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3.4 The Soil System Soil Profiles Soils perform a number of vital func3ons for humans: • 
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Medium for plant growth Major store of fresh water Filter materials added to the soil thereby maintaining water quality Some recycling of nutrients through breakdown of dead organic ma?er Habitat for billions of microorganisms and larger animals Provide raw materials (peat, clay, sand, gravel and minerals) Soil is a non-­‐renewable resource Soil is natural capital: –  Maintains CO2 –  Stores water –  Stores nutrients Soil Profiles – 
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Soils are formed of: Weathered bedrock (mineral par3cles) Organic ma?er (dead & alive) Air (within soil grains) Water (within spaces between soil grains) Soils are divided into horizons (layers) with dis3nct physical and chemical features (boundaries are blurred by earthworm ac3vity): – 
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Organic horizon: top layer of vegeta3on A horizon: mixed mineral-­‐organic. Dark color due to organic ma?er (humus) E horizon: eluvial or leached B horizon: deposited or illuvial. Material comes from E horizon (Fe, humus, clay) C horizon: bedrock or parent material Humus: parCally decomposed organic maFer mixed with fine mineral parCcles Soil formaCon processes Main processes: •  Weathering (C horizon): ini3al bases and nutrients (fer3lity), structure and texture (drainage) •  Transloca3on: –  Leaching: downward movement of soluble material –  EluviaCon: physical downwashing of small par3cles (clay & humus) –  CalcificaCon: in arid and semiarid environments when evotranspira3on is greater than precipita3on. Movement of soil solu3on is upwards (water drawn to drying surface by capillary ac3on. CaCO3-­‐ remains in soil (in grasslands) –  SalinizaCon: capillary rise of water (from a saline water table close to surface) produce excessive concentra3on of Na HumificaCon: plant liFer decomposiCon, degraded by fungi, algae, bacteria, worms & small insects (organic changes). Occurs mostly at or near surface Soil formaCom takes a long Cme (1cm can take up to 100 years to form) Soil structures & properCes •  Loam soil: ideal for cul3va3on. Balance between water-­‐holding ability and freely draining areated condi3ons (influenced by soil texture).Well balanced soil with significant propor3ons of sand, silt & clay •  Soil texture: refers to the propor3on of different sized materials (sand, silt & clay) present in a soil The agricultural poten3al of a soil depends on: •  Porosity & permeability •  Surface area of the soil peds (par3cles) Porosity: amount of space between par3cles. Determines the rate at which water drains through a soil Permeability: ease at which gases 6 liquids can pass through Surface area: determines the amount of water and nutriens in solu3on that can be retained against the force of gravity ( a heavy clay soil can hold twice as much water than a light soil) Soil structures & properCes •  Light soils (80% sand approx): coarse-­‐textured and easily drained. Warm up more quickly than heavy (clay) soils and so allow early growth in spring •  Heavy soils (>25% clay): fine-­‐textured. Water and nutrient reten3ve. Clay absorbs water: soil swells when wet and shrinks when dry. Low permeability: can lock dissolved minerals between pores difficul3ng plant roo3ng. Rich in minerals but low fer3lity. •  Soil structure: refers to the shape & arrangement of individual soil peds (par3cles). Ideal structure is a crumb structure in which peds are small and porous. Result of both the texture & the processes that have formed the soil such as root ac3on. Soil structure depends on: •  Soil texture •  Dead organic ma?er •  Earthworm a3vity •  Root ac3on Soil structures & properCes •  Different soil types have different levels of primary produc3vity: – 
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Primary produc3vity depends on: – 
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Clay soil: quite low Sandy soil: low Loam soil: high Mineral content Drainage Water-­‐holding capacity Air spaces Biota Poten3al to hold organic ma?er Clay soils: fer3le in temperate loca3ons but nutrient defficient and easily leached in the tropics Workability of a soil depends on the amount of clay present (more clay content –> harder to work) Field capacity: maximum amount of water that a soil can hold Soil degradaCon • 
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Soil degradaCon is the decline in quanCty & quality of soils Includes: – 
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Erosion (wind / water) Biological degradaCon (loss of humus and plant or animal life) Physical degradaCon (loss of structure, changes in permeability) Chemical degradaCon (acidifica3on, declining fer3lity, changes in pH / salinity) Causes of soil degradaCon •  Water erosion (60% erosion) •  Wind erosion •  Acidifica3on (toxifica3on): change in chemical composi3on. It may trigger circula3on of toxic metals •  Eutrophica3on (nutrient enrichment) may degrade quality of ground water – over-­‐abstrac3on may lead to dry soils •  Saliniza3on: capillary ac3on brings salts to upper soil (coastal loca3ons, hot arid areas) •  Atmospheric deposi3on of heavy metals and persistent organic pollutants (DDT) •  Deser3fica3on •  Climate change •  Human ac3vity Water erosion Gully erosion Soil degradaCon • 
Climate change has 3 direct impacts on soils: –  Rise of temperature provokes high decomposi3on rates of organic ma?er: nutrient deple3on and less moisture reten3on capacity –  More precipita3ons and floodings: more water erosion –  More droughts: more wind erosion • 
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Indirect impacts of climate change on soils: –  Need for more agriculture land to compensate the loss of degraded land –  Higher yields for european grain crops due to CO2 fer3liza3on effect Human ac3vity impacts on soils: –  Removal of wodland or ploughing stablished pastures leads to erosion –  Cul3va3on leads to exposure of bare soil. Increased run-­‐off when on slopes: can create rills and gullies. Irriga3on in hot areas can lead to saliniza3on –  Overgrazing can reduce vegeta3on cover and leave surface vulnerable to erosion –  Roads or tracks collect water due to reduced infiltra3on and can lead to rills & gullies forma3on –  Mining leads to exposure of bