INTRODUCTORY SOILS INTRODUCTORY SOILS

Subject OutlineSubject Outline

1. Introduction

2. What is Soil

3. Functions of Soils

4. Composition of Soils

5. Soil Formation

Why Soil Science?

Soil is an essential part, and some would argue, the most important component of the terrestrial ecosystem

Soil is an environmental interface

Lithosphere -rocks

Atmosphere -air

Hydrosphere -water

Biosphere –living organisms

Beyond that,

Concepts of soil differ greatly among users of soil

Mining engineer Civil engineer Home owner Farmer

What is soil

… … natural product formed from natural product formed from weathered rock by the action of climate weathered rock by the action of climate

and living organisms modified by and living organisms modified by topography over a period of time.topography over a period of time.

Definition

The Soil ProfileThe Soil ProfileAlabama –Bama Soil

How different can they look like?How different can they look like?

Tanzania Quebec Sri Lanka BrazilVirginia Montana

Functions of soilsFunctions of soils

Functions of Soils

1. Medium for plant growth

2. Regulator of water supplies

3. Recycler of raw materials

4. Habitat for soil organisms

5. Engineering medium

Soil as Medium for plant growth

Physical support

Air

Water

Temperature moderation

Nutrient elements

Protection

Soil as Regulator of Water Supplies

Soil regulates both the quality and quantity of water in rivers, lakes, and underground aquifers

Quantity of water suppliesSome of the water may be stored in the soil and used by trees and other plants

Quality of water suppliesWater is purified and cleansed as it soaks through the upper layers of soil.

Soil as Recycler of Raw Materials

Soils play a role in geochemical cycles Assimilate organic waste

Turn it into beneficial humus

Convert the mineral nutrients into plant and animal usable forms

Returning carbon to the atmosphere to be used for photosynthesis again

Soil as Habitat for soil organisms

Handful of soil is home to billions of organisms in thousands of species

How does this happen? Micro-environment differences

• Pore spaces

• Moisture

• Temperature

• Organic matter

Soil as Engineering Medium

Soil is firm and solidGood base to build structures

Soils differ in stability

Designs for structures are different for soils

Physical properties influence engineering uses

Composition of SoilsComposition of Soils

Soil consists of three major phases OR four components:

Solid phase (Soil Minerals)

Liquid phase (Soil Water)

Gas phase (Soil Air)

Components of SoilComponents of Soil

Soil Solid Phase (two components)

Inorganic mineralsSoil minerals are either primary or secondary mineralsPrimary minerals were formed in the original igneous rocks. Secondary minerals formed in soil by weathering of the primary minerals.

Organic matter (humus). Humus is the product of the decay of organic residues such as wood, leaves, and other biological materials.

Soil water (soil solution)

Water is vital to the ecological functioning of the soil.

Soil solution which contains water, dissolved ions, molecules and gases.Soil water is different from drinking water in 2 ways:

Soil water is held by many types of forces within the pores of the soil. Soil water is never pure but contains hundreds of dissolved organic and inorganic compounds.

Soil Atmosphere

contains similar gases as found in the atmosphere above the soil

But often in very different proportions.

Usually higher in carbon dioxide and lower in oxygen than the atmosphere.

Interaction of the Interaction of the ComponentsComponents

The components interact to determine the nature of a soil e.g.,

soil moisture controls air and nutrient supply

mineral particles control water movement

Organic matter controls arrangement of minerals which influence pores that determine water and air relationships

Soil FormationSoil Formation

Soil FormationSoil Formation

Five factors of soil formation

Weathering of rocks and minerals

Processes of soil formation

Where: s = any soil property cl = climate (rainfall & temperature)o = organisms (biota) p = parent materialr = relief (slope aspect and position)t = time (relative age of soil formation)

Five factors of soil formation

s = f(cl, o, r, p, t…)

1. Parent MaterialNature of parent material influences the characteristics of soils

E.g., texture of sandy soils is determined by parent material movement of water is controlled by texture of the parent materialInfluences the chemistry of the soilInfluences the type of clay minerals present in soil.

Weathering of Rocks and Minerals

What is weathering?

.. the modification or breakdown and destruction of the physical and chemical characteristics of rocks and minerals and carrying away the soluble products.

..the nature of the breakdown depends on the type of material

Types of Rocks

Igneous rocks Form from molten magma

Granite and diorite

Sedimentary rocks Compacted or cemented weathering products from

preexisting rocks Sandstone and shale

Metamorphic rocks Formed by change in the form of other rocks

Gneiss, marble, and slate

Two main types of weathering

a) Physical (Mechanical) weatheringa) Physical (Mechanical) weathering

.. Causes rocks to disintegrate into smaller pieces without affecting their composition

Factors:Temperature

Abrasion by water, ice and wind

Plants and animals

b) Chemical weatheringb) Chemical weathering

.. Degradation of rocks and minerals by the chemical activities of water, oxygen, and microbial action

Factors of Chemical weathering: Hydration

Hydrolysis

Dissolution

Carbonation

Oxidation-reduction

Complexation

HydrationProcess of binding of water molecules to a mineral

Hydrolysis

Splitting of water molecules into its components. Split components in turn attack the minerals.

Dissolution

Process of hydrating of ions until they become dissociated

OHOFeOHOFe 21510232 9.95

OHKOHAlSiOHOKAlSi

OHHOH

83283

2

OHSOCaOHOHCaSO 22

42

224 422.

