

applied before plastic deformation of the soil. Some soils are sufficiently strong to resist

all applied loads, whereas others are so weak that they compact upon small loads. If the

structural strength is exceeded, the inter-aggregate pores disappear and the soil

compacts. A good indicator of the structural soil strength is the precompression stress

(see further).

4.1.1.2. Soil shear strength

The soil shear strength is a measure for the maximum shear loads (lateral pressure)

that a soil can bear before it is deformed. In case the shear strength is exceeded, not

only the inter-aggregate pores, but also the intra-aggregate pores reduce, leading to a

destruction of soil structure and a decrease of the bearing capacity. The shear strength

of a soil is characterised by its cohesion and internal friction. Measurement of the

cohesion and angle of internal friction is laborious and expensive and data for these

properties are not common in research.

4.1.2. Factors affecting soil strength

The degree of deformation and rearrangement of the particles depends on soil structure

and aggregation, and on the extent to which soil particles can change position by rolling

or sliding. Particle movement under stress is affected by (1) frictional forces between

particles and (2) interparticle bonds (mineral-mineral, mineral-organic-mineral, …).

Soil texture, clay mineralogy, soil organic matter content affect soil strength through

changes in aggregation, bulk density, and moisture content. Clayey soils have more

strength and cohesiveness than sandy soils. This high cohesion of clay soils results in a

high shear strength which prevents shear failure of clayey subsoils. Displacement and

rearrangement of solid particles to a closer packing becomes more difficult as the bulk

density increases. Soil strength increases with a decrease in soil moisture content. Soil

organic matter influences soil strength through its effects on aggregation and porosity.

4.1.3. Propagation of external stress with depth

Detailed assessments of soil compaction study and model the propagation of the

externally applied stresses with depth. Analytical models (e.g. SOCOMO) can be used to

calculate the soil stresses in the subsoil exerted by a series of relevant wheel-load/tyre-

dimensions/tyre-inflation combinations and compare them with the strength of the soil.


4.2. SUSCEPTIBILITY TO COMPACTION

Soil susceptibility to compaction is the probability that soil becomes compacted when

exposed to compaction risk.

Several factors influence the intrinsic susceptibility of soils to compaction:

4.2.1.1. Soil texture

Well graded soils can be compacted to a higher bulk density than poorly graded soils

since the smaller particles can easily fill up spaces in between larger particles. Texture

also influences the resilience of soils to compaction (see further).

4.2.1.2. Soil packing density

The packing density can be readily determined from:

PD = BD + 0.009C

Where :

PD is the packing density (g cm-3 or t m-3)

BD is the bulk density (g cm-3 or t m-3)

C is the clay content (%, w w-1)

Packing density has proved to be a very useful parameter for spatial interpretations

that require a measure of the compactive state of soils and of its susceptibility to

compaction.

Three classes of packing density are recognized:

− low < 1.40 g cm-3;

− medium 1.40 to 1.75 g cm-3; and

− high > 1.75 g cm-3.

4.2.1.3. Soil structure and organic carbon content

Soils rich in organic matter are characterised by a more stable soil structure and

consequently are more resistant to compaction. Yet, on the other hand, high organic

carbon contents reduce the bulk density of these soils, leading to a lower bearing

capacity. In these cases, the susceptibility to soil compaction will be larger than when


applied to soils with weak soil structure, low organic carbon contents and high bulk

densities (probably originating from previous compaction).

Table 4.1 summarises some soil susceptibility assessment to compaction, based on soil

texture and packing density.

Table 4.1. Soil susceptibility assessment to compaction, based on soil texture and packing

density (Jones et al., 2002)

FAO texture class Packing density (t m-3)

low (< 1.40 t m-3) medium (1.40 to 1.75 t

m-3)

high (> 1.75 t m-3)

Coarse very high high medium1

Medium high medium medium

Medium fine medium to high medium low3

Fine medium2 low4 low3

Very fine medium2 low4 low3

organic very high high 1 except for naturally compacted or cemented coarse materials that have a very low susceptibility 2 found in alluvial soils with BD of 0.8 – 1.0 t m-3, or in topsoils with > 5% organic carbon 3 these soils are already compact 4 Fluvisols in this category have a moderate susceptibility

Besides these intrinsic factors, the susceptibility to soil compaction is also strongly

dependent on the soil moisture content.

4.2.1.4. Soil moisture content

A dry soil in which there is some friction between soil particles and aggregates is less

susceptible to compaction. In moist soils however, the water acts as a lubricant between

particles, reducing its structural load and resulting in a higher susceptibly to

compaction. With increasing soil moisture contents, the pores will finally be filled

mainly with water, which increases the structural load (Figure 5.1).

Figure 4.1 only reflects the impact of soil moisture content on changes in structural load.

However, in wet soils, soil deformation can also occur as the soil shear stress will be

exceeded!

These effects of soil moisture content are more pronounced in heavy textured soils than

in coarse textured or cohesionless soils.


Figure 4.1. Impact of soil moisture content on susceptibility for soil compaction

(~exceedence of soil structural load)

4.3. RESILIENCE TO COMPACTION

Through swelling and shrinking processes with changes in soil moisture content (and

freezing and thawing processes in temperate zones), compacted soils with >10% clay can

decompact to some extent, whereas this is not the case in compacted sandy soils (lower

clay contents, lower water holding capacity). Furthermore, sandy soils are also generally

characterised by a lower biological activity. Consequently, once compacted, sandy soils

recover only slowly.

A distinction needs also to be made between compaction of the topsoil (upper 20 - 35 cm

of the soil profile) and subsoil.

Topsoil compaction considers the compaction of the A horizon (the ploughed layer under

arable and the humus enriched horizon under other land uses). In most cases the topsoil

has greater organic matter content, contains many more roots and supports a much

greater biological activity than the subsoil. Also, physical processes such as wetting,

drying, freezing and thawing are more intense in the topsoil than in the subsoil.

Consequently, natural loosening processes are much more active and stronger in the

topsoil than in the subsoil. This makes topsoil more resilient to compaction than the

subsoil. Topsoil compaction can be easily removed through normal soil management

practices. The resilience of topsoils to soil compaction is especially important in forest

soils, and less so in cropped fields that are regularly tilled.


The subsoil can include a plough pan in its upper part. Such a pan layer is caused by

tractor tyres driving directly on the subsoil during ploughing or by heavy wheel loads

that transmit the pressure through the topsoil into the subsoil. The pan layer is

characterised by greatly reduced rootability and permeability for water and oxygen. It

acts as a „bottleneck‟ for the functioning of the subsoil. Consequently, subsoil compaction

can persist if no specific remediation measures are taken.

4.4. CAUSES OF SOIL COMPACTION

Soil compaction can have both natural as well as human-induced causes. Soil sealing

and crusting under the impact of raindrops can be a cause of soil compaction in years of

high rainfall intensities. Yet, generally, soil compaction in cropland and forests is often

caused by human action. Also other activities such as recreation and tourism might lead

to compaction problems in nature reserves or recreation areas.

The main threats in agricultural and forest land are related to the use of heavy

machinery. The ground pressure exerted by wheeled or tracked vehicles is a direct cause

of compaction and structural degradation.

Forest soils, especially during works with wood, like skidding after cutting down the

trees, are exposed to crossings by heavy machinery in any type of weather and under

any soil moisture content condition.

Heavy machinery is also used during seeding, tilling and harvesting operations in large-

scale farms. Ploughing of soil to the same depth for several years accelerates plough pan

genesis even if done in proper soil moisture content.

The degree of soil compaction strongly differs with the type of machinery (wheel load,

tyre dimension and tyre inflation) which depends on the farming/exploitation system, as

well as on the moisture content when the soil is worked. Also the type of soil

management (tillage) and the number of passes play a role. The resulting compaction is

often found at 10 to 60 cm depth.

In grazing land, trampling animals can cause topsoil compaction problems, especially at

high soil moisture content. Stock density and type of animals can give an indication of

possible causes of soil compaction.


4.5. CONSEQUENCES OF SOIL COMPACTION

Soil compaction can be defined as: “The densification and distortion of soil by which total

and air-filled porosity are reduced, causing a deterioration or loss of one or more soil

functions".

The major part of the decrease in pore volume, as a result of compaction, is the

reduction in macro-pores, and the resulting soil deformation strongly affects pore

continuity. Both these conditions reduce the ability of the soil to conduct water and air,

causing anaerobic conditions and reduced infiltration rates and capacity. The reduced

infiltration increases surface runoff and leads to more erosion.

4.5.1. Influence on soil physical properties

4.5.1.1. Change in pore volume and distribution

The total pore volume decreases upon compaction; leads to a relative increase in micro-

pores and to a relative decrease in macro-pores. These changes affect water movement,

oxygen diffusion and soil chemistry.

4.5.1.2. Destruction of soil aggregates and change in soil structure

Upon compaction, first the intra-aggregate pores are reduced, followed by a more dense

re-organisation of the soil aggregates and even reductions in the inter-aggregate pores.

This can finally lead to a platy or massive soil structure.

4.5.1.3. Increase in bulk density and soil strength

Soil compaction leads to an increase in bulk density. The dry bulk density consequently

is often used to characterise the soil compaction status. However, comparison of bulk

density values among different soil types is difficult (Figure 4.2). Compaction also

changes the mechanical properties of a soil: the soil strength increases.

4.5.1.4. Reduced infiltration rate and drainage

Changes in pore volume and pore continuity lead to a reduction in the infiltration rate

and hydraulic conductivity. In compacted topsoil layers, soil pores are mainly oriented

horizontally (platy structure), often leading to ponding water at the surface. With

compaction of the subsoil, leading to a reduced internal drainage, this problem of

ponding water will even be increased. This also increases surface run-off and risk for soil


erosion. Even on slightly sloping terrain (2-4% slope gradient), surface run-off due to the

presence of a compacted soil can cause significant soil erosion losses (> 20 ton/ha.year).

Figure 4.2. Bulk densities typical for a variety of soils and soil materials (Brady and

Weil, 2008)

4.5.2. Influence on soil chemical properties

Changes in pore size distribution change the moisture and air regime of soils, affecting

organic matter decomposition and nitrogen mineralisation. The higher the soil moisture

content of the compacted soils, the larger the negative impact.

4.5.3. Influence on soil biological properties

Soil biota - micro-, meso- and macro-organisms - are influenced by the change of soil

properties due to compaction. More specifically they are sensitive to changes in moisture

and air regime, amount and redistribution of available nutrients, pH and Eh, and soil

density.

The macro-fauna (e.g. earthworms) are particularly sensitive to reduced macro-porosity.

Nematodes generally show increased activity around plant roots, causing plant damage

and reducing yields. Soil compaction also affects the microbial activity through changes

in the air management, leading to anaerobic conditions and processes (e.g. microbial

denitrification).


4.5.4. Influence on soil functions

The main soil functions affected by soil compaction are :

− Food and biomass production (soil productivity for agricultural and forest cropping);

− Environmental interaction (water filtration and storage capacity);

− Physical and cultural heritage (compaction, and its alleviation by subsoiling, have

been shown to destroy cultural artefacts in soil).

4.6. ASSESSING SOIL COMPACTION

4.6.1. Field observations

Possible symptoms of soil compaction that can be readily observed in the field are:

− Former wheel tracks becoming visible in the growing crop

− Ponding soil water or excessive erosion

− Need for higher drafting power to work the field (due to increased soil strength)

− Misshapen roots

− Dwarf growth of the crops/plants; early drought stress

− Yield decline

It is however clear that these symptoms are not exclusively related to soil compaction

and can have also other causes.

4.6.2. Soil profile descriptions

In field experiments on soil compaction, the intensity of soil compaction can be studied

through morphological analysis of the soil profiles (structure and rooting). The ground

pressure exerted by wheeled or tracked vehicles is a direct cause of compaction and

structural degradation. This is a direct and complete determination of compaction and

structural degradation and the effect on rooting, biota, structure, and air and water

regime in the whole profile.

Such visual assessments of soil structure and rooting patterns need experienced

researchers. Interpretation is furthermore difficult because seasonal, climatic and

drainage conditions can have a large impact on rooting and other visible aspects of

compaction.


4.6.3. Direct laboratory methods

The soil compaction process leads to an increase in bulk density and soil strength, both

soil properties can be measured. The degree or severity of soil compaction is expressed

in terms of soil bulk density, total porosity, aeration porosity, and void ratio.

4.6.3.1. Bulk density and packing density

Soil bulk density (BD) refers to the density of the soil as a whole, including solid

particles as well as pores. It is determined on oven-dry undisturbed samples.

Undisturbed soil samples are taken in the field using Kopecky rings. Interpretation of

the obtained values strongly depends on the soil type (Figure 5.2).

If both BD and clay content are know, alternatively, the packing density can be

calculated, providing a measure of the apparent density of soil. Soils with high packing

density (> 1.75 g cm-3) can be regarded as compact and almost always have an air

capacity (air-filled pores at 5kPa) less than 10% and often less than 5%. Most plant roots

have problems to penetrate soils with a total pore volume less than 40%.

4.6.3.2. Penetrometer resistance

The soil strength or penetration resistance can be measured using a penetrometer which

measures the resistance exerted by soil for vertical penetration. This resistance is a

measure for the bearing capacity and the rootability of the soil.

There is a strong relationship between mechanical resistance and the shape and size of

the penetrometer cone used. The „small‟ ASAE(B) cone, which has a top angle of 30° and

a base area of 1.3 cm2, is recommended.

Limiting values of penetrometer resistance (Pr) for root growth range between 2 and 5

MPa. A threshold value of Pr < 3 MPa has been suggested. However, in well-structured

subsoils a higher threshold value Pr < 4 or 5 MPa may be acceptable. Nevertheless, it

must be appreciated that in certain circumstances a well-structured soil can have a high

penetration resistance whilst rootability still remains good.

There is a strong inverse relationship between resistance to penetration and soil

moisture condition, especially for clayey and loamy soils. Measurements should

preferably be taken when the soil is at „field capacity‟ and soil moisture content should

be determined simultaneously.


A high penetration resistance and bulk density is typical for sandy soils. In addition, the

cohesion between the sand grains is always low, in both dry and wet conditions.

4.6.3.3. Pre-compaction stress

Compaction by failure of the pre-compaction stress plays an important role in both

sandy and clay soils.

Pre-compaction stress = since soil compaction is an essentially irreversible process, the

soil remembers the max level of stress to which it has been previously exposed (= pre-

compaction stress). Further compaction will not occur untill the precompaction stress is

exceeded.

The pre-compaction stress can be measured with a uni-axial test. This test is moderately

laborious but not very expensive. Measurement of the pre-compaction stress can be a

good option for a systematic assessment and monitoring of the vulnerability of subsoils

to compaction.

A classification of pre-compaction stress is given in Table 4.2.

Table 4.2. Classification of pre-compaction stress

Pre-compaction stress (kPa) Classification

< 30 Very low

30 – 60 Low

60 – 90 Medium

90 – 120 High

> 120 Very high

This classification is a measure of compactability, the higher the pre-compaction stress

the lower the vulnerability of the soil to compaction.

4.6.4. Indirect methods

4.6.4.1. Air-capacity

Thresholds for the air-filled pore volume are given in table 28. It should be noted that

this air-filled pore volume is not coupled to any specified soil water suction. The values

in Table 4.3 tell us that a soil with an excellent soil structure can be almost saturated

before anaerobic conditions occur whereas a soil with a poor structure must be rather


dry and have a higher air-filled pore volume than a better structured soil to avoid

anaerobic conditions.

Table 4.3 Determined minimum and preferred air-filled pore volumes to avoid (severe)

anaerobic conditions for plant root growth

Soil structure Air-filled pore volume (%)

At least Preferably

Excellent 2 >14

Good 5 >15

Moderate 8 >17

Poor or structureless 12 >21

An air capacity of at least 10% (air-filled pore volume at a specified suction) is required

for a satisfactory medium for plant growth. In table 28 a required air-filled pore volume

> 5% agrees with a well-structured soil.

The next question is which soil water suction should be used to measure the water

content to determine air capacity. We propose a soil water suction that agrees with a

condition of a wet but well drained soil in early spring. This agrees with a soil water

suction of 5 kPa. Our conclusion is that a threshold value of 10% for an air capacity (air-

filled pores) at a soil water suction of 5 kPa is a reasonable threshold.

4.6.4.2. Permeability

The saturated conductivity (Ksat) of a soil can be considered as one of the best indicators

of its physical quality because Ksat has a direct relationship with the quality of the soil

structure and the existence of continuous macro-pores. The major part of the decrease in

pore volume caused by compaction is at the expense of the macro-pores and soil

deformation strongly affects pore continuity. Both macro-pores and pore-continuity

affect the ability of the soil to conduct water and air and any reduction in either results

in slow saturated hydraulic conductivity, that increases surface runoff and erosion.

A saturated hydraulic conductivity of 10 cm d-1 is a suitable threshold for permeability

in the subsoil, which is the same value used for classifying a soil as „bad‟ in the

Netherlands. In the UK, soils with air capacities (air-filled pores at 5 kPa) < 5% are

considered to be very slightly porous. Very slight porosity is normally associated with a

saturated hydraulic conductivity of < 10 cm d-1 and is indicative of poor soil structure.


4.7. PREVENTION AND REMEDIATION

4.7.1. Preventing soil compaction

Too heavy machines with a too high tyre pressure lead to soil compaction. Avoiding the

use of these machines is a straightforward measure to prevent soil compaction. Also the

access of machines to the field can be reduced, either by combining different soil

management practices, or by choosing for reduced tillage. The ploughing technique can

be optimised.

Critical to the prevention of soil compaction during field practices is the consideration of

the bearing capacity of the soil. It is important to access the field when it is at the most

optimal soil moisture content. In order to increase the structural stability of the soil, it is

further important to keep the organic matter content of the soil at an optimal level.

Likewise, it is also important to monitor the pH of the soil. Low pH reduces aggregate

formation and biological activity.

4.7.2. Remediating soil compaction

4.7.2.1. Mechanical decompaction

A compacted soil layer can be remediated by deep soil cultivation (subsoiling). The main

aim of this operation is to rip the compacted layer, thereby creating new fissures and

pores, without loosening the other soil material too much (which would lead to a

decrease in bearing capacity and increased sensitivity to compaction). These operations

will remediate the rootability and permeability of the compacted layer.

4.7.2.2. Biological decompaction

Some deep rooting crops (alfalfa, yellow mustard) have the capacity to break the

compacted layer. The rooting channels created as such can be used by the following

crops in the rotation.

References



Jones RJA., Spoor G., Thomasson AJ. 2003. Vulnerability of subsoils in Europe to

compaction: a preliminary analysis. Soil and Tillage Research 73 (1-2): 131-143.


Lal R., and Shukla MK. 2004. Principles of Soil Physics. Marcel Decker, New York.

Soil Degradation | Soil surface sealing and crusting 58

5. Soil surface sealing and crusting

Crusting and surface seal formation are often consequences of soil structural

degradation and are, in their turn, the precursors to surface compaction, low infiltration,

and high soil evaporation.

5.1. SEALING VERSUS CRUSTING

The terms soil crusting and soil sealing are sometimes used synonymously. Both refer to

a thin dense layer on the soil surface characterized by low porosity, high density, and

low permeability to air and water. Seals and crusts are differentiated by their moisture

contents: seals are wet and crusts are dry.

More specifically, soil sealing is the name for the disconnection between the soil surface

and the soil interior for water transport and air. Pores are closed by the rearrangement

of particles through collapse of the soil surface structure, the swelling of the wetted clay

or mechanical compaction. A seal is usually thin (l-5 mm) and does not crack (Bergsma,

1996).

A soil crust can be defined as a surface layer on the soil, characterised by a platy

structure, ranging in thickness from a few mm to as much as a few cm, and which is

more compact, hard and brittle when dry, than the material immediately beneath it. A

crust does crack and peels off (Bergsma, 1996).

Hard setting is a name for the dense structure caused by repeated wetting and drying of

soils with a lower content of non-swelling clays, moderate silt and/or fine sand and

maybe with some dispersion. It concerns quite thick surface layers of several cm even to

about 20-30 cm.

5.2. TYPES OF CRUSTS

There are three principal categories of crusts: physical, chemical and biological crusts

(Lal and Shukla, 2004).

Physical crusts are formed due to alteration in structural properties of the soil.

Generally, two main types of crusts are distinguished by their mode of formation (Chen

et al. 1980). Structural crusts develop in situ. Depositional crusts are formed of particles


which have been transported from their original location. Depositional crusts are thicker

than structural crusts.

Chemical crusts are formed due to salt incrustations on soil surface in arid and semi-

arid regions.

Biological or microbiotic crusts are primarily formed by algal growth. Ponded water

on the surface of slowly permeable soils in arid and semi-arid tropics leads to formation

of algal crusts. Such crusts are extremely hydrophobic, and drastically reduce the rate of

water infiltration into a soil.

5.3. SURFACE SEALING AND CRUSTING PROCESS

Several scientists tried to describe the different steps to seal and crust formation.

According to Agassi et al. (1981), seal formation is the result of two complementary

mechanisms:

(i) physical disintegration of surface aggregates caused by wetting of dry

aggregates and/or the beating action of the rain drops, and subsequent

compaction of the disintegrated aggregates by raindrop impact; and

(ii) physico-chemical dispersion of soil clays which migrate downward with the

infiltrating water clogging pores immediately beneath the surface.

Surface crusting is due to the breakdown of surface aggregates into finer fragments

and/or primary particles, which are then redistributed on the surface or within the top

few mm. This redistribution occurs both laterally on the surface by splash and/or

overland flow, or vertically within the top few mm by the infiltrating water. These small

particles tend to clog the soil pores, increasing the bulk density. This re-orientation and

development of a close packing arrangement of particles leads to the formation of a thin

layer of fine, structureless material, the surface seal. If this dispersion and reorientation

of dispersed particles is followed by rapid drying and desiccation, a thin crust on the soil

surface is formed.

5.3.1. Morphology of structural crusts

In most studies, researchers found that structural crusts consist of one or several layers

which are only few mm thick. The different layers were attributed to different

mechanisms in the formation of a crust and were identified as skin seal, disruptional


layer, washed-in layer and washed-out layer. Yet these mechanisms were not always

supported by micromorphological evidence.

Skin seal

An upper skin of the order of 0.1 mm thick consisting of small soil particles with high

density and no visible pores. McIntyre (1958) attributed skin seal formation to

compaction due to raindrop impact, Norton (1987) reported skin seal formation due to

the deposition of clay particles at the end of a rainfall event.

Disruptional layer

A layer at the soil surface that has a higher bulk density, smaller and fewer pores than

the underlying horizons. This layer is the result of raindrop impact, leading to the

destruction of aggregates and formation of smaller aggregates.

Washed-in layer

Typical for this layer are small particles clogging the larger pores below the soil surface.

This "washed in" layer is a few mm thick with decreased porosity and formed mainly on

soils which are easily dispersed.

Washed-out layer

This layer is found right beneath the skin seal and is devoid of smaller particles since

they were migrating with infiltrating water.

5.3.2. Morphology of depositional crusts

Depositional crusts consist of a stack of several, very thin soil layers which are formed

upon deposition of soil particles that were suspended in surface water flow. They are

deposited at the moment the water flow speed reduces in, for instance, micro-

depressions in a field. Each layer is the results of 1 rainfall event. Figure 5.1 illustrates

the structure of a sedimentary crust.

5.3.3. Formation of soil crusts

West et al. (1992) proposed a four stage model of the formation of crust:

Stage 0

Stage 0 represents the condition of the freshly tilled soil before any rainfall. Prominent

micro-relief, high surface roughness determined by large clods, and lack of crustation


are characteristics of this stage.

Stage 1

Stage 1 or the initial stage of crust development involves breakdown of aggregates and

particle re-arrangement due to raindrop impact and slaking. The aggregate disruptions

result in formation of a disruptional layer.

Figure 5.1. Multiple layer crust formed due to successive rainfall events

Stage 2

Stage 2 may involve two pathways. For a soil of high aggregate stability and low

susceptibility to dispersion, this stage represents continued development of the

disruptional layer. For a soil with weak aggregation and high potential for dispersion,

the particle disfunction is more extensive, and the released micro-mass may move

downward to form a washed-in layer.

Stage 3

Stage 3 represents the maximum development of the crust, leading to maximal runoff

and erosion of the washed-out layer. There may be further thickening of the disruptional

layer and formation of a secondary washed-out layer. However, the released micro-mass

may be washed out in the runoff. The micro-relief may flatten during this stage, and soil

surface may be covered by a sedimentary crust (Figure 5.2.).

Consequently, structural and depositional crusts can represent different successive

stages in the formation of a soil crust (Figure 5.2).


Figure 5.2. Time- and space-dependent sequence of crusts in a loamy cultivated field: (1)

sealing of the soil surface by a structural crust, then (2) development of a depositional

crust (Bresson and Valentin, 1994)

5.4. FACTORS AFFECTING SLAKING AND

DEFLOCCULATION

Sealing and physical soil crusting are soil surface phenomena caused by susceptibility of

aggregates at the soil-air interface to disruptive forces of climatic elements and

perturbations caused by agricultural practices (e.g., tillage and traffic).

There are three principal influencing factors: kinetic energy of rainfall, soil properties,

and anthropogenic factors.

5.4.1. Rainfall factor

The rate and intensity of crust formation increase with increase in energy of the

raindrop impact. The energy of flowing water may also have indirect impact, probably

due to its influence on transport and deposition of sediments.

5.4.2. Weather

Since wetting-drying and freeze-thaw cycles affect aggregation, they also influence crust

formation and strength. Crust formation is weak or it completely breaks down if the

weather conditions favour freeze-thaw or wet-dry cycles. Crust strength increases when

heavy rain is followed by dry and hot weather that desiccates the crust.

5.4.3. Soil Properties

Susceptibility to crust formation depends on numerous soil properties. Important among

these are texture, clay mineralogy, soil organic matter content, and degree and strength


of aggregates. Indicators of structural stability, such as the mean weight diameter are

also correlated with susceptibility to crusting. Also the properties of the double layer

and stability of the colloidal system are important.

5.4.3.1. Effect of soil texture on seal and crust formation

The tendency of a soil to form a seal and crust depends to some degree on soil structural

stability, which tends to increase with increasing clay content. Thus, stability of

aggregates also increases with an increase in clay content.

Ben Hur et al. (1985) found that soils with 20–30 % clay were the most susceptible to

seal formation and had the lowest infiltration rate. With increasing clay content above

40 %, soil structure became more stable, seal formation was diminished and infiltration

rate increased. In soils with clay contents <10 %, the amount of clay was too low for

clogging the inter-particle pores and seal development.

Clay content also played an important role in sealing of sandy soils in the Sahel where it

was found that soils with < 5 % clay did not seal and those with > 5 % formed seals (Heil

et al., 1997). Low organic matter also contributed to the tendency to form seals.

5.4.3.2. Effect of clay mineralogy

Soils with low-activity clays are less prone to dispersion than those with high-activity

clays. As such, smectitic soils are known to be very susceptible to seal formation,

whereas kaolinitic soils are much less susceptible to seal formation.

5.4.3.3. Properties of the Electric Double Layer

The ease of dispersion is also affected by the effective thickness of the double layer,

which depends on the surface charge, the nature of the cations on the exchange

complex, and degree of hydration. Predominance of monovalent cations (e.g., Na+)

increases the thickness of the double layer.

5.4.3.4. Effect of soil organic matter content

Seal formation diminishes as soil structure and aggregate stability increase. Aggregate

stability increases with increasing organic matter content. Soils with low concentration

of soil organic matter are thus more prone to crusting than those with higher

concentrations.


5.4.4. Antecedent soil moisture and wetting rate

5.4.4.1. Antecedent soil moisture

The antecedent soil moisture content or soil wetness at the beginning of the rainfall

influences aggregate strength, slaking or dispersion, infiltration rate, and the rate of

overland flow.

Under initial dry soil conditions, slaking causes rapid aggregate breakdown of relatively

stable aggregates, quickly filling the intra-aggregate pore space with micro-aggregates

or dispersed primary particles. The aggregate breakdown depends more on rainfall rate

than on its kinetic energy or momentum.

Under wet soil conditions, aggregates are less prone to slaking but more to the raindrop

impact. The surface seal formation is caused by the kinetic energy or momentum of the

rain and overland flow.

Figure 5.3 shows the impact of antecedent soil moisture on the infiltration rate of soils

with clay content >30% from California (Le Bissonnais and Singer, 1992).

Figure 5.3. Impact of antecedent soil moisture on the infiltration rate of soils with clay

content >30% from California (Le Bissonnais and Singer, 1992)


5.4.4.2. Wetting rate

The mechanical process that breaks soil aggregates upon wetting is also affected by the

wetting rate of the surface aggregates. Rapid wetting of aggregates leads to their

slaking because of stresses produced by differential swelling, and entrapped air

explosion, whereas slow wetting of the aggregates reduces the susceptibility of the soils

to seal formation.

Shainberg et al. (2003) showed how runoff and erosion increased as rain kinetic energy

and wetting rate increase; however, the magnitude of change depends on clay content:

In the loamy soils, the effect of rain KE on seal formation and runoff was significant and

the effect of WR was small. Conversely, in the clayey soils (>50% clay), the effect of WR

on seal formation was significant and the effect of rain KE was negligible. They

concluded that for seal formation and runoff production, rain KE predominates in

medium- and light-textured soils and WR predominates in heavy-textured soils.

In low clay and high silt soils with low aggregate stability, neither antecedent moisture

content nor WR play an important role. In these soils, aggregate stability is low in both

wet and dry aggregates and raindrop impact energy determines seal formation.

Conversely, in clay soils with stable aggregates, disintegration of the aggregate by fast

wetting of the dry aggregates is essential for seal formation. When the antecedent

moisture content of the stable aggregates is high, fast wetting does not disintegrate the

aggregates and a seal is not formed.

5.4.5. Microrelief

Micro-relief is defined by surface cloddiness, clod size, and geometry. The micro-relief is

prominent soon after ploughing. Rough seedbed decreases susceptibility to crust

formation. Micro-relief also controls the physical processes occurring at the soil surface,

e.g., micro-rills, surface depressions, infiltration rate, etc (Lal and Shukla, 2004).

5.5. CHARACTERISATION OF SOIL CRUSTS

The crust is primarily characterized by reduction in total volume, size, shape, and

continuity of pores. The magnitude of reduction in porosity of the crust may range from

30 to 90%, with corresponding decrease in pore size. The pore diameter in the crust may

be as small as 0.075mm (Valentin and Figueroa, 1987). There may be no relationship


between crust and infiltration rate or hydraulic conductivity due to other interacting

factors.

The crusted layer is more dense but may be of similar textural makeup than the

unaffected soil beneath it. The stratification of particles within a depositional crusted

layer are indicative of the differences in settling velocity as governed by Stokes law. A

crust formed upon drying of a ponded area receiving runoff is characterized by clay layer

on the top followed by silt and sand. The clay skin cracks on drying and generally curls

upward.

There are several methods to characterize the crust properties (Figure 5.4).

Figure 5.4. Methods to determine crust properties (Lal and Shukla, 2004)

5.5.1. Direct methods

The soil crust is due to a modification in the soil particle arrangement at the microscopic

scale. For that reason, micromorphology has been widely used to study the changes in

the soil surface properties. The soil crusts have been qualitatively described by optical or

scanning electron microscope.

In the field, the soil crust thickness is studied using a calliper. The most commonly used

instrument to determine the strength of a crust, is the penetrometer, which is forced

into the soil to measure its resistance to vertical penetration.

Determination of crust properties

Direct methods Indirect methods

Visual Crust characteristics Index properties Effects on seedlings

thickness

micromorphology

strength

density

Crust condition

Modulus of

rupture

Infiltration rate

emergence

growth


Penetrometers fall into three main groups:

Those which record the pressure necessary to push the tip a specific distance into

the soil volume (static-tip penetrometers);

Those which measure the pressure or (force) required to move the tip through the

soil at a more or less constant rate (moving-tip penetrometers); and

Those which record the number of blows required to drive the penetrometer tip

through a specific depth of soil (impact penetrometers) (Taylor, 1980).

Comparison of results from different investigations is difficult if not impossible if

different types of penetration probes and different test procedures are used.

5.5.2. Indirect methods

The soil susceptibility to sealing and crusting has been related to different indices.

FAO experimental index of crusting

The FAO developed a simple experimental index of crusting (FAO, 1979) at the soil

surface:

15 0.75Index of crusting

10

Zf Zc

C O M

where Zf = fine silt % (2 – 20 µm)

Zc = coarse silt % (20 – 50 µm)

C = clay %

OM = organic matter %

Such empirical indices should be checked and calibrated according to the situation

where the evaluation is carried out. They may need to be modified to suit local

conditions.

De Ploey consistency index

The consistency index by De Ploey (1981) is the difference in moisture percentage of a

soil at the point of 5 and the point of 10 strokes by the Casagrande apparatus:

With SMC5 and SMC10 being the soil water contents (% dry weight) for which the two

sections of a part of the soil in the Casagrande cup touch each other over a distance of 1


cm, after 5 and 10 blows, respectively.

For certain soils (from the temperate regions) an index greater than 3 indicates stability

against sealing, an index equal to or lower than 2.5 indicates sensitivity to sealing.

Stable soils in this area have higher clay and organic matter content; they show higher

biological activity, larger variation in pore size diameter, and presence of cementing

agents such as carbonates.

Absolute sealing index

ASI is the minimum value of the hydraulic conductivity in the seal formed by impact of

water drops. The test measures the changes of saturated hydraulic conductivity of the

seal formed by raindrop impact obtained from a layer of soil aggregates of approximately

1.5 cm thick and 2 to 4 mm in diameter that receive a simulated rainfall during 60

minutes with an intensity around 90 mm/hour. Graphs of the hydraulic conductivity

plotted against the time (Figure 5.5), the cumulative rainfall energy or cumulative

kinetic energy of the falling water drops can be used to determine the minimum

hydraulic conductivity.

Graphs of this relationship plotted for different soils show distinct differences. Fine

sand, silt and clay promote sealing, while coarse sand resists compact sealing.

This index is similar to the index of sealing proposed by Poesen (1986) to be the relative

decrease in percolation rate with time.

5.6. SEALING AND CRUSTING IN DIFFERENT CLIMATES

It is in the subhumid and semi-arid tropics that the problem of sealing and crusting

is the most serious. Many soils of the semi-arid savannahs have a sandy topsoil. A

strong textural differentiation between topsoil and subsoil can occur as a result of the

formation of an argillic B-horizon. Such sandy topsoils may be prone to crusting, i.e., the

formation of a thin layer of a few mm at the surface of the soil that is very dense and

hard when dry, without or with very low porosity and sometimes even water repellent

with algae growth.


Figure 5.5. Hydraulic conductivity (K) of the seal versus time of laboratory simulated

rainfall at 90 mm/hr in six different agricultural soils of Venezuela; (X1= silt + very fine

sand + fine sand (%); X2= clay (%) (Pla, 1985).

In the humid tropics, soils with low iron content and high silt content have problems

of sealing and surface compaction. Also heavily textured Oxisols, once they are cleared,

show this feature, especially in a climate with some dry months.

In arid and semiarid regions, soil sealing and crusting can have disastrous

consequences because high runoff losses leave little water available to support plant

growth. Crusting can be minimized by keeping some vegetative or mulch cover on the

land to reduce the impact of raindrops.

Sealing has always been a problem on a number of temperate zone soils in a humid

climate: The problem has re-emerged especially on the undulating uplands, because of

lower applications of organic matter in the soil, stronger degrees of mechanization and

the introduction of late spring sowing.


Both hard setting and slaking are induced by a too intensive seed bed preparation which

in itself results e.g., in an intensive increasing biological activity, rapid decline in

organic material, and a chemical leaching because of an improved accessibility of

particle surfaces for percolating water. Soil strength due to biological activity is reduced

because increasing tillage intensity reduces e.g., earth worm abundance, number of

earth worm channels, net consumption of organic matter, etc. All these facts finally

result in a lower site productivity, higher susceptibility for compressibility during moist

conditions, more pronounced water as well as wind erosion. If the chemical aspects of

filtering and buffering are also considered, both are reduced resulting in a more

pronounced pollution of surface water.

5.7. IMPACT ON CROPS

The effects of a seal or a crust on the agricultural properties of a soil are both direct, in

that the crust inhibits seedling emergence, plant and root growth, and indirect in that

desirable soil properties and soil processes are adversely affected.

Mechanical resistance linked to the soil acts directly when it disturbs seed emergence,

when it affects root development, or when, due to the effect of desiccation, shrinkage and

superficial hardening phenomena cause damage at the root collar, thus allowing

parasite and insect penetration.

