Soil Degradation and Conservation

Soil Degradation – the reduction in the quality and usefulness of soil

Soil Conservation – maintaining and/or restoring soil to optimal quality

Agriculture today

We have converted 38% of Earth’s surface for agriculture, the practice of cultivating soil, producing crops, and raising livestock for human use and consumption.

Croplands (for growing plant crops) and rangelands (for grazing animal livestock) depend on healthy soil.

Croplands

• Help maintain water flow and soil infiltration

• Provide partial erosion protection • Can build soil organic matter

• Store atmospheric carbon

• Provide wildlife habitat for some species

Ecological Services Economic Services

• Food crops

• Fiber crops

• Crop genetic resources

• Jobs

Natural Capital

Croplands

Ecological Services

Economic Services

•Help maintain water flow and soil infiltration

•Provide partial erosion protection

•Can build soil organic matter

•Store atmospheric carbon

•Provide wildlife habitat for some species

•Food crops

•Fiber crops

•Crop genetic resources

•Jobs

World soil conditions• Soils are becoming degraded in many regions.

Figure 8.1a

Soil degradation by continent• Europe’s land is

most degraded because of its long history of intensive agriculture.

• But Asia’s and Africa’s soils are fast becoming degraded.

Figure 8.1b

Causes of soil degradation

• Most soil degradation is caused by:

– • livestock overgrazing

– • deforestation

– • cropland agriculture.

Figure 8.2

The Importance of Humus

• AKA – Organic Matter

• Dark, crumbly mass of undifferentiated material made up of complex organic compounds

• Soils with high humus content

• hold moisture better and

• are more productive for plant life.

Soil characterization• Soil can be characterized by color and several other traits:– Texture (percentage sand, silt, clay)– Structure– Porosity– Cation exchange capacity – the ability of clay and humus

particles to attract K, Mg, Ca, Fe, and other metal minerals necessary for plant growth

– pH– Parent Material– Infiltration rate / Permeability– Nutrient concentrations

– Best for plant growth is loam, an even mix of sand, silt and clay.

Erosion and deposition• Erosion = removal of material from one place and

its transport elsewhere by wind or water

• Deposition = arrival of eroded material at a new location

• These processes are natural, and can build up fertile soil.

• But where artificially sped up, they are a big problem for farming.

Erosion

• Commonly caused by:

– • Overcultivating, too much plowing, poor planning

– • Overgrazing rangeland with livestock

– • Deforestation, especially on slopes

Types of soil erosion

Figure 8.11

Splash erosion

Rill erosion

Gully erosion

Sheet erosion

Erosion: A global problem

• Over 19 billion ha (47 billion acres) suffer from erosion or other soil degradation.

• Much of the erosion ends up in the water shed– Mississippi River…to thin to plow to thick to drink (Sam

Clemens)

• Soil integrity and fertility is reduced.

Desertification• A loss of more than 10% productivity due to:– • Erosion– • Soil compaction– • Forest removal– • Overgrazing– • Drought– • Salinization– • Climate change– • Depletion of water resources When severe, there is

expansion of desert areas, or creation of new ones, e.g., the Middle East, formerly, “Fertile Crescent”.

The Dust Bowl• Drought and degraded

farmland produced the 1930s Dust Bowl.

• Storms brought dust from the U.S. Great Plains all the way to New York and Washington, and wrecked many lives.

Figure 8.14

Colorado Kansas

DustBowl

Oklahoma

New Mexico

Texas

MEXICO

ConsequencesCauses

Worsening drought

Famine

Economic losses

Lower living standards

Environmentalrefugees

Overgrazing

Deforestation

Erosion

Salinization

Soil compaction

Natural climate change

Soil conservation• As a result of the Dust Bowl,

• the U.S. Soil Conservation Act of 1935 and

• the Soil Conservation Service (SCS) werecreated.

• SCS: Local agents in conservation districts worked with farmers to disseminate scientific knowledge and help them conserve their soil.

Preventing soil degradation

• Several farming strategies to prevent soil degradation:– • Crop rotation– • Contour farming– • Intercropping– • Terracing– • Shelterbelts– • Conservation tillage

Crop rotation• Alternating the crop planted (e.g., between corn and

soybeans) can restore nutrients to soil and fight pests and disease.