CarbonationCarbon dioxide dissolves in water to form carbonic acid which accelerates chemical breakdown of materials

Oxidation-ReductionMinerals that contain Fe, Mn,or sulfur are susceptible to this reaction when exposed to environments different from the ones in which they formed. This destabilizes the mineral.

ComplexationBiological processes produce organic acids that can form complexes with elements within the structure of a mineral thereby pooling the element from the mineral and destabilizing it.

FeOOHOHOFeO 424 22

32

332

3222

2HCOCaCaCOCOH

COHOHCO

Classification of Parent Materials

Organic Deposits

Weathering of rocks in place

Deposition of weathered rock materials from elsewhere

Parent materials are classified by their mode of placement at their current location.

1. Gravity• colluvium

2. Ice transport• glacial till, moraine, outwash

3. Wind transport• Eolian (dune sand, loss, dust)

4. Water transport• Lakes -lacustrine

• Streams –alluvium (floodplain, alluvial fans, delta)

• Oceans –marine

5. Volcanic ash

Types of Parent Material

2. ClimateMay be the most influential of the four factors acting on the parent material

• Determines the nature and intensity of weathering (precipitation and temperature)

Both affect the physical, chemical and biological processes

Climate also exerts influence indirectly through a second soil forming factor, the living organisms (natural vegetation).

Climate is so important in soil formation that certain evidence of climatic change could be found in the soil

Precipitation1. Water is essential for all the major chemical

weathering reactions.

2. The deeper water penetrates the parent material, the more effective it is in soil development.

3. Water percolating through the soil profile transports soluble and suspended material from the upper to the lower layers.

4. Thus percolating water stimulates weathering reactions and helps differentiate soil horizons.

Temperature1. Every 10 deg C, the rate of chemical reaction

doubles

2. If warm temperatures and abundant water are present in soil at the same time, the processes of weathering, leaching, and plant growth will be maximum and lead to deep soil profiles.

3. Compare this to very modest soil profile development processes that are obtained in cold areas

3. Organisms (Biota)

a) Role of natural vegetationOrganic matter accumulation

Cation cycling by trees

Heterogeneous rangelands

Soil organisms, both the animals (fauna) and the plants (flora) physically churn the soil and help stabilize the soil structure

Heterogeneous Rangelands (Plants & soil formation)

b) Role of animals

Animals such as gophers, moles, prairie dogs bore into lower soil horizons and bring materials to the surface –tunnels.

Earthworms and termites• Bring about considerable soil mixing

Human influence• destruction of natural vegetation

• Soil tillage for crop production

• Irrigation

• Fertilizer application

4. Topography

Relates to the configuration of the land surface

It is described in terms of differences in elevation, slope and landscape position

Steep slopes encourage soil loss by erosion and allow less rainfall to enter the soil

Thus prevents formation of soils from getting ahead of soil destruction

In the depressions where runoff tends to concentrate, the soil is usually more deep

Role of Topography in Soil Formation

5. TimeTime that materials have been subjected to weathering is important because soil forming processes take time to show their effects.

Clock of soil formation starts when e.g.Landslide exposes a new rockFlooding river deposits sediment on floodplainGlacier melts and dumps load of debrisBulldozer cuts and fills landscape, etc.

Rates of weathering and soil developmentThis is a function of the interaction of time and the other factors of soil formation.

Role of Time in Soil Formation

Processes of Soil formation

Processes that are involved in soil formation can be placed in four main groups

1. Additions

2. Transformations

3. Transfers or Translocations

4. Losses

Additions

Additions entail the inputs of materials to the developing soil profile from outside sources. E.g.

Addition of organic matter from Plant leaves and sloughed-off rootsAddition of water by precipitationAddition of dust particles that fall on the soil surfaceAddition of salts and silica dissolved in groundwater and deposited near or at soil surface

TransformationsTransformations entail disintegration and altering of

composition and form of primary minerals

Physical weathering to smaller particles

Decomposition of organic residues

Recombination of decomposition products to form new minerals such as silicate clays and oxides

Synthesis of organic acids, humus and other products

Aggregation of mineral particles

Translocations involve the movement of organic and inorganic materials laterally within a horizon or vertically from one horizon to the another.

Movement of water

Movement of dispersed fine clay particles

Movement of dissolved organic substances

The most common translocation agents are water and soil organisms.

Translocations

Materials are lost from the soil by the following:

Drainage and leaching to groundwaterErosion of surface materialsEvaporationPlant uptakeMicrobial decompositionAnimals and humans

Losses

Combination of Processes

LossesLosses

TranslocationsTranslocationsTranslocationsTranslocations

AdditionsAdditions

TransformationsTransformations

Example of the soil forming processes in action

Addition of litter and root residues

Transformation of plant substances into humus

Accumulation of humus enhances water and nutrient capacity (feedback for plant growth)

Microorganisms attracted by humus buildup accelerate transformations and cause translocations

Soil Horizon DevelopmentA-Horizon development

Accumulation of organic matter

Clumping of individual soil particles

Distinct from parent material and other layers

B and C horizon developmentCarbonic and organic acids are carried by water into soil where dissolve various minerals (transformations)

Soluble materials (ions –Ca2+, CO32-, SO42-, etc) are carried by water and precipitate in the soil from upper to lower horizons (translocation)

Weathering of primary minerals into secondary minerals

Wetting and drying cracks soils and makes structures.

Soil Horizon Development

Soil-Landscape-Ecosystem Relationships

The above processes of soil genesis, operating under the influence of the five environmental factors discussed previously gives us a framework for understanding the relationships between particular soils and the landscapes and ecosystems in which the soils exist.