The indirect effects include the decrease of water intake rate, the increase of erosion and

runoff hazards, the restriction of air capacity and internal aeration and the increase of

mechanical strength as the seal or crust dries out. It affects yield indirectly by reducing

water consumption or use of essential nutrients.

Emergence

Taylor et al. (1966) determined the relationship between crust strength and emergence

of corn, onion, barley, wheat, switch-grass and rye seedlings by means of a laboratory

penetrometer. A slight decrease in emergence percentage was observed for crust

strengths in the range of 0.6-0.9 MPa, with no emergence occurring above the 1.2-1.8

MPa range.

Earlier experiments of Parker and Taylor (1965) on emergence of sorghum seedlings,

showed values of 0.3 MPa where emergence decreased and 1.3-1.8 MPa above which no

emergence occurred.


Most of the experiments showing mechanical impedance reducing root growth and

seedling emergence are conducted under controlled laboratory conditions and there are

doubts about the application of laboratory results to field practices.

Arndt (1965 a,b) made use of a balance to measure impedance of soil surface seals under

natural conditions on field plots and produced a model which provides a quantitative

assessment of the mechanical impedance met by seedlings.

5.7.1.1. Crop production and yield

For several reasons the relationship between mechanical impedance and crop yield does

not easily show a direct cause-effect relationship (Taylor, 1980).

First, mechanical impedance does not, itself, reduce yield. Plants require water,

essential minerals, and anchorage from the soil. If the impeding layers do not increase

plant stresses at any time between emergence and physiological maturity, mechanical

impedance will not affect yield.

In general, the used strength-sensing devices integrate their measurements over soil

volumes substantially larger than the size of the plant root or the seedling. In addition,

the devices either follow a rigid path (penetrometers) or cause a pre-or-dained failure

pattern (shear vanes and compressive strength machines). The small and flexible plant

roots are able to penetrate soil layers through soil cracks, worm holes, root channels,

and other voids that do not substantially affect results obtained with the strength-

sensing devices (Nash and Baligar, 1974; Davis et al., 1968).

Despite the difficulties, many experiments have shown that crop yields are reduced as

the strength of soil layers or volumes increases.

In an irrigated experiment, Carter et al. (1965) found that seed cotton yield decreased

linearly from 3,600 kg/ha, where penetrometer resistance measured at field capacity

(cone penetrometer) was 3 bars, to 1,450 kg/ha where resistance was 40 bars.

In the semi-arid environment of the Southern Great Plains, the yield of non-irrigated

cotton was reduced as the penetrometer resistance (static-tip penetrometer) increased.

The yield of lint cotton was 560 kg/ha and 280 kg/ha where penetrometer resistance was


25 bars. Yields were no further reduced as penetrometer resistance was increased above

25 bars.

In the sub-humid environment of Alabama, seed cotton yield was reduced from 180

glcylinder (an oil drum buried in field soil) when no pan existed, to about 70,45, and 35 g

when soil pans with 80 bars penetrometer resistance existed at 30,20 and 10 cm (Lowry

et al., 1970). Soybean followed the same general pattern of reduced yields as

penetrometer resistance increased (Rogers and Thurlow, 1973). Plant water stresses

induced by soil pans were thought to be the reason for reduced yields in both cases.

5.7.1.2. Harvest

Mechanical resistance can also pose problems when certain cropping operations related

to the success of the crop are to be performed. In the harvest of groundnut, for example,

lifting out can be hindered by superficial soil hardening, which can lead to considerable

loss in harvest (Nicou and Charreau, 1980).

5.8. CRUST MANAGEMENT

There are several technological options for crust management (Figure 4.5), and the

choice of technology also depends on the causes of crust formation: impact of raindrops

on an unprotected soil, trampling action of livestock or humans, or vehicular traffic of

farm operations.

5.8.1. Preventive measures

Preventative measures are based on strategies of enhancing aggregation, improving soil

structure, and minimizing the disruptive effects of raindrop impact through crop residue

management to improve the resilience to structural degradation.

The key factor is the application of various mulches on the part of the land exposed to

rains. The mulches prevent direct impact of raindrops on the soil, eliminate particle

detachment.

Planting seeds on soil ridges may also be advantageous because the crusts formed on

ridges are likely to be thinner than crusts formed on a horizontal flat surface. An added

advantage is that a ridge also creates stress concentration in a crust just above the row

of seedlings. Seeds can be planted on the sloping side of furrows as soil crusting is not


severe on this portion of the furrow. Also, the soil crust strength is less for the sloping

side of the furrow. Seeds can be planted in groups (hill-dropping), as this would generate

greater emergence force to break through the crusted soil. The seed rate per meter down

the row can be increased for increased seedling emergence in crusted soils.

In traditional agriculture, planting on heaps of soil is a common practice. Although

heaping may in part be purely traditional, it frequently serves a specific purpose, as in

the construction of high heaps on hydromorphic soils, or in the cultivation of yam on

soils with gravel layers at shallow depth. Surface erosion, if it takes place, transports

soil from the top to the bottom of individual heaps, but runoff and erosion are effectively

stopped because of the mulch cover normally present in the hollows between the heaps.

This is so, even though in clean weeded lands, heaps have been shown to accelerate

erosion in the exposed and connected hollows.

Use of inorganic (gypsum) and organic amendments (compost, farmyard manure) helps

to maintain clay in an aggregated or flocculated state. In arid regions the reclamation of

dispersed soils by using gypsum to create the flocculated condition should be considered

only as a preliminary step to the establishment of the desired soil structure and should

be followed by addition and a build-up of organic matter.

The field (seeded row) can also be covered with straw or plastic or a soil stabilizer film to

prevent the soil surface from developing high strength through compaction by rainfall

impact and rapid drying. This technique seems to be applicable only if economically

justified.

5.8.2. Curative measures

The curative measures involve strategies of managing crust once it has been formed.

Tertiary tillage (harrowing or rotary hoe) can be used to disrupt a depositional crust and

produce a rough soil surface.

Tillage to break the seal is not effective when the seals reforms quickly. In these cases,

tillage may even reduce the remaining continuous pores that are available for

infiltration of rainwater, and systems of limited tillage are to be preferred in some sandy

soils (Valentin, 1986) and in clays (Pagliai and Guidi, 1986).

Better spacing of plants in the row (Metzer, 2002) can also improve stand establishment

in crust-prone soils. Choice of appropriate planters and sowing depth are also critical to


reducing adverse impact of crust on stand establishment (Nabi et al., 2001; Hellllnat

and Khashoei, 2003). Seedling vigor is an important genetic characteristic that should

be considered in the selection of planting material and in the breeding programs where

crusting is a serious problem.

Management and enhancement of soil organic matter content is a useful strategy to

increase aggregate strength and stability and minimizes risks of structural crust

formation. Cover on the soil surface, canopy cover or crop residue mulch, is an effective

measure to reduce the raindrop impact. Use of conservation tillage and residue mulch

minimizes crust formation because of the protection against raindrop impact. Soil

conditioners and polymers have also been found useful to improve aggregation and

minimize crusting (Shainberg et al., 1989). Application of soil conditioners, manure, or

mulch on the seed row can reduce the risks of crusting.

References

Agassi M., Shainberg I., and Morin. J. 1981. Effect of electrolyte concentration and soil

sodicity on infiltration rate and crust formation. Soil Sci. Soc. Am. J. 45:848-851.

Arndt, W. 1965a. The nature of the mechanical impedance to seedlings by soil surface

seals. Aus. J. Soil Res. 3:45-54.

Ben-Hur M., Shainberg I., Bakker D., Keren R. 1985. Effect of soil texture and calcium

carbonate content on water infiltration in crusted soil as related to water salinity. Irrig.

Sci. 6: 281-294.

Bergsma E. 1996. Terminology for soil erosion and conservation. ISSS. Special

Publication.

Bresson LM., and Valentin C. 1994. Soil surface crust formation: contribution of

micromorphology. In Ringrose-Voase AJ. and Humphreys GS (Ed), Soil

micromorphology: studies in management and genesis. Proc. IX Int. Working Meeting

on Soil Micromorphology, Townsville, Australia, July 1992. Developments in Soil

Science 22. Elsevier, Amsterdam, p 737-762.

Carter, L.M., Stockton JR, Tavernetti JR., and Colwick RF. 1965. Precision tillage for

cotton production. Trans. Am. Soc. Agric. Eng. 8 (2):177-179.


Chen Y., Tarchitzky I., Brouwer J., Morin J., and Banin I. 1980. Scanning electron

microscope observations on soil crusts and their formation. Soil Sci. 130:49-55.

Heil, J.W., A.S.R. Juo and K.J. McInnes (1997) „Soil properties influencing surface

sealing of some sandy soils in the Sahel‟. Soil Science 162 (7): 459-469.

Le Bissonnais, Y., and M. Singer. 1992. Crusting, runoff and erosion wetting and drying

in seedbeds of a hardsetting soil with contrasting response to soil water content and

successive rainfall events. Soil Sci. Soc. Am. J. 56:1898–1903.

Lowry, F.E., Taylor H.M., and Huck MG. 1970. Growth rate and yield of cotton as

influenced by depth and bulk density of soil pans. Soil Sci. Soc. Am. Proc. 34:306-309.

McIntyre, DS. 1958a. Permeability measurements of soil crusts formed by raindrop

impact. Soil Sci. 85:185-189

McIntyre, DS. 1958b. Soil splash and the formation of surface crusts formed by raindrop

impact. Soil Sci. 85:261-266.

Nash, V.E. and V.C. Baligar.1974. The growth of soybean (Glycine max.) roots in

relation to soil micromorphology. Plant and Soil 41:81-89.

Nicou, R. and Charreau C. 1980. Mechanical impedance to land preparation as a

constramt to food production in the tropics. (with special reference to fine sandy soils in

West Africa). p. 371-388. In: Priorities for Alleviating Soil-Related Constraints to Food

Prediction in the Tropics. International Rice Research Institute and New York State

College of Agriculture and Life Science, Cornell University, Ithaca, N.Y.

Norton, LD. 1987. Micromorphological study of surface seals developed under simulated

rainfall. Geoderma 40: 127-140.

Parker JJ, and Taylor HM. 1965. Soil strength and seedling emergence relations/ I. Soil

type, moisture tension, temperature and planting depth effect. Agronomy Journal 57:

289-291.


Pla, I. 1985. A routine laboratory index to predict the effects of soil sealing on soil and

water conservation. p. 154-162. In: F. Callebaut, D. Gabriels, and M. De Boodt (eds.),

Assessment of Soil Surface Sealing and Crusting. Symposium Proceedings, Ghent,

Belgium, 1985.

Poesen J. 1986. Surface sealing as influenced by slope angle and position of simulated

stones in the top layer of loose sediments. Earth Surface Processes and Landforms 11

(1): 1-10.

Rogers, H.T. and D.L. Thurlow. 1973. Soybeans restricted by soil compaction. Highlights

Agric. Res. 20:10. Auburn University Agricultural Experiment Station, Auburn, AL.

Shainberg et al., 1989

Taylor, H.M. 1980. Mechanical impedance to root growth. p. 389-405. In: Priorities for

Alleviating Soil-Related Constraints to Food Production in the Tropics, International

Rice Research Institute and New York State College of Agriculture and Life Science,

Cornell University, Ithaca, N.Y.

Taylor, H.M., G.M. Robertson, and J.J. Parker, Jr. 1966. Soil strength-root penetration

relations for medium to coarse-textured soil materials. Soil Sci. 102:18~22

Valentin, C. 1986. Effects of soil moisture and kinetic energy on the mechanical

resistance of surface crusts. p. 367-369. In: F. Callebaut, D. Gabriels, and M. De Boodt

(eds.), Assessment of Soil Surface Sealing and Crusting. Symposium Proceedings,

Ghent, Belgium.

West LT., Chiang SC., and Norton LD. 1992. The morphology of surface crusts. In: M.E.

Sumner and B.A. Stewart (eds) “Soil Crusting: Chemical and Physical Processes”, Lewis

Publishers, Boca Raton,73-92.

Soil Degradation | Soil erosion 77

6. Soil erosion

6.1. SOIL EROSION PROCESSES

Erosion that takes place under natural conditions (i.e., when the land surface and native

vegetative cover have not been disturbed by human activities) is called natural or

geologic soil erosion. The natural process of soil erosion can be increased horrendously

by human action. These actions have generally been through stripping of natural

vegetation for cultivation, indirect changes in land cover through grazing and controlled

burning or wildfires, through regrading of the land surface and/or a change in the

intensity of land management, for example through poor maintenance of terrace

structures.

Resulting changes to the soil cover allow natural forces of erosion to remove the soil

much more rapidly than soil-forming processes can replace it. Any soil loss > 1 t ha-1 yr-1

can be considered irreversible within a span of 50-100 years.

Soil erosion is regarded as the major and most widespread form of soil degradation. Soil

can be eroded away by wind and water. Hillslope processes are of two very broad types,

first the weathering and second the transport of the regolith, with a number of separate

processes within each type (Table 6.1); many of these processes occur in combination.

Most slope processes are greatly assisted by the presence of water, which helps chemical

reactions, makes masses slide more easily, and carries debris as it flows. For both

weathering and transport, the processes can conveniently be categorised as chemical,

physical and biological (Table 6.1).

6.1.1. Soil erosion by water

Material may be detached by two processes, "raindrop impact" and "flow traction", and

transported either by saltation through the air or by overland water flow (surface

runoff) (Figure 6.1).

Combinations of these detachment and transport processes give rise to the main

processes, "Rainsplash", "Rainwash", "Rillwash", and "Sheet wash" as indicated in table

6.2.


Figure 6.1. Process of water erosion (Brady and Weil, 2008)

Table 6.1. Classification of most important hillslope processes

Types : T = Transport Limited; S = Supply Limited removal (Gobin et al., 2002)

Rainsplash and rainflow are most significant in areas between small channels, or rills,

which form on a rapidly eroding surface, and are commonly grouped together as inter-

rill erosion processes.


Table 6.2. Types of soil erosion by water

Detachment by Transportation Mode

Raindrop impact Saltation Rainsplash

Overland Flow Traction Rainflow

Snowmelt

Rillwash

Gully Erosion

Sheet wash/slope wash

Mud flows/earth flows

Bank erosion

The total rate of detachment increases rapidly with rainfall intensity. Raindrop impact

is also effective in breaking down soil aggregates into constituent soil particles. These

particles are re-deposited between aggregates on and close to the surface, forming "soil

crusts", which seal the surface, and limit infiltration by filling the macro-pores between

the aggregates. These crusts initially may make the surface more resistant to erosion.

However, their greatest importance is in increasing runoff from storm rainfall.

Runoff is the portion of precipitation or irrigation on an area which does not infiltrate,

but instead is discharged from the area, usually into stream channels. Surface run-off is

lost without entering the soil. It is the most important direct driver of severe soil

erosion. Processes, which influence runoff, must therefore play an important role in any

analysis of soil erosion intensity, and measures, which reduce runoff, are critical to

effective soil conservation.

Perhaps the most important control on runoff is the degree of crusting of the soil

surface. Of secondary, but still major importance, are the micro-relief of the soil surface

and the sub-surface soil structure, particularly the presence or absence of macro-pores

in the form of cracks and/or voids between soil aggregates.

6.1.2. Soil erosion by wind

Wind erosion is accelerated when soil is dry, weakly aggregated, bare and smooth, and

the winds are strong. Moderate wind velocities can keep individual particles of humus

clay, and silt (particles < 0.05 mm in diameter) in suspension simultaneously (Figure

6.2). Very fine, fine and medium sands (0.05-0.5 mm in diameter) are moved by wind in

a succession of bounces known as saltation (Figure 6.2). Coarse sand (0.5-1.0 mm in

diameter) is not usually airborne but rather is rolled along the soil surface. This kind of

erosion is called surface creep. Very coarse sand (1-2 mm in diameter), gravels, peds,


and clods are too large and too dense to be rolled by the wind, so wind-eroded soils have

surfaces covered with coarse fragments larger than 1 mm in diameter. This kind of arid

soil surface is known as desert pavement. Eroding winds deplete soil productivity and

change the soil texture toward coarseness.

Figure 6.2. Processes involved in wind erosion (Brady and Weil, 2008)

6.1.3. Disturbance, translocation or tillage erosion

Soil erosion can be initiated and/or exacerbated by disturbance through tillage,

excavation and animal activity.

An important anthropogenic process is "tillage erosion", which is the result of ploughing,

either up and down slope or along the contour. Each time the soil is turned over, there is

a substantial movement of soil.

Soil loss by harvesting root crops : it is well known that soil particles attached to the

surface of root crops, can result in significant removal of soil from cultivated fields.

Erosion by trampling and burrowing animals : the surface pressure exerted by the

hooves of grazing animals can destroy vegetation leaving a bare soil surface, vulnerable

to erosion by water. Burrowing animals, such as rabbits, moles, etc. can also initiate or

exacerbate soil erosion.


6.2. ON-SITE AND OFF-SITE EFFECTS OF ACCELERATED

SOIL EROSION

Erosion damages the site on which it occurs and also has undesirable effects off-site in

the larger environment. The off-site costs relate to the effects of excess water, sediment,

and associated chemicals on downhill and downstream environments. While the costs

associated with either or both of these types of damages may not be immediately

apparent, they are real and grow with time.

6.2.1. On-site damage

The most obviously damaging aspect of erosion is the loss of soil itself. In reality, the

damage done to the soil is greater than the amount of soil lost would suggest because

the soil material eroded away is almost always more valuable than that left behind. Not

only are surface horizons eroded while subsurface horizons (which are usually less

useful) remain untouched, but the quality of the remaining topsoil is also impaired.

Erosion selectively removes organic matter and fine mineral particles, while leaving

behind mainly relatively less active, coarser fractions.

6.2.2. Off-site damage

Erosion moves sediment and nutrients off the land, creating the two most widespread

water pollution problems in our rivers and lakes. The nutrients impact water quality

largely through the process of eutrophication caused by excessive nitrogen and

phosphorus. In addition to nutrients, sediment and runoff water may also carry toxic

metals and organic compounds, such as pesticides. The sediment itself is a major water

pollutant, causing a wide range of environmental damages.

DAMAGE FROM SEDIMENT. Sediment that washes into streams makes the water

cloudy or turbid. High turbidity prevents sunlight from penetrating the water and thus

reduces photosynthesis and survival of the submerged aquatic vegetation (SAV). The

demise of the SAV, in turn, degrades the fish habitat and upsets the aquatic food chain.

WINDBLOWN DUST. Wind erosion also, has its off-site effects. The blowing sands may

bury roads and fill in drainage ditches, necessitating expensive maintenance. The

sandblasting effect of wind-borne soil particles may damage the fruits and foliage of


crops in neighbouring fields, as well as the paint on vehicles and buildings many

kilometres downwind from the eroding site.

HEALTH HAZARD. The finest particles blow the farthest, and may present a major

human health hazard. While silt-sized particles are generally filtered by nose hairs or

trapped in the mucous of the windpipe and bronchial tubes, smaller clay-sized particles

often pass through these defences and lodge in the air sacs (alveoli) of the lungs. The

particles themselves cause inflammation of the lungs, but they may also often carry

toxic substances that cause further lung damage.

6.3. ASSESSING SOIL EROSION RISK AND LOSSES

6.3.1. Field observations

6.3.1.1. Water erosion

Three major types of soil erosion are shown in Figure 6.3. Sheet erosion is relatively

uniform erosion from the entire surface. Only perched stones and pebbles protect the

soil underneath from raindrop impact and sheet erosion. Rill erosion is initiated when

the water concentrates in small channels as it runs of the soil. Rills are channels small

enough to be smoothed by normal tillage. When sheet erosion takes place primarily

between irregularly spaced rills, it is called interrill erosion. Gully erosion creates deep

channels that cannot be erased by cultivation. Gullies on cropland are obstacles for

tractors and cannot be removed by ordinary tillage practices (Brady and Weil, 2008).

Also sediment deposition sites within or below the field provide evidence of water

erosion.

6.3.1.2. Wind erosion

Visual evidence of air-borne soil movement onto or off the field. Such evidence may

include dust storms, crop damage, wind-blown soil deposits, and smooth soil surfaces.

6.3.2. Field experiments

Data on soil erosion and its controlling factors can be collected in the field or, for

simulated conditions, in the laboratory. For realistic data on soil loss, field

measurements are the most reliable. But, because conditions vary in space and time, it

is often difficult to determine the causes and controlling factors, and understand the


processes at work. These latter issues are easier revealed in lab experiments where

many factors can be controlled (Morgan, 2006).

Figure 6.3. Three major types of soil erosion

Field measurements can be classified in 2 groups: those designed to determine soil

erosion from a relatively small sample area (erosion plots) and those designed to assess

erosion over larger areas, such as a watershed.

6.3.2.1. Soil erosion plots

Field plots to measure soil erosion allow the direct measurement of real world conditions

and facilitate the understanding of different factors, soil treatments and land uses. Each


plot is a physically isolated piece of land of known size, slope gradient, slope length and

soil type from which both runoff and soil loss are monitored. The number of plots

depends on the experiment, but usually allows for at least some replicates. The data

obtained give a measure of the soil loss from the entire plot that may be reasonably

realistic of losses from fields under similar conditions. The experiments don‟t provide

information on the redistribution of soil within the field, along the slope (Morgan, 2006).

Boix-Fayos et al. (2006) provide a review of (i) the advantages and limitations of the use

of field plots to measure soil erosion; and (ii) the potential sources of variation in the

results obtained due to a lack of harmony between methodological conditions and the

processes operating in the environment at different scales.

6.3.2.2. Measured soil loss by suspended sediment load in rivers and streams

Eroded soil may be deposited few meters downslope or might end up in rivers and

streams.

The sediment yield of a catchment is obtained from measurements of the quantity of

sediment leaving a catchment along a river over time. Recording stations are typically

installed at the exit point (outlet) for automatic recording of the discharge and sediment

concentrations (water samples or turbidity recording) in the river water (Morgan, 2006).

The ratio of the amount of soil delivered to a stream to the total amount of soil that was

eroded is termed the delivery ratio.

As much as 60% of the eroded soil may reach a stream in certain watersheds with steep

sloped; whereas as little as 1% may reach the rivers in a gently sloping plain. Typically

larger watersheds are characterised by lower delivery ratios since there are more

opportunities for deposition (Brady and Weil, 2008).

6.3.2.3. Sand traps

Techniques for measuring wind erosion are less well established. They are based on the

design and installation of an aerodynamically sound trap that catches the soil particles,

while allowing the air to pass through freely. The collected particles can be weighted

either afterwards, at the end of the collecting period, or simultaneously, providing a

continuous records of information during for instance a storm. A large number of

sampling points are needed to provide a reliable estimate of the soil losses by wind

erosion (Morgan, 2006).


6.3.3. Laboratory measurements

6.3.3.1. Rainfall simulation

A rainfall simulator is designed to produce a storm of known energy, intensity and drop-

size characteristics that can be repeated on demand. It is crucial to be able to reproduce

the drop-size distribution, crop velocity at impact, and the intensity of natural rainfall

with a uniform spatial distribution.

Studies on splash and sheet erosion are made by filling containers (soil pans) with soil,

weighing them dry and subjecting them to a simulated rainstorm or pre-defined

intensity. The soil pans at the research unit of soil erosion and conservation have a

length of about 0.55 m and a width of 0.20 m with a false bottom at 0.16 m depth. This

false bottom has perforations of 3 mm and is covered with a cloth to allow drainage.

These pans can be placed at a pre-defined slope gradient. A splash board of 0.5 m can be

affixed to them for the collection of soil lost by splash erosion. Two pluviometers placed

on each side of the soil pans measure of the rainfall intensity. The sediment is collected

and evaporated to obtain the rain splash and sheet erosion. Measurements of rain

splash, runoff, and infiltration are typically made at 5-min intervals for 90 min (Arthur

et al., 2011; Gabriels et al., 1973; Vermang, 2009).

6.3.3.2. Wind tunnels

Laboratory studies of wind erosion are carried out in wind tunnels. Wind is supplied by

a fan and the tunnel is shaped so that the wind flow is first straightened, then flows into

a convergence zone where flow is constricted before it passes through the test section. A

mesh screen at the outlet traps most of the sand particles. Soil trays are placed on the

floor of the tunnel test section and can be removed for weighing. The amount of wind

erosion is thus determined by weight loss. The combined wind tunnel and rainfall

simulator facility at the International Centre for Eremology (UGent) allows to study

wind and water erosion simultaneously.

6.3.4. Modelling soil losses by water erosion

6.3.4.1. Water Erosion Prediction Project (WEPP)

The WEPP is the most ambitious and sophisticated of all erosion models developed so

far. It is a complex, process-based computer programme based on an understanding of

the fundamental mechanisms involved with each process leading to soil erosion.


The WEPP is a simulation model that computes, on a daily basis, the rates of hydrologic,

plant-growth, and even litter-decay processes. Theoretically, it can predict exactly how

rainfall will interact with the soil on a site during a particular rainstorm or during the

course of an entire year. Raindrop impact, splash erosion, interrill flow, rill formation,

channelized flow, gully formation, and sediment deposition both on- and off-site can all

be predicted – if sufficient data are available to feed into the model.

The programme and some databases can be downloaded from the WEPP home page :

http://soils.ecn.pur-due.edu:20002/-wepp/wepp.html. For details about the availability

and uses of the WEPP materials, see Laflen et al. (1997):

6.3.4.2. Universal Soil Loss Equation (USLE)

In contrast to the process-based operation of WEPP, most predictions of soil erosion

continue to rely on much simpler models that statistically relate soil erosion to a

number of easily observed factors. Scientists can make such empirical models if they

know that certain conditions are associated with soil erosion, even if they do not

understand the details of why this is so. At the heart of these models is the realization

that water-induced erosion results from the interaction of rain and soil. Decades of

erosion research have clearly identified the major factors affecting this interaction.

These factors are quantified in the universal soil-loss equation (USLE) :

A = R K L S C P

A : the predicted soil loss, is the product of :

R = rainfall erositivity Rain-related factor

K = soil erodibility

L = slope length Soil-related factors

S = slope gradient or steepness

C = cover and management Land-management factors

P = erosion-control practices

Working together, these factors determine how much water enters the soil, how much

runs off, how much soil is transported, and when and where it is re-deposited. Unlike

the WEPP programme, the USLE was designed to predict only the amount of soil loss by

sheet and rill erosion in an average year for a given location. It cannot predict erosion

from a specific year or storm, nor can it predict the extent of gully erosion and sediment

delivery to streams. It can, however, show how varying any combination of the soil- and


land-management-related factors might be expected to influence soil erosion, and

therefore can be used as a decision-making aid in choosing the most effective strategies

to conserve soil. For discussions of the original USLE, see Wischmeier and Smith (1978).

6.3.4.3. Revised Universal Soil Loss Equation (RUSLE)

The USLE has been used widely since the 1970s. In the early 1990s, the basic USLE

was updated and computerized to create an erosion-prediction tool called the revised

universal soil-loss equation (RUSLE). The RUSLE uses the same basic factors of the

USLE, although some are better defined and interrelationships among them improve

the accuracy of soil-loss prediction. The RUSLE is a computer software package that is

constantly being improved and modified as experience is gained from its use around the

world. For discussions of the RUSLE, see Renard et al. (1997).

6.3.5. Modelling soil losses by wind erosion

The detachment, transport, and deposition processes of soil erosion can be predicted

mathematically by soil erosion models. These are equations – or sets of linked equations

– that interrelate information about the rainfall, soil, topography, vegetation and

management of a site with the amount of soil likely to be lost by erosion.

A wind erosion prediction equation (WEQ) has been in use since the late 1960s :

E = f(ICKLV)

The predicted wind erosion E is a function f of :

I = soil erodibility factor

C = climate factor

K = soil-ridge-roughness factor

L = width of field factor

V = vegetative cover factor.

The WEQ involves the major factors that determine the severity of the erosion, but it

also considers how these factors interact with each other. Consequently, it is not as

simple as is the USLE for water erosion.

The soil erodibility factor I relates to the properties of the soil and to the degree of slope

of the site in question.


The soil-ridge-roughness factor K takes into consideration the cloddiness of the soil

surface, vegetative cover V, and ridges on the soil surface.

The climatic factor C involves wind velocity, soil temperature, and precipitation (which

helps control soil moisture).

The width of field factor L is the width of a field in the downwind direction. Naturally,

the width changes as the direction of the wind changes, so the prevailing wind direction

is generally used.

The vegetative cover V relates not only to the degree of soil surface covered with

residues, but to the nature of the cover - whether it is living or dead, still standing, or

flat on the ground.

A revised, more complex, and more accurate computer-based prediction model has been

developed, and is known as the Revised Wind Erosion Equation (RWEQ). It is still an

empirical model based on many years of research to characterize the relationship

between observable conditions and resulting wind erosion severity.

Scientists and engineers around the world are also cooperating in the development of a

much more complex process-based model known as the Wind Erosion Prediction System

(WEPS). Like its water erosion sister, WEPP, this computer programme simulates all

the basic processes of wind interaction with soil. Scientists are continually improving

the model and testing its predictions against data observed in the real world.

The factors of the wind erosion equation give clues to methods of reducing wind erosion.

For example, since soil moisture increases cohesiveness, the wind speed required to

detach soil particles increases dramatically as soil moisture increases.

6.4. SOIL LOSS TOLERANCE

There has been much discussion in the literature about thresholds above which soil

erosion should be regarded as a serious problem. This has given rise to the concept of

„tolerable‟ rates of soil erosion that should be based on reliable estimates of natural rates

of soil formation.


A tolerable soil loss is the maximum amount of soil that can be lost annually by the

combination of wind and water erosion on a particular soil without degrading that soil‟s

long-term productivity.

There has been little research done on the quantitative assessment of soil loss tolerance.

There are two approaches to determine soil loss tolerance limit (SLTL).

The first approach is an economic assessment that considers both on-site and off-site

effects. The second approach is based on the bio-physical issues determined by

weathering of the parent rocks and other factors of soil formation. The bio-physical

approach considers 11.2 Mg ha−1 yr−1 as the worldwide upper permissible soil loss

tolerance limit (SLTL) (Wischmeier and Smith, 1978) since it approximates the

maximum rate of a horizon development under optimum conditions.

However, numerous criteria should be considered in evaluating SLTL, including not

only rate of weathering, but also changes in soil quality, impact on water quality, etc.

Cole and Higgins (1985) proposed that SLTL should be assessed in terms of a specific

range with an acceptable degree of risk associated with a specific soil type. USDA-NRSC

(1999) proposed a typical range of SLTL in integer steps from 2.5 to 12.5 Mg ha−1 yr−1. As

such, soils with shallow layers of infertile, impermeable, or rocky material are generally

assigned lower T-values.

In Europe, the threshold values for tolerable soil loss have been set at 1 to 2 Mg ha−1

yr−1. It may however be realistic to propose different rates of soil erosion that are

tolerable in different parts of Europe.

Alternative quantitative approaches to estimate the tolerable soil loss appear regularly

in international literature (Bhattacharyya et al., 2008; Mandal et al., 2010).

References

Boix-Fayos C., Martínez-Mena M., Arnau-Rosalén E., Calvo-Cases A., Castillo V.,

Albaladejo J. 2006. Measuring soil erosion by field plots: Understanding the sources of

variation. Earth-Science Reviews 78: 267–285.

Bhattacharyya P., Bhatt V.K., Mandal D. 2008. Soil loss tolerance limits for planning of

soil conservation measures in Shivalik–Himalayan region of India. Catena 73: 117-124.


Cole, G.W., Higgins, J.J., 1985. A probability criterion for acceptable soil erosion.

Transactions of ASAE 28 (6), 1921–1926.

Gabriels, D., De Boodt M. and Minjauw W.1973. Description of a rainfall simulator for

soil erosion studies, Med. Fac. Landbouw. RUG nr. 3.

Laflen, J.M., W.J. Elliot, D.C. Flanagan, C.R. Meyers and M.A. Nearing. 1997. "WEPP-

predicting water erosion using a process-based model" J. Soil Water Cons., 52:96-102.

Mandal, D., Sharda V.N., and Tripathi K.P. 2010. Relative efficacy of two biophysical

approaches to assess soil loss tolerance for Doon Valley soils of India. Journal of Soil and

Water Conservation 65 (1): 42 – 49.

Renard, K.G., G. Foster, G. Weesies, D. McCool, and D. Yoder. 1997. Predicting Soil

Erosion by Water : A Guide to Conservation Planning with the Revised Universal Soil

Loss Equation (RUSLE). Agricultural Handbook n° 703 (Washington, D.C., USDA).

U.S. Department of Agriculture Natural Resources Conservation Service (USDA-NRCS),

1999. National Soil Survey Handbook: Title430-VI, U.S. Government Printing Office,

Washington, D.C.

Vermang et al., 2009. Aggregate stability and erosion response to antecedent water

content of a loess soil, Soil Science Society of America Journal 73, pp. 718–726.

Wischmeier, W.H., Smith, D.D., 1978. Predicting Rainfall Erosion Losses: a Guide to

Conservation Planning. U.S. Department of Agriculture. Agricultural Handbook, vol.

537. USDA, Washington, D.C., p. 85. Washington, D.C.

Soil Degradation | Salinisation 91

7. Salinisation

Salt affected soils cover extensive areas on each continent of the earth. Salinity,

alkalinity and sodicity are among the most important and widespread soil degradation

processes and environmental/ecological stresses in the biosphere. They limit the agro-

ecological potential and represent a considerable ecological and socio-economical risk for

sustainable development.

Soil salinisation is a process that leads to an excessive increase of water soluble salts in

soil. Soil sodification is a process that leads to an accumulation of Na+ in the solid and/or

liquid phases of the soil as crystallised NaHCO3 or Na2CO3 salts (salt „efflorescence‟), in

highly alkaline soil solution (alkalisation), or as exchangeable Na+ ion in the soil

absorption complex.

A distinction can be made between primary and secondary salinisation processes:

− Primary salinisation involves accumulation of salts through natural processes such

as physical or chemical weathering and transport from saline geological deposits or

groundwater.

− Secondary salinisation is caused by human interventions such as inappropriate

irrigation practices, use of salt-rich irrigation water and/or poor drainage conditions.

Figure 7.1 shows the global distribution of salt-affected soils. Salt-affected soils occur

most often in arid and semiarid climates, but they can also be found in areas where the

climate and mobility of salts cause saline waters and soils for short periods of time.

7.1. CAUSES OF SOIL SALINISATION

7.1.1. Soluble salts

In arid and semiarid climates, there is not enough water to leach soluble salts from the

soil. Consequently, the soluble salts accumulate, resulting in salt-affected soils.

The major cations and anions of concern in saline soils and waters are Na+, Ca2+, Mg2+

and K+ and the primary anions are Cl-, SO42-, HCO3

-, CO32- and NO3

-. In hypersaline

waters or brines B, Sr, Li, SiO2, Rb, F, Mo, Mn, Ba and Al (since the pH is high, Al

would be in the Al(OH)4- form) may also be present.


Bicarbonate ions result from the reaction of carbon dioxide in water. The source of the

carbon dioxide is either the atmosphere or respiration from plant roots or other soil

organisms. Carbonate ions are normally found only at pH > 9.5.

Boron results from weathering of boron-containing minerals such as tourmaline.

When soluble salts accumulate, Na+ often becomes the dominant counter-ion on the soil

exchanger phase, causing the soil to become dispersed. This results in a number of

physical problems such as poor drainage. The predominance of Na+ on the exchanger

phase may occur due to replacing most of exchangeable Ca2+ and Mg2+ that precipitate

as CaSO4, CaCO3 and CaMg(CO3)2.

Figure 7.1. Global distribution of salt-affected soils

7.1.2. Evapotranspiration

An additional factor in causing salt-affected soils is the high potential

evapotranspiration in arid and semi-arid areas, which increases the concentration of

salts in both soils and surface waters.


7.1.3. Drainage

Poor drainage can also cause salinity and may be due to a high water table or to low soil

permeability caused by sodicity (high sodium content) of water. Soil permeability is "the

ease with which gases, liquids or plant roots penetrate or pass through a bulk mass of

soil or a layer of soil".

7.1.4. Irrigation water quality

An important factor affecting soil salinity is the quality of irrigation water. If the

irrigation water contains high levels of soluble salts, Na, B and trace elements, serious

effects on plants and animals can result.

7.2. SOURCES OF SOLUBLE SALTS

The major sources of soluble salts in soils are weathering of primary minerals and

native rocks, residual fossil salts, atmospheric deposition, saline irrigation and drainage

waters, saline groundwater, seawater intrusion, additions of inorganic and organic

fertilizers, sludges and sewage effluents, brines from natural salt deposits, and brines

from oil and gas fields and mining.

