1. introduction 2. what is soil 3. functions of soils 4. composition of soils 5. soil formation
TRANSCRIPT
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
How different can they look like?How different can they look like?
Tanzania Quebec Sri Lanka BrazilVirginia Montana
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)
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
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
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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
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
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.
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-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.