Figure 8.16a

Contour farming• Planting along contour lines of slopes helps reduce

erosion on hillsides.

Figure 8.16b

Intercropping• Mixing crops such as in strip cropping can provide

nutrients and reduce erosion.

Figure 8.16c

(c) Alley cropping

Terracing• Cutting stairsteps or terraces is the only way to

farm extremely steep hillsides without causing massive erosion. It is labor-intensive to create, but has been a mainstay for centuries in the Himalayas and the Andes.

Figure 8.16d

Shelterbelts• Rows of fast-growing trees around crop plantings provide

windbreaks, reducing erosion by wind.

Figure 8.16e

Conservation tillage• No-till and reduced-tillage farming leaves old crop residue

on the ground instead of plowing it into soil. This covers the soil, keeping it in place.

• Here, corn grows up out of a “cover crop.”

Figure 8.16f

Conservation Tillage Advantages

• Reduces erosion• Increases humus• Saves fuel• Reduces carbon dioxide emissions• Reduces irrigation because soil holds water• Reduces soil compaction• Allows for several crops per season• High crop yield

Conservation tillage• Issues with Conservation tillage

• More chemical herbicides (because weeds are not plowed under)

• More fertilizer (because other plants compete with crops for nutrients)

• Fungal growth due to soil moisture

• Breeding ground for pest insects and rodents

• Expensive new equipment

• Time and cost to train farmers

• Legume cover crops can keep weeds at bay while nourishing soil, and green manures can be used as organic fertilizers.

Soil Enhancement

• Crop rotation• Addition of aeration organisms such as

earthworms• Addition of compost, manure, other organic

matter - • Mulch on top of soil

Irrigation

• The artificial provision of water to support agriculture

• 70% of all freshwater used by humans is used for irrigation.

• Irrigated land globally covers more area than all of Mexico and Central America combined.

• Irrigation has boosted productivity in many places… but too much can cause problems.

Waterlogging and salinization

• Overirrigation can raise the water table high enough to suffocate plant roots with waterlogging.

• Salinization (buildup of salts in surface soil layers) is a more widespread problem.

• Evaporation in arid areas draws water up through the soil, bringing salts with it. Irrigation causes repeated evaporation, bringing more salts up.

Improved irrigation

• In conventional irrigation, only 40% of the water reaches plants.

• Efficient drip irrigation targeted to plants conserves water, saves money, and reduces problems like salinization.

Figure 8.17

Reduce irrigation

Switch to salt-tolerant crops(such as barley, cotton, sugar beet)

Prevention

Flushing soil(expensive andwastes water)

Not growing crops for 2-5 years

Installing under- ground drainagesystems (expensive)

Cleanup

Solutions

Soil Salinization

Fertilizers• Supply nutrients to

crops

• Inorganic fertilizers = mined or synthetically manufactured mineral supplements

• Organic fertilizers = animal manure, crop residues, compost, etc.

Figure 8.18

Global fertilizer usages• Fertilizer use has risen dramatically in the past 50 years.

Figure 8.19b

Trade-OffsInorganic Commercial Fertilizers

Advantages Disadvantages

Do not add humus to soil

Reduce organic matter in soil

Reduce ability of soil to hold water

Lower oxygen content of soil

Require large amounts ofenergy to produce,transport, and apply

Release the greenhouse gas nitrous oxide (N2O)

Runoff can overfertilizenearby lakes and kill fish

Easy to transport

Easy to store

Easy to apply

Inexpensive to produce

Help feed one of every three people in theworld

Without commercialinorganic fertilizers,world food output coulddrop by 40%

Overgrazing

• When livestock eat too much plant cover on rangelands, impeding plant regrowth

• The contrast between ungrazed and overgrazed land on either side of a fenceline can be striking.

• Overgrazing reduced by using paddocks – grazing rotation

Figure 8.22

Overgrazing

• Overgrazing can set in motion a series of positive feedback loops.

Figure 8.21

Recent soil conservation laws• The U.S. has continued to pass soil conservation

legislation in recent years:– • Food Security Act of 1985– • Conservation Reserve Program, 1985– • Freedom to Farm Act, 1996– • Low-Input Sustainable Agriculture Program, 1998

• Internationally, there is the UN’s “FAR” program in Asia.