Salts from atmospheric deposition, both as dry and wet deposition, can range from 100

to 200 kg year-1 ha-1 along sea coasts and from 10 to 20 kg year-1 ha-1 in interior areas of

low rainfall.

7.2.1. Weathering

The formation of naturally occurring salt-affected soils is a part of the continued

geochemical processes that have existed since ancient geologic time. The ultimate source

of soluble salts in soils comes from the weathering of primary minerals in the rocks

forming continents.

These weathering products (soluble salts) move with infiltrating rainfall water through

the groundwater systems and eventually into streams and oceans. The oceans thus very

slowly gain salinity at the expense of the land masses. Rainfall in humid climates is

usually sufficient to transport soluble salts out of a region as fast as they are formed. In

arid regions, the weathering products remain localized, as the rainfall is not sufficient to


provide the leaching and transport of the salts from the area. This is enhanced by the

high evaporative demands of the region.

Salt accumulation in soil is related to definite types of relief and geomorphologic

conditions. Since salt moves with water, saline conditions are linked to lowlands or

depressions in the topography, where water naturally drains and accumulates. This

situation, oftentimes associated with restricted internal drainage of the soil, is

conducive to high water table conditions. Thus excess salts accumulate in areas that

receive surface or subsurface drainage from surrounding lands, with subsequent

evaporation leaving the transported salt in the soil. These drainage basins may

encompass only a few hectares or they may include thousands of square kilometers.

7.2.2. Secondary deposits

Throughout geologic history, large portions of the earth have been covered by saline

seas. The marine sediments that were laid down during extended periods of large

inundation have since been uplifted and have served as parent material for large areas

of soils in arid regions. These secondary deposits include shale's, sandstones, mudstones

and conglomerates, and can serve as substantial sources of salt. It must be recognised,

however, that the original source of these sedimentary materials was the weathering of

continental rock. Saline marine shale's are notorious sources of salt.

Salt-affected soils that occur naturally present a problem when new lands are developed

and brought under irrigation. Before crop production is economically feasible, salt-

affected soils must be reclaimed to reduce the amount of salt existing in the root zone

and/or the exchangeable sodium percentage on the exchange complex.

7.2.3. Irrigation water

Man-induced salt-affected soils, which result from the application of irrigation water

without proper management to control salt accumulation, are however more important,

both historically and economically.

All irrigation waters contain dissolved salts. These salts accumulate in irrigated soils

because evapotranspiration removed essentially pure water from the soil, and the salt is

left behind. The salt that is introduced to the soil by irrigation water is added to any salt

that occurs naturally in the profiles.


For every 100 mg/l salt existing in a given irrigation water, 1 metric ton of salt is added

to the soil per hectare-meter (ha-m) of applied water. Thus if a water with a salt

concentration of 1000 mg/l is used for irrigation and the annual application rate is 1 ha-

m, up to 10 metric tons of salt per hectare can accumulate in the soil unless proper

management is used.

7.3. MEASURING SALINITY AND SODICITY

The parameters that are determined to characterize salt-affected soils depend primarily

on the concentrations of salts in the soil solution and the amount of exchangeable Na+ in

the soil.

Ideally, the salt present in the soil should be determined by analysis of the soil water

under field moisture conditions. In many instances this cannot be done conveniently and

the salinity status of the soil is evaluated from a sample under laboratory conditions.

The concentration of salts in the solution phase can be characterized by several indices

(Table 7.1).

Table 7.1. Salinity parameters

7.3.1. Total dissolved solids (TDS)

In concept, the simplest way to determine the total amount of dissolved salts in a

sample of water is to heat the solution in a container, until all water has evaporated and

only a dry residue remains. The residue can then be weighted and the total dissolved

solids can be expressed as mg l-1.


In water to be used for irrigation, TDS typically ranges from about 5 to 1000 mg/l, while

in the solution extracted from a soil samples, TDS may range from about 500 to 12,000

mg/l (Brady and Weil, 2008).

The TDS can also be estimated by measuring the electrical conductivity (EC) to

determine the effects of salts on plant growth.

7.3.2. Electrical conductivity (EC)

7.3.2.1. Concept

The preferred index to assess soil salinity is electrical conductivity. It is based on the

fact that electrolytes dissociate into charged ions in the presence of water. Pure water is

a poor conductor of electricity, but conductivity increases as more salts are dissolved in

the water. The ions carry electric current; the greater the concentration of ions, the

greater the current-conducting capacity or electrical conductivity of the solution. The

electrical current carried by a salt solution under standard conditions thus increases as

the salt concentration of the solution increases. The electrical conductivity of the soil

solution gives us an indirect measurement of the salt content.

7.3.2.2. Units and methods

The conductivity is expressed in reciprocal ohms, that is ohms-1, and referred to as mho

(ohm spelled backwards). In SI units the reciprocal of the ohm is the siemen (S). For

most natural systems, the value of a Siemens is too large for convenience. The working

unit of electrical conductivity is micromhos per centimeter (µmho cm-1) or in millimhos

per centimeter (mmho cm-1). In SI units, EC is given as S m-1 or as deciSiemens per

meter (dS m-1). One dS m-1 is one mmho cm-1.

The EC can be measured both on soil samples (using an EC meter) or on the bulk soil in

situ (using electrodes or surface transmitter & receiving coils) (Table 7.2).

Table 7.2. Different measurement methods for expressing the EC

Symbol Description

On soil sample

ECe Conductivity of the solution extracted from a water-saturated soil paste

ECp Conductivity of the water-saturated soil paste

ECw Conductivity of the solution extracted from a soil-water mixture (eg 1:2)

On bulk soil in place

ECa Apparent conductivity of bulk soil sensed by metal electrodes in soil

ECa* Electromagnetic induction of an electric current using surface transmitter

and receiving coils


In principle the electrical conductivity of the soil extract should allow some

approximation of the conditions that exist in the soil solution under field conditions. It is

obvious that conductivity measurements can be made on soil extracts obtained using

any soil-to-water ratio. However, a major problem exists when extract data are

interpreted in terms of field conditions.

There are several important parameters that are commonly used to assess the status of

Na+ in the solution and on the exchange phases. These are the sodium adsorption ratio

(SAR), the exchangeable sodium ratio (ESR), and the exchangeable sodium percentage

(ESP).

7.3.2.3. Saturation paste extract

The saturation paste extract method is the most commonly used procedure and standard

to which the others are usually compared. A soil sample is saturated with distilled water

and mixed to the consistency of a paste that glistens with water and flows slightly when

jarred. After standing overnight to thoroughly dissolve the salts, the solution is

extracted by vacuum filtration and its EC is measured. The extract is referred to as the

saturation extract and simulates the soil water under saturated conditions.

Approximate relationships exist between the ECe and other chemical parameters of the

saturation extract or any other solution:

Osmotic potential

The approximate relation of the osmotic potential (the negative of the osmotic pressure)

of a solution to its EC is:

Where = osmotic potential (bar)

ECe = electrical conductivity of the saturated paste at 25°C (mS cm-1)

The osmotic potential is a component of the total water potential and is a function of the

concentration of salt in solution. An increase in solution salinity decreases the osmotic

potential, which in turn decreases the availability of water to plants.

Salt content

Based on large datasets, relating the EC to the TDS, the following relationships have

been developed:


− For soils with an EC between 0.1 and 5.0 dS m-1: TDS = 640 x TDS

− For soils with an EC between > 5.0 dS m-1: TDS = 800 x TDS

To obtain the total concentration of soluble cations (TSC) or total concentration of

soluble anions (TSA), EC (dS m-1) is usually multiplied by a factor of 0.1 for mol liter-1

and a factor of 10 for mmol liter-1.

Example calculations:

From a very large number of data, a relationship between the ECe and the salt

concentration has been observed:

1 mS cm-1 ~ 10 meq l-1

The most common cations in the soil solution (Na+, Ca2+) have an average equivalent

mass of 22 g; the most common anions in the soil solution (Cl- and SO42-) have an

average equivalent mass of 42 g. This means that a solution with an electrical

conductivity of 1 mS cm-1 will contain 0.64 g l-1 of salt.

1 mS cm-1 ~ 0.64 g l-1

The salt content can also be expressed as a mass percentage of the soil itself:

The diagram in Figure 7.2 shows these relations. The diagram has been based on the

assumptions that a soil with an ECe of 10 mS cm-1 and a saturation percentage (SP) of

100% contains 0.64% of salt. This value of 0.64% is one point on the diagram that is

connected to the zero point. The connecting line reflects the salt % for all ECe-values of a

soil with a SP-value of 100%. One can construct this relation for the other SP-values as

well.

Since the saturation percentage varies with texture, the diagonal lines help in

correlating the conductivity of the saturation extract with the percent salt content for

various soil textures. For example for a soil with a saturation percentage of 75 and an

ECe of 4 mS cm-1 (for which nearly all crops make good growth), this corresponds to a

salt content of about 0.2%. On the other hand, 0.2% salt in a sandy soil for which the


saturation percentage is 25 corresponds to an ECe of 12 mS cm-1, which is too saline for

good growth of most crop plants.

A decrease in soil moisture content (e.g. from saturation to the PWP) will lead to an

increase in the measured EC of the soil solution. Due to the great proportion of large

pores in coarse-textured soils, a relatively greater salt-concentrating effect will be

reported for sandy soils compared to clayey soils.

Figure 7.2. Relation of the % salt in the soil to the osmotic pressure and electrical

conductivity of the saturation extract

A partial list of crop plants in their order of tolerance to salinity is given in Table 7.2.

7.3.2.4. Other soil-water extract ratios

Since no convenient, rapid method exists for determining directly the composition of the

soil water, extraction procedures have tended to use relatively low soil-to-water ratios

for simplicity. The soil-to-water ratios commonly used are 1:1, 1:5 and 1:10. The greater

the amount of water used to obtain an extract, the greater the difficulty in making

inferences regarding the salinity of the soil solution in the field.


All salts can be considered, for convenience, as soluble or relatively insoluble. How these

two classes of salts respond to increasing water content for obtaining the soil extract will

be shown.

For soluble salts (e.g., NaCl, CaCl2, Na2SO4, etc.) the total amount of salt will be

independent of water content; however, the concentration of soluble salts decreases with

increasing water content according to the simple dilution process.

Table 7.2. Classification of crops according to their salt tolerance (ECe)

Relatively insoluble salts (e.g., CaCO3, CaMg(CO3)2, CaSO4.2H2O) will follow the

solubility-product principle. The concentration (activity) of these salts will remain

constant as the amount of water increases; however, the amount of salt brought into

solution as the water content increases will be proportional to the water content, that is,

the amount of salt will increase proportionally to the water content (assuming excess

solid phase present). If sufficient water is added to dissolve the solid phase, then in the

presence of additional increments of moisture, the curves representing the solubility of


relatively insoluble salts will behave in a manner similar to that of a soluble salt. The

effect of moisture content on the ionic composition of the extract is qualitatively

summarised in Figure 7.3.

Figure 7.3. Influence of moisture content on the concentration of salt in solution (a) and

the amount of salt in solution (b) for a completely soluble salt and a relatively insoluble

salt, respectively. The relative position of the curves has no significance.

Sometimes a 1/5 ratio extract (20 g of air dry soil and 100 ml water) is used as a fast

method and also when not enough soil material is available.

Since the dilution is higher than with the saturation extract, some less soluble salts

such as CaCO3 and CaSO4.2H2O, can be dissolved, resulting in a higher ECe value when

calculated as such.

Using a 1/5 extract one obtains, as the amount of salt per 100 g of soil, the ash of 0.5

liter extract. This means for an EC-value of 1 mS cm-1 0.32 g per 100 g of soil or 0.32%.

7.3.2.5. Mapping EC in the field

Advances in instrumentation now allow rapid, continuous field measurements of bulk

soil conductivity.

A first method uses a 4-electrode conductivity apparatus that can be driven across a

field. In this method, 4 carefully spaced electrodes are inserted in the soil. The data can

be transformed into a map showing the spatial variation of soil salinity across a field.


A second method employs electromagnetic induction (EM) of electrical current in a soil

which can be related to EC and salinity. This technique can measure EC to considerable

depth in the soil profile, without mechanically probing the soil.

7.3.3. Exchangeable sodium percentage

Exchangeable Na+ is determined by exchanging the Na+ from the soil with another ion

such as Ca2+ and then measuring the Na+ in solution.

It is often convenient to express the relative amounts of various exchangeable cations

present in a soil as a percentage of the cation exchange capacity. As such, the

exchangeable-sodium-percentage (ESP) is equal to 100 times the exchangeable-sodium

content divided by the CEC, both expressed in the same units:

ESP-levels greater than 15 are associated with severely deteriorated soil physical

properties and sometimes with pH values above 8.5. Soils with an ESP > 30 are very

impermeable, which seriously affects plant growth.

7.3.4. Sodium adsorption ratio

The SAR is a second, more easily measured property to indicate sodicity (alkalinity). It

gives information on the relative concentrations of Na+, Ca2+, and Mg2+ in soil solutions:

where [Na+], [Ca2+] and [Mg2+] are the concentrations (mmol l-1) of the Na+, Ca2+ and

Mg2+-ions in the soil solution.

The SAR of a soil extract takes into consideration that the adverse affect of sodium is

moderated by the presence of calcium and magnesium ions. It is also used to

characterise irrigation water applied to soils.

The value of the SAR is calculated on the assumption that equilibrium exists between

the ions in the saturation extract and the exchange complex of a given soil. This value


has been statistically related to the measured ESP of the given soil and is given by the

equation:

A SAR value of 13 for the solution extracted from a saturated soil paste is approximately

equivalent to an ESP of 15. For many soils the numerical values of the ESP of the soil

and the SAR of the soil solution are approximately equal up to ESP levels of 25 to 30.

A nomogram, which relates soluble sodium and soluble calcium and magnesium

concentrations to the SAR can be used (Figure 7.4). It includes also a scale for

estimating the corresponding ESP based on the linear equation of the ESP with SAR.

Figure 7.4. Nomogram for determining the SAR value of a saturation extract and for

estimating the corresponding ESP value of soil at equilibrium with the extract.


To use this nomogram, lay a straightedge across the figure so that the line coincides

with the sodium concentration on scale A and with the calcium plus magnesium

concentration on scale B. The SAR and the estimated ESP are then read on scales C and

D, respectively.

A logical question to ask is whether the SAR initially calculated from the analysis of the

saturation extract has the same value as that of the soil solution. Since the

concentration of the soil solution usually increases as the moisture content decreases

from saturation to the field moisture level, the question can be rephrased to ask whether

salinity affects the SAR value. Assuming that as the moisture content of the soil

decreases, no chemical precipitation occurs and plant uptake of salt is minimal, that is,

the cation ratio of the solution remains constant, it can be shown that:

In this case the SAR value increases directly as the square root of the total salt

concentration in the soil solution. Any factor that increases soil salinity tends to

increase the SAR.

7.4. EFFECTS OF SALINITY AND SODICITY ON PHYSICAL

SOIL DEGRADATION

Soil salinity and sodicity can have a major effect on the structure of soils.

In the presence of high amounts of exchangeable Na+, swelling and eventual dispersion

of the clay fraction, and slaking of the aggregates, may lead to the breakdown of the soil

structure. Swelling causes the soil pores to become more narrow and slaking reduces the

number of macro-pores through which water and solutes can flow, resulting in plugging

of pores by the dispersed clay. Blocking of the conducting soil pore system, reduces the

soil hydraulic conductivity to a very low value.

The mechanism by which Na+ promotes swelling-dispersion is based on the repulsion of

the clay particles when their Na+-dominated diffuse double layers interact. The theory of

the electrical double layer quantitatively describes this process. Swelling is affected by

clay mineralogy, the kind of ions adsorbed on the clays, and the electrolyte concentration

in solution. The tendency to swell and disperse is more important in soils with high clay


contents and when the clay is composed of 2:1 expanding minerals (smectite) that are

Na+-saturated. As the electrolyte concentration decreases, clay swelling increases.

Excessive amounts of salts in solution however compress the existing double layer,

thereby reducing the repulsion between clay particles. Thus at field management level,

the presence of salt reduces the sodic hazard by maintaining adequate permeability.

Figure 7.5 shows schematically this ameliorating effect of salinity on the degradation of

a soil as the ESP or SAR increases.

Figure 7.5. Salt (EC) effect on reducing soil degradation resulting from high Na (ESP or

SAR)

Consequently, if a soil has high quantities of exchangeable Na+ and the EC is low, soil

permeability, hydraulic conductivity, and the infiltration rate are decreased. Typically,

soil infiltration rates are initially high, if the soil is dry, and then they decrease until a

steady state is reached.

In the presence of a high salt concentration in the soil, the sodic hazard is minimised

and the infiltration and permeability of the soil remain adequate in spite of the increase

in ESP. The problem arises when the salt is removed, for example, by reclamation and

the high ESP manifests itself in the deterioration of the physical condition of the soil.

It is important to remember that the physical condition of all soils is not affected equally

as the ESP increases. Soils containing non-swelling clay, for example, kaolinite or

hydrous oxides, can have large ESP values without suffering effects compared with soils

containing the highly swelling clay montmorillonite.


The soil structure certainly in the upper part of the profile, can under some

circumstances strongly deteriorate. Such soil structure degradation is often due to

changes in the soil/water chemical composition as a result of the quality of the irrigation

water such as a too low salt concentration coupled with high sodium content.

7.5. CLASSIFICATION OF SALT AFFECTED SOILS

The composition of the saturation extract is used to classify soils into normal, saline,

sodic, and saline-sodic categories. The major criteria used to classify salt-affected soils

are:

− salinity of the saturation extract as measured by the electrical conductivity (ECe) at

25°C

− exchangeable-sodium percentage (ESP) as measured by the sodium-adsorption ratio

(SAR).

While the ESP is used as a criterion for classification of sodic soils, the accuracy of the

number is often a problem due to errors that may arise in measurement of CEC and

exchangeable Na+. Therefore, the more easily obtained SAR of the saturation extract

should be used to diagnose the sodic hazard of soils.

The limits for the various classes of salts in terms of ECe and SAR are given in Table

7.3.

At best any attempt to classify natural systems by a set of single-value parameters

cannot be totally satisfying because of the many factors that affect how the parameters

respond under field conditions. The values presented should be used as a guide,

realising that a transition between categories represents a range of values, not a defined

point.

Table 7.3. Classification of Salt-Affected Soils

Class ECe (dS/m) ESP (%) pH

normal < 4 < 15 < 8.5

saline > 4 < 15 < 8.5

sodic <4 > 15 > 8.5

saline-sodic > 4 > 15 > 8.5


7.5.1. Saline soils

Saline soils have traditionally been classified as those in which the ECe of the

saturation extract is > 4 dS m-1, ESP < 15% and SAR < 13. The SAR-value reflects a

relatively low level of adsorbed sodium, whereas the high salt concentration gives the

saturation extract an ECe value of greater than 4 mS/cm.

The major problem with saline soils is thus the presence of soluble salts. The Cl- and

SO42- ions are the principal anions, sometimes NO3

-, and with smaller amounts of HCO3-.

The principal cations are Ca2+ and Mg2+ with lesser amounts of Na+. As with most salt

affected soils, the concentration of K+ ion is low. Salts of low solubility, such as CaSO4

(CaSO4.2H2O) and lime (CaCO3 and CaMg(CO3)2), may also be present.

The high salt concentration keeps the soil in a flocculated condition, and water

permeability is high. The soil pH is less than 8.5. Reclamation is usually required for

satisfactory crop yields.

Saline soils are often recognised by the presence of a white crust on the surface and

often referred to as "white alkali" soils. The absence of a salt crust however does not

preclude the existence of a saline soil.

7.5.2. Sodic soils

Sodic soils have an ESP > 15, the ECe is < 4 dS m-1, and the lower limit of the saturation

extract SAR is 13. Sodic soils are characterised by a soil water of low salinity. The SAR

values are larger than 13, indicate that a greater amount of exchangeable Na+ exists in

sodic soils than in saline soils. Consequently, Na+ is the major problem in these soils.

Sodic soils have a pH between 8.5 and 10. The exchangeable Na+ hydrolyses, forming

NaOH in the soil solution. The NaOH can then react with dissolved CO2 to form NaCO3.

Under these conditions the pH of sodic soil can increase to 10.

The major anions in the soil solution of sodic soils are Cl-, SO42- and HCO3

-, with lesser

amounts of CO32-. Since the pH is high and CO3

2- is present, Ca2+ and Mg2+ are

precipitated, and therefore soil solution Ca2+ and Mg2+ are low. Besides Na+, another

exchangeable and soluble cation that may occur in these soils is K+.


The presence of sodium ion represents a potential hazard to the soil of the arid and

semi-arid regions. When the ESP of a soil reaches 15% or greater, a deterioration of the

physical structure of the soil may occur. The high amount of Na+ in these soils, along

with the low ECe, results in dispersion, which greatly reduces both air and water entry

into the soil.

Sodicity is particularly serious in heavy-textured soils that contain 2:1 expanding clay

minerals. Sandy soils are correspondingly less affected by increasing sodium on the

exchange complex because of their low clay content. The quantity and type of clay

present in the soil thus are considerations in assessing how SAR and ESP values affect

soil sodicity. A higher SAR value may be of less concern if the soil has a low clay content

or contains low quantities of smectite.

These soils are often referred as " black alkali". The term black alkali arises from the

dissolution of soil organic matter and its subsequent deposition of soil surface by

evaporation. This process produces a characteristic darkening of the soil surface.

7.5.3. Saline-sodic soils

Saline-sodic soils have an ECe > 4 dS m-1 and an ESP > 15. Thus, both soluble salts and

exchangeable Na+ are high in these soils. Since electrolyte concentration is high, the soil

pH is usually < 8.5 and the soil is flocculated. However, if the soluble salts are leached

out, usually Na+ becomes an even greater problem and the soil pH rises to > 8.5 and the

soil can become dispersed.

Table 7.4 Analyses of saturation extracts of arid region soils

ECe Cations (meq/l) Anions (meq/l) SAR

(dS/m) Ca Mg Na K Total HCO3 SO4 Cl CO3 Total (-)

Normal soils

0.60 2.71 2.26 1.20 0.91 7.08 2.60 2.09 0.87 0 5.56 0.8

1.68 3.33 1.94 12.20 0.70 18.17 6.14 4.28 4.93 0 15.35 7.5

0.84 2.76 1.69 5.22 0.18 9.85 6.63 2.67 0.44 0 9.74 3.5

Saline soils

12.0 37.0 34.0 79.0 0.40 150.4 7.20 62.2 47.0 0 148.4 13.3

8.8 28.4 32.8 53.0 1.10 105.3 5.20 74.0 29.0 0 108.2 10.5

Sodic soils

1.74 1.10 1.42 15.6 0.42 18.5 6.51 8.48 2.86 0 17.9 13.9

2.53 1.41 1.01 21.5 0.28 24.2 3.29 3.80 16.70 0 23.8 19.6

3.16 1.10 0.30 29.2 4.10 34.7 18.70 4.60 7.50 8.4 39.2 35.0

Saline-sodic soils

9.19 6.73 9.85 79.5 0.48 96.6 2.35 20.10 72.0 0 94.5 27.6

16.70 32.40 38.30 145.0 0.51 216.2 3.29 105.00 105.0 0 213.4 24.4

5.60 0.60 0.90 58.5 1.60 61.6 19.90 21.50 16.30 5.0 62.7 67.4


7.6. IMPACT ON PLANT GROWTH

Soil salinisation causes harm to plant life (soil fertility, agricultural productivity, growth

of cultivated crops and their biomass yield); natural vegetation (ecosystems); life and

function of soil biota (biodiversity); soil functions (increased erosion potential,

desertification, soil structure degradation, aggregate failure, compaction); the

hydrological cycle (changed moisture regime, increasing hazard – frequency, duration,

severity – of extreme moisture events as flood, water logging, and drought); and

biogeochemical cycles (affecting availability of plant nutrients, pollutants, potentially

harmful elements and compounds).

7.6.1. How salts affect plant growth

The salt content above which plant growth is affected depends on several factors. The

presence of salinity in the soil solution, resulting from either indigenous salt or that

added by irrigation, could affect plant growth directly in two ways:

− Osmotic effects : reduction of the osmotic potential and hence the water potential,

thereby reducing water availability; and

− Specific-ion effects: increased concentration of certain ions that have a characteristic

toxic effect on plant metabolism.

Sodicity also affects plant growth indirectly through its impact on soil structural

degradation and nutrient imbalances.

7.6.1.1. Osmotic Effect

The osmotic potential Ψo and the matric potential Ψm are additive and under field

conditions determine the soil water potential Ψw, where Ψw = Ψm + Ψo. Soluble salts

lower the osmotic potential of the soil water, making it more difficult for roots to remove

water from the soil. The term physiological drought has been applied to the apparent

shortage of water within a plant when growing in a moist saline soil or saline solution.

Plants are most susceptible to salt damage in the early stages of growth. Many plants,

for example, barley, wheat, and corn, are sensitive to the osmotic effect during

germination and early seedling stages. Effort should be made to keep the salinity as low

as feasible during these stages even though the crop has greater tolerance at later

stages.


For established plants this rarely results in wilting or even reduced water uptake, but it

does require the crops to expend more energy making osmotic adjustments to counteract

the low osmotic potential of the soil solution. This lost energy results in decreased

growth. The plant functions that are affected include photosynthesis, hormone

production, stomatal opening and respiration.

The inhibiting effect of soil salinity on plant growth can be decreased by increasing the

frequency of irrigation, thereby maintaining the total water potential as high as

possible. Even at field capacity the presence of salt in the soil water requires that the

plants exert enough energy to overcome the value of' Ψw to obtain adequate water.

The relation between ECe and ECiw (irrigation water) can be approximated by assuming

that the irrigation water concentrates three times as it becomes soil water (3ECiw =

ECsw) and that the salinity of the saturation extract is half that of the soil water (2ECe =

ECsw) or ECe = 3/2ECiw. For example, the yield of sugar beets is reduced 10% when the

ECe reaches 8.6 mS/cm. This corresponds to ECiw = 5.7 mS/cm, which is the EC of the

irrigation water that would result in a 10% yield decrement if the assumptions are valid.

Also incorporated in this calculation is the concept that salt concentration is the only

process involved, and all salinity is derived from irrigation water.

7.6.1.2. Specific-Ion Effect

Knowledge of total salinity is not always adequate to define a problem. The kind of salt

can make a big difference in how plants respond to salinity.

In addition, possible toxic effects must be considered. Certain ions, including Na+, Cl-,

H3BO4- are quite toxic to many plants. Other ions may have regional importance. Toxic

levels of ions differ greatly between plant species, reflecting the selective ion adsorption

and nutrient requirements of the plant.

Chloride

The chloride ion exists in a wide range of concentrations in irrigation waters and soils. A

typical symptom of chloride toxicity is the burning of the leaf margins and early drop in

citrus. Stone fruit trees, citrus, avocado, grapes, almonds, and ornamentals are sensitive

to chloride. Toxicity symptoms of chloride appear in sensitive plants when leaves

accumulate about 0.50% on a dry weight basis. Most agronomic crops can tolerate up to

5-10% without developing injury symptoms. Fruit tree variety and rootstock show large

differences in tolerance for chloride.


Sodium

Sodium and chloride toxicity are usually discussed together because they affect the

same type of plants and they are often associated in the soil solution. Leaf burn

symptoms occur when leaves of sensitive plants accumulate about 0.25% on a dry weight

basis. More information on the tolerance of various crops to sodium is discussed in

section 7.6.2.2.

Boron

Boron tends to accumulate in soils to a greater extent than other soluble salts. Thus

water with even low concentrations may produce toxic effects if accumulation of boron

occurs.

Boron toxicity, unlike chloride and sodium toxicity, affects all crops when even

moderately low levels of boron are present in the soil solution. Although boron is an

essential plant element, it becomes toxic to plants only slightly above optimum

concentrations in the soil. Foliar analysis is the preferred method of detecting boron

toxicity. Normal foliage, depending on plant variety, contains 25-100 ppm boron,

whereas boron levels of > 200 ppm are associated with boron toxicity. Crop tolerance to

boron varies.

7.6.1.3. Physico-chemical effects of sodicity

Deterioration of physical properties may also be a factor in determining which plants

can grow on sodic soils. The colloidal dispersion caused by sodicity may harm plants in

at least 2 ways:

(1) Oxygen becomes deficient due to the breakdown of soil structure and the very

limited air movement; and

(2) Water relations are poor due to the very slow infiltration and percolation rates

(Brady and Weil, 2008).

Besides specific toxicities, high levels of Na+ can also cause imbalances in the uptake

and utilisation of other cations, eg K+.


7.6.2. Tolerance to salinity and sodicity

Plants vary considerably in their ability to tolerate soil salinity and sodicity.

Satisfactory plant growth on saline/sodic soils depends on a number of interrelated

factors, including the physiological constitution of the plant/variety or rootstock type,

the growth stage, and the rooting pattern. The proper choice of crop therefore becomes

an important management option that can be used to reduce field salinity problems.

7.6.2.1. Tolerance to salinity

Table 7.5 presents a classification of saline soils, and their relation with crop tolerances

to salinity is proposed. In Table 7.6 examples of crops with variable tolerances to

salinity are shown.

Table 7.5. Classification of saline soils based on ECe, and their relation with crop

tolerances to salinity

ECe

(dS/m)

TDS

(%)

Classification Crop tolerance/response

<2 <0.1 Non-saline Salinity effects or generally negligible

2-4 0.1-0.15 Slightly saline Yields of very sensitive crops restricted

4-8 0.15-0.35 Moderately saline Yields of sensitive to moderately sensitive crops

restricted

8-16 0.35-0.70 Very saline Only tolerant crops yield satisfactorily

>16 >0.70 Extremely saline Only a very few very tolerant crops yield satisfactorily

Evaluation of plant salt-tolerance data suggested that for each crop a certain threshold

value exist beyond which crop yields decrease linearly with increasing salinity. When

the ECe value is less than some prescribed threshold value, crop yields are unaffected

and represent 100% relative yield (0% decrement) in terms of soil salinity (Figure 7.6).

Approximate yield reductions in relationship to increasing salinity of the soil extract

ECe for various crops are shown in Figure 7.7.

Many interpretations of ECe values have been devised, but no universal, precise

interpretation is possible because the effects of salinity are modified greatly by other

factors. The response of a crop to salinity and/or sodicity depends on a number of

interrelated factors, including their sensitivity to salinity and alkalinity, described

above, and how this interacts with the soil moisture management, irrigation methods

and irrigation water quality, soil texture, salt types present and their distribution, soil

fertility, and climate. For instance, hot, dry conditions aggravate salinity effects,

whereas cool, humid conditions reduce salinity problems. So it is obvious that salt-


tolerance data for any given crop cannot be considered fixed values; but they should

serve as guidelines in management decisions.

Table 7.6. Classification of crops according to their salt tolerance

Figure 7.6. Relative productivity of 5 groups of plants classified by their sensitivity to

salinity as measured by the ECe


Figure 7.7. Salt tolerance of field and vegetable crops (USDA, 1964)

7.6.2.2. Tolerance to sodicity

The tolerances of various crops to the ESP of the soil are presented in Table 7.7. Note

that sensitive crops suffer from toxicity at ESP values that do not affect the physical

properties of the soil. As the ESP increases, the more tolerant crops show effects of both

poor soil conditions and unbalanced nutrition.


Table 7.7. Tolerance of various crops to ESP

ESP (%) Crop tolerance Impact on growth Example crops

2 – 10 Extremely sensitive Na-toxicity symptoms even at

low ESP values

Deciduous fruits, nuts,

citrus, avocado

10 – 20 Sensitive Stunted growth at low ESP

values even though the

physical condition of the soil

may be good

Beans

20 – 40 Moderately tolerant Stunted growth due to both

nutritional factors and

adverse soil conditions

Clover, oats, tall fescue, rice,

dallisgrass

40 – 60 Tolerant Stunted growth usually due

to adverse physical

conditions of the soil

Wheat, cotton, alfalfa,

barley, tomatoes, beets

>60 Most tolerant Stunted growth usually due

to adverse physical

conditions of the soil

Crested and fairway

wheatgrass, tall wheatgrass,

Rhodes grass

Table 7.8 summarises some optimal and marginal ESP values. The greater tolerance for

alkalinity by the coarse and medium textured soils is due to their good permeability and

porosity, allowing some dispersion of the clay particles, without seriously affecting water

and oxygen supply.

Table 7.8. Optimal and marginal ESP values for different types of crops or land

utilisation types

The leaves of the stone fruits, almonds, and citrus trees absorb both sodium and chloride

readily. Thus irrigation sprinklers that wet foliage can result in sodium or chloride

damage even though the water used would not cause any damage if applied to the soil.

As little as 4 meq/l of sodium or chloride (about 70 ppm or 100 ppm respectively) may

induce toxicity symptoms in sensitive fruit trees. Foliar absorption restricts the use of

sprinkler irrigation for some fruit crops.


7.7. RECLAMATION OF SALINE AND SODIC SOILS

Management of irrigated soils should aim to simultaneously minimize drainage water

and protect the root zone from damaging levels of salt accumulation. Since these two

goals are in conflict, the land manager will have to find an optimal compromise through

adapted soil management techniques.

One option can be to grow salt-tolerant crops or select the varieties that are most

tolerant. Also the timing of the irrigation is important on saline soils. Especially

emerging and young crops are sensitive to salinity and thus irrigation should precede or

immediately follow planting in order to move salts away from the seedling roots.

The location of the salts in or outside the root zone is also a critical factor that can be

influenced by soil management:

- Conservation tillage or surface residue management reduce evaporation through

the soil surface and therefore reduce the upward transport of soluble salts;

- Irrigation techniques that direct salt concentrations away from the root zone of

young plants can allow higher levels of salt to accumulate without damage to the

crop; and

- Frequent but slowly applied irrigation water using sprinkler- or drip-irrigation

systems dissolve salts and move them away, towards the border of the wetting

front (should be outside the root zone)

The restoration of soil physical and chemical properties conducive to high productivity is

referred to as soil reclamation.

7.7.1. Reclamation of saline soils

Saline soils can be reclaimed by leaching them with good-quality (low electrolyte

concentration) water. The water causes dissolution of the salts and their removal from

the root zone. For successful reclamation, salinity should be reduced in the top 45 to 60

cm of the soil to below the threshold values for the particular crop being grown.

Reclamation can be hampered by several factors : restricted drainage caused by a high

water table, low soil hydraulic conductivity due to restrictive soil layers, lack of good-

quality water, and the high cost of good-quality water. If the natural soil drainage is

inadequate to accommodate the leaching water, an artificial drainage network must be

installed. Intermittent applications of excess irrigation water may be required to


effectively reduce the salt content to a desired level. The process can be monitored by

measuring the soil‟s EC (Brady and Weil, 2008).

The amount of water needed to remove the excess salts from saline soils is called the

leaching requirement. The LR is determined by the characteristics of the crop, the

irrigation water, and the soil. An approximation of the LR for relatively uniform salinity

conditions is given by the ratio of the salinity of the irrigation water to the maximum

acceptable salinity of the soil solution for the crop to be grown:

The LR indicates water that needs to be added in excess of that needed to thoroughly

wet the soil and meet the crop‟s evaporative transpiration needs. Multiplication of LR

with the amount of water needed to wet the root zone yields the minimum amount of

water that needs to be leached through the water-saturated zone to maintain the root

zone salinity to an acceptable level.

This leaching requirement approach to manage irrigated soils is only an approximation

and has several inherent weaknesses:

- Additional leaching might be needed to reduce excess concentration of specific

toxic elements (B);

- it does not take into account the rise in water table that is likely to result from

increased leaching; and

- it leads to an over-application of water if the entire field is treated whereas the

problem might be restricted to some saline spots.

An alternative approach would be to closely monitor the salinity of the soil profile

(Figure 7.8) by taking repeated measurements in the field. The sufficiency of leaching

could then be judged by the type of salinity profile observed. This can be combined with

site-specific management techniques.


Figure 7.8. Soil salinity profile curves showing the levels of salts throughout the root zone

7.7.2. Reclamation of sodic soils

In sodic soils reclamation is effected by applying gypsum (CaSO4.2H2O) or CaCl2 to

remove the exchangeable Na+. The Ca2+ exchanges with the Na+ which is then leached

out as a soluble salt, Na2SO4 or NaCl. The CaSO4 and CaCl2 also increase permeability

by increasing electrolyte concentration. Sulfur can also be applied to correct a sodium

problem in calcareous soils (where CaCO3 is present).

7.7.3. Reclamation of saline-sodic soils

In saline-sodic soils a saltwater-dilution method is usually effective in reclamation. In

this method the soil is rapidly leached with water that has a high electrolyte

concentration with large quantities of Ca2+ and Mg2+ to remove excess soluble salts and

exchange Na+ from the soil as a soluble salt. After leaching, and the removal of Na+ from

the exchanger phase of the soil, the soil is leached with water of lower electrolyte

concentration to remove the excess salts.

7.8. CONTROLLING SALT-BUILD UP IN SOILS

To develop a reliable method for the prediction and control of salinization and

alkalization, the following problems must be solved in the course of surveying and

monitoring:


− the main sources of water-soluble salts (irrigation water, groundwater, surface

waters, salty deep layers, etc.) must be identified and quantitatively characterised;

− the main features of the salt regime (salt balance) must be characterized;

− the whole range of natural factors influencing the salt regime must be analyzed; and

− the impact of human activity has to be studied and accurately determined.

Consequently, an exact salinity and/or alkalinity prognosis must be based on the

evaluation of many natural and human factors and a thorough knowledge of the soil

processes in progress.

These data will provide guidelines for adopting preventive and control measures. Such a

system is a precondition for identifying the early warning signals of the onset of

degradative processes. Identification is a key to successfully combat salinization.

Numerous methods and approaches exist to characterize the dynamics of salts in the

soil at different scales and levels of detail.

7.8.1. Salt balance

The salt balance expresses not only the present salt content of soils but also its changes,

taking into consideration input and output factors. Whether estimated in a single field

or in a large watershed, understanding the salt balance is a basic requisite for wise

management of salt-affected soils.

It is possible to set up the salt balance of a large area, e.g., a river valley, but the

information obtained by this method is much more concrete if a smaller area, such as an

irrigated field, is considered.

To achieve the salt balance, the amount of salt coming in, must be matched with the

amount being removed. Meeting this condition is a fundamental challenge to the long-

term sustainability of irrigated agriculture.

The general form of a salt balance is:

With DS = the change in salt or sodium content of the soil (ton/ha)


S2 = quantity of salts/exchangeable sodium at the end of the reference

period (ton/ha)

S1 = quantity of salts/exchangeable sodium at the beginning of the

reference period (ton/ha)

The type of the balance and the length of the balance period can be chosen, depending

on the goal of the study or the type of the problem that needs to be assessed.

Salt balances can be calculated for:

− The total salt content or for specific ions

− For the whole soil profile from surface to water table, for various layers or specific

horizons, or for the root zone

− For vegetation periods, cropping/irrigation seasons, years.

Besides these general salt balances, it is also very important to establish more detailed,

factorial balances that also reveal the partial contribution of various factors. The factors

in the general form of the salt balance are:

Where

DS = change in the salt content of the soil at a given depth and over a given period

W = salts derived from local weathering products

P = salts derived from the atmosphere (airborne salts, rainfall, wind action, etc.)

R = horizontal inflow of salts transported by surface waters,

G = horizontal inflow of salts transported by subsurface waters,

Y = quantity of salts added with irrigation water,

F = salts added as chemical amendments,

lp = salts leached out by precipitation,

r = horizontal outflow of salts transported by surface waters,

g = horizontal outflow of salts transported-by subsurface waters

li = quantity of salts leached out by irrigation water, and

u = salts assimilated by plants and transported from the area with the yield.

All factors can be expressed for instance in ton/ha or mg/100 g.


Three types of salt balance can be distinguished:

− Stable: The salt content of the soil at the depth does not change during the period of

observation;

− Accumulation: The salt balance is positive and the total salt content of the soil at the

depth increases during the period;

− Leaching: The salt balance is negative and the salt content of the soil at the depth

decreases during the period.

7.8.2. Salinity hazard

Maps of the salinity hazard show the potential for salt mobilisation in various

catchments and basins. An area with a high salinity hazard will become saline only if

there is a change in management practices that affect the water balance and mobilise

salt in the landscape.

Salinity hazard maps can inform catchment and land managers of those parts of the

landscape that are most vulnerable to salinity. It is an indication of the vulnerability of

the landscape to salinity due to the inherent characteristics of the landscape. Such areas

require a more rigorous assessment of the potential impact of changes in land

management on groundwater systems. While areas with a high hazard rating should be

treated with most caution, areas of moderate or moderate to high salinity hazard should

not be ignored in any planning decisions.

For mitigating salinization and/or alkalization hazards in irrigated areas, or areas to be

irrigated, the following factors should be evaluated:

− climatic parameters,

− geological and landscape factors,

− soil profile and pedogenesis factors,

− agrotechnological factors, and

− irrigation techniques.

These factors determine the aims and methods of the preliminary survey, and define the

existence or the degree of potential salinity and/or alkalinity in soils.

Example:

Dryland salinity can occur when a rising water table brings natural salts buried deep in the

soil to the surface. The salt remains in the soil and becomes progressively concentrated as


the water evaporates or is used by plants. One of the main causes for rising water tables is

the removal of deep rooted plants, perennial trees, shrubs and grasses and their replacement

with annual crops and pastures that do not use as much water (Figure 7.9).

Figure 7.9: Dryland salinity process

The natural, inherent vulnerability to dryland salinity in this case depends on the

presence of salts in the soil, the recharge potential and the discharge sensitivity (Figure

7.10).

Figure 7.10. Model showing recharge, transmission and discharge zones

The amount of salts that accumulate in a soil depend on their possible sources (see

earlier, atmospheric deposition, weathering), and the amount of leaching which is

dependent on the amount of rainfall and soil properties. The groundwater recharge

potential depends on the frequency and volume of rainfall events, the soil moisture

balance, and soil properties such as permeability. The sensitivity to discharge can be

related to a particular topographic position, or the presence of impermeable soil/rock

leading to a rise in groundwater table.


The salinity hazard is determined by a number of relatively stable, long term conditions

that affect salinity processes. These include climate (rainfall and evaporation),

topography, catchment shape, groundwater system properties, geomorphology, drainage,

and others. Based on these datasets, different classes of salinity hazard can be defined:

− High salt storage tends to contain soil materials (e.g. deep clays) that by their very

nature accumulate high concentrations of salts. They tend to allow only very slow

movement of water through them. In addition, their position in the landscape is such

that it is difficult for salts to be flushed free. They therefore accumulate salt over

millennia.

− Moderate salt storage contains soil materials that permit slightly better

through‐flow of groundwater. There is some potential for substantial salts to

accumulate in parts of the landscape.

− Low salt storage has soil material (e.g. sands) which has a very limited capacity to

accumulate salts.

Salinity hazard and risk are distinctly different, but interrelated, concepts that are

frequently confused. Salinity hazard is a function of the inherent characteristics of the

landscape that predispose it to land or water salinity. Salinity risk is the probability

that certain management practices will contribute to the expression of land or water

salinity in the landscape when changes occur over time.

It is important to note that salinity risk is more difficult to determine than salinity

hazard. In order to determine salinity risk, the potential for salinity must be weighed up

against the likelihood and the consequence of salinity occurring within a specified time

period. It is determined by overlaying salinity hazard with conditions affecting salinity

processes that can change over time (e.g. land use, vegetation condition, soil condition).

7.8.3. Preliminary surveys and monitoring schemes

7.8.3.1. Preliminary surveys

Soil properties, groundwater depth and chemical composition as well as the salt balance

and hazard can be represented on maps with recommendations for techniques of

irrigation and water use. Evidently, the environmental conditions and land use methods

should also be considered. Accordingly, different limit values and different methods,

based on uniform principles, should be selected in the course of this procedure.


The series of maps include maps for the pedological, hydrological and farming factors of

salt build-up as well as maps with recommendations for land use, irrigation, drainage,

cropping, etc.

Mapping the results of preliminary and subsequent surveys constitutes not only a good

display of soil and environmental conditions of the irrigated areas or areas to be

irrigated, but can also provide guidelines for proper irrigation and land protection.

Table 7.9 shows that the prediction and prevention of secondary salinization and

alkalization of the soils to be irrigated should be based on a preliminary survey of the

landscape and soils before the construction of the irrigation system.

Table 7.9. Scheme of methods recommended for the control of salt build-up in irrigated

soils

Before construction of an irrigation system: Preliminary survey of:

Soils Landscape Planned irrigation

Genesis Climate Available irrigation water

Spatial distribution Hydrology Groundwater depth and

quality

Properties Hydrogeology Irrigation technology

Salinity/alkalinity geomorphology Salt tolerance of crops

During irrigation: Monitoring of:

Physical soil properties Salinity and alkalinity of the GWT

Water infiltration Chemical composition GW

Toxic elements in soil and

water, if any

Chemical composition of the irrigation water

7.8.3.2. Monitoring irrigation schemes

During irrigation, a well-organized monitoring of soil and water properties is to be

conducted in order to record changes, if any, and to undertake precautions, when

necessary. Monitoring methods as well as the timing and location of sampling depend

upon local conditions. The following factors have to be outlined and thoroughly

determined:

− aim and subject of investigations (the factors and features to be examined)

− sampling (time, frequency, intervals, methods, etc.)

− field and laboratory examinations (mathematical procedures, statistical analysis,

possibly a computer program, etc.)


7.8.3.3. Monitoring threat of salinisation at European scale

Within the context of the European Strategy for Soil Protection, 3 key issues for the

threat “Soil Salinisation” have been selected:

− Salinisation : accumulation of salts in the upper soil horizons resulting in

physiological or physical deterioration and extreme moisture regime.

− Sodification : accumulation of exchangeable sodium in the upper soil horizons

resulting in physiological or physical deterioration and extreme moisture regime.

− Potential salinisation : risk of salt accumulation from either a rising saline water

table or one with an unfavourable ion composition, or salt movement from lower to

upper horizons and the active root zone, or salt water intrusion from the sea, or the

incorrect use of saline or brackish irrigation water

The selected indicators in the European Soil Strategy (Figure 7.11) call attention to the

actual or potential state of salinisation/sodification and serve as guidelines for the

identification of hot spots where these environmental hazards are important.

The indicator ‟salt profile‟ was selected because it gives a complete picture on the

salinity state of the soil, or more exactly the salt-affected area. Salt profiles give a

vertical picture of the distribution of water soluble salts.

Exchangeable sodium percentage (ESP) was chosen as main indicator, as ESP and SAR

(Sodium Absorption Ratio) are the best characteristics for Solonetz (soil with a natric

horizon) formation.

The indicator „potential salt sources‟ gives a good picture of the characteristics of the

potential salt source (irrigation water or groundwater) as well as the vulnerability of

soils to salinisation/sodification.


Figure 7.11. Key issues and indicator selection for soil salinisation

Table 7.10 summarises the baseline and threshold values for the indicators selected for

soil salinisation.

Table 7.10. Baseline and threshold values for soil threat soil salinisation in Europe


7.8.4. Soil salinity modelling

Darab et al. (1994) designed a computation model to predict the conditions of irrigation,

taking into account the depth of groundwater at a stable salt balance (critical depth) and

the use of irrigation waters with different salinity.

The model was applied to soils where the regime of soil compounds and the factors

affecting it were known, and their relevant data had been used previously to determine

limit values for the salt content of irrigation water (Darab and Ferencz, 1969). These

authors made three case studies of characteristic regions in the Tisza-Valley which are

typified by the following features:

− Low groundwater table, a slight decrease in the average salt content, considerable

drainage effect, and seasonal salt accumulation,

− Accumulation - as a rule - prevails over leaching processes, salt accumulation, and

seasonal salt leaching resulting from irrigation, and

− Salt-affected rice field with poor drainage, and salt accumulation in the summer.

More salts accumulate from irrigation and groundwater than are leached out from

the soils.

According to the calculated salt balance of the three cases studied, the possibilities of

irrigation and leaching are found to be different for different places.

The majority of the computer models currently available for water and solute transport

in the soil e.g.:

- SWAP (http://www.swap.alterra.nl/),

- DrainMod-S (http://www.bae.ncsu.edu/soil_water/drainmod/models.html),

- UnSatChem (http://www.ars.usda.gov/Services/docs.htm?docid=8966))

are based on Richard's differential equation for the movement of water in unsaturated

soil in combination with a differential salinity dispersion equation.

The models require input of soil characteristics like the relation between unsaturated

soil moisture content, water tension, hydraulic conductivity and dispersivity. These

relations vary to a great extent from place to place and are not easy to measure. The

models use short time steps and need at least a daily data base of hydrological

http://www.swap.alterra.nl/

http://www.bae.ncsu.edu/soil_water/drainmod/models.html


phenomena. Altogether this makes model application to a fairly large project the job of a

team of specialists with ample facilities.

Simpler models, like SaltMod based on seasonal water and soil balances and an

empirical capillary rise function, are also available. They are useful for long-term

salinity predictions in relation to irrigation and drainage practices.

Spatial variations owing to variations in topography can be simulated and predicted

using salinity cum groundwater models, like SahysMod.

References



Darab, K., Rédly, M., Csillag, J. 1994. Salt balance in sustainable irrigated farming.

Agrokémia és Talajtan 43 (1-2): 196-210.

USDA 1964. Agricultural Information Bulletin n° 283. US Dept. of Agriculture,

Washington D.C.

Soil Degradation | Decline in OM 129

8. Decline in OM

8.1. DEFINITION OF SOIL ORGANIC MATTER

Soil organic matter (SOM) and humus can be thought as synonyms, and include the

total organic compounds in soils excluding undecayed plant and animal tissues, their

"partial decomposition" products, and the soil biomass. Soil organic matter contents

generally range from 0.5 to 5% on a weight basis in the surface horizon of mineral soils

to 100% in organic soils. The quantity of soil organic matter in a soil depends on climate,

vegetation, parent material, topography and time.

8.2. COMPOSITION OF SOIL ORGANIC MATTER

Humic substances (HS) can be defined as "a general category of naturally occurring,

biogenic, heterogeneous organic substances that can generally be characterized as being

yellow to black in color, of high molecular weight (MW), and refractory". Humic

substances can be subdivided into humic acid (HA), fulvic acid (FA), and humin.

Definitions of HS are classically based on their solubility in acid or base.

The main constituents of SOM are C(52-58%), O(34-39%), H(3.3-4.8%), and N(3.7-4.1%).

As shown in Table 8.1, the elemental composition of HA from several soils is similar.

Other prominent elements in SOM are P and S. The major organic matter groups are

lignin-like compounds and proteins with other groups, in decreasing quantities, being

hemicellulose, cellulose, and ether and alcohol soluble compounds. While most of these

constituents are not water soluble, they are soluble in strong bases.

Table 8.1. Elementary composition of humic acids from different soils


Soil organic matter consists of non-humic and humic substances. The non-humic

substances have recognizable physical and chemical properties and consist of

carbohydrates, proteins, peptides, amino acids, fats, waxes, and low-molecular-weight

acids. These compounds are attacked easily by soil microorganisms and persist in the

soil only for a brief time.

Humic acids are extremely common and are found in soils, waters, sewage, compost

heaps, marine and lake sediments, peat bags, carbonaceous shales, lignites, and brown

coals. While they are not harmful, they are not desirable in potable water.

Table 8.2 shows the average elemental composition for HA and FA. The major elements

composing HA and FA are C and O. The C content varies from 41 to 59% and the O

content varies from 33 to 50%. Fulvic acids have lower C (41 to 51%) but higher O (40 to

50%) contents than HA. Percentages of H, N, and S vary from 3 to 7, 1 to 4, and 0.1 to

4%, respectively. Humic acids tend to be higher in N than FA, while S is somewhat

higher in FA. Soil HA have O/C ratios around 0.50, while FA have O/C ratios around

0.70.

Table 8.2. Average values for elemental composition of soil HS

The main acidic (functional) groups of HS are carboxyl (R-C=O-OH) and acidic phenolic

OH groups (presumed to be phenolic OH), with carboxyls being the most important

group (Table 8.3). The total acidities of FA are higher than those for HA. Smaller

amounts of alcoholic OH, quinonic, and ketonic groups are also found. Fulvic acids are

high in carboxyls, while alcoholic OH groups are higher in humin than in FA or HA and

carbonyls (C=0) are highest in FA (Table 8.4).

While we know the elemental and functional group compositions of HS, definitive

knowledge of the basic "backbone structure" is still an enigma.


Table 8.3. Some important functional groups of SOM

R is an aliphatic (a broad category of carbon compounds having only a straight, or branched, open

chain arrangement of the constituent carbon atoms; the carbon - carbon bonds may be saturated

or unsaturated; backbone) and Ar is an aromatic ring.

Table 8.4. Functional groups in HS in cmol kg-1

8.3. INFLUENCE ON SOIL PROPERTIES AND THE

ENVIRONMENT

Soil organic matter affects so many soil properties and processes that a complete

discussion of this topic is beyond the scope of this course.

Some of the general properties of SOM and its effects on soil chemical and physical

properties are given in table 8.5. It improves soil structure, water-holding capacity,

aeration, and aggregation. It is an important source of macronutrients such as N, P and


S and of micronutrients such as B and Mo. It also contains large quantities of C which

provides an energy source for soil macroflora and microflora.

Table 8.5. General properties of SOM and associated effects in the soil

Soil organic matter has a high specific surface (as great as 800-900 m² g-1) and a cation

exchange capacity (CEC) that ranges from 150 to 300 cmol kg-1. Thus, the majority of a

surface soil's CEC is in fact attributable to SOM. Due to the high specific surface and

CEC of SOM, it is an important sorbent of plant macronutrients and micronutrients,

heavy metal cations, and organic materials such as pesticides. The uptake and

availability of plant nutrients, particularly micronutrients such as Cu and Mn, and the

effectiveness of herbicides are greatly affected by SOM.

Figure 8.1 summarizes some of the more important effects of organic matter on soil

properties and on soil-environment interactions.


Figure 8.1. Some of the ways in which soil organic matter influences soil properties, plant

productivity, and environmental quality. Many of the effects are indirect, the arrows

indicating the cause-and-effect relationships (the thicker line shows the sequence referred

to in the text).


Often one effect leads to another, so that a complex chain of multiple benefits results

from the addition of organic matter to soils. Many of the effects are indirect, the arrows

indicating the cause-and-effect relationships. For example (beginning at the upper left

in Figure 8.1; see thicker line), adding organic mulch to the soil surface encourages

earthworm activity, which in turn leads to the production of burrows and other biopores,

which in turn increases the infiltration of water and decreases its loss as runoff, a result

that finally leads to less pollution of streams and lakes.

It can readily be seen that the influences of soil organic matter on soil properties, plant

productivity and environmental quality are far out of proportion to the relatively small

amounts present in most soils.

8.4. MANAGEMENT OF SOIL ORGANIC MATTER

Perhaps the most useful approach to defining soil organic matter quality is to recognize

different pools of organic carbon that vary in their susceptibility to microbial

metabolism. A model identifying five such pools of carbon in plant residues and soil

organic matter is illustrated in Figure 8.2.

Plant residues contain some components (metabolic carbon), such as sugars, proteins,

and starches, that are quite readily metabolized by soil microbes. Other components

(structural carbon) exist mostly in the structure of the plant cell walls, including lignin,

polyphenols, cellulose, and waxes, and are resistant to decomposition. The model in

Figure 8.2 denotes these groups as metabolic and structural pools within the plant

residues.

The total organic matter content of a soil is the sum of several different pools of soil

organic matter, namely the active, slow and passive fractions.

− The ACTIVE FRACTION of soil organic matter consists of materials with relatively

high C/N ratios (about 15 to 30) and short half-lives (half of these materials can be

metabolized in a matter of a few months to a few years). Components probably

include the living biomass, some of the fine particulate detritus (referred to as

particulate organic matter or POM), most of the polysaccharides, and other

nonhumic substances as well as some of the more labile (easily decomposed) fulvic

acids. This active fraction provides most of the readily accessible food for the soil


organisms and most of the readily mineralizable nitrogen. It is responsible for most

of the beneficial effects on structural stability. This fraction rarely comprises more

that 10 to 20% of the total soil organic matter.

Figure 8.2. A conceptual model that recognizes various pools of soil organic matter (SOM)

differing by their susceptibility to microbial metabolism. Models that incorporate active,

slow and passive fractions of soil organic matter have proven very useful in explaining

and predicting real changes in soil organic matter levels and in attendant soil properties

− The PASSIVE FRACTION of soil organic matter consists of very stable materials

remaining in the soil for hundreds or even thousands of years. This fraction includes

most of the humus physically protected in clay-humus complexes, most of the humin,

and much of the humic acids. The passive fraction accounts for 60 to 90% of the

organic matter in most soils, and its quantity is increased or diminished only slowly.

The passive fraction is most closely associated with the colloidal properties of soil

humus, and it is responsible for most of the CEC and water-holding capacity

contributed to the soil by organic matter.

− The SLOW FRACTION is intermediate in properties between the active and

passive fractions of soil organic matter. This fraction probably includes very finely

divided plant tissues, high in lignin and other slowly decomposable and chemically


resistant components. The half-lives of these materials are typically measured in

decades.

Soil Degradation | Decline in biodiversity 137

9. Decline in biodiversity

9.1. DEFINITION

The soil biota play many fundamental roles in delivering key ecosystem goods and

services, and are both directly and indirectly responsible for delivering many important

functions such as releasing nutrients from soil organic matter, forming and maintaining

soil structure and contributing to water storage and transfer in soil.

The soil biodiversity is generally defined as the variability of living organisms in soil

and the ecological complexes of which they are part; this includes diversity within

species, between species and of ecosystems. Decline in soil biodiversity is generally

considered as the reduction of forms of life living in soils, both in terms of quantity and

variety.

Little is known about how soil life reacts to human activities but there is evidence that

soil organisms are affected by the :

− soil organic matter content;

− chemical properties of soils (e.g. amount of soil contaminants or salts); and

− physical properties of soils such as porosity (affected by compaction or sealing).

Biological organisms and related activities are central to most soil functions. As it is

known that many of the soil threats will affect soil biodiversity, monitoring its decline is

crucial to maintain soil sustainability.

9.2. ASSESSMENT OF BIODIVERSITY

For soil biodiversity two key issues are considered :

− species diversity; and

− biological functions (e.g. organic matter decomposition and mineralization or release

of nutrients in mineral form).


When monitoring the decline of soil biodiversity both aspects should be considered. The

indicators related to the two key issues and regrouped according to classical soil

definitions are given in Figure 9.1.

Figure 9.1. Key issues and indicators for decline in soil biodiversity

Whatever the indicator, it is not possible to define single baseline or threshold values for

all soils within all land uses because the diversity and activity of soil organisms are

strongly dependent on climate (e.g. dryness), land use (e.g. forest, grassland, crops), soil

properties (e.g. pH, SOM) and management practices (e.g. tillage, use of pesticide and of

fertilizers). Consequently, the following information is needed for data interpretation :

(1) General habitat characterization :

− Detailed geographical characterization (including georeferencing of monitoring

sites),

− Land use (e.g. forest, grassland, crop sites, urban sites) and land practices (including

vegetation),


− Climate data (annual means and minimum and maximum of temperature and

precipitation), and

− Groundwater level and, if appropriate, distance to nearest surface water.

(2) Soil properties, differentiated by soil horizon :

− pH-Value (CaCl2),

− Soil organic carbon content,

− Total nitrogen, C/N-ratio,

− Texture (sand, silt, clay),

− Cation-Exchange Capacity (CEC), and

− Assessment of the usable field capacity of the root layer.

(3) Contamination and anthropogenic stresses :

− Concentration of heavy metals and organics (e.g. persistent organic pollutants and

pesticides), and

− Any other kind of anthropogenic stress like soil compaction.

Soil Degradation | Aridity, drought and climate change 140

10. Aridity, drought and climate change

10.1. ARIDITY

Aridity is defined as an overall moisture deficit in air and soil, under average climatic

conditions. It is tied to climatic data such as rainfall, insolation, elevated temperature,

low air humidity and strong evapotranspiration. Aridity thus refers to a long-term

climatic phenomenon of permanent pluviometric deficit.

An aridity index (AI) is a numerical indicator of the degree of dryness of the climate at a

given location. A number of aridity indices have been proposed (see below); these

indicators serve to identify, locate or delimit regions that suffer from a deficit of

available water, a condition that can severely affect the effective use of the land for

activities such as agriculture or stock-farming.

Aridity has been defined by various indicators, and some include both temperature and

precipitation, such as De Martonne‟s aridity index, which is the ratio between the mean

annual values of precipitation (P) and temperature (T) plus 10°C (De Martonne, 1926).

Since then, other more complex aridity indexes, for instance involving reference

evapotranspiration (ETo), have been described e.g. Thornthwaite (1948). At the same

time, there was no standard method to calculate ETo until the late XXth century when

the Penman-Monteith (Monteith, 1965; Jensen et al., 1990) method got international

recognition. Based on this method (PM-ETo), crop evapotranspiration (ETc) and

irrigation water requirements (IWRs) could be calculated worldwide (Allen et al., 1998).

10.1.1. Lang factor (1915)

Richard Lang (1915) established a climate classification based on a ratio factor between

precipitation and temperature, from which six climate types are proposed. The Lang

climate factor (L) is obtained using the following formula:

Where L = Lang Factor

P = mean annual precipitation (mm)

T = mean annual temperature (°C)


Table 10.1 summarises the corresponding climate types.

Table 10.1. Climate types proposed by Richard Lang (1915)

Climate Aridity index

Desert 0 – 20

Arid 20 – 40

Semi-arid 40 – 60

Subhumid 60 – 100

Humid 100 – 160

Very humid >160

10.1.2. De Martonne Aridity Index (1926)

The De Martonne aridity index is the ratio between the mean values of precipitation (P)

and temperature (T) plus 10°C (De Martonne, 1926) and can be calculated using long-

term annual or monthly climatic data.

With P = annual average rainfall (mm)

T = annual average temperature (°C)

Based on this index, 7 different climatic types have been distinguished (Table 10.2).

Table 10.2. Climatic types based on the De Martonne aridity index


Arid 0 – 10

Semi-arid 10 – 20

Mediterranean 20 – 24

Semi-humid 24 – 28

Humid 28 – 35

Very humid 35 – 55

Extremely humid > 55

It recognizes the significance of temperature in allowing colder places such as northern

Canada to be seen as humid with the same level of precipitation as some tropical deserts

because of lower levels of potential evapotranspiration in colder places.


10.1.3. Emberger Aridity Index (1932)

In 1932, Emberger elaborated a synthetic expression for the Mediterranean climate

(Daget, 1977), classifying this kind of climate on the base of three important climatic

parameters: precipitation, temperature, and evaporation.

Precipitation (P) is represented by the annual precipitation (mm).

For temperature, the mean of the maximum temperatures of the hottest month in the

year (M) and the mean of the minimum temperatures of the coldest month (m) in the

year were considered because vegetation growth is strictly related to these thermal

limits. The temperature parameter is represented by the quotient (M+m)/2.

Evaporation is represented by the temperature range (M-m) because evaporation

frequently increases with it; this parameter expresses the continentality of a climate

(Calvet, 1964).

All these climatic parameters are represented in the pluviothermic quotient of

Emberger, calculated with the following formula:


M = average temperature of the hottest month (°C)

m = average temperature of the coldest month (°C)

Emberger classified Mediterenean region into 4 different climate types (Table 10.3).

Table 10.3. Climate types of Emberger (1932)


Arid < 30

Semi-arid 30 – 50

Subhumid 50 – 90

Humid > 90


10.1.4. Thornthwaite Classification

The Thornthwaite (1948) precipitation effectiveness index PE is based on temperature

and precipitation, defined by the formula:

With P = monthly precipitation (inches)

T = monthly average temperature (°F)

i = identifier of the month

Table 10.4. Thornthwaite climate classification (1948)

Climate PE index

Arid < 16

Semi-arid 16 – 31

Subhumid 32 – 63

Humid 63 – 127

Wet >127

10.1.5. Gaussen-Bagnouls Classification

This classification is based on the average monthly temperature and precipitation. It

gives a more precise climate classification and the climate identification is obtained by

determining separately the numbers of dry and wet months.

A dry, or arid month, corresponds to the month having the ratio between precipitation

(P) and temperature (T) less than two.

The Bagnouls-Gaussen Aridity index (BGI) is calculated as

With t = average air temperature of the month i (i=1 to 12)

k = coefficient indicating the number of months in which 2t > p

pi = average rainfall of the month i (i=1 to 12).


Table 10.5. BGI climate classification (1952)

Climate BG index

Very dry >130

Dry 50 – 130

Moist 0 – 50

Humid 0

The Bagnouls – Gaussen index thus allows to determine dry, semi-humid and humid

periods along the year:

P > 3t humid

3t > P> 2t semi-humid

P < 2t dry

The BGI dry period is comparable to the months where precipitation is half the ETo,

which is considered as the level sufficient to meet water requirements of dryland crops

(FAO, 1983).

These two parameters are plotted as an omberothermic chart on the same graph

doubling the values on the scale of precipitation. The dry months are those of which the

mean temperature curve is higher than the precipitation one.

10.1.6. UNEP Aridity Index

UNEP (1997) has defined the aridity index as the ratio of precipitation (P) to potential

evapotranspiration (ET0):


ET0 = annual potential evapotranspiration (mm)

Five climate types have been defined according to the annual ratio P/PET.

Table 10.6. UNEP (1997) Climate classification


Hyper-arid <0.03

Arid 0.03 – 0.2

Semi-arid 0.2 – 0.5

Dry sub-humid 0.5 – 0.65

Humid >0.65


The calculation of the aridity index requires determination of the moisture loss or

potential evapotranspiration (PET) values from the climatic surface data. PET can be

determined in three ways, each having its own limitations and advantages.

First, it can be established through direct measurement using lysimeters, or evaporation

pans. At global scale such an approach is impractical while the availability of existing

data is poor, and in any case the equipment used in reported studies is not standardized

and therefore not always directly comparable.

Second PET can be calculated using empirical formulae. The method of Penman or

Penman-Monteith is commonly cited, but its calculation requires a large body of directly

measured meteorological data, including solar radiation, wind velocity, relative

humidity and temperature. Its application at global scale is therefore again constrained

by data availability.

A more practical approach utilizes knowledge and understanding of the empirical

relationship between measured PET and values of more readily gained environmental

variables. This approach (Thornthwaite method) allows PET to be calculated from just

two parameters: mean monthly temperature data and the average number of daylight

hours per month. The Thornthwaite method is known systematically to underestimate

PET for dry conditions and to overestimate values for moister and cold environments.

Consequently an empirical adjustment factor was applied to the data to bring the values

closely in line with those of the Penman method:

With ET0 = annual potential evapotranspiration (mm)

Nm = adjustment factor related to hours of daylight (-)

Ti = mean monthly temperature (°C)

I = heat index (-)

a = exponent, being a function of I (-)

Figure 10.1 shows a global map of the aridity zones. Aridity zones are based on an

evaluation of the relationship between key climatic variables, creating an index of


moisture deficit, termed the aridity index. The extent (Table 10.7) and distribution

(Figure 10.1) of aridity zones were determined using data for specified time periods or

'timebands'. As such, the aridity index (AI) was derived from the climatic surface maps

and calculated on a monthly basis, as the ratio P/PET for the period 1950 -2000. Aridity

index values were classified so that climatic zones could be delimited.

Figure 10.1. Global aridity index

Table 10.7. Global extent of aridity zones

Aridity zone Aridity Index Area (million ha) Area (%)

Cold (1) >0.65 1765 14

Humid (2) >0.65 5100 39

Dry sub-humid (3) 0.50 – 0.65 1296 10

Semi-arid (4) 0.20 – 0.50 2305 18

Arid (5) 0.05 – 0.20 1571 12

Hyper-arid (6) <0.05 978 8

Drylands (3 to 6) 0 – 0.65 6150 47

Susceptible drylands (3 to 5) 0.05 – 0.65 5172 40

Aridity indices have been developed in order to delineate zones prone to desertification

(see further). Zones prone to desertification are characterized by low rainfall and by high

summer temperatures, so that the vegetation has little opportunity to restore after

destruction by human impact or prolonged droughts.


10.2. RAINFALL DISTRIBUTION

10.2.1. Precipitation Concentration Index (PCI)

The Precipitation Concentration Index (PCI) was proposed by Oliver (1980) to define

temporal aspects of the rainfall distribution within a year. It is expressed in % according

to the formula:

With pi = monthly precipitation (mm)

P = annual precipitation (mm)

The PCI permits grouping of data sets according to the derived value, with increasing

values indicating increasing monthly rainfall concentration (Table 10.8).

Table 10.8. Precipitation Concentration Index classification

Concept PCI

Hyper-arid 8.3 – 10

Arid 10 – 15

Semi-arid 15 – 20

Humid 20 – 50

Irregular 50 – 100

10.2.2. Modified Fournier Index (MFI)

The Fournier index is used to estimate the aggressivity of the climate.

Arnoldus (1980) modified the (FI) index into a Modified Fournier Index (MFI)

considering the rainfall amounts of all months in the year:

With pi = monthly precipitation (mm)

P = annual precipitation (mm)


Table 10.9. Modified Fournier Index scale

MFI Description Class

<60 Very low 1

60 – 90 Low 2

90 – 120 Moderate 3

120 – 160 High 4

>160 Very high 5

10.3. DROUGHT

Droughts result from a temporary pluviometric (short-term phenomenon) deficit in

relation to the normal precipitation and is generally perceived as the incidence of below

average availability of natural water.

Below average has a physical aspect, that is, below the long-term mean (normal), and a

social aspect, that is, below the expected volume that would satisfy the needs of

agriculture, livestock, domestic use, etc.

Manifestations include annual rainfall less than normal; month/season rainfall less than

normal; reduced river flow; reduced groundwater availability.

Incidents of drought prevail in all climate zones of the world, but its impacts are acutely

felt in drylands. Analysis of past droughts allows us to understand how failure of

rainfall, which under natural conditions would have little effect, becomes a disaster

under some socio-economic changes.

Management of drought, similar to management of the other natural hazards, comprises

three principal elements: an early warning (forecast) mechanism, societal preparedness

(society organized and drilled to face the event), and an enabling mechanism that would

provide menaced communities with support and relief.

Drought is to be seen in the light of ecosystem change, including the climatic changes

and feedback processes of land and resource use. The demographic explosion in arid,

semi-arid and sub-humid ecosystems increases accordingly the needs for food, fibre and

energy. The demands made on vegetation, soils, and water exceed the carrying capacity

of land and water availability. In the 20th century, population growth, accompanied by

very lengthy drought, has unbalanced the traditional adaptation; nomadic pastoralism


has declined and drought has forced the pastoralists into cultivated zones with resulting

tension. Shifting cultivation has declined as a system of livelihood.

10.3.1. Different types of drought

Drought is not purely a physical phenomenon that can be defined by the weather.

Rather, at its most essential level, drought is defined by the delicate balance between

water supply and demand. Whenever human demands for water exceed the natural

availability of water, the result is drought.

Consequently, different types of drought can be identified.

10.3.1.1. Meteorological drought

Meteorological drought occurs when precipitation is significantly below expectations

(average) over one or several successive years.

This has been a notable characteristic of the Sahel region in the decades of the 1960s,

19705 and 1980s.

The quantity of rain alone controls the productivity of the vegetation only in patio. Next

to fertility and soil structure, the distribution of the precipitation in time and space

plays a crucial role and if this is satisfactory, even "below average rains" may permit

entirely sufficient crops. However, "average" or "below average" precipitation is not

synonymous with "average" or "above average" harvests when these rains are scattered

and when dry periods alternate with periods of excessive precipitation.

10.3.1.2. Hydrological drought

Occurs when water resources used for industry, for human or animal consumption or to

support agriculture (e.g. by irrigation) reach low levels.

Hydrological drought is usually reflected in low levels of rivers or lakes, reservoirs or

groundwater. These levels are determined not only by precipitation but also by water

usage and by evaporation and evapotranspiration. Consequently, hydrological drought

can happen even during times of average or above average precipitation, if human

demand for water is high and increased usage has lowered the water reserves.


10.3.1.3. Agricultural drought

Agricultural drought occurs when the available soil moisture is inadequate to sustain

agricultural production. This type of drought is influenced not only by the amount of

rainfall, but also by the inefficient use of that water.

Although agricultural drought often occurs during dry, hot periods of low precipitation,

it can also occur during periods of average precipitation when soil conditions or

agricultural techniques require extra water.

It is a useful term, particularly with the expansion of rainfed cropping in drylands, as it

is a way of assessing the impact of a moisture deficit on production systems.

Agricultural drought can be considered to be the soil moisture deficit during the growing

season. This can be analyzed using precipitation, potential evapotranspiration and soil

moisture data together with coefficients reflecting the moisture requirements of

different crops.

Another important aspect of agricultural drought is that an absolute annual or seasonal

deficit of precipitation may not necessarily be the problem, since the timing of the rains

is also of crucial importance. Where meteorological drought leads to stress for the plant

life, agricultural drought is defined by needs resulting from human activities and

consists of a deficit of water in the ground during the period of plant growth. It then

leads to a disequilibrium in the agricultural economy. This may result from

demographic pressures or from the introduction of unadapted plant varieties, which are

more demanding of water. In Kenya, for instance, the replacement of millet by maize

has created such a disequilibrium in certain areas prone to droughts.

Drought is thus not simply a period of low rainfall. Rather, it is the interaction between

climatic events (precipitation, winds, temperature and humidity), land-management

practices and various water uses that creates economic, social and environmental

impacts. Figure 10.2 illustrates the relationship between various types of drought and

duration of drought events.

10.3.1.4. Edaphic drought

This is defined as a decrease of the infiltrability of the soils and thus as an accentuation

of the arid character of a certain landscape. Field rain simulators have shown that in


the Sahelian and Sudanian-Sahelian zones the infiltrability depends almost exclusively

on the condition of the soil surface.

Figure 10.2. Relationship between various types of drought and duration of drought

events

Under the influence of droughts the density of the gramineous and bushy stratum

decreases, faunal activity disappears, and the glazed surfaces and areas eroded by

runoff and gullying increase in size. This increase in the physical mechanism leads to a

severe deterioration of the environment, which is sometimes irreversible, at least on a

human time scale. Edaphic drought may be the consequence of a meteorological drought

but depends also on the mode of exploitation of the soils.


10.3.2. Drought and desiccation

Distinction is made between drought (period of 1-2 years with below-average rainfall)

and desiccation (dry period lasting for a decade or more).

The often quoted example is the failure of rainfall in the Sahel (Africa) region. The Sahel

region had a long period of declining rainfall from the early 1950s to the 1980s, with

increasing rainfall since 1985, as shown in Figure 10.3 showing deviation from mean

precipitation for 1900 to 2010.

Figure 10.3. Sahel precipitation anomalies 1900-2010

To construct the graph, the long-term mean values was first subtracted from each

annual value in order to normalise the annual rainfall series. The difference was then

divided by the long-term standard deviation. The figure shows June through October

averages of the Sahel rainfall series. Sahel rainfall is characterized by year to year and

decadal time scale variability, with extended wet periods in 1905-09 and 1950-69, and

extended dry periods in 1910-14 and 1970-1997.

Drought is an aspect of inter-annual variations that is usual attribute of low rainfall

climates, but protracted drought (desiccation) may herald a degree of climate change.


10.3.3. Early warning systems

Droughts are harsh climatic events and natural disasters. Their impact on the

environment, their socio-economic and political effects lead to disturbances of the

equilibrium, to crises of the production systems, to a drop in foodstuff production and to

social upheavals.

Drylands are affected in an irregular manner by droughts. The disturbing effects of

these droughts became actually more severe as a result of an increasing demography

and the growing inability of the states both to make provision especially for the food and

socio-economic aspects of these disasters in the short term and to adopt anti-drought

strategies in the long term.

Early warning is a key element. FAO and WMO have programs that provide countries

with information derived from meteo-satellite imagery that could be used as rainfall

forecast. US-AID initiated a Famine Early Warning System (FEWS) project for Africa.

National early warning-systems are available in a few countries of Africa (e.g. Ethiopia).

Drought and climatic variability are parameters inherent to drylands to which are

added the pressures resulting from demographic growth and modern economics, which

in the framework of development are placing more emphasis on cash crops than on

subsidence crops and thereby increasing the risks. The ecological disequilibrium caused

by the droughts is felt all the more, as with the modernisation of the ways of life, viz.

sedentarism of nomads and the extension of agriculture into marginal areas, the

demand for water increases.

10.4. DRYLAND ZONES

10.4.1. Delineation of dryland zones

Drylands are territories where water income (precipitation) is less than the potential

water expenditure (evapotranspiration) during part of, or the whole year. This shortfall

is the measure of aridity. Drylands thus comprise 4 zones, delineated based on the

aridity index: hyper-arid, arid, semi-arid and dry sub-humid zones.


The global extent of drylands is shown in Table 10.10. More than 6.1 billion ha, 47% of

the Earth's land surface is dryland. Nearly 1 billion ha of this area are naturally hyper-

arid deserts, with very low biological productivity. The remaining 5.1 billion ha are

made up of arid, semi-arid and dry subhumid areas, part of which have been degraded

since the dawn of civilisation while other parts of these areas are still being degraded

today. These lands are the habitat and source of livelihood for about a fifth of the world

population. They are areas experiencing pressures on the environment caused by human

mismanagement, problems that are accentuated by the persistent menace of recurrent

drought.

Table 10.10. World drylands in millions of hectares

Zone Africa Asia Australia Europe N-

America

S-

America

World

Hyper-Arid 672 277 0 0 3 26 978

Arid 504 626 303 11 82 45 1571

Semi-arid 514 693 309 105 419 265 2305

Dry subhumid 269 353 51 184 232 207 1296

Total 1959 1949 663 300 736 543 6150

% world drylands 32 32 11 5 12 9 100

% continent/world 66 46 75 32 34 31 47

The hyper-arid territories (978 million ha) are natural (climatic) deserts that extremely

arid (practically rainless). The arid and semi-arid territories (1571 million and 2305

million ha) are rangelands and rainfed farmlands, the dry sub-humid territories (1296

million ha) are more bio-productive (woodlands, farmlands and pasturelands). The total

area of these world drylands (6150 million ha) is 47 % of the total area of the world. It

may be noted that wherever additional water is available, irrigated farmland prevails

even in hyper-arid territories (e.g. oases, Nile valley in Egypt, etc.).

10.4.2. General characteristics of dryland zones

10.4.2.1. Hyperarid environments (zones)

They have very limited and highly variable rainfall amounts both inter-annually (up to

100%) and on a monthly basis such that there is no seasonal rainfall regime. Year-long

periods without rainfall have been recorded. These areas are the true deserts and are

not considered to be prone to desertification because of their naturally very low

biological productivity. Therefore they are excluded in the definition “Desertification”.


10.4.2.2. Arid areas

Mean annual precipitation values up to about 200 mm in winter rainfall areas and 300

mm in summer rainfall areas but more importantly inter-annual variability in the 50 -

100% range. Pastoralism is possible but without mobility or the use of groundwater

resources is highly susceptible to climatic variability.

10.4.2.3. Semi-arid areas

Distinctly highly seasonal rainfall regimes and mean annual values up to about 800 mm

in summer rainfall areas and 500 mm in winter regimes. Inter-annual variability is

nonetheless high (25 -50 %) so despite the apparent suitability for grazing of semi-arid

grasslands, this and other sedentary agricultural activities are susceptible to seasonal

and inter-annual moisture deficiency.

10.4.2.4. Dry subhumid areas

Have highly seasonal rainfall regimes with less than 25% inter-annual rainfall

variability and rainfed agriculture is widely practiced. UNESCO found such areas very

susceptible to degradation, probably enhanced by the seasonally of rainfall, drought

periods and the increasing intensity of human use. It is therefore appropriate to retain

areas designated as dry sub-humid in this survey and to regard them as part of the

overall global arid realm. To this effect, dry sub-humid areas are included in the

definition of desertification.

10.4.3. Problems delineating dryland boundaries

Dryland zones are generally delineated based on climatic and environmental attributes.

However, these boundaries are neither static, nor abrupt. This is not surprising given

the high inter-annual variability in mean rainfall and the occurrence of drought, which

may last for periods of several years at a time. Attempts to locate these climatically

derived boundaries on the ground or identify them in terms of features such as soil type

or natural vegetation are likely to fail. Physical changes are likely to be gradual and

human-induced processes such as grazing, deforestation and burning will modify them.

Identification of climatic changes from shifts in boundaries therefore needs to proceed

with caution given the dynamism that is inherent in dryland climatic regimes and the

fact that drought can cause spectacular but temporary changes in natural vegetation.


It should also be recognized that individual aridity zones do not represent homogeneous

climates, either in the long term or during a particular time band. As already be noted

specific P/PET ratios can be derived from a wide range of values for individual

meteorological parameters.

10.4.4. Climatic variability and change in drylands

The study of desertification is complicated by the natural environmental variability that

occurs in dryland areas. Accurate identification of the causes of desertification at any

particular location, and thus suitable strategies for its treatment can only be made by

paying close attention both to the human use and possible mismanagement of resources

and also to the way in which dryland ecosystems and their resources respond to climatic

variations.

The inherent natural dynamism of dryland ecosystems is very largely governed by the

variability in climatic parameters (especially precipitation) that characterize such

regions. While many dryland areas receive important inputs of moisture from dew and

some others from fog, rainfall is still the key source of moisture in most of the world's

dryland regions, however sporadic and unreliable its occurrence may be. It is not

sufficient to consider rainfall inputs alone, however, since the effectiveness of rainfall,

that is 'the amount available for plant growth or other uses‟ is also dependent on

outputs. The main output route, evapotranspiration, is influenced by parameters such

as vegetation cover and type, wind speeds and perhaps most importantly temperature.

Therefore in this section, the scales of climate change and variability will be explained,

and the relationships between climate change and desertification will be considered.

10.4.4.1. Climatic variability

Climate varies over all spatial and temporal scales: from the diurnal cycle to El Niño to

multi-decadal and millennial variations. Climatic variability refers to the year to year

variability of individual climatic parameters around longer-term mean values, and is

inherent in dryland areas.

This climatic variability may be linked to variations in climate forcing mechanisms, e.g.

cycles of change in solar input, identified by sun spot changes, operate at scales of 11,

60, 80 and 180 years, and are believed to impact on climate variability.


A closer look will be made at the variability of rainfall in the world's drylands.

Rainfall

Rainfall variability is an important aspect of dryland climates and is described under

each aridity zone. Figures 10.4 shows the time series graphs of annual rainfall for

various dryland regions across the world.

Figure 10.4. Time series of annual precipitation (% of mean, with the mean given at top

for 1961 to 1990) from 1900 to 2005 for different dryland regions (bars). The continuous

lines show decadal variations based on 2 different datasets (GHCN precipitation from

NCDC, CRU decadal variations).


All graphs clearly illustrate that dryland regions experience prolonged periods of above

and below average rainfall, as well as individual years of marked rainfall excess and

deficit (drought). The range is +30 to –30% except for the two Australian panels.

Western Africa has known relatively high annual rainfall amounts from the 1920 to the

1970s, followed by a desiccation period. It is unclear whether this marks the impact of

climate change since rainfall tends to increase again in the last years. Nevertheless,

given this trend, it is sensible to regard the rainfall conditions of the last 30 years as a

more appropriate basis for current planning decisions than a longer-term mean value.

Other rainfall graphs (e.g. Southern America, South Africa, Australia) show more

dramatic variations in rainfall anomalies over short periods. Years of high positive

rainfall anomalies can be closely juxta-posed to severe deficits. Marked runs of years

with rainfall above and below mean values are noted as well. Care should however be

exercised in extrapolating region-wide impacts of these tends on human activities, since

many dryland regions are subjected to a very high spatial variability of rainfall,

especially if they are subjected to the impact of a wide range of climatic systems.

Comparison of some rainfall anomaly graphs indicates that certain times of extreme

rainfall anomaly correspond inversely. This inverse relationship may not be

coincidental. Tele-connections - the tendency for variations in climate in one part of the

world to be linked to or to precede change in other areas - may be an important factor in

explaining aspects of climatic variations in dryland areas. Several researchers have

noted that extremes of rainfall can be related to anomalous sea surface temperatures

over certain parts of the tropical Atlantic Ocean.

While aridity results from low average moisture availability, human activities can be

compromised further by the uncertainty that exists about rainfall occurring in any

particular location in anyone season or year.

Inter-annual rainfall variations have great relevance to issues of land degradation, and

the failure of rains in one location can increase the pressures on neighbouring lands that

have been better watered. Significantly, periods of better rainfall may contribute to

desertification as well, by increasing the pressure of cultivation, which then creates

difficulties during subsequent drier periods.


It are particularly the years of rainfall deficiency that have relevance to the study of

desertification and dryland development. The case studies presented here demonstrate

that such years are common, both as shorter-term periods of drought and more extended

periods of desiccation.

It must not be forgotten that as well as overall rainfall deficiency, the timing of rainfall

events is highly important to human activities. The focus here has been on

meteorological drought, but the severity of the impact of drought to some extent depends

on the types of land use being practiced and varieties of crops being grown in an area. A

certain level of rainfall deficiency at a particular time of the year may mean the critical

loss of a sorghum crop for example, but be more advantageous for yields of millet in a

neighbouring field. Understanding the causes of drought and linkages with controlling

factors is undoubtedly important for future attempts to forecast its occurrence, but the

ability to predict may differ markedly from area to area depending on how strongly

linkages with causal mechanisms and tele-connections can be established.

The variability of dryland climates (being mean annual precipitation and mean annual

temperature) makes it likely that the extent of different dryland zones will vary over

time. Figure 10.5 gives a spatial perspective on this variability, showing changes in the

global climatic surfaces for precipitation from 1901 to 2005, and from 1979 to 2005.

Across South America, increasingly wet conditions were observed over the Amazon

Basin and southeastern South America, including Patagonia, while negative trends in

annual precipitation were observed over Chile and parts of the western coast of the

continent. The largest negative trends in annual precipitation were observed over

western Africa and the Sahel. Dai et al. (2004b) noted that Sahel rainfall in the 1990s

has recovered considerably from the severe dry years in the early 1980s. A drying trend

is also evident over southern Africa since 1901. Northwestern Australia shows areas

with moderate to strong increases in annual precipitation over both periods.

Mean annual temperature changes are shown in Figure 10.6. Increases of up to 0.5°C

have occurred in the dryland areas of southern Africa, parts of the Sahara and the

eastern Sahel, SE Europe, southern Asia including India, northern China and Australia.


Figure 10.5. Trend of annual precipitation amounts for 1901 to 2005 (top, % per century)

and 1979 to 2005 (bottom, % per decade), using the GHCN precipitation data set from

NCDC. The percentage is based on the means for the 1961 to 1990 period. Areas in grey

have insufficient data to produce reliable trends. Trends significant at the 5% level are

indicated by black + marks.


Figure 10.6. Global and hemispheric annual combined land-surface air temperature and

SST anomalies (°C) (red) for 1850 to 2006 relative to the 1961 to 1990 mean, along with 5

to 95% error bar ranges, from HadCRUT3 (adapted from Brohan et al., 2006). The

smooth blue curves show decadal variations (IPCC).


Figure 10.7 shows the change in the Palmer Drought Severity Index. This index uses

precipitation, temperature and local available water content data to assess soil

moisture. Although the PDSI is not an optimal index, since it does not include variables

such as wind speed, solar radiation, cloudiness and water vapour, it is widely used and

can be calculated across many climates as it requires only precipitation and temperature

data for the calculation of potential evapotranspiration (PET) using Thornthwaite‟s

(1948) method. Because these data are readily available for most parts of the globe, the

PDSI provides a measure of drought for comparison across many regions.

Figure 10.7. Most important spatial and temporal patterns of the monthly Palmer

Drought Severity Index (PDSI) for 1900 to 2002. The smooth black curve shows decadal

variations.

The time series shows how the sign and strength of the PDSI have changed since 1900,

indicating increasing drought over the global land area. It mainly reflects widespread

increasing African drought, especially in the Sahel, for instance. Yet, there are also

wetter areas, especially in eastern North and South America and northern Eurasia.


10.4.4.2. Climate change

In contrast with climatic variations that occur within the scale of human life-spans

which have a more direct and immediate effect on society-environment relationships,

climate change is a significant and lasting change in the statistical distribution of

weather patterns over periods ranging from decades to millions of years. It may be a

change in average weather conditions or the distribution of events around that average

(e.g., more or fewer extreme weather events).

Marked global climatic changes at the millennia and greater time scales have affected

arid zones, leading to both their expansion and contraction in the past. These long-term

changes, driven particularly by changes in earth-sun relationships, have had important

effects on the development of landscapes, soils and animal and plant distributions in

drylands. Current research suggests that besides natural climate change impacts, the

current and future climatic conditions are also affected by human-induced climate

change.

Global Warming

Regional climates may be significantly changed as a result of increasing level of carbon

dioxide and other greenhouse gases in the atmosphere. Figure 10.6 indicates the

phenomenon of global warming for the two hemispheres and the whole globe.

While not all climatologists are convinced that the warming trends in these graphs can

be conclusively attributed to human activities, the implications for dryland climates

should this trend continue are great.

Changes in precipitation

Temperature changes are one of the more obvious and easily measured changes in

climate, but atmospheric moisture, precipitation and atmospheric circulation also

change, as the whole system is affected. Radiative forcing alters heating, and at the

Earth‟s surface this directly affects evaporation. Further, increases in temperature lead

to increases in the moisture-holding capacity of the atmosphere. Together these effects

alter the hydrological cycle, especially characteristics of precipitation (amount,

frequency, intensity, duration, type) and extremes (Trenberth et al., 2003).

Computer simulations from Global Circulation Models for Sahelian climate in a

"warmer world" for example disagree, some suggesting a drier climate others a wetter

one.


Regional effects

At regional and local scales, it has long been suggested that human-induced soil and

vegetation changes in drylands may modify rainfall amounts. While there have been

numerous studies to investigate this phenomenon and the principles involved, the

possible contribution it makes to climate changes in drylands remains debated. Overall,

climate change is clearly an important issue related in a number of ways and at a range

of temporal and spatial scales to desertification.

Whatever the effects of global warming on the regional climates of drylands, many

scientists are now arguing that the anthropogenic impact can no longer be ignored as a

new input to the global and regional climate systems. What remains uncertain are the

likely magnitudes of human-induced climate changes, and their impacts on areas

susceptible to desertification. The resulting changes will also have to be incorporated

into future efforts to ameliorate the problems of land degradation/desertification in the

world's drylands.

10.4.4.3. Interaction between climate change and desertification

It is noticed that desertification can affect climate as well as climate affects

desertification.

Climatic changes, regardless of their cause, can impact on desertification processes and

human activities that lead to land degradation. Any changes of climate in the future are

likely to be relevant to issues of desertification and land degradation, particularly if the

balance between moisture gain and moisture loss through evapotranspiration is altered.

Such changes will alter the extent and distribution of drylands and may intensify, or

even perhaps reduce, problems of moisture availability and drought occurrence, by

changes in climatic variability.

Even if absolute changes in moisture availability are limited, changes in the seasonality

of climate can have significant impacts on the distribution of vegetation communities, on

soil moisture availability, and on the potential for specific human, especially

agricultural, activities. Desertification may even be triggered or exacerbated in

productive areas by pressures brought to bear in other locations. The occurrence of a

prolonged drought, or an increase in aridity in one area, will inevitably increase the

pressures to cultivate and produce more food in other areas.

Soil Degradation | Desertification 165

11. Desertification

Desertification is not a new environmental issue, similar environmental problems had

been confronted by dryland populations earlier in the twentieth century and in previous

centuries and millennia.

Desertification is not only affecting developing nations. Environmental degradation also

occurs in drylands in the economically-developed world, for example in Australia and

the USA. The so-called Dust Bowl of the American mid-west was perhaps the first well

documented case of human mismanagement contributing to dryland degradation. But it

were events during the 1970s, especially in the Sahel, that placed the plight of dryland

peoples and environments squarely on the world stage.

11.1. EVOLUTION OF THE CONCEPT

The term desertification prompts an immediate image of regions with serious problems

of land damage or with difficult conditions for the development of life. Nevertheless,

certain confusion has been caused by using this word. Confusion comes partly from the

extent of the biological concept of a desert and from the different conceptual approaches

in which the term is used. This problem is basically due to the complex nature and

multifactor character of the processes involved.

As commonly used the word "desertification" means an environmental crisis, which

produces desert-like conditions in any ecosystem. The word desertification is therefore a

label for the low biophysical landscapes in which it can develop, mainly in dryland

areas.

It is basically a process of soil destruction or degradation, which also affects other

components of the ecosystem (vegetation, water cycle, fauna). Secondly, it is a process

that is - either directly or indirectly - induced by human action. Most definitions contain

explicit reference to drought as the basic cause, although drought in fact only plays the

role of reveller and intensifier of the phenomenon. The consequences of desertification

are serious, there is a risk of irreversibility and threat to the general biospheric

potential of the territory affected.


In addition to these basic concepts, different variables, referring to various causes and

factors, have been introduced over time: the geographical boundary where the process

develops, the different weather conditions, the variety of soil degradation processes

considered, the different agents involved, and the magnitude of repercussions.

The concept and the content of the term “desertification" thus have been evolving and

becoming refined over time until reaching an encouraging level of consensus and

acceptance.

11.1.1. UNCOD (70s and 80s)

The UN Conference on Desertification (UNCOD) held in Nairobi, Kenya in 1977

represented the beginning of an in-depth analysis of the problem of desertification

involving all continents. UNCOD initially established simple definitions: desertification

deals with the "process of conversion of regions which climatically are no deserts to

desert conditions" or "the reduction or destruction of the biological potential of the

ground which could lead ultimately to the formation of desert-like conditions". This

conceptual scope was generally well accepted for its global view or its synthetic nature.

However, UNCOD's definition is not typified by its accuracy and can include natural

and man induced processes. Neither is it limited to certain climatic regions, and it can

include areas from humid tropical to arid regions.

11.1.2. UNEP (1991)

In 1991, in order to have better and more scientific acceptable guide for programmatic

activities, UNEP redefined desertification to “land degradation in arid, semi-arid and

dry-subhumid areas as a basic result of adverse human conduct”.

For the UNEP, "land" in this definition includes soil, water resources, crops and natural

vegetation. "Land degradation" involves the reduction of resource potential by one or

various combined processes acting on soil. These processes include water erosion and

sedimentation, long-term reduction of the quality and diversity of natural vegetation or

a considerable decrease in crop production, salinisation and sodification.

This definition is more explicit than those of 1977 because it establishes the biophysical

or geographical limits where the phenomenon occurs, includes the specific processes

acting, and heavily stresses the role of man as inducer of the process.


11.1.3. Earth summit (1992)

During the UN Conference on Environment and Development, the Agenda 21 (UNCED,

1992), an apparent light but important variation was introduced as compared to the

previous definition: "Desertification is land degradation in arid, semi-arid and dry-

subhumid areas resulting from climatic variations and human activities.

The change of emphasis is now in the sense of reducing the strong responsibility

attributed to human factors and stick out the obvious implications of climatic variations.

The temporal and spatial variability of precipitations is one of the intrinsic

characteristics of drylands which influence the processes of land degradation. Yet,

besides the natural climatic oscillations of the lands prone to desertification, this

definition also considers the link between global climatic change and desertification.

11.1.4. Convention on desertification (1994)

During the Earth Summit (UNCED, 1992), the General Assembly of the UN adopted a

resolution aimed at curbing the worldwide degradation of drylands through a "United

Nations Convention to Combat Desertification in those countries experiencing serious

drought and/or desertification, particularly in Africa" (UNCED, 1994).

This Convention was approved in June 1994 in Paris and includes the above quoted

definition of Agenda 21, in the chapter dedicated to use of terms and definitions. This

part of the Convention on Desertification also includes the following definitions:

Combating desertification includes activities which are part of the integrated

development of land in arid, semi-arid and dry sub-humid areas for sustainable

development which are aimed at:

− prevention and/or reduction of land degradation;

− rehabilitation of partly degraded land; and

− reclamation of desertified land.

Land means the terrestrial bio-productivity systems that comprises soil, vegetation,

other biota, and the ecological and hydrological processes that operate within the

system.


Land degradation means the reduction or loss in arid, semi-arid and dry sub-humid

areas, of the biological or economic productivity and complexity of rainfed cropland (dry

farming), irrigated cropland, or range pasture, forest and woodlands resulting from land

uses or from a process or combination of processes, including processes arising from

human activities and habitation patterns, such as:

− soil erosion by wind and/or water;

− deterioration of the physical, chemical and biological or economic properties of soil;

and

− long-term loss of natural vegetation.

Arid, semi-arid and dry sub-humid areas means areas, other than polar and sub-

polar regions, in which the ratio of annual precipitation to potential evapotranspiration

falls within the range from 0.05 to 0.65.

In this course the term "susceptible drylands" covers these regions. It excludes the

hyper-arid regions, the true deserts that are considered to be prone to desertification

because of their naturally very low biological productivity. As such, the Sahara Deserts

belong to the hyper-arid regions, where very low rainfall and very high rates of potential

evaporation restrict plant growth to a minimum and preclude other than transient or

extremely sparse and localised occupation.

The UN Convention on Desertification considers also the links of this process with

important aspects such as integrated development and the sustainable development,

which fully establishes the relationship of land degradation and socio-economic aspects.

The most widely accepted definition of desertification is that of the United Nations

Convention to Combat Desertification which defines it as "land degradation in arid,

semi-arid and dry sub-humid areas resulting from various factors, including climatic

variations and human activities".

As such, desertification is not a unique form of land degradation in terms of the actual

processes through which it takes effect. Some specific processes such as salinisation and

wind erosion might be more prevalent in drylands than in other environments, but the

physical principles are the same and their solution requires the same physical remedies.

On this basis desertification is only a distinct form of land degradation by spatial

definition.


The UN Convention on Desertification (UNCED, 1994) has brought to a first level of

consideration the global dimension of the issues, in parallel with the other two UN

Conventions: Bio-diversity and Global Climatic Change.

11.2. CHARACTERISTICS OF DRYLAND DEGRADATION

11.2.1. Soils in drylands

Soils commonly found in dryland areas are distinctive in a number of ways. They have

low organic matter contents, due to the generally low plant biomass, and are therefore

dominantly mineral soils of an immature or skeletal type. Low precipitation levels also

mean that dryland soils are little affected by leaching, so that soluble salts tend to

accumulate at a certain level in the soil profile dependent upon the local water table or

the depth of moisture infiltration. The general deficiency in moisture means that salts,

once accumulated, tend to remain in situ. Moisture deficiency also discourages

recolonisation by plants that have been removed or damaged, and the build-up of

organic matter is slow since it is usually consumed rapidly under warm conditions. Re-

establishment rates for soil bacterial and fungal populations have been little studied,

but the nitrogen fixation capability of dryland soil is thought to require at least 50 years

to recover from a serious disturbance.

Selected characteristics of some dryland soils are presented in Table 11.1 and their

effect on human land-use.

11.2.2. Susceptibility to degradation

An important characteristic that makes dryland soils especially vulnerable to

degradation is the slowness of their recovery from a disturbance. Since water is often

not available, or only in very little amounts in a few places, new soil is formed slowly.

Actual data on dryland soil formation rates are relatively hard to find in literature. For

the loess soils of the US Great Plains, a soil formation rate of 0.2 mm per year is

suggested, but this figure drops to 0.02 mm per year in the arid south-west. The ability

of soils in dry areas to recover from negative changes in the soil/land surface is generally

lower than in humid areas. In other words, dryland soils have a low resilience.

In drylands, susceptibility by specific processes varies spatially and is affected by a

range of environmental characteristics.


Table 11.1. Selected characteristics of some dryland soils and their influence on human

land use

Characteristics Reasons for occurrence Effect on land use

Low levels of organic matter Poor organic matter

production and rapid oxidation

due to high temperature and

low precipitation.

In part, low organic matter

results in poor aggregation

and low aggregate stability,

leading to a high potential for

wind and water erosion.

Accumulation of salts on soil

surface or in a subsurface

horizon. Salts include calcium

carbonate, gypsum and

sodium chloride. Horizons may

or may not be

indurated/cemented.

Evapotranspiration greatly

exceeds precipitation,

dominant direction of water

flow is upwards in the profile.

Irrigation may cause

secondary salinization.

Soils with a natric horizon are

easily dispersed due to

predominance of Na+ on

exchange complex and are

thus susceptible to erosion.

Other effects may be iron and

zinc deficiency, low chemical

fertility, and low available

water. These factors will affect

crop growth and yields.

Limited biological activity Low moisture levels Micro-organisms affect the formation and stability of soil

aggregates, soil texture and

fertility.

Poor structural stability Low organic matter and poor

aggregation leads to poor

structural stability. The

presence of Na+ also

contributes

Poor structural stability

increases susceptibility to

erosion.

Susceptibility to formation of

crusts and scales. These may

be chemical, biological,

structural or depositional.

Structural crust/seals formed

by raindrop impact.

Depositional crusts form when

fine textured material is

suspended in water and

allowed to settle. Biological

crusts refer to microphytic

crusts.

Some authors believe crusts

and seals decrease infiltration,

increase runoff and generate

overland flow and erosion.

Others suggest soil crusts

such as the microphytic types

may reduce susceptibility to

erosion.

Coarse to medium texture May be accurate if describing

sandy soils alone which are

relatively extensive in

drylands. However, a range of

particle sizes are found in

dryland soils.

Coarse textured soils

generally have low moisture

retention, low fertility, and are

susceptible to erosion. Sandy

soils have much higher rates

of erosion than clay soils.

Hydrophobicity Due to the large contact angle

between the solid/liquid

interface, thin inorganic and

organic coating, and oxidation

of OM may impart repellence

Generates high run-off and

encourages erosion. Water

cannot penetrate pores and

overland flow may occur.

11.2.3. Desertification processes

The processes that result in dryland degradation include:


− compaction and crusting of soils, leading to decreased infiltration and increased

runoff,

− accelerated erosion of soil by wind or water and the consequent exposure of subsoil

or rock,

− soil salinization associated with vegetation loss,

− waterlogging and salinization associated with irrigation,

− alkalinization of soils,

− declining soil fertility,

− desert encroachment or desert creep,

− the destruction or removal of the vegetation cover and its non-replacement in

drylands, and

− alteration of atmospheric micro-climates.

Most of these processes occur as a result of many causes, both natural and human-

induced, some of which are interactive.

11.2.3.1. Wind erosion

Drylands are more susceptible to wind erosion than any other biome because soils tend

to be dry, poorly cemented and sparsely covered by protective vegetation.

Susceptibility to wind erosion once any protective vegetation cover has been disturbed is

affected by the particle size distribution of the soil. Soils with a high clay content will

naturally offer greater resilience to deflation than silty or sandy soils because of their

increased particle cohesion. With mixed grain sizes, selective wind blowing of the

deflatable component can dramatically reduce soil fertility and resilience.

Extensive areas of ancient wind-lain sediments in dry lands are particularly vulnerable

to deflation following damage to the protective vegetation cover. Such reactivation of

Quaternary aeolian sediments has been noted in many dry lands, including the African

Sahel, the Great Plaines of the USA, northwest India and China.

11.2.3.2. Water erosion

Compared to other world climate zones, dryland areas have long been recognized as

particularly susceptible to water erosion because, although the precipitation levels are

relatively low and do not support a year-round cover of protective vegetation, they are

often highly erosive when rain does fall. This generic relationship highlights the

importance of soil degradation in drylands.


In areas that receive an effective mean annual precipitation above 300 mm, increased

precipitation encourages greater vegetation growth which protects the soil from water

erosion, while below the 300 mm-mark, erosion declines as precipitation levels decline.

Topography affects susceptibility to water erosion. High relative relief encourages runoff

even when only moderate areas of the ground surface have lost the protection of a

vegetation cover. Water erosion is rarely spatially continuous and often manifests itself

through rilling and gullying.

11.2.3.3. Salinization

Salt-affected soils occur mostly in dryland regions where evapotranspiration exceeds

rainfall and hence leaching and transportation of salts to the oceans is not as effective

as in more humid regions.

Salinization occurs locally where water is available, either from groundwater sources or

from perennial rivers with headwaters in more humid areas, as in the case of parts of

the Colorado River basin of the USA and the Indus valley in Pakistan.

While salt-affected soils are found extensively under natural conditions, salinity

problems of great concern to agriculturalists arise when previously productive soil

becomes salinized as a result of irrigation. Salinity problems on poorly managed

irrigation schemes are also often associated with physical deterioration of dryland soils

through waterlogging. This combination of degradation processes, so ironic in dry lands

in that they are intimately linked to excessive application of irrigation water combined

with poor drainage, has been widely reported from all over the dryland realm.

11.2.3.4. Nutrient loss

The soils with most potential for loss of nutrients are those that naturally contain

higher nutrient concentrations in the first place. These are the very soils which offer

some of the greatest potential for agricultural development.

11.2.3.5. Desert creep

Terms like "encroachment of the Sahara” and desert creep suggest that deserts extend

beyond their natural (climatic) limits into bordering territories. This perception is valid

in case of mobile sand bodies that move from their origin in the desert and overwhelm


farmlands and settlements in oases or lands out skirting the desert. This accounts for

only a small part (±10%) of the desertification process.

Overall, susceptibility to degradation, in total or with regard to specific processes, is by

no means even throughout the drylands. The occurrence of degradation reflects a range

of natural contemporary and antecedent conditions and processes and the wide variety

of land uses practiced by the human occupants of such regions (see Assessment of Soil

Degradation).

Degradation means a reduction in the overall productivity of dryland ecosystems with

an attendant impoverishment of the human communities dependent on these

ecosystems. A combination of climatic stress and dryland degradation can lead in turn to

extreme social disruption, migrations and famine.

11.2.4. Desertification within different land use systems

Land degradation relates to land use systems. In desertification prone territories, four

systems prevail (single or in combination):

− use of woodland (cutting, gum tapping etc..),

− pasture/rangeland,

− rainfed agriculture, and

− irrigated farmland

The last 3 land uses are the most important. Symptoms of desertification are different in

these different land-use forms.

Deterioration in irrigated farmlands is often related to the rise of water table

(waterlogging), primarily due to imbalance between excessive irrigation and inefficient

drainage. Waterlogging often entails salinization and other forms of chemical damage of

the soil.

Degradation of rainfed farmlands is often manifest as soil erosion, loss of organic matter

and depletion of nutrients, compaction and crust formation, and excessive invasion of

weeds.


Degradation of rangelands includes reduction of bio-productivity, invasion of non-

palatable species including succulents and thorn-bushes, soil erosion, poorer livestock,

etc.

11.3. CAUSES OF DESERTIFICATION

Degradation of agriculturally used dryland relates to a combination of:

− excessive human exploitation that oversteps the natural carrying capacity of the

land resource system in a inherent ecological fragile system (human factor) ; and

− periodical prolonged drought (climatic factor).

11.3.1. Population growth and emigration

In many arid zones, especially in North Africa, population densities are far beyond

carrying capacity.

Many areas that lack both irrigated land and industry of any sort have

population densities of 80 -150 inhabitants per km². To give some examples, this is

the case of the Island Djerba, in the Matmata Mountains in Tunisia, of the leffera

coastal plain of Tunisia and Lybia, of the Hodna basin in Algeria and the Sous

and Baous regions of Morocco where rainfall is usually below 250 mm/year.

The causes of this population explosion are medical advances, increased food production,

international aid and solidarity etc ...

The results of this population explosion include: increased cultivation and clearing of

steppe; increased numbers of livestock; overgrazing; destruction of forests and woody

vegetation in general (for firewood, charcoal, distillation); salinization of large areas as a

result of faulty irrigation projects; inadequate human settlements; emigration, etc.

One of the main responses of the population to this situation is emigration. In most of

these areas about 60 to 80% of the potential labor force emigrates, either to other

regions within the country or to Europe. The money sent home by emigrants represents

in many cases much more than all other aggregate resources. Some of this money is

used to buy tractors and mechanical equipment to replace the emigrated labour force. As

the area is subjected to mechanical ploughing and cultivation, the most "effective" cause

of desertification is expanded. Each labourer can till about 20 times more land per unit


time. Moreover the disc plough is generally used: this uproots all perennial vegetation

on one single run, whereas the traditional swing-plough takes several years to destroy

the perennials. Emigration prompted by desertification has thus a strong feedback and

multiplying effect.

11.3.2. Over-cultivation

An increasing demand for food is the most common root cause of over-intensive

cultivation in drylands. Population, e.g. in the drylands of North Africa and the Middle

East has increased approximately six- to seven-fold since the beginning of the 20th

century (means more mouths to feed), and the area cultivated has increased by similar

amount. More food can be obtained either by increasing the area under cultivation or by

increasing yields, the latter approach usually being the more expensive of the two.

In the Sahel, for example, a study of ten countries over the period 1960-87

indicated that population had increased from 60 million to 107 million, with

growth rates accelerating over the period. Although food production had not kept

pace with the population increase, it had also grown, despite continuing rainfall

deficits, decreasing soil water and periodic droughts, and this growth in

production had been achieved primarily by increasing the area under cultivation.

In the absence of animal production, an average adult individual requires an

"income" of about 500 kg of grain per year to meet his minimum basic needs.

Official statistics, clearly show that the increase in the area of cultivated land is

almost strictly proportional to the increase in population.

The new cultivated land is usually cleared from prime pasture land, thereby increasing

almost exponentially the pressure of livestock on mediocre pastures, since one hectare of

good pasture produces about as much as ten hectares of mediocre rangeland. Wheat and

especially barley crops are now grown extensively in areas receiving only 150-200 mm of

annual rainfall, where the average yields do not exceed 50-200 kg/ha/year, and where

crop expectancy is extremely low (one crop out often cultivation's).

Over-cultivation and the creep of agriculture into climatically marginal dryland areas

frequently contribute to their later degradation. This has proved to have a two-pronged

detrimental effect in some places. It was noted that one effect has been to reduce the

areas available to livestock systems, to which can be added the displacement of

subsistence farmers to new, more marginal lands in the face of expanding cash crop

production. A second aspect of the quest for new lands to cultivate has been the


expansion on to steeper erosion-susceptible slopes, and also into areas that are

extremely marginal in terms of rainfall, even for the production of staple crops such as

sorghum and millet.

Cultivation of virgin territory, leading to degradation because the technique or crops are

unsuitable for the newly cultivated areas, has been effected in many parts of the world,

not always solely in response to pressure from increasing population. Also fears over

food security played a prominent role in the expansion of cereal cultivation into

grasslands. Inappropriate agricultural machinery being applied to semi-arid former

grasslands goes often hand in hand with widespread problems of wind erosion.

Poor agricultural practices may also be one result of the decline or debilitation of labor

caused by poverty, malnutrition or diseases.

Such over-cultivation has led to severe land degradation in many dryland regions of the

world. Land left barren after cropping (or due to crop failure) is prone to wind and water

erosion, and this is often exacerbated when stock are allowed to graze freely on stubble

and crop residues.

Losses of topsoil of 10 tons/ha/month from wind erosion have been measured in

southern Tunisia. Losses of 200 to 250 tons/ha/year have been estimated on the

fringe of the Sahara. As a result of this reservoirs have been silted up in ten years

and sometimes less, and costly irrigation schemes representing investments of

several thousands of dollars per hectare have in many cases remained operative

for less than five years.

Some authorities believe that over-cultivation is the principal cause of dryland

degradation. The World Bank considers that such degradation has led to declining yields

of staple food crops, especially in Sub-Saharan Africa and some South American

countries. Over-cultivation is a consequence of changes in the application of traditional

dryland rainfed agricultural methods and the introduction of inappropriate methods

developed in other environments.

Several attributes of cultivation are seen to create problems. Shorter fallow periods lead

to nutrient depletion, which is a serious problem in African drylands. This lowers the

potential' for production and reduced yields result. Soil erosion by wind and water may

result from the weaker soil structure but more significant in tins respect may be the


growth of mechanized agriculture with its attendant large fields and the ability to deep

plough, further damaging soil structure. High erosion rates are not just due to the direct

effects of deep ploughing but also to increased runoff caused by surface compaction by

machinery.

In Sudan tractors numbers trebled in six years up to 1973 contributing to serious

wind erosion on the lands subject to mechanized methods in the east of the

country. The wind-liberated dust particles also lead to direct crop damage.

The worldwide total annual production of dust by deflation of soils and sediments

was estimated to be 61 to 366 x 106 tons. For Africa alone it is estimated that more

than 100 million tons of dust per annum is blown westward over the Atlantic. The

major world dust producing regions are located in the broad band of arid and

semi-arid lands stretching from West Africa to North China, with the majority

being in the Northern Hemisphere.

In practice it can be difficult to distinguish the impacts on yields of drought from those

of mismanagement and inappropriate expansion, not least because of inadequate data.

Nonetheless one factor that has been regarded as significant is the expansion in crops

grown for export, although again not all the evidence points in the same direction.

Different scientists note how cash crops, particularly the often-cited groundnuts in

Niger and Sudan, are very demanding on nutrients and rapidly contribute to soil

depletion. In the context of desertification this can contribute to reduced ground

cover through declining biomass. Even if fields are abandoned, natural vegetation

can be slow to return because of the preceding nutrient depletion and topsoil loss.

11.3.3. Overgrazing

Overgrazing is frequently cited as a significant desertification cause in rangelands. It

has been attributed to a combination of overstocking and poor land management

aggravated by periodic drought. The number of livestock may have increased or the

state of the grazing may have deteriorated. This could be because the mobility of

livestock has been curtailed or those new, less nutritious pastures are being used.

Stock grazing is the most widespread use of drylands, from the Mediterranean steppes

to the Sahelian savanna and the cerrado of South America and it is widely regarded as

a major cause of desertification owing to its increased intensity. The animal products

needed to meet the growing human consumption requirements have been obtained for


the past 40 years by increasing the numbers of animals. This has been made possible by

progress in the field of veterinary medicine (vaccinations, etc.): productivity has

remained stable or has slightly decreased (again with the usual exceptions).

The result is overgrazing, i.e. livestock consumes more feed than the pasture produces,

leading to depletion of plant cover, replacement of good forage species by others which

are not consumed by livestock, and a general downward trend of ecosystem productivity.

The reasons for the replacement of traditional pastoralism by ranching are numerous.

These trends have partly arisen through the desire to "westernise" in the post-

independence period. Partly through the reapportionment of the traditional grazing

lands to a new political elite and also due to entry into world meat markets, with their

strict disease controls and subsidies from the west demanding modifications to

traditional systems.

The expansion of bore water supplies; a notable trend in African dry lands in recent

decades, is one factor in the sedentarisation of nomadic or transhumant pastoralists (see

further). Sedentarisation is often encouraged by central governments for political

reasons, to make control of the population easier, to modernize practices, to make

education and medical facilities more easily available, and to bring the pastoralists'

products into the wider economy in a manageable way. Another important pressure on

nomadic pastoralism has been competition for land, which reduces the areas available

for livestock. Pastures have been lost to the expansion of cropland resulting in an

increase in livestock densities on the remaining pastureland. Additionally in many areas

the number and sizes of herds have also increased in response to high rates of human

population growth. Other factors such as improved animal husbandry and veterinary

methods and the eradication of pests such as the Tseste fly have encouraged the growth

in animal numbers in traditional societies in recent decades.

Overgrazing is particularly damaging where livestock congregate around watering

points, and rangelands are typically destroyed within a radius of 5 to 6 Ian around

villages and wells (corresponding to the maximum distance that sheep and goats can

walk per day for water during the dry season). The addition of new boreholes in dryland

areas limited by a lack of surface water, leading to excessive grazing pressure and

subsequent land degradation in the proximity of these new water sources.

Overstocking, expressed in terms of the concept of potential carrying capacity (PCC) has

been widely used as an expression of the way in which herders mistreat the


environment. PCC is simply the number of livestock a unit of land in a given

environment can support without detrimental impacts to the environmental system and

without density-dependent mortalities in the livestock system. And is therefore linked to

the concept of sustainable production. PCC values can be calculated with respect to a

simple parameter such as mean annual rainfall or in a more rigorous manner using a

basket of environmental parameters.

Exceeding the potential carrying capacity has been seen as a common cause of

overgrazing leading to general environmental degradation. Increasing opportunities to

produce livestock in drylands have been linked to the sinking of boreholes and wells to

tap groundwater supplies. A major consequence of this has been to place continual

pressure on a restricted area of rangeland around each watering point, zoning

vegetation use and grazing into circular zones. As cattle need to drink every second day

the circular grazing zone has a maximum radius that in theory is equivalent to one day's

walk from the water source.

Studies suggested that problems only begin to arise when external factors upset

traditional systems. Boreholes might be one such factor, because they could be seen as

improving the chances of large livestock herds surviving through droughts. However, in

such circumstances shortage of grazing probably leads to herd decline through

starvation rather than water shortage, which. is a further feedback mechanism that

regulates additional environmental pressure.

The combined effects of trampling, which destroys the structure of soil and excessive

grazing, which reduces the vegetation cover, can expose topsoil to .accelerated wind or

water erosion. The end result is always the same: soil denudation, erosion,

desertification.

Nomadism and Transhumance: scapegoats.

Nomadism occurs mostly in marginal regions bordering deserts, which are characterized

by marked rainfall seasonality and a tendency towards drought, creating a precarious

balance between livestock and resources. Nomads will generally follow fixed patterns of

seasonal movement, which are dependent on rainfall, and the locations of water holes

and areas of vegetation. For example during the dry season in the Sahel zone, nomads'

herds are at the southernmost limit and as the rain begins to fall they are moved

northwards to graze on new grass.


The transition to semi-nomadic patterns of land use that harvest the land, limit

movement and promote concentration around water points in permanent settlements,

has promoted the destruction of rangeland, further exacerbating the impact of climatic

variability, particularly that of drought.

Nomadism has often been accused of being an important cause of desertification. Such

an accusation is totally unjustified: nomadism is merely the scapegoat. The population

growth rates of the nomads or transhumants are clearly much lower than those of the

settled communities. The pressure from man and animals, the main cause of

desertification, is almost always much higher in sedentary areas of the arid zone. In

areas of nomadism and transhumance, population is very thinly spread, usually less

than from five to ten people per km², whereas in similar areas with sedentarized

populations, densities are up to ten times higher.

11.3.4. Deforestation and fuel gathering

The cutting and feeling of trees for fuelwood, to clear land for cultivation, for fodder and

various other uses such as construction is at its most critical around dryland urban

areas in developing countries.

Urban populations tend to use more fuelwood per capita than their rural counterparts.

The rapid growth of urban areas in many dryland developing countries has put

increasing strains on the ability of the local environment to provide increasing amounts

of wood.

Drought in the Sahel has pushed rural inhabitants towards cities in the hope of

finding employment and food. In Mauritania as a whole fuel wood consumption is

eight times higher than the natural growth of accessible forests.

Clearing forest and woodland to create agricultural and pasture land has been a human

activity since time immemorial. It has been a characteristic of drylands too but in recent

decades has accelerated in line with the expansions of ranges and cultivated land. The

scale of clearance has increased as modern agricultural methods have been

implemented, both for mechanized ploughing, where large fields are most cost effective,

and for the application of irrigation schemes.

In Sudan, for example to the establishment of center-pivot schemes, increasing the

potential for wind erosion.


Naturally forested upland areas are particularly susceptible to water erosion following

clearance.

The scale of clearance has been considerable: it is estimated that only 4-6% of

Ethiopia is now forested where 40% once was.

Perhaps the principal cause of deforestation that leads to desertification is wood

collection for domestic use. This includes extraction of timber for construction but is

dominated by what has become known as the fuelwood crisis. The potential scale of the

problem is all the more alarming when it is realized that in many Sahel countries,

fuelwood and charcoal account for over 90% of all energy use.

Wood has long been utilized as a source of fuel for heat and for cooking, but there are

three factors, which explain why a resource crisis has arisen. First, there is the simple

factor of population growth and an increase in absolute demand. Second, high rates of

rural-urban migration creates spatial pressures of demand.

The case of Khartoum in Sudan is often quoted, where hardly a tree survives

within 90 km of the city, but a. similar picture applies to many other Sahel urban

centers, for example Dakar in Senegal.

Once the fuelwood reserve is exhausted, people may resort to burning dung, which

further deprives soils of nutrients.

One study in Ethiopia estimated the fertilizer value of manure diverted from the

field to the cooking stove as $123 million a year, which could increase grain

harvests by 1-1.5 million tons annually.

The third factor is that taking wood from living trees has often now replaced the

traditional collection of dead wood. The consequences of this do not need spelling out,

but the need to cut wood for fuel from living trees is in part due to the general reduction

in woodland areas, resulting from clearance for cultivation, reducing the sustainable

base for collection.

The World Bank (1985) has estimated the scale of the fuelwood problem in human terms

for the Sudan-Sahelian region, suggesting an enormous resource shortfall (Table 8).


Many authorities have spoken of the need for reforestation programs as a multifaced

step to dealing with several attributes of land degradation in drylands. Even where this

is occurring the rate generally still falls well short of the annual rate of loss.

Deforestation for fuelwood, agriculture, industry and homecraft to lesser extent is

increasing at an alarming rate and this is causing serious land degradation. Even

though a lot of states are promoting afforestation, deforestation is going on at a faster

rate than the former. The major constraints are inadequate funding and shortage of

trained manpower to support the activity.

It is generally agreed that the basic fuel needs are 1.0-2.0 kg of wood (dry matter)

per person and per day. The impact on the environment is enormous.

Woody species in a reasonably Mediterranean steppe represents an above-ground

phytomass of about 1000 kg/ha. Assuming an average wood consumption of 1.5

kg/person/day and assuming that 60% of the population in the arid zone rely on

this sort of fuel (plus 10% on cow-dung fuel), we have an annual consumption of

about 59 million tons or a theoretical denudation of 59 million ha out of 400

million ha in the arid Mediterranean zone. This represents an annual denudation

rate of 14%. Fortunately there is some regeneration during the rainy years, so that

it might take another 100 years, instead of 50 as suggested by the above figures, to

convert the whole arid Mediterranean zone into desert.

11.3.5. Unsound development projects

Irrigation

Irrigation systems have been a feature of dryland agriculture for centuries (and in some

cases millennia), offering the potential to overcome some of the problems that rainfall

deficiencies in drylands pose for crop production. Not only are yields potentially

increased but also the range of crops that it is possible to produce. Some dryland

countries have little option but to irrigate new areas in the face of rising food demands

and a small national cultivable area. For instance just 3% of the national land area in

Egypt is cultivable and in that case most of this area is obtained with irrigation.

It is not surprising therefore that irrigation schemes have dramatically increased in dry

lands, with a 34% increase in the area of dryland irrigated cropland between 1961~1978.

Irrigation projects have been a popular target for aid schemes funded by the World

Bank. However, their actual contribution to the economies of recipient nations has to be

viewed in the light of the environmental problems that accompany them.


At present some 200 million ha are irrigated in the world. According to the

Russian expert Kovda over 200.000 ha of irrigated land are lost annually, of

which probably 30.000 to 40.000 ha are in the Mediterranean arid zone. One

source has estimated that annually the equivalent of 12% of the new area of

drylands brought into irrigation is desertified. According to UNEP roughly 30% of

all irrigated drylands are considered to be degraded to varying degrees. Although

food production from the irrigated lands of the Nile and Indus valleys is vital to

the economies of Sudan, Egypt and Pakistan the associated environmental

problems are considerable. Similar problems exist in the San Joaquin basin in

California. Irrigation schemes have contributed to very high productivity levels

but over 800 000 ha are now lost each year to salinization and associated

problems in that area.

The major problems associated with irrigation schemes are their wasteful use of water,

with application rates exceeding possible plant uptake and leading to problems linked to

waterlogging, salinization and alkalinisation.

For example leakage from improperly-lined supply canals has been a major

problem in the Punjab where the surrounding water table has risen by up to 9 m

in 10 years, but many difficulties arise from over-application of water, poor

drainage and waterlogging. These combined with the naturally high evaporation

rates of drylands, result in the concentration of salts in the soil and at the soil

surface. The salt tolerance of most cultivated plants is relatively low so that

productivity rapidly declines.

The success of the irrigation schemes largely depends on continued availability capital,

experienced management, maintenance, spare parts and fuel. Also complex water laws

and tenure systems developed to regulate water use in traditional systems have often

not been successfully reproduced in large-scale irrigation schemes set up in modern

times. Use of faulty technology leads to salinisation and sodification.

Degradation by salinization is not only caused by irrigation, nor are salinization and its

attendant problems the only ones linked to irrigation. For instance a significant cause of

salinization occurs in the Middle East where seawater incursion has affected coastal

drylands due to excessive groundwater pumping.


The potential for degradation off site due to overuse of source of irrigation water is no

better illustrated than by the ecological devastation caused around the Aral Sea.

Water development

Water development for livestock, whether in the form of new boreholes, wells, ponds,

cisterns etc. has, when carried out without pasture management regulations, resulted in

concentrations of animals far beyond the carrying capacity of the pastures. In many

places such water development has created desert conditions within a radius of 5 -20 km

around water points.

11.3.6. Urban and industrial activities

Waste disposal and pollution from industrial activities are not major causes of land

degradation in drylands but they can take on local significance.

Waste disposal is a particular problem for dryland industries and urban centers.

Salinization can occur through the direct dumping of wastes onto soil, a procedure that

has been used for brackish waters pumped from oil wells in the Americas and Middle

East.

Urbanization has been a major feature of drylands since World War II. The indirect

effects of growing urban populations on demand for food and fuel are very significant

factors in rural land use change and processes. This has contributed to the need for

practices such as irrigation, notably in drylands of the USA, Mexico, Egypt and in the

Indian subcontinent but also increasingly in Africa.

Several authors have also noted the deleterious effect of recreational vehicle use by

urban dwellers on the desert environment, by damaging vegetation and destabilizing

soil surfaces. Similar effects have been noted during desert military campaigns.

Ignorance, poor planning based on inadequate understanding of the dryland

environment, and the adoption of inappropriate techniques go a long way towards

explaining many of the examples of degradation.

11.4. EXTENT OF DESERTIFICATION

Susceptible drylands group those zones that are prone to desertification. They include

all dryland zones, except for the hyper-arid zone which is characterised by naturally


occurring environmental conditions that severely restrict biomass production. Hyperarid

areas are therefore excluded from the main consideration: they generally have very

strong desert characteristics and nutrient-deficient soils, giving limited potential for

degradation by many processes and they offer only limited opportunities for

degradation-inducing human land uses.

11.4.1. Aerial extent

On world scale, 40% of its area belongs to susceptible drylands being 5.172 x 104 km2

(Table 11.2).

Table 11.2. World susceptible drylands in millions of hectares

Zone Africa Asia Australia Europe N-

America

S-

America

World

Arid 504 626 303 11 82 45 1571

Semi-arid 514 693 309 105 419 265 2305

Dry subhumid 269 353 51 184 232 207 1296

Total 1287 1672 663 300 733 517 5172

% world drylands 25 32 13 6 14 10 100

% continent/world 43 39 75 32 34 30 40

It is obvious that the greatest extensions of susceptible drylands exist in Asia and

Africa, but in percentage it is Australia that shows the highest extension. The other

continents have something between ± 30 and ± 35% of their surface belonging to

susceptible drylands.

Of these susceptible drylands, almost 20% is experiencing soil degradation, which

means 1035 million hectares of the world's susceptible drylands.

11.4.2. Economic impact

The costs of land degradation and desertification are most often measured in terms of

lost productivity. This could mean reduced crop yields, grazing intensities, etc.

Secondary costs include loss of ecosystem services, and indirect costs are those

associated with mitigating desertification. Desertification costs are often borne by the

poorest of subsistence farmers and herders.

Globally, UNEP estimated that annual economic losses from desertification are more

than $42 billion. Table 11.3 provides an assessment of the economic loss (income

foregone/year) due to desertification (at least moderately degraded). These estimates


were based on the following figures (1990 prices): $ 250 per hectare of irrigated land, $

38 per hectare of rainfed cropland and $ 7 per hectare of rangeland.

Table 11.3. Annual average income foregone (in millions of US$)

Continent Irrigated land Rainfed cropland rangeland total

Africa 475 1855 6966 9296

Asia 7953 4647 8313 20913

Australia 63 544 2529 3136

Europe 474 450 564 1488

N. America 1465 441 2878 4784

S. America 355 252 2084 2691

Total 10785 8189 23234 42308

Dregne and Chou (1992) provide criteria for a global assessment of desertification,

classifying land according to usage and range of desertification severity, and offer a cost-

benefit analysis for recapturing lands. They also present tables on the annual income

foregone due to desertification and on the cost of rehabilitation over a 20-year period.

In the United States, loss of crop yield due to soil degradation has been a subject of an

extensive study. Langdale and Shrader (1982) provide tables on crop yield estimates

associated with various levels of soil erosion. Mokma and Sietz (1992) report on a study

of soil erosion's effects on crop yields in South-central Michigan. The authors found that

production in severely eroded plots averaged 21 percent less than production in normal

or slightly eroded soils.

Indirect costs of land degradation and desertification generally include the effects of

damages due to sediments in streams, canals, dams, and reservoirs. These are usually

harder to assess, and many of the impacts may not be felt directly by the farmer.

Reynolds and Stafford-Smith (2002) suggest that a distinction needs to be made between

the local costs and the regional impacts. Gully erosion to the local land manager may not

result in any discernible decrease in income, yet to the manager of the hydroelectric dam

downstream, the increased sedimentation may represent real costs in terms of

reductions in power generation. Should it be determined that the gully erosion is having

an impact on hydroelectric power generation, it still remains to be determined if the

gullies can be economically rehabilitated and who should bear the cost. These issues

make desertification mitigation complicated.


11.5. COMBATTING DESERTIFICATION

Dryland environments have attributes that make them particularly vulnerable to

degradation. They are also subject to natural phenomena, particularly drought, which

induce stresses in plant, animal and human populations alike. Although dryland soils

are as variable as soils elsewhere, as a result of their generally low soil organic matter

content they are generally more liable to soil structure breakdown than their

counterparts in wetter regions.

People cause desertification and it is necessary to know the circumstances that cause

unfriendly actions to be put into practice. It is not usually the specific land-use activities

that cause degradation. Rather, it is their intensity, the specific localities that they

affect and their occurrence relative to other environmental attributes that lead to

degradation to understand this, the political, social, and demographic underpinnings of

land-use pressure require consideration. Some scientists have argued that the extent of

desertification has been exaggerated and that the influence of recent prolonged intervals

of low rainfall and the political, economic and market factors in dryland countries are

far more potential contributors to dryland deterioration and regional famine than

overgrazing and deforestation.

Drylands often contain societies that, for a range of social, political and technical

reasons, are especially vulnerable to disturbances in the environment. In some cases,

like in the Sudano-Sahelian region and many of the world's poorest nations, they are

subjected to high rates of population growth.

If viable solutions are to be found to the problems of desertification, the focus of

attention must be on human actions. Irrespective of whether human action or nature, or

some combination of the two, is believed to be responsible for desertification, it is the

human role that we have the ability to modify, since as yet we are unable, significantly

to control such natural phenomena as drought.

References

Dregne, H. E. and Chou, N. T. 1992. Global desertification dimensions and costs. In

Degradation and restoration of arid lands. Lubbock: Texas Tech. University.


Langdale, G. W., and W. D. Schrader. 1982. Soil erosion effects on soil productivity of

cultivated cropland. Chapter 4 in Determinants of soil loss tolerance. American Society

of Agronomy special publication no. 45. Madison, WI: American Society of Agronomy,

Soil Science Society of America.

Mokma, D. J. and Sietz, M. A. 1992. Effects of soil erosion on corn yields on Marlette

soils in South-central Michigan. Journal of Soil and Water Conservation 47(4):325-327.

Reynolds, JF & DM Stafford Smith (2002) Do humans cause deserts? In: JF Reynolds

and DM Stafford Smith (eds), Global Desertification: Do Humans Cause Deserts? pp 1-

21 Berlin: Dahlem Workshop Report 88, Dahlem University.

Soil Degradation | Global land degradation assessments 189

12. Global land degradation assessments

12.1. GLASOD

The United Nations Environment Programma (UNEP)-funded GLASOD project has

produced a world map of human-induced soil degradation, using a expert-based

approach. This project was coordinated by ISRIC, assisted by scientists of the

International Society of Soil Science (ISSS), the Winand Staring Centre (the

Netherlands), the FAO, and the International Institute for Aerospace Survey and Earth

Sciences (ITC). Data were complied in cooperation with a large number of soil scientists

throughout the world, using uniform guidelines and international correlation. The

status of soil degradation was mapped within loosely defined physiographic units

(polygons), based on expert judgment.

12.1.1. General Approach

Soil degradation was defined as a process that describes human-induced phenomena

that lower the current and/or future capacity of the soil to support human life. The

assessment is thus essentially focusing on degradation of soils, although it was

inevitable to also include some aspects such as degradation of vegetation.

Regional correlators (institutes or individuals) were designated to give their expert

opinion on the status of human-induced soil degradation in close consultation with

national and environmental scientists. The world was divided into 21 regions and over

250 scientists were consulted. The use of this 'expert-system' approach implied the need

to prepare general guidelines (ISRIC, 1988) for the assessment of the status of human-

induced soil degradation. The guidelines indicated the procedures of mapping and

reporting of human-induced soil degradation and were needed to ensure uniformity in

reporting and delineating on maps the seriousness of various soil degradation processes.

The first step involved delineation of physiographic units on the provided base map.

These units should show certain homogeneity of topography, climate, soils, vegetation,

and land use. Correlators would make maximum use of existing land inventory maps

and reports and remote sensing materials.

The second step was an evaluation of the status of human-induced soil degradation that

may have occurred in the mapped unit. The soil degradation status was to be

characterized by the degree to which the soil was degraded, by the percentage of the


mapped area that was affected, and by the apparent rapidity of the soil degradation

process estimated over the past 5 to 10 years (this latter parameter was seldom

reported). Also the kind of human intervention that had caused soil degradation had to

be indicated. All data on the status of soil degradation was incorporated in matrix tables

and returned to the coordinating institution.

All 21 segments of the world map were then combined and presented at a generalized

scale of 1:10 million.

12.1.2. Soil degradation categories and types

Two categories of human-induced soil degradation processes were recognized:

Soil degradation by displacement of soil material:

The two major types of soil degradation in this category are water erosion and

wind erosion. Displacement of soil material will lead to on- and off-site effects.

Internal soil physical and chemical deterioration:

In this category only on-site effects are recognized of soil that has been abandoned

or is forced into less intensive usages. It does neither refer to cyclic fluctuations of

soil chemical and physical conditions of relatively stable agricultural systems, in

which the soil is actively managed to maintain its productivity, nor to gradual

changes in the chemical composition as a result of soil forming processes.

Various soil degradation types belong to these two categories. In total, 12 soil

degradation types have been recognized and mapped. They have been organized into 4

large groups of soil degradation processes: water erosion, wind erosion, physical soil

degradation and chemical soil degradation.

12.1.2.1. Water Erosion

Loss of topsoil through water erosion is generally known as sheet erosion. On very steep

slopes, natural loss of topsoil may occur frequently. This 'geologic erosion' is not

indicated on the degradation map, unless it is accelerated by human intervention.

Terrain definition/mass movement. The most common phenomena of this degradation

type are rill and gully formation. Other phenomena of this degradation type are

riverbank destruction and mass movement (landslides).


Offsite effects include sedimentation of reservoirs, harbours and lakes, flooding,

destruction of coral, shellfish beds and seaweed.

12.1.2.2. Wind Erosion

Loss of topsoil. This degradation type is defined as the uniform displacement of topsoil

by wind action. In (semi-) arid climates natural wind erosion is often difficult to

distinguish from human-induced wind erosion, but natural wind erosion is often

aggravated by human activities.

Terrain deformation by wind erosion is defined as the uneven displacement of soil

material by wind action and leads to deflation hollows and dunes. It can be considered

as an extreme form of loss of topsoil, with which it usually occurs in combination.

Overblowing, which is defined as the coverage of the land surface by wind-carried

particles, is an off-site effect of the wind erosion types mentioned above.

12.1.2.3. Physical Deterioration

Compaction, sealing and crusting occur in all continents, under nearly all climatic and

soil physical conditions.

Alkalinisation or sodication, leading to a deterioration of the soil structure. This type of

soil degradation was not retained for mapping at global scale.

Waterlogging includes flooding by river water and submergence by rain water caused by

human intervention in natural drainage systems. The construction of paddy fields is not

included, as this is considered to be an improvement rather than a degradation of the

soil.

Aridification refers to human-induced changes of the soil moisture regime towards an

aridic regime, caused for instance by lowering of the local base ground water level. This

type of soil degradation was not retained for mapping at global scale.

Subsidence of organic soils, as caused by drainage and/or oxidation, is only recognized if

the agricultural potential of the land is negatively affected. In many cases however,

drainage of organic soils will lead to an increase in agricultural potential, and is not

mentioned on the map.


12.1.2.4. Chemical Deterioration

Loss of nutrients and/or organic matter occurs if agriculture is practiced on poor or

moderately fertile soils, without sufficient application of manure or fertilizer. It causes a

general depletion of the soils and leads to decreased production. The rapid loss of

organic matter after clearing the natural vegetation is also included in this type of soil

degradation. The loss of nutrients by erosion of fertile topsoil is considered to be a side-

effect of erosion, and not distinguished separately.

Human-induced salinization can be the result of three causes: (1) poor management of

irrigation schemes, (2) intrusion of seawater or fossil saline ground water bodies in

ground water reserves of good quality, and (3) human activities fostering an increase in

evapo(transpi)ration of soil moisture in soils on salt-containing parent material or with

saline groundwater.

Acidification may occur in coastal regions, upon drainage of pyrite-containing soils or by

over-application of acidifying fertilizer, which may also lead to strong acidification and

reduced agricultural potential.

Many types of pollution can be recognized, best known is probably industrial or urban

waste accumulation. Other types of pollution are excessive use of pesticides,

acidification by airborne pollutants, excessive manuring, oil spills, etc. Degree and

distribution of these individual types vary strongly.

12.1.2.5. Biological deterioration

This form of soil degradation referred to an imbalance of the microbiological activity in

the topsoil, caused for instance by deforestation or by overemphasis of chemical fertilizer

applications. In the final GLASOD map, this type of soil degradation has not been

retained for mapping.

12.1.3. Degree of soil degradation

The status of soil degradation is an expression of the severity of the process. The

severity of the process is characterized by the degree in which the soil is degraded and

by the relative extent of the degraded area within a delineated physiographic unit.


The degree to which the soil is presently degraded is estimated in relation to changes in

agricultural suitability, in relation to declined productivity and in some cases in relation

to its biotic functions. Four levels are recognized (Table 12.1).

Table 12.1. Criteria for the determination of the degree of soil degradation

Soil degradation degree

Indicators slight moderate severe extreme

Agricultural

suitability

suitable still suitable marginal unsuitable

Agricultural

productivity

somewhat

reduced

greatly reduced almost nil nihil

Restoration

potential

modification of

the management

structural

alterations

major engineering no restoration

possible

Biotic

function

largely intact partly destroyed largely destroyed fully destroyed

More detailed guidelines related to some specific soil degradation processes have been

formulated for water and wind erosion, salinisation and nutrient decline.

For the assessment of soil erosion, a distinction is made between cropping and pastoral

use of land. In the former case, a further distinction of criteria is made, depending on

the soil depth (deep soils versus shallow soils). Criteria included are (1) extent to which

the top- and subsoil have been removed, (2) the depth and spacing of rills and gullies (in

case of water erosion) and (3) the depth and frequency of hollows or windblown (in case

of wind erosion). The degree of soil erosion under pastoral used has been estimated

using the cover of the perennial, original or optimal vegetation. An overview of the

criteria is given in Tables 12.2 and 12.3.

Table 12.2. Criteria for assessing the degree of water erosion

Degree Criteria

Deep cropland soils (>50 cm) Shallow cropland soils Pastoral

use

Soil losses Rill

spacing

(m)

Gully

spacing

(m)

Soil losses Rill

spacing

(m)

Plant

cover*

(%)

slight Topsoil

partly

20-50 - - >50 >70

moderate Topsoil

completely

< 20 20 – 50 Topsoil partly 20 – 50 30 – 70

severe Topsoil

completely

& subsoil

partly

- < 20 Topsoil

completely,

exposes contact

- <30

extreme Land is unreclaimable

* Perennial/original/optimal plant cover


Table 12.3. Criteria for assessing the degree of wind erosion

Degree Criteria

Deep cropland soils (>50 cm) Shallow cropland soils Pastoral

use

Soil losses Hollows/Windblows*

Soil losses Hollows/Windblows*

Plant

cover*

(%)

slight Topsoil

partly

Few, shallow

- Very few, shallow

>70

moderate Topsoil

completely

Common, shallow or

few, moderately deep

Topsoil

partly

few, shallow 30 – 70

severe Topsoil

completely

& subsoil

partly

Many shallow or

common moderately

deep or few deep

Topsoil

completely,

exposes

contact

- <30

extreme Land is unreclaimable

* frequency classification of hollows: very few (<10%), few (10-40%), common (40-70%); depth

classification: shallow (0-5 cm), moderate (5-15 cm), deep (>15 cm)

The evaluation of the degree of salinisation is based on relative changes in the

salinisation class of the land unit, reported during the last 50 years. Four different

classes of salinity have been determined, based on the electrical conductivity of the

saturation extract (Table 12.4 and 12.5). In all cases, the ESP is lower than 15% and the

pH is lower than 8.5. Table 12.6 summarised the criteria used to identify the degree of

nutrient decline.

Table 12.4. Salinisation class

Class ECe (dS/m)

Non-saline < 4

Slightly saline 4 – 8

Moderately saline 8 – 16

Severely saline > 16

Table 12.5. Criteria for assessing the degree of salinisation

Degree ECe

Slight Change of 1 class

Moderate Change of 2 classes

Strong Change of 3 classes

extreme Change from non-saline to unproductive


Criteria to assess the degree of nutrient decline are (1) the organic matter content, (2)

the parent material, (3) the climatic conditions. Nutrient decline by leaching or

extraction by plant roots without adequate replacement is identified by a decline in OM,

P, exchangeable Ca, Mg and K.

Table 12.6. Criteria for assessing the degree of nutrient decline

Degree ECe

Slight Cleared and cultivated grassland or savanna, in tropical regions, on inherently

poor parent materials

Cleared or cultivated formerly forestland in tropical regions, on soils with rich

parent materials

Cleared or cultivated formerly forestland in temperate regions, on sandy soils

Moderate Cleared and cultivated grassland or savanna on soils high in inherent organic

matter which has declined markedly by mineralization, in temperate regions

Cleared or cultivated formerly forestland in tropical regions, on soils with rich

parent materials where subsequent crop growth is poor or non-existing

Strong Change of 3 classes

extreme Change from non-saline to unproductive

12.1.4. Relative extent of the degradation type

At the chosen scale it is not possible to separate areas of soil degradation individually on

the map. It is however possible to estimate the relative extent of each type of soil

degradation within the mapped unit. Five categories are recognized:

− infrequent: up to 5% of the unit are affected

− common: 6 to 10% of the unit is affected

− frequent: 11 to 25% of the unit is affected

− very frequent: 26 to 50% of the unit is affected

− Dominant: over 50% of the unit is affected.

12.1.5. The severity of soil degradation

The severity of soil degradation is indicated by a combination of the degree and the

relative extent of the process. Since there are four degrees specified and the relative

extent is given in five categories 20 combinations are possible. These 20 combinations

were then grouped into four severity classes as illustrated in Table 12.7. Each severity

class is given a different shading of the colour of the dominant soil degradation process

occurring in a given mapped unit.


Table 12.7. Severity classes of human induced soil degradation

Degree of

degradation

Percentage of

mapping unit

affected

0 - 5 5 - 10 10 - 25 25 - 50 50 – 100

Light 1.1 1.2 1.3 1.4 1.5

Moderate 2.1 2.2 2.3 2.4 2.5

Strong 3.1 3.2 3.3 3.4 3.5

Extreme 4.1 4.2 4.3 4.4 4.5

The above mentioned interpretation of degradation severity becomes more complicated

if two types of soil degradation are recognized in one mapped unit. When both types

have the same weight of importance and also the correlator in the field did not indicate

a difference in importance, a colour mosaic is shown on the map. The shade of the

mosaic is determined by the so called 'aggregate severity', which may be higher than the

severity of the individual degradation types (see also below). In case one of the two

degradation types is subordinate, it is still possible that the aggregate severity is one

class higher than the severity of the most important type. This will occur if the severity

of the second type is significant enough to have a bearing on the overall severity.

12.1.6. Causes of soil degradation

For each mapped unit with some form of degradation, one or two of the following

causative factors are given:

12.1.6.1. Deforestation and removal of the natural vegetation

This causative factor is defined as removal of the natural vegetation (usually forest) of

stretches of land. Reason for this clearing may be the reclamation of land for

agricultural purposes (cropping or cattle raising), large scale commercial forestry, road

construction, urban development, etc.

12.1.6.2. Overgrazing

Besides the actual overgrazing of the vegetation by livestock, this causative factor also

includes other effects of livestock, such as trampling. Overgrazing usually leads to a

decrease of the soil cover, which increases the water and Wind erosion hazard.

Trampling may cause compaction of the soil. A widespread effect of overgrazing is the

encroachment of unfavourable (unpalatable or noxious) shrub species. Although this

phenomenon certainly influences grazing potential, it is not distinguished as soil

degradation, as the soil itself is not affected.


12.1.6.3. Agricultural activities

This causative factor is defined as improper management of agricultural land. It

includes a wide variety of practices, such as insufficient or excessive use of fertilizers,

shortening of the fallow period in shifting cultivation, use of poor quality irrigation

water, absence of anti-erosion measures, improperly timed use of heavy machinery, etc.

12.1.6.4. Overexploitation of vegetation for domestic use

This causative factor deals with the use of the vegetation for fuel wood, fencing, etc.

Contrary to deforestation and removal of the natural vegetation, it usually does not lead

to complete removal of all vegetation. However, the remaining vegetation does not any

more provide sufficient protection to soil erosion.

12.1.6.5. (Bio-) industrial activities

This causative factor usually leads to degradation type 'Cp: pollution'.

12.1.7. Rate of soil degradation

Instances of soil degradation during critical periods should be totalled and averaged

over the last 5 to 10 years in order to define whether the rate is slow, medium or rapid.

The reliability of these estimates are however rather low.

12.1.8. Output

The type, extent, degree, rate and main causes of degradation have been printed on a

global map, at a scale of 1:10 million, and documented in a downloadable database.

Information about the areal extent of human-induced soil degradation can be found in

an explanatory note.

The colour of a mapped unit is determined by the dominant degradation type occurring

in the unit.

The status of soil degradation is indicated by its degree, relative extent in a mapped

unit, and recent-past rate. This last element in the assessment was the most difficult

and was not always reported on, while reasons for giving a degradation type a certain

degree and indicating how widespread it occurred within a mapped unit was in most

cases well documented. After careful consideration a decision was made indicating the

seriousness of a type of soil degradation by a combination of the degree and relative


extent. Since there are four degrees specified (light, moderate, strong, extreme) and the

relative extent is given in five categories (infrequent, common, frequent, very frequent,

and dominant) a total of 20 combinations are possible.

The recent past rate and the type of human intervention that has caused soil

degradation -the causative factors -are indicated as symbols in the mapped unit.

12.1.8.1. Extent of human induced soil degradation

GLASOD indicated that, in 1991, 15% of the land was degraded. The highest

proportions were reported for Europe (25%), Asia (18%) and Africa (16%), the least in

North America (5%).

Figure 12.1. Global assessment of the status of human-induced soil degradation (USDA,

1999)

Soil erosion was by far the most important type of land degradation. It affected 83% of

the global degraded area. The other types of land degradation are less important,


though there are large regional differences. Nutrient depletion affected only 4% globally,

but represents 28% of the degraded land in South America. Salinity affects less than 4%

worldwide, but is relatively important (18%) in West Asia. Soil physical degradation and

chemical contamination are globally less important, but represent 4 and 8% of the

degraded land in Europe.

Table 12.8 gives some GLASOD estimates of the extent of different types of human-

induced land degradation.

Table 12.8. GLASOD estimates of human-induced soil degradation (million ha)

12.1.8.2. GLASOD applications:

− International policy makers and planners (e.g. UNEP, FAO, WRI)

− National policy makers and planners

− International conventions and programmes (CCD, Kyoto protocol, UN-CPB, IGBP)

− Researchers at national and international level (NARI‟s, CGIAR, universities)

− Education professionals (teachers, professors, etc.) and students

− Environmental organisations (general public awareness)

12.1.8.3. GLASOD limitations:

− Small scale: not appropriate for national breakdowns

− Expert judgement: qualitative and (potentially) subjective

− Limited number of attributes due to cartographic restrictions

− Visual exaggeration: each polygon which is not 100% stable shows a degradation

colour, even if only 1 to 5% of the polygon is actually affected

− Extent classes (5) rather than percentages

− Complex legend: combined extent and degree (severity) for four major degradation

types (water and wind erosion, physical and chemical deterioration)


− Only “dominant” main type of degradation is shown in colour

− Degradation sub-types only shown by codes

− Only “bad news”

12.1.8.4. Follow-up assessments:

− Regional assessments of soil degradation status - South and Southeast Asia

(ASSOD), Central and Eastern Europe (SOVEUR)

− Land Degradation Assessment in Drylands (LADA) - Global project, under

UNEP/FAO. ISRIC and WUR Centre for Geo Information are responsible for the

spearpoint Global Assessment of Land Degradation and Improvement based on 22

years of fortnightly NDVI data

12.2. GLADA

The only harmonized assessment, the Global Assessment of Human-induced Soil

Degradation is a qualitative assessment of global degradation based on perceptions of

the type and degree of degradation. Dating from 1991, it is now out-of-date. There is

pressing need for an up-to-date, quantitative and reproducible assessment to support

policy development for food and water security, environmental integrity, and national

strategies for economic development and resource conservation.

In response, within the GEF-UNEP-FAO program Land Degradation in Drylands, the

Global Assessment of Land Degradation and Improvement (GLADA) will identify:

− the status and trends of land degradation,

− hotspots suffering extreme constraints or at severe risk, and,

− bright spots, areas where degradation has been arrested or reversed.

In GLADA, land degradation is defined as the long-term loss of ecosystem function and

it is measured in terms of changes in net primary productivity using remotely sensed

data and existing datasets.

12.2.1. Net primary productivity as land degradation indi-

cator

Above ground biomass is an integrated measure of biological productivity, as the result

of below ground activities in the soil and the prevailing climatic conditions. Because

NDVI, a greenness indicator of the earth surface, is available in long time series and


correlates well with the above ground biomass (Tucker et al., 1985) and net primary

productivity (Running and Nemani, 1988; Diallo et al., 1991; Carlson and Ripley, 1997),

its change has been used as an indicator for change in biomass and therefore as an

indirect indicator for land degradation or rehabilitation. This prompted researchers such

as Wessels et al. (2004, 2007) and Metternicht et al. (2010) to use NDVI trends as a

proxy for land degradation.

But also this methodology has its shortcomings. The difficulty is to discount false alarms

raised by other factors, notably fluctuations in rainfall, rising temperatures,

atmospheric CO2 concentration, nitrate precipitation, and land use change. The

assessment procedure actually includes some correction for the impact of climatic

variability. The effect of soil, land use types and management is not explicitly included

at this moment. With respect to land use types and management it is clear that there

are no reliable global datasets. The project however includes validation procedures in

several smaller pilot zones where land use and management data are available.

Eventually, in future, the NDVI will be related to soil and weather conditions,

eventually though linkage with soil and crop models. The method needs to be validated

by national observations of land degradation.

12.2.2. General methodology

12.2.2.1. Step 1. Development of simple indicators: mean annual sum NDVI and

NPP

The base maps for the assessment are GIMMS (Global Inventory Modelling and

Mapping Studies) radiometer (AVHRR) data, which are available for the period July

1981 – December 2003. These satellite images are corrected for calibration, variations in

solar and view zenith angle, stratospheric aerosols, and other effects not related to

vegetation change, and then generalized to 8 km grids for 15-day periods.

Simple NDVI (normalized vegetation index) indicators are then calculated by year. In

the northern hemisphere, this is done per calendar year (Jan-Dec), in the southern

hemisphere, the calculations are done from Oct to Sep, as to encompass complete

growing seasons. The indicators calculated are simple descriptive statistics: minimum,

maximum, range, mean, standard deviation, coefficient of variation, sum).


It is the annual sum NDVI - the aggregate greenness over the growing season - that is

used as a surrogate for annual biomass productivity.

The observed NDVI can be translated into estimates of NPP, based on a comparison of

the GIMMS data with the MODIS dataset, which provides regionally validated

estimates of the NPP at a 1-km spatial resolution and 8-day temporal resolution. The

observed, empirical relationship is:

NPP = 1106.37 x sum NDVI – 564.55 (r=0.83; n = 3,128,207)

with NPP in kg C ha-1 year-1.

These annual datasets allow to determine trends in the NPP with time from 1981 to

2003. Figure 12.2 illustrates the change in NPP during this period. Trends of NDVI and

its derivatives were determined by linear regression. The absolute change is the slope of

the regression. The data were tested for temporal and spatial independence and a T-test

was used to arrange the obtained slope values in classes showing strong or weak

positive or negative trends.

Figure 12.2. Global change in NPP from 1981 to 2003


12.2.2.2. Step 2. Correction of the observed trends for climate effects

The observed trends have to be corrected for the impact of variations in climatic

conditions:

− Impact of rainfall and irrigation

− Impact of temperature

Calculation of a rain-use-efficiency-adjusted NPP

Rain-use efficiency is the production per unit of rainfall:

RUE = annual sum NDVI / annual rainfall

It may fluctuate dramatically in the short term: often there is a sharp decline in RUE

when rainfall increases. We assume that the vegetation (cultivated or semi-natural)

cannot make immediate use of the additional rain. But, where rainfall is the main

limiting factor to biomass productivity, the long-term trend of RUE is a good indicator of

land degradation or improvement. RUE analysis on a pixel resolution to some extent

also reflects effects of local variations in terrain, soil and vegetation.

In areas where rainfall determines the NPP (in dryland zones), there is a positive

relationship between rainfall and NPP (Figure 12.3). Humid and cold regions, irrigated

areas and some wetlands, NPP is not or negatively related to rainfall. Figure 12.4 shows

the change in rainfall use efficiency.

RUE was used to adjust the NDVI/NPP values as follows:

For areas where there is a positive relationship between rainfall and NPP:

o If NPP declines but RUE increases: the decline in productivity can be at-

tributed to a decline in rainfall, and consequently those areas don‟t indi-

cate land degradation

o If RUE declines, NDVI trends were calculated as RUE-adjusted NDVI

For areas where there is a negative relationship between rainfall and NPP:

o the observed NDVI trend has been considered as RUE-adjusted NDVI


Consequently, a decline in RUE-adjusted NDVI values (Figure 12.5) indicates land

degradation. These results have been validated by field observation in North China (Bai

et al. 2005) and independently by Chen & Rao (2008); Kenya (Bai & Dent 2006); and

Bangladesh (Bai 2006).

Figure 12.3. Correlation between annual sum NDVI and annual rainfall, 1981 – 2003

Figure 12.4. Change in rain-use-efficiency, 1981 to 2003


Figure 12.5. Global negative trend in RUE-adjusted NDVI and confidence levels of these

negative trends

As a quantitative measure of land degradation, loss of NPP has been calculated for those

areas where both NPP and RUE are declining. This is likely to be a conservative

estimate since globally, NPP has increased over the period. Also, where NPP is

increasing but RUE is declining, some process of land degradation may have begun that

is reducing NPP but is not yet reflected in declining NPP.


By the same reasoning, RUE should be used alone for early warning of land

degradation, or a herald of improvement. Where NPP is rising but RUE declining, some

process of land degradation might be under way that is not yet reflected in declining

NPP; it will remain undetected if we consider only those areas where both indices are

declining. The reverse also holds true: we might forgo promising interventions that

increase RUE but have not yet brought about increasing NPP.

Use of residual trends

An alternative procedure to distinguish land degradation effects from rainfall variability

consists of the use of residual trends (Wessels et al., 2007). According to this procedure,

a correlation was drawn between the annual sum NDVI and annual rainfall for each

pixel. The resulting regression equation allowed prediction of sum NDVI according to

rainfall in any one year. Residuals of sum NDVI (differences between the observed and

predicted sum NDVI) were calculated for each pixel and residual trend (RESTREND)

was analysed by linear regression; its significance was assessed by the T-test (Figure

12.6).

RESTREND points in the same direction as RUE: negative values may indicate human-

induced land degradation and positive values, improvement.

Calculation of an energy-use-efficiency corrected NDVI

To take into account the significant lengthening and warming of the growing season at

high latitudes and altitudes, an energy-use efficiency factor was calculated for all pixels.

Energy use efficiency (EUE) is calculated as the ratio of annual sum NDVI to

accumulated temperature (day-degrees Celsius above zero):

EUE = annual sum NDVI / annual accumulated temperature (day degrees above 0°C)

The global increase in temperatures, especially at high latitudes, has been accompanied

by a marked increase in NDVI but not, in general, in the EUE of either natural

vegetation or farmland (Figure 12.7).

Combination of negative EUE indicator with negative RUE-adjusted NDVI makes

virtually no difference to the delineation of land degradation – areas of negative EUE

are also areas of negative RUE-adjusted NDVI. However, addition of the EUE indicator

does make a big difference to the assessment of land improvement: Figure 12.8 maps the


areas that exhibit both a positive trend in RUE-adjusted NDVI and positive EUE as

climate-adjusted NDVI.

Figure 12.6. Residual trends of sum NDVI and their confidence levels


Figure 12.7. Trends in EUE

Areas with a positive trend in both RUE-adjusted NPP and EUE reflect land

improvements and are quantified as climate-adjusted NPP (Figure 12.8).

12.2.2.3. Step 3. Stratification of the landscape using land cover and soil and

terrain data to enable a more localised analysis of the NDVI data.

Lack of consistent time series data at the global level currently prohibit

analyzing/removing the impact of changes in land use and management, but this issue

will be analysed in more detailed at the local level.

Nevertheless, the obtained indices of land degradation and improvement have been

compared with land cover, soil and terrain data, rural population density, and indices of

aridity and poverty. Maps of the climate-adjusted NDVI index were overlaid with these

other global maps and a correlation analysis performed.


Figure 12.8. Areas with a positive trend in climate-adjusted sum NDVI and the

corresponding confidence level


12.2.2.4. Step 4. Localised analysis of the hotspots and bright spots

At the next stage, the identified hotspots will be characterised manually, using 30m-

resolution Landsat data, to identify the probable kinds of land degradation, preliminary

to field examination by national teams within the wider LADA program.

12.2.3. Results

The results are very different from the previous global assessment of land degradation

(GLASOD) and challenge conventional wisdom. The global values are presented in Table

12.9; detailed values by country are given in Bai et al. (2008).

Table 12.9. Global statistics of degrading areas 1981-2003

Area (km²) Area (%) NPP loss (t C/23 year) Population (%) Population affected

(number)

35058104 23.54 955221418 23.89 1537679148

Areas severely affected include:

- Africa south of the Equator (13% of global degrading area and 18% of lost global

NPP);

- Indo-China, Myanmar, Malaysia and Indonesia (6% of the degrading area and

14% of lost NPP;

- S China (5% of the degrading area and 5% of lost NPP);

- N-central Australia and parts of the western slopes of the Great Dividing Range

(5% of the degrading area and 4% of lost NPP);

- The Pampas (3.5% of the degrading area and 3% of lost NPP);

- Swaths of the high-latitude forest belt in North America and Siberia.

The usual suspects – the dry lands around the Mediterranean, Middle East, South and

Central Asia - are represented by only relatively small areas of degradation in southern

Spain, the Maghreb, Nile delta, Iraqi marshes, and the Turgay steppe. Probably, many

differences from the previous assessment arise because GLASOD compounded current

land degradation with the legacy of centuries past. These are two different things; both

are important; but most areas of historical land degradation have become stable

landscapes – with a stubbornly low level of productivity. The present assessment deals

only with 1981-2003 and we have no comparable data for earlier periods.


12.2.3.1. Is land degradation a global issue?

Over the last 25 years, 24% of the land area has been degrading; this is on top of the

legacy of thousands of years of mismanagement in some long-settled areas. GLASOD

estimated that 15% of the land was degraded, and those areas are, by and large, not the

same as the areas highlighted by the new analysis; land degradation is cumulative - this

is the global issue.

Degrading areas currently support 1.5 billion rural people. In terms of C fixation, these

areas represent a loss of NPP of 9.56 x 108 t C relative to the 1981-2003 mean; that is

9.56 x 108 t C not removed from the atmosphere - equivalent to 20% of the global CO2

emissions for 1980. At the shadow price for carbon used by the British Treasury in

February 2008 ($50/t C, Montbiot 2008) this amounts to $US 48 billion in terms of lost C

fixation. But the cost of land degradation is at least an order of magnitude greater in

terms of C emissions from loss of soil organic carbon: as much as one third of the

human-induced increase in atmospheric CO2 and 20% of global carbon emissions over

the period 1989- 1998 is related to land use change (IPCC 2000, Houghton 2008).

12.2.3.2. Is land degradation mainly associated with farming?

Comparison of degrading areas with global land cover (JRC 2003) reveals that 18 % of

degrading land is cropland, 23 % is broadleaved forest, and 19 % needle-leaved forest.

Comparison of degrading areas with global land use systems (FAO 2008, Tables 12.10)

indicates that 48 % of degrading land is forest. At this stage, we can only speculate

about the reasons: apart from land degradation as it is commonly understood, high-

latitude taiga is subject to catastrophic fires and pest outbreaks that affect huge areas

and, since the rate of recovery is slow, the 23-year period may encompass a whole cycle;

and, surely, some of the recorded degradation is, in fact clearance for cropland and

grazing. Twenty six % of the degradation is in grasslands (herbaceous vegetation in the

FAO legend), where the natural and protected areas seem to be faring better than

grazed areas.

Degrading farmland makes up 18 % of the total degrading area shown in both the

LC2000 land cover map and the FAO Land Use Systems map. Interestingly, irrigated

areas fare no better than the average.

Change of land use and management may generate false alarms about land degradation.

Conversion of forest or grassland to arable, pasture or even perennial crops will usually


result in an immediate reduction in NPP (and NDVI) but may well be profitable and

sustainable, depending on management. Lack of consistent time series data for land use

and management precludes a generalised analysis of land use change but this can be

undertaken manually for the potential hot spots of land degradation.

Table 12.10. Global degrading/improving lands in the aggregated land use systems

Land use class % of the land use class % of the degrading area

degrading improving degrading improving

forestry 29.4 9.9 48.1 23.9

grassland 15.8 16.7 26.1 40.8

agricultural land 22.2 20.0 18.1 24.1

urban 17.3 11.7 2.3 2.3

wetlands 25.0 11.2 2.6 1.7

bare areas 2.9 4.9 2.9 7.2

undefined 2.8 2.2 0.0 0.0

Total land

12.2.3.3. Is land degradation a dryland issue?

Drylands do not figure strongly in ongoing land degradation, apart from in Australia.

Indeed, the recovery of the Sahel from the droughts of the 1980s is notable (Figure 12.2

and Olsson et al., 2005). Globally, there is little between land degradation and Turc‟s

aridity index correlation (r = -0.12); 78 % of degradation by area is in humid regions, 8 %

in the dry sub-humid, 9 % in the semi-arid, and 5 % in arid and hyper-arid regions.

12.2.3.4. Land improvement

Land improvement is identified by: 1) a positive trend in RUE-adjusted sum NDVI and

2) a positive trend in energy-use efficiency.

These areas account for 12.7 % of the land area; 24 % is cropland (20 % of the total

croplands), 24 % is forest, and 41 % rangeland. Many gains in cropland are associated

with irrigation but there are also swaths of improvement in rain-fed cropland and

pastures in the Great Plains of North America, and in western India. Some of the NDVI

gains are a result of increasing tree cover, either through forest plantations, especially

in Europe and North America (FAO 2006), and some significant land reclamation

projects, for Instance in North China. However, some of the biomass gains may

represent woodland and bush encroachment into rangeland and farmland - which is not

generally regarded as land improvement.


We may attribute the general global increase in greenness to the increasing trends of

atmospheric CO2 concentrations and nitrate deposition. The increasing trend production

across the Sahel probably includes an element of recovery from the devastating drought

of the early 1980s, in spite of the adjustment for RUE and application of RESTREND.

Increases in biomass in the Amazon basin may be related to lower rainfall, accompanied

by decrease in growth-limiting cloudiness, but global data for net incoming radiation are

not available to check this.

12.2.4. Limitations

GLADA is an interpretation of GIMMS NDVI data – which is taken as a proxy for NPP.

The proxy is several steps removed from the symptoms of land degradation as it is

commonly understood – such as soil erosion, salinity, or nutrient depletion. The various

kinds of land degradation and improvement are not distinguished. The same goes for

land improvement.

Some inherent limitations of the NDVI data have already been flagged: saturation of the

NDVI signal by dense vegetation - leading to a lack of precision for forest mapping;

interference by cloud in perennially cloudy areas; and the scant rainfall observations in

many parts of the world.

Any particular trend in NDVI may mean something very different in Central Africa

from a quantitatively similar trend in South China or South Africa – both in terms of

the kind of land degradation and the underlying causes.

A declining trend of NPP, even allowing for climatic variability, may not even be

reckoned as land degradation:

− urban development is generally considered to be development - although it brings a

loss of ecosystem function;

− land use change from forest or grassland to cropland of lesser biological productivity

may or may not be accompanied by soil erosion, compaction and nutrient depletion -

and it may well be sustainable and profitable, depending on management.

Similarly, an increasing trend of NPP means greater biological production but may

reflect, for instance, bush encroachment in rangeland or cropland - which is not land

improvement as commonly understood.


The 8-km resolution of the GIMMS data is a limitation in two senses. First, an 8-km

pixel integrates the signal from a wider surrounding area. Many symptoms of even very

severe land degradation, such as gullies, rarely extend over such a large area; they must

be severe indeed to be seen against the signal of the surrounding unaffected areas. More

detailed analysis is possible for those areas that have higher resolution time series data,

notably South Africa (Wessels et al., 2004). Secondly, an 8-km pixel or even a 1-km pixel

cannot be checked by a windscreen survey; and a 23-year trend cannot be checked by a

single snapshot.

As already mentioned, the lack of consistent time series data on land use prevents a

general accounting of land use change in the global assessment.

12.3. GLOBAL DIMENSION OF DESERTIFICATION

The danger of potential desertification or drought-prone areas necessitates the

knowledge of where the more vulnerable areas are and the formulation of global theories

about their location. Those areas are characterized mostly by low rainfall and high

summer temperatures, so that the vegetation has little opportunity to restore after

destruction by human impact or prolonged droughts. Such areas occur mainly in two

belts on both sides of the equator; 15-30° NL and 15-30° SL, and are places mostly

protected against the main wind direction by mountain chains.

Even if it has not been scientifically demonstrated that deserts are expending at their

margins, even if the idea of encroachment must now be abandoned, a certain

relationship between deserts, their semi-arid and dry-subhumid margins, and

vulnerability exists. Our first question will be: where are the deserts and their margins?

Where are the drought prone areas, the areas with rainfall deficit?

1.2.1. USDA approach

The purpose of their study was to define and locate desertification tension zones around

the world where the potential decline in land quality is so severe as to trigger a whole

range of negative socioeconomic conditions that could threaten political stability,

sustainability, and the general quality of life.

The formal definition of desertification adopted by the United Nations Convention on

Desertification is, "land degradation in arid, semi-arid, and dry sub-humid areas


resulting from various factors, including climatic variations and human activities."

Excluded in the definition are areas which have a "hyper-arid or a humid" climate.

Under low-input agricultural systems, tension zones occur in areas where the productive

capacity of the land is stressed by mismanagement, generally by resource poor farmers.

The situation arises when the population supporting capacity is exceeded. In high-input

systems, tension zones arise due to excessive use of agri-chemicals, uncontrolled use of

irrigation, and monoclonal plantations with minimal genetic diversity. In either case,

probability of failure of the system is high; the difference between the two systems is a

matter of time.

Tension zones result from:

− Excessive and continuous soil erosion resulting from over and improper use of lands

especially marginal and sloping lands;

− Nutrient depletion and/or soil acidification due to inadequate replenishment of

nutrients or soil pollution from excessive use of organic and inorganic agri-chemicals;

− Reduced water holding capacity of soils due to reduced volume of soil and reduced

organic matter content, both a consequence of erosion and reduced infiltration due to

crusting and compaction;

− Salinization and water-logging from over-irrigation without adequate drainage; and

− Unavailability of water stemming from decreased supply of aquifers and drainage

bodies.

12.3.1.1. Vulnerability to desertification

Two databases provided the biophysical basis for the assessment. The FAO/UNESCO

Soil Map of the World at a scale of 1:5,000,000 (FAO, 1991), whose units were converted

to taxa of Soil Taxonomy. Second, a climate database with records for about 25,000

stations globally, which was used in computing the soil moisture and temperature

regimes. The resulting pedoclimate map was then superimposed on the soil map using

Geographic Information Science (GIS).

The soil and pedoclimate information was used to place each map unit into one of nine

land quality classes with class one having the most favorable and class nine the least

desirable attributes for grain production (Eswaran et al., 1999). This map is shown in

Figure 12.9. To facilitate placement into these classes, a list of 24 land stresses that

constrain grain production was developed.


Figure 12.9. Inherent land quality assessment (USDA, 1998)

Figure 12.9 shows the global distribution of the LQCs. LQCs 1, 2 and 3 have the highest

potentials and least constraints for sustainable agriculture. They occupy 13.3% of the

ice-free land surface and about 1.4 billion people (24.2%) live on these lands. Class 4, 5

and 6 lands occupy 33.4% of the land surface and are present mostly in the inter-tropical

areas. Most of the developing countries have large areas of such lands. About 3 billion

people (52% of global population) live on these lands. They are mostly poor and practice

low-input low-output agriculture. Large areas of these lands have long periods of soil

moisture stress, which is the main cause of reduced soil quality. In the areas with a

humid climate, plantation agriculture provides the wealth of the country. Land quality

classes (LQC) 7, 8 and 9 occur in the fragile ecosystems and are excluded in the

following discussions due to inherent difficulties of implementing sustainable

agriculture programs and also because they are excluded by the narrow definition of

„desertification‟.

An assessment of vulnerability to desertification was then made using the procedure of

Eswaran and Reich (1998). The land qualities and climatic properties without

considering availability of irrigation were employed to make the assessment of

vulnerability to desertification. To evaluate the number of people affected, the map of


vulnerability to desertification was superimposed on an interpolated population density

map developed by Tobler et al. (1995).

Figure 12.10 and Table 12.11 show the results of this analysis. Comparing Figures 12.9

and 12.10, it is clear that many of the lands that are vulnerable belong to LQC 4, 5, and

6. The high to very high desertification vulnerability classes occupy about 11.6% of the

global land surface.

Figure 12.10. Desertification vulnerability (USDA, 1998)

Desertification processes impact about 2.6 billion people or 44% of the world‟s

population (Table 12.11). Many of them are probably contributing to the process as they

live in the developing countries of the world where good land management is not the

rule. There are, of course, considerable differences between countries with respect to

impacts of high populations to land degradation.

Cleaver and Schreiber (1994) estimate that about 50% of Sub-Saharan agricultural land

has lost its productivity due to degradation and about 80% of rangeland show signs of

degradation. Shifting cultivation with long fallow periods and transhuman pastoralism

was appropriate in the past when populations were low. However, in many countries

this steady state is being tilted towards exploitation of the resource base. The slow


evolution to more intensive and permanent systems without appropriate inputs is

contributing to the decline of land quality. A similar process is also operating in many

countries of Asia.

Table 12.11. Estimates of land area belonging to vulnerability classes and corresponding

number of impacted population

Vulnerability

class

Area subjected to desertification Population affected

Area

(million km²)

Percent

(land area)

Number

(Millions)

Percent

(global pop.)

Low 14.60 11.2 1,085 18.9

Moderate 13.61 10.5 915 15.9

High 7.12 5.5 393 6.8

Very High 7.91 6.1 255 4.4

TOTAL 44.24 34.0 2,648 44.0

12.3.1.2. Risk of human-induced desertification

In a second analysis, classes of population density were superimposed on the

desertification map. Accelerated desertification takes place with increasing population

density and particularly under low-input systems. In some situations, this

generalization may not be true, but this assumption was made to evaluate risk of

human-induced desertification.

Three classes of population density were used and a map of these three classes was

superimposed on the vulnerability to desertification map (Table 12.12). This matrix was

developed to relate vulnerability and population density to risk of human-induced

desertification. A high population density in an area that is highly vulnerable to

desertification poses a very high risk for further land degradation. Conversely, a low

population density in an area where the vulnerability is also low poses in principle a low

risk.

Table 12.12. Matrix for assessment of risk of human-induced desertification. The tension

zones are: 1=Low risk; 2,3=Moderate risk; 4,5,6=High risk; 7,8,9=Very high risk.

Vulnerability class Population density

< 10 10 – 40 > 41

Low 1 3 6

Moderate 2 5 8

High/very high 4 7 9

Figure 12.11 shows the distribution of the risk of human-induced desertification. The

Mediterranean countries of North Africa are very highly prone to desertification. In

Morocco, for example, erosion is so extensive that the petrocalcic horizon of some


Palexeralfs is exposed at the surface. In the Sahel, there are pockets of very high-risk

areas. The West African countries, with their dense populations, have major problems

containing the processes of desertification. There are large areas of Central and

Southern Asia, which are highly vulnerable. And in South America, the northeast corner

of Brazil (the province of Pernambuco) is highly vulnerable.

Figure 12.11. Risk of human induced desertification (USDA, 1999)

There are about 7.1 million km² of land at low risk of human-induced desertification, 8.6

million km² at moderate risk, 15.6 million km² at high risk, and 11.9 million km² at very

high risk. Each of these classes represents a tension zone. In the very high risk zone

about 1.413 billion people are involved.

The concept of desertification suggests some or all of the following negative effects and

the probability of their occurrence is highest in the tension zones:

− Systematic reduction in crop performance even leading to failure in rainfed and

irrigated systems;

− Reduction in land cover and biomass production in rangeland with an accompanying

reduction in quality of feed for livestock;

− Reduction of available woody plants for fuel and increased distances to harvest

them;


− Significant reduction in water from overland flows or aquifers and a concomitant

reduction in water quality;

− Encroachment of sand and crop damage by sand-blasting and wind erosion;

− Increased gully and sheet erosion by torrential rain.

1.2.2. UNEP – UNCCD approach

Only four global assessments of the extent and severity of desertification have been

published.

12.3.1.3. First World Map of Desertification (1977)

The first World Map of Desertification was prepared by FAO, UNESCO, and WMO

(1977) at a scale of 1:25 million showing the extent and degree of desertification hazards

by bio-climatic zones. It was an estimate of the global areas affected by desertification.

The degree of hazard was assessed on the basis of severity of climate, sensitivity to

degradation of terrain, soils and vegetation cover, and pressure of human and livestock

populations. Although the map depicted the threat, rather than the actuality of

desertification, it may be taken as expressive of the magnitude of the problem

nevertheless. It was however based on very little data or experience and has only

historical significance.

12.3.1.4. Second world map of desertification (1983)

A few years later, an attempt was made to produce the first country-by-country

assessment of land degradation, including 91 countries with land areas inside the

climatic limits of terrain susceptible to desertification. Again, the database that could be

collected was poor. Field experiment data were mainly restricted to water effects on crop

yields. Informed opinion, anecdotal evidence, observations by travelers, and published

and unpublished reports formed the basis for the numerical estimates. The classification

of the desertification status was refined:

For rainfed and irrigated cropland, desertification was deemed :

o moderate if there was wide-spread erosion or salinisation and waterlog-

ging with losses of up to 25% of crop production

o severe with losses between 25-50%

o very severe with losses> 50%

For rangelands, desertification was identified as:

o moderate when 25% decline in carrying capacity


o severe when decline in carrying capacity was between 25-50%

o very severe when decline in carrying capacity was > 50%

12.3.1.5. Third world map of desertification (1992)

The 1992 assessment was considered by its authors to be considerably better than the

1983 estimates, due to additional field experiments and better literature searches. Its

accuracy was still low, presumably, but there was no way to check that because there

were no good base data against which to compare estimates.

This third global survey, conducted under UNEP sponsorship used a soil degradation

database, GLASOD (Global Assessment of Soil Degradation), which had not been

developed specifically for desertification studies but for human-induced soil degradation

in all environments. The expert based methodology used by GLASOD is based on a more

rigorous and consistent set of guidelines than previous assessments and draws its

estimates from a much larger pool of regional soil degradation experts. Assessment of

the degree and extent of human-induced soil degradation has been made using a

detailed and consistent set of indicators for each of four main degradation processes:

water erosion, wind erosion, physical deterioration and chemical deterioration.

It provided virtually all the data for the first edition of the World Atlas of Desertification

(UNEP, 1992), which is not an atlas of land degradation but an atlas of soil degradation,

ignoring the most widespread degradation process of all, vegetation degradation.

Table 12.13. summarises the extent of land affected by desertification based on

GLASOD.

Table 12.13. UNEP estimates of areas affected by desertification in the form of human-

induced soil degradation (GLASOD), stratified into cropland (rainfaed/irrigated) and

rangeland

Land use Million Ha % of total drylands

Degraded irrigated lands

Degraded rainfed croplands

Degraded rangelands

Total

43

216

757

1016

0.8

4.1

14.6

19.5


12.3.1.6. Fourth world map of desertification (1997)

Degradation of vegetation, which might not necessarily also involve degradation of soil,

is regarded as another important aspect of desertification. Therefore a second

assessment of vegetation degradation desertification was also commissioned by UNEP

and was carried out by ICASALS (International Centre for Arid and Semi-Arid Land

Studies).The greatly revised second edition of the atlas attempts to rectify the

overemphasis on soil degradation in the first atlas (UNEP, 1997).

12.3.1.7. Towards a new world map of desertification (2011-2012)

In response to the interest expressed by the UNCCD for an updated World Atlas on

Desertification, the European Commission's Joint Research Centre (JRC) in partnership

with UNEP, is coordinating the compilation of a new atlas. On the data side progress

has been made through longer time series of observations and through increased

availability and quality of national, regional and global datasets. Earth observation now

offers possibilities not at hand in the past, through long time series, a wide variety of

sensors and a range of value added products. International achievements from projects

such as the Millennium Ecosystem Assessment (MA) and Land Degradation Assessment

in Drylands (LADA) will be fully integrated as a valuable basis for producing the

enhanced World Atlas on Desertification. Experts will agree on a suite of methodologies

for the assessment and mapping of desertification, land degradation and drought,

adapted to the various scales and perception.

(http://desert.jrc.ec.europa.eu/action/php/index.php?action=view&id=-1)

To better address and solve the global desertification challenges there is an urgent need

for improved baseline and trend information using properly agreed assessment

methodologies and functional maps on the status of desertification, land degradation

and drought.

12.3.2. Special patterns

Some 1035 million ha of the world's susceptible drylands are affected by soil

degradation, of which 87% are in the light and moderate categories. The distribution by

continent and by aridity zone is given in Table 12.14. High and very high severity

degradation can be seen to be a phenomenon that is particularly significant in Africa,

whether considered in relative or absolute terms. As noted earlier soil degradation

affects some 320 million ha in Africa out of a total of 1286 million, or about a quarter of

the susceptible drylands.

http://desert.jrc.ec.europa.eu/action/php/index.php?action=view&id=-1


Table 12.14. Soil degradation degree by region in susceptible dryland areas (millions of

ha)

Region Aridity zone Soil degradation degree

Light and

moderate

Strong and

extreme

Total

Africa Dry sub-humid 25.2 12.1 37.3

Semi-arid 69.9 39.6 109.5

Arid 150.2 22.3 172.5

Asia Dry sub-humid 70.6 7.7 78.3

Semi-arid 124.2 17.2 141.4

Arid 131.9 18.8 150.7

Australia Dry sub-humid 4.2 0.6 4.8

Semi-arid 32.9 1.0 33.9

Arid 48.9 0.0 48.9

Europe Dry sub-humid 59.0 2.3 61.3

Semi-arid 30.8 2.6 33.4

Arid 4.8 0.0 4.8

North America Dry sub-humid 15.0 3.2 18.2

Semi-arid 50.9 2.3 53.2

Arid 6.3 1.6 7.9

South America Dry sub-humid 21.4 2.3 23.7

Semi-arid 43.9 4.0 47.9

Arid 7.5 0.0 7.5

Regardless of how much of the areas are affected the distribution of the severest

degradation in Africa, Asia and North America occurs on the wetter margins of the

drylands. Scrutiny of Table 12.14 however, suggests a rather different picture.

Degradation in Africa and Asia is largely concentrated in semi-arid and arid areas:

actual areas affected increase from dry sub-humid zones, through the semiarid zones, to

peak in their extent in arid areas, although Figure 12.1 indicates that degradation

appears to be minimal in the northern Sahel. A similar pattern is identifiable in

Australia, albeit that the level of overall severity is consistently lower than in Africa and

Asia. In the Americas, by contrast, greater areas of land are degraded in the semiarid

zone than in the dry sub-humid and arid zones combined. The situation is different

again in Europe, where degradation in the dry sub-humid zone greatly exceeds that in

the semiarid and arid parts of the continent.

12.3.3. Relative importance of different degradation types

The GLASOD database indicates that dryland degradation is dominated by soil erosion

(Table 12.15). Water erosion accounts for 45% of the degraded area and wind erosion for

42%. Chemical deterioration accounts for 10% and physical deterioration just 3%.

This situation however varies according to aridity zone. Given likely variations in

vegetation cover, it is not surprising that 60% of soil degradation in arid zones is by


wind erosion, a figure which falls to 21 % in dry sub-humid areas. The reverse trend is

found for water erosion to which the wetter dryland areas are inevitably going to be

more susceptible. Water erosion is the dominant form of degradation in dry sub-humid

and semiarid zones, accounting for 63% and 51% of the degradation in these zones

respectively. In arid areas, water erosion accounts for 29% of the degradation.

Table 12.15. Soil degradation type by susceptible dryland climate zone (million ha)

Climate zone Type of soil degradation

Water erosion Wind erosion Chemical

deterioration

Physical

deterioration

Total

Dry sub-humid 141.0 46.8 22.5 13.2

Semi-arid 213.2 150.3 40.9 15.1

Arid 113.3 235.3 37.3 6.5

Total 467.4 432.4 100.7 34.7 1035.2

The relative importance of water erosion for all continents except for Africa is shown by

the data in Table 12.16.

Table 12.16. Soil degradation type by region in susceptible dryland climate zone (million

ha)



deterioration

Physical

deterioration

Total

Africa 119.1 159.9 26.5 13.9 319.4

Asia 157.5 153.2 50.2 9.6 370.5

Australasia 69.6 16.0 0.6 1.2 87.5

Europe 48.1 38.6 4.1 8.6 99.4

N. America 38.4 37.8 2.2 1.0 79.4

S. America 34.7 26.9 17.0 0.4 79.0

Total 467.4 432.4 100.7 34.7 1035.2

In Asia, Europe North America and South America water erosion is responsible for

degrading slightly more land than wind erosion, while in Australasia water erosion is by

far the most dominant form, affecting 80% of the susceptible dry lands to wind erosion's

18%. In Africa, however, more land is degraded by wind erosion than by water .erosion.

Only in Asia is the proportion of dry lands affected by chemical deterioration greater

than the global average. The global average proportion of land affected by physical

deterioration is exceeded only in Europe, where 9% of the susceptible drylands are

affected.

For all types of deterioration except for chemical deterioration, the greatest areas

susceptible dryland affected are degraded to a moderate degree (Table 12.17). For wind

erosion, 95% of all degradation is in the light and moderate categories. The proportion of


land degraded by water erosion to these degrees is 82%, while for physical deterioration

it is 74%. Of all types of degradation, the greatest proportion of land affected to a strong

degree is by in situ processes: 24% of the susceptible drylands degraded by chemical

deterioration, and 26% of the susceptible drylands degraded by physical deterioration.

No soils affected by physical deterioration are degraded to an extreme degree.

Table 12.17. Soil degradation type by degree in susceptible dryland climate zone (million

ha)



deterioration

Physical

deterioration

Total

Light 175.1 197.2 44.3 10.8 427.3

Moderate 208.5 215.4 31.4 15.0 470.3

Strong 79.0 18.0 24.2 8.9 130.1

Extreme 4.8 1.8 0.8 0.0 7.5

Total 467.4 432.4 100.7 34.7 1035.2

The global patterns are investigated further on specific degradation types; followed by

analysis of the human activities causing degradation and an appraisal of the

relationships between soil degradation and vegetation.

References

Bai ZG, Dent DL, Olsson L and Schaepman ME 2008. Global assessment of land

degradation and improvement. 1. Identification by remote sensing. Report 2008/01,

ISRIC – World Soil Information, Wageningen

Eswaran, H. and Reich, P.F. 1998. Desertification: a global assessment and risks to

sustainability. Proceedings of the 16th International Congress of Soil Science,

Montpellier, France.

Oldeman LR., Hakkeling R.T.A., Sombroek W.G. 1991. World map of the status of

human-induced soil degradation: an explanatory note. Wageningen: International Soil

Reference and Information Centre.

USDA-NRCS 1999. Human-Induced Desertification Map. USDA.

Wessels, KJ, et al. 2004. Assessing the effects of human-induced land degradation in the

former homelands of northern South Africa with a 1 km AVHRR NDVI time-series.

Remote Sensing of Environment, vol 91(1), pp 47-67.

Soil Degradation | Soil protection and conservation 226

13. Soil protection and conservation

Land degradation, resulting from unsustainable land management practices, is a threat

to the environment as well as to livelihoods. Land degradation is further exacerbated by

climate change and climate variability. Land degradation negatively affects the state

and the management of the natural resources – water, soil, plants and animals - and

hence reduces agricultural production.

Especially in regions where the majority of people directly depend on agricultural

production, there is a potentially devastating downward spiral of overexploitation and

degradation, enhanced by the negative impacts of climate change - leading in turn to

reduced availability of natural resources and declining productivity. This jeopardises

food security and increases poverty.

13.1. PRINCIPLES OF SUSTAINABLE LAND MANAGEMENT

Sustainable Land Management has been defined by TerrAfrica as: „the adoption of land

use systems that, through appropriate management practices, enables land users to

maximize the economic and social benefits from the land while maintaining or

enhancing the ecological support functions of the land resources‟.

The main objective of SLM is to integrate people‟s coexistence with nature over the long-

term, so that the provisioning, regulating, cultural and supporting services of

ecosystems are ensured. This means SLM has to focus on increasing productivity of

agro-ecosystems while adapting to the socio-economic context, improving resilience to

environmental variability, including climate change and at the same time preventing

degradation of natural resources.

Concerted efforts to deal with land degradation through SLM must address water

scarcity, soil fertility, organic matter and biodiversity. Improving the water productivity

and water cycle, soil fertility and plant management are important in raising land

productivity. There is huge potential for SLM in climate change mitigation and

adaption.

SLM thus seeks to increase production including traditional and innovative systems and

to improve resilience to food insecurity, land degradation, loss of biodiversity, drought

and climate change. SLM includes management of soil, water, vegetation and animal


resources. SLM also includes ecological, economic and socio-cultural dimensions (Hurni,

1997).

13.1.1. Increasing land productivity

The primary target of SLM is thus to increase land productivity, improve food security

and also provide for other goods and services. There are three ways to achieve this: (1)

expansion, (2) intensification and (3) diversification of land use.

In many areas of the world, there is very limited scope for further expansion without

highly detrimental impacts on natural resources (e.g. deforestation).

Agricultural intensification during the last 50 years is largely as a result of the „Green

Revolution‟ which was based on improved crop varieties, synthetic fertilizers, pesticides,

irrigation, and mechanisation.

Diversification implies an enrichment of the production system related to species and

varieties, land use types, and management practices. It includes an adjustment in farm

enterprises in order to increase farm income or reduce income variability. This is

achieved by exploiting new market opportunities and existing market niches,

diversifying not only production, but also on-farm processing and other farm-based,

income-generating activities (Dixon et al., 2001). Diversified farming systems (such as

crop–livestock integration, agroforestry, intercropping, crop rotation etc.) enable farmers

to broaden the base of agriculture, to reduce the risk of production failure, to attain a

better balanced diet, to use labour more efficiently, to procure cash for purchasing farm

inputs, and to add value to produce.

Expansion, intensification and diversification to increase agricultural productivity

imply:

− increasing water productivity (water use efficiency)

− enhancing soil organic matter and soil fertility (carbon and nutrient cycling)

− improving plant material (species and varieties), and

− producing more favourable micro-climates.

Table 13.1 gives an overview of different strategies and practices, aimed at improving

productivity and yields.


Table 13.1. Strategies and practices to increase yields

13.1.2. Improved livelihoods

Increased and sustained agricultural production, the provision and securing of clean

water and maintaining a healthy environment are essential for improved livelihoods.

Despite the constraints and problems land users have, they are willing to adopt SLM

practices that provide them with higher net returns, lower risks or a combination of

both.

13.1.2.1. Costs and benefits

For improved livelihoods and for adoption and spreading of SLM, costs and benefits play

a central role. Investments in SLM should aim at both short-term (rapid) and long-term

(sustained) paybacks. Thus inputs for both initial establishment and continued

maintenance afterwards need to be compared with benefits.


Experiences with implementation of SLM, show the need for accurate assessment of

benefits and costs (in monetary and non-monetary terms) and short- and long-term

gains. However, this is seldom done and data are few. Assessments of benefits and costs

are very site specific and therefore pose a great challenge for the spread of SLM in SSA.

Without proper assessments, land users and development agencies cannot make

informed decisions about which technologies and approaches are the most viable options

for a particular natural and human environment - and where incentives for land users

are needed.

Figure 13.1: Benefits and costs of SLM over time, short-term establishment phase and

long-term maintenance phase.

13.1.2.2. Input challenges

Land users may require additional inputs to take up SLM practices. These are related to

materials (machinery, seeds, fertilizers, equipment, etc.), labour, markets, and

knowledge. Some of the SLM practices require few extra or different inputs and little

change compared to current practices; others mean a complete change in machinery,

inputs and management. Some considerations are:

− Small-scale land users in subsistence agriculture have fewer options and resources

to invest than commercial or large-scale farmers with a high level of mechanisation.

− A clear distinction between initial investment for the establishment and the

maintenance of SLM practices is essential. Any material and financial support


should build on currently available resources. Special attention needs to be given to

poor and marginalised land users.

− Labour availability is a major concern and depends on the health of people and

competition with other income generating activities.

− Access to inputs and equipment such as machinery, seeds / seedlings, fertilizers, etc.

is essential. Introduction of SLM is only possible if markets for inputs and products

are secured.

− Access to knowledge related to SLM practices and their introduction is a prerequisite

for all land users. Practices that are easy to learn, and build on existing experiences

and knowledge, have the best chance of being taken up.

Apart from the costs, benefits, access to inputs, markets and knowledge, there are other

elements related to improved livelihoods such as the need for practices to be:

− socially and culturally acceptable: aesthetics and beliefs, norms and values;

− flexible enough to allow (and even encourage) local adaptation and innovation;

− clearly seen to add value to the land and to the quality of life.

13.1.3. Improved ecosystems

The principles of increased production presented above, to be truly sustainable should

also aim at improving ecosystem functions and services. Best practices must be

environmentally friendly, reduce current land degradation, improve biodiversity and

increase resilience to climate variation and change.

13.1.3.1. Prevent, mitigate or rehabilitate land degradation

Depending on what stage of land degradation has been reached, SLM interventions can

be differentiated into prevention and mitigation of land degradation or rehabilitation of

already degraded land (WOCAT, 2007).

− Prevention implies employment of SLM measures that maintain natural resources

and their environmental and productive function on land that may be prone to

degradation.

− Mitigation is intervention intended to reduce ongoing degradation. This comes in at

a stage when degradation has already begun.

− Rehabilitation is required when the land is already degraded to such an extent that

the original use is no longer possible, and land has become practically unproductive

and the ecosystem seriously disturbed. Rehabilitation usually implies high

investment costs with medium- to long-term benefits.


The measures for prevention, mitigation and rehabilitation of land degradation and

restoration of ecosystems services can be classified into four categories (WOCAT, 2007)

of increasing efforts and investments (Figure 13.2). Combinations of measures that lead

to integrated soil and water, crop-livestock, fertility and pest managements are

promising as they increase both ecosystem - and livelihood - resilience.

Figure 13.2. Categories of SLM practices

13.1.3.2. Improve biodiversity

A key concern in sustainable land management and protecting ecosystem functions in

SSA is conserving biodiversity. Sustainable management of natural forests, woodlands,

wetlands, grasslands, savannas and deserts results in the protection of biodiversity and

environmental quality and at the same time offers opportunities for food security and

poverty alleviation.

Agricultural biodiversity encompasses domesticated crop plants, livestock and fish (etc.),

wild cr op relatives, wild food sources, and „associated‟ biodiversity that supports

agricultural production through nutrient recycling, pest control and pollination.

13.1.3.3. Climate change

SLM practices not only contribute to building up carbon in the land but can also give

protection against climate variability. There is evidence of current adaptations and

innovation in SLM technologies and approaches, demonstrating response to climate


change: this experience needs to be acknowledged, investigated and tapped (Woodfine,

2009).

Mitigation in the context of climate change means reducing greenhouse gases and thus

their impacts, while adaptation means amending practices to cope with the impacts of

changing climate (FAO, 2009). SLM is concerned with both. With respect to mitigation,

SLM practices can help sequester carbon in the vegetation as well as in the soil; in

terms of adaptation suitably versatile and „climate proof‟ SLM technologies and

approaches are key to maintaining productive land and ecosystem function.

While mitigation of climate change is not a priority for poor farmers, the same SLM

practices that benefit them directly, can help sequester carbon and reduce emissions.

Sequestering carbon above and below ground can be achieved through:

− afforestation, reforestation and improved forest management practices;

− agroforestry and silvopastoral systems, integrated crop-livestock systems which

combine crops, grazing lands and trees;

− improved management of pastures and grazing practices on natural grasslands,

including optimising stock numbers and utilising rotational grazing to maintain

ground cover and plant biodiversity;

− improved tillage practices – such as conservation agriculture – to increase soil

organic carbon (SOC) content through permanent soil cover with crops and mulch,

minimum soil disturbance, fallows, green manures, and crop rotations; and

− micro-dosing with fertilizer to increase biomass production, yields and SOC.

Reducing emissions of carbon dioxide through:

− reduced land degradation and deforestation, loss of biomass and OM;

− reduced use of fire in rangeland and forest management;

− reduced machine hours for agriculture by adoption of conservation tillage practices /

conservation agriculture systems; and

− practices requiring lower doses of agrochemicals.

Reducing emissions of methane and nitrous oxide through:

− improved nutrition for ruminant livestock;

− more efficient management of livestock waste (manure);

− more efficient management of irrigation water on rice paddies; and

− more efficient nitrogen management on cultivated fields, reducing volatile losses

through better agronomic practices (rotations, fallows, manuring and micro-dosing).


Adaptation to climate change means dealing with its impacts and this can be achieved

by adopting more versatile and climate change resilient technologies – but also through

approaches which involve flexibility and responsiveness to change.

Implementing SLM practices which increase soil organic matter will be beneficial in

adapting to climate change. These will increase „the resilience of the land‟, and thus

„climate proofing‟ through enhanced fertility, soil structure, water infiltration and

retention, soil life and biomass production (Scherr and Sthapit, 2009).

Surface mulch or plant cover established under several SLM practices generally protect

soil from wind, excess temperatures and evaporation losses, reduce crop water

requirements and extend the growing period. This could prove critical in many areas of

SSA affected by climate change. All practices improving water management increase

resilience to climate change. This can be achieved through reducing water losses and

harvesting of rainwater to improve water storage in the soil but also in reservoirs.

Practices diversifying incomes and reducing risks of production failure, for example

integrated crop-livestock systems and improved or more appropriate plant varieties

provide additional opportunities for adaptation.

13.2. TECHNOLOGIES

There is no one miracle solution („silver bullet‟) to solve the problems which land users

in SSA face. The choice of the most appropriate SLM practice in a particular situation

will be determined by local stakeholders, based on the local topographic, soil and

vegetation conditions and socioeconomic context, such as farm size and assets which

may make certain practices ill-advised or not feasible.

13.2.1. Integrated soil fertility management

13.2.1.1. Definition

Integrated Soil Fertility Management (ISFM) aims at managing soil by combining

different methods of soil fertility amendment together with soil and water conservation.

It takes into account all farm resources and is based on 3 principles: (1) maximising the

use of organic sources of fertilizer; (2) minimising the loss of nutrients; (3) judiciously

using inorganic fertilizer according to needs and economic availability.


In Sub-Saharan Africa, soil fertility depletion is reaching a critical level, especially

under small-scale land use. ISFM techniques can regenerate degraded soils and then

maintain soil fertility by using available nutrient resources in an efficient and

sustainable way. ISFM aims at making use of techniques without much additional cost

to the farmer, such as organic fertilizer, crop residues and nitrogen-fixing crops, in

combination with seed priming and water harvesting. A next step is the use of inorganic

fertilizer, which requires financial input; however micro-fertilization can provide a cost-

saving entry point.

Low cost ISFM techniques include: micro-dosing with inorganic fertilizers, manuring

and composting, rock phosphate application, etc. SLM practices such as conservation

agriculture or agroforestry include supplementary aspects of fertility management.

13.2.1.2. Applicability

ISFM is required in areas with low and rapidly declining soil fertility. Due to the wide

variety of ISFM techniques, there is no specific climatic restriction for application apart

from arid areas where water is constantly a limiting factor. ISFM is particularly

applicable in mixed crop-livestock systems.

The technology addresses the following types of land degradation:

− Chemical soil deterioration: fertility decline through reduced soil organic matter

content and nutrient loss

− Physical soil deterioration: compaction, sealing and crusting

− Water degradation: aridification

− Soil erosion by water: loss of topsoil / soil surface

13.2.1.3. Resilience to climate variability

ISFM leads to an increase in soil organic matter (SOM) and biomass, and thus to soils

with better water holding capacity that can support more drought-tolerant cropping

systems. Main benefits: Increased nutrient replenishment and soil fertility maintenance

will enhance crop yields and thus increase food security, improve household income and

hence improved livelihoods and well-being.

13.2.1.4. Adoption and upscaling

Land users‟ attitudes and rationale behind adoption of ISFM are influenced by the

availability and access to inputs such as organic fertilizers (compost, manure) and the


affordability of inorganic fertilizers. Access to financial services and micro-credit must

be provided to land users to enable investment in fertility management. Awareness

raising and capacity building on suitable options of ISFM techniques and appropriate

application is needed.

13.2.2. Conservation agriculture


Conservation Agriculture (CA) is a farming system that conserves, improves, and makes

more efficient use of natural resources through integrated management of soil, water

and biological resources. It is a way to combine profitable agricultural production with

environmental concerns and sustainability. The three fundamental principles behind

the CA concept are: minimum soil disturbance, permanent soil cover, and crop rotation.

Each of the principles can serve as an entry point to the technology; however, only the

simultaneous application of all three results in full benefits. CA covers a wide range of

agricultural practices based on no-till (also known as zero tillage) or reduced tillage

(minimum tillage).

These require direct drilling of crop seeds into cover crops or mulch. Weeds are

suppressed by mulch and / or cover crops and need to be further controlled either

through herbicide application or pulling by hand.


CA has been proven to work in a variety of agro-ecological zones and farming systems:

high or low rainfall areas; in degraded soils; multiple cropping systems; and in systems

with labour shortages or low external-input agriculture.

CA has good potential for spread in dry environments due to its watersaving ability,

though the major challenge here is to grow sufficient vegetation to provide soil cover.


− Physical soil deterioration: reduction in soil‟s capacity to absorb and hold water due

to degradation of soil structure (sealing, crusting, compaction, pulverization) in

drought-prone situations

− Water degradation: aridification due to runoff and evaporation loss

− Chemical soil deterioration and biological degradation: reduction in soil


− organic matter and fertility decline due to soil loss and nutrient mining, reduction of

biodiversity and pest risk (in tropical and subtropical conditions)

− Erosion by water and wind


CA increases tolerance to changes in temperature and rainfall including incidences of

drought and flooding.

13.2.2.4. Main benefits

CA is considered a major component of a „new green revolution‟ in SSA which will help

to make intensive farming sustainable through increased crop yields / yield reliability

and reduced labour requirements; will cut fossil fuel needs through reduced machine

use; will decrease agrochemical contamination of the environment through reduced

reliance on mineral fertilizers; and will reduce greenhouse gas emissions, minimise run-

off and soil erosion, and improve fresh water supplies. CA can thus increase food

security; reduce off-site damage; reduce foreign exchange required to purchase fuel and

agrochemicals; and create employment by producing CA equipment locally. The

potential to mitigate and to adapt to climate change is high.


Change of land user‟s mind-set, support for specific material inputs and good technical

know-how increase the potential for adoption.

A main aim is to phase out or minimise herbicide use - because of the potential risk to

the environment. Alternative methods of weed control with minimum soil disturbance

are needed. Pioneer farmers in regions of new adoption require support for access to no-

till tools / equipment, cover crop seed and technical guidance. Critical constraints to

adoption appear to be competing uses for crop residues (as mulch), increased labour

demand for weeding, and lack of access to, and use of, external inputs.

13.2.3. Rainwater harvesting


Rainwater Harvesting (RWH) refers to all technologies where rainwater is collected to

make it available for agricultural production or domestic purposes. RWH aims to

minimise effects of seasonal variations in water availability due to droughts and dry

periods and to enhance the reliability of agricultural production.


A RWH system usually consists of three components: (1) a catchment / collection area

which produces runoff because the surface is impermeable or infiltration is low; (2) a

conveyance system through which the runoff is directed e.g. by bunds, ditches, channels

(though not always necessary); (3) a storage system (target area) where water is

accumulated or held for use - in the soil, in pits, ponds, tanks or dams. When water is

stored in the soil - and used for plant production there - RWH often needs additional

measures to increase infiltration in this zone, and to reduce evaporation loss, for

example by mulching. Furthermore soil fertility needs to be improved by composting /

manuring, or micro-dosing with inorganic fertilizers. Commonly used RWH techniques

can be divided into microcatchments collecting water within the field and macro-

catchments collecting water from a larger catchment further away.


RWH is applicable in semi-arid areas with common seasonal droughts. It is mainly used

for supplementary watering of cereals, vegetables, fodder crops and trees but also to

provide water for domestic and stock use, and sometimes for fish ponds. RWH can be

applied on highly degraded soils.


− Water degradation: aridification through decrease of average soil moisture content

and change in the quantity of surface water

− Erosion by water: loss of fertile topsoil through capturing sediment from catchment

and conserving within cropped area


− Chemical soil deterioration and biological degradation: fertility decline and reduced

organic matter content


RWH reduces risks of production failure due to water shortage associated with rainfall

variability in semi-arid regions, and helps cope with more extreme events, it enhances

aquifer recharge, and it enables crop growth (including trees) in areas where rainfall is

normally not sufficient or unreliable.


RWH is beneficial due to increased water availability, reduced risk of production failure,

enhanced crop and livestock productivity, improved water use efficiency, access to water


(for drinking and irrigation), reduced off-site damage including flooding, reduced

erosion, and improved surface and groundwater recharge. Improved rainwater

management contributes to food security and health through households having access

to sufficient, safe supplies of water for domestic use.


The RWH techniques recommended must be profitable for land users and local

communities, and techniques must be simple, inexpensive and easily manageable.

Incentives for the construction of macrocatchments, small dams and roof catchments

might be needed, since they often require high investment costs. The greater the

maintenance needs, the less successfully the land users and / or the local community

will adopt the technique.

13.2.4. Smallholder irrigation management


A Smallholder Irrigation Management (SIM) unit is typically a plot covering an area

less than 0.5 ha. SIM schemes may be managed either by an individual land user or by

groups / communities.

The guiding principle of sustainable SIM is „more crop per drop‟, in other words

efficiency of water use. This can be achieved through more efficient (1) water collection

and abstraction; (2) water storage; (3) distribution and; (4) water application in the field.

Two main categories of SIM can be distinguished, traditional surface irrigation systems

and recent micro-irrigation systems including drip irrigation.

Micro-irrigation systems are commonly used for, and are very important in, the

production of vegetables, fruits and flowers. More efficient water use can enhance

production benefits remarkably. However, additional measures including soil fertility

management, introduction of high value crops and appropriate pest and disease control

are necessary for a substantial increase in production. As water resources in SSA are

generally scarce and very unevenly distributed, any dream of widespread irrigation

schemes is unrealistic. However, there is scope for improved irrigation management -

making the most efficient use of precious water resources, especially for small-scale

farming. Priority areas for SIM in SSA are in semi-arid and subhumid areas, where a

small amount of irrigation water leads to a significant increase in yield - or at least a

reduction in crop failure. Often there are possible synergies to be made by basing such


schemes on water collected through rainwater harvesting. Therefore, SIM builds on the

principles of supplementary irrigation, with rainfall as the principle source of water, and

supplementary irrigation helping during dry spells and extending the growing period.


SIM is most applicable to arid, semi-arid and subhumid areas. In water-scarce regions,

the amount of irrigation water is limited and irrigation competes with other water

demands.


− Water degradation: aridification – decrease of average soil moisture content,

overuse/over-abstraction of surface and groundwater / aquifer level due to inefficient

− water use and too high demand on irrigation water

− Physical soil deterioration: waterlogging, sealing and crusting through inappropriate

irrigation management

− Chemical soil deterioration: salinisation of soil through inappropriate irrigation

management and through bad quality of irrigation water

− Unsuitable for areas prone to salinisation where salts cannot be washed out by

drainage


SIM systems can enhance the resilience to droughts and temperature increase. The

storage of excess rainfall and the efficient use of irrigation are critical in view of growing

water scarcity, rising temperatures and climatic variability.


This system can increase incomes of the farmers by producing more, and higher-value,

crops. Helping land users to move from subsistence farming to producing cash crops

contributes to poverty reduction, primarily by enhancing the productivity of both labour

and land. Agricultural production risks can be reduced, and food security enhanced.


The major constraint to smallholder irrigation is the availability of water. Financing

(high costs of equipment), and the lack of a functioning

market system to sell products, are further constraints. Therefore it is important that

access to financial services is provided to land users. Land user group organisations can

be a means to pool land users and resources and develop irrigation schemes.


13.2.5. Agroforestry


Agroforestry (AF) is a collective name for land use systems and practices in which woody

perennials are deliberately integrated with agricultural crops and / or livestock for a

variety of benefits and services. The integration can be either in a spatial mixture (e.g.

crops with trees) or in a temporal sequence (e.g. improved fallows, rotation). AF ranges

from very simple and sparse to very complex and dense systems. It embraces a wide

range of practices: alley cropping, farming with trees on contours, or perimeter fencing

with trees, multi-storey cropping, relay cropping, intercropping, multiple cropping, bush

and tree fallows, parkland systems, homegardens etc.; many of them are traditional

land-use systems.

AF is thus not a single technology but covers the broad concept of trees being integrated

into cropping and livestock systems in order to achieve multifunctionality.

There is no clear boundary between AF and forestry, nor between AF and agriculture.


On subhumid mountain slopes AF can be practiced on a whole farm as around Mt.

Kilimanjaro (Chagga system) and Mt. Kenya (Grevillea system). In the drylands AF is

rarely practiced on whole farms (except under parkland systems in the Sahel). It is more

common for trees to be used in various productive niches within a farm. AF is mainly

applicable to small-scale land users and in small-to large-scale tea / coffee plantations.


− Chemical soil deterioration: declining soil fertility and organic matter content (due to

continuous cropping and few inputs)

− Erosion by water and wind: loss of fertile topsoil


− Water degradation: namely high water losses by non-productive surface evaporation,

extreme heavy events causing runoff and erosion


AF is tolerant to climate variability. AF systems are characterised by creating their own

micro-climates, and buffering extremes (excessive storms or dry and hot periods). AF is


recognised as a greenhouse gas mitigation strategy through its ability to sequester

carbon biologically. The adaptation and mitigation potential depends on the AF system

applied.


Agroforestry systems have great potential to diversify food and income sources, improve

land productivity and to stop and reverse land degradation via their ability to provide a

favourable micro-climate, provide permanent cover, improve organic carbon content,

improve soil structure, increase infiltration, and to enhance fertility and biological

activity of soils .


There is a lack of quantitative and predictive understanding

about traditional and innovative agroforestry practices and their importance in order to

make them more adoptable. Long term field research / monitoring are needed because of

the complex nature of tree / crop systems.

13.2.6. Pastoralism and rangeland management


Pastoralism and rangeland management refer to extensive production of livestock using

pastures and browse, and is mainly found in arid and semi-arid areas. In SSA the term

„pastoralism„ is usually associated with the use of common property resources subject to

some group agreements rather than „open access‟.

„Ranching‟ on the other hand implies individual, privatised land ownership. Pastoralism

is based on open grazing lands, e.g. savannas, grasslands, prairies, steppes, and

shrublands, managed through herding. Pastoralists adopt opportunistic land use

strategies, that is they follow resources of grazing / browsing and water, destock in times

of drought (often de facto through livestock mortality rather than stock sales) but have

rapid response post-drought restocking strategies (commonly based first on the high

reproduction rates amongst indigenous sheep and goats).

There are many types and degrees of pastoral mobility, which vary according to

environmental conditions or the given household situation. Mobility can be seasonal,

regular between two well-defined pasture areas, or following erratic rain. It is rarely the


same from one year to another. Movement is not necessarily undertaken only for

resource-based reasons; it can be for trade or because of conflict.

Pastoral activities have conventionally been considered uneconomic and ecologically

destructive. Current thinking increasingly recognises these strategies as economically

viable, environmentally sustainable, and compatible with development. The challenge is

to adapt traditional pastoralism to today‟s changing environmental conditions.

Establishment of feed banks, improvement of herd composition and health, a more

dense distribution of wells, collection and storage of surface water by, for example,

„charco dams‟, adaptive grazing, land use plans, access to markets, and empowerment

are such opportunities.


A production system for marginal, dry lands: relatively low inherent productivity due to

aridity, altitude, temperature and / or a combination of all factors. Pastoralism is

becoming increasingly constrained because of weakening of traditional governance over

communal natural resources, restricted mobility, sedentarisation, boundaries and

advancing agriculture.

Resilience to climate variability: By definition pastoralism is based on continuous

adaptation to highly uncertain environments, especially climate. Traditional

pastoralism has / is losing flexibility and options for coping with drought (e.g. loss in

mobility due to encroachment of cropping and growing human populations) leading to

increased risk.


Mobile herding systems combine economic production in marginalised land and

environmental protection (biodiversity) of vulnerable ecosystems, which have been

modified over time by pastoralism itself; improved food security and livelihood of

marginalised and disadvantaged people. The vast areas of degraded rangeland play a

vital role in sequestering carbon. Dry soils are better longer term sinks for C than soils

in more humid environments.



Effective pastoral management of the drylands depends on livestock mobility (access to

dry season grazing sites and water points), effective communal tenure and governance

systems, and herd adaptation.

13.3. APPROACHES

For upscaling sustainable land management, an enabling environment is of paramount

importance; this includes institutional, policy and legal framework, local participation

as well as regional planning (landscape or watershed), capacity building, monitoring and

evaluation, and research.

SLM technologies need approaches that enable and empower people to implement,

adopt, spread and adapt best practices.

Current promising approaches underlie the following principles:

− People-centred approaches: People and their actions are a central cause of land

degradation, and thus need to be at the centre of SLM. There must be genuine

involvement of land users throughout all phases.

− Multi-stakeholder involvement: This includes all actors, with their various

interests and needs, with respect to the same resources. It includes local, technical

and scientific knowledge and mechanisms to create a negotiation platform.

− Gender consideration: Gender roles and responsibilities need to be considered

seriously, since in smallholder agriculture women are taking over more of the

agricultural tasks once done only by men such as land preparation, and they are

investing more work in cash crop production.

− Multi-sectorial approaches: Successful SLM implementation brings together all

the available knowledge in different disciplines, institutions and agencies including

government, non-governmental and private sectors.

− Multi-scale integration: This unifies local, community but also the landscape,

watershed or transboundary level, and up to the national and international level

also. It implies that not only are local on-site interests considered, but off-site

concerns and benefits also. This means that the concept of „freedom of local land

users‟ might be narrowed down in the interest of a larger community.However, it

also opens up possibilities for additional markets, as well as compensation or

funding mechanisms. While local benefits from investments in SLM already might


be a sufficient incentive for land users, off-site concerns and benefits need to be

negotiated.

− Integrated land use planning: This assesses and assigns the use of resources,

taking into account demands from different users and uses, including all agricultural

sectors - pastoral, crop and forests - as well as industry and other interested parties

also.

13.4. DECISION SUPPORT

Land users, agricultural advisors and decision makers are faced with the challenge of

finding the best land management practices for particular conditions. Thus they have

the same questions to answer (see Figure 13):

− Which SLM technology and approach should be chosen?

− Where to apply them?

− How to apply them?

− Who plays what roles?

− What are the costs?

− What are the impacts?

− Do they improve food security, and alleviate poverty?

− Do they combat land degradation / desertification?

− How well are they matched to a changing climate?

Another fundamental question is where and when to invest: prevention before land

degradation processes start, or rather mitigation / „cure‟ after degradation has started -

or rehabilitation when degradation is most severe? The costs vary considerably

depending on the stage of SLM intervention (Figure 13).

Inputs and achievements depend very much on the stage of degradation at which SLM

interventions are made. The best benefit-cost ratio will normally be achieved through

measures for prevention, followed by mitigation, and then rehabilitation. In prevention,

the „benefit‟ of maintaining the high level land productivity and ecosystem services has

to be measured compared to the potential loss without any intervention. While the

impacts of (and measures involved in) rehabilitation efforts can be highly visible, the

related achievements need to be critically considered in terms of the cost and associated

benefits.


Step 1 – Identification of SLM best practices involving all stakeholders:

The first step for better decision support is the initial involvement of all stakeholders in

SLM (e.g. through a stakeholder workshop). The aim is to identify existing prevention

and mitigation strategies against land degradation and desertification. The methodology

brings together scientific and local knowledge while simultaneously supporting a co-

learning process oriented towards sustainable development. The objectives are:

(1) to reflect on current and potential problems and solutions related to land degra-

dation and desertification;

(2) to create a common understanding of problems, potentials and opportunities;

(3) to strengthen trust and collaboration among concerned stakeholders;

(4) to identify existing and new SLM practices; and

(5) to select a set of these identified strategies for further evaluation and documenta-

tion in the next step.

Step 2 – Participatory decision-making for selection and implementation of SLM best

practices:

After documentation and assessment of existing SLM practices, the challenge is to

decide on best practices and where to implement them. This again involves all

stakeholders (e.g. in a second stakeholder workshop) and recently developed decision

support tools to evaluate the best options and set priorities. These tools allow selection

of SLM options, comparison and ranking of them, negotiation and finally a decision

regarding which is (or are) the best-bets for specific conditions (Schwilch et al. 2009).

The choice of the most appropriate SLM practice in a particular situation will be

determined by local stakeholders, based on the local topographic, soil and vegetation

conditions and socioeconomic context, such as farm size and assets which may make

certain practices ill-advised or not feasible.

Whether such SLM practices are accepted or not depends on cost-effectiveness, severity

of degradation, knowledge, enabling framework conditions (e.g. policies and subsidies)

and on other socio-cultural and economic issues.

The key to success lies in a concerted effort by all, where special attention needs to be

paid to the participatory process of selecting potential SLM interventions. Otherwise

land users will neither accept nor properly implement the practice, and project success

will be threatened. Stakeholder involvement is crucial at all stages.


Step 3 - Monitoring and assessment – improve SLM and justify investments

Monitoring and assessment (M&A) of SLM practices and their impacts is needed to

learn from the wealth of knowledge available including traditional, innovative, project

and research experiences and lessons learnt – both successes and failures. M&A can

lead to important changes and modifications in approaches and technologies (WOCAT,

2007). SLM is constantly evolving, which means M&A must be ongoing and responsive.

Land users have to take an active role as key actors in M&A: their knowledge and

judgement of the pros and cons of SLM interventions is crucial. More investment in

training and capacity building is needed for M&A generally, and specifically to improve

skills in knowledge management and decision support.

The major research challenges are:

− M&A of the local impacts of SLM and land degradation (ecological, economic and

social);

− proper cost and benefit analysis of SLM intervention measures;

− M&A of regional impacts at watershed and landscape levels (including off-site and

trans-boundary effects);

− mapping and monitoring of land degradation and the extent and effectiveness of

SLM practices; and

− use of knowledge about SLM for improved decision making at all levels (developing

tools and methods for improved knowledge management and decision support).

References

Dixon, J., Gulliver, A., Gibbon, D., Hall, M. (2001). Farming Systems and Poverty.

Improving Farmer‟s Livelihoods in a Changing World. FAO and World Bank, Rome,

Italy and Washington D.C. USA.

FAO, 2009. FAO - Profile for climate change.

ftp://ftp.fao.org/docrep/fao/012/i1323e/i1323e00.pdf

Liniger, HP., Mekdaschi Studer, R. , Hauert C. and Gurtner M. 2011. Sustainable Land

Management in Practice – Guidelines and Best Practices for Sub-Saharan Africa.

TerrAfrica, World Overview of Conservation Approaches and Technologies (WOCAT)

and Food and Agriculture Organization of the United Nations (FAO).

ftp://ftp.fao.org/docrep/fao/012/i1323e/i1323e00.pdf


SoCo Project Team. 2009. Addressing soil degradation in EU agriculture: relevant

processes, practices and policies. JRC Scientific and Technical reports. EUR 23767.

WOCAT 2007. Where the land is greener – case studies and analysis of soil and water

conservation initiatives worldwide.

soil degradation - ugentaverdood/cursus land degradation...soil degradation ann verdoodt academic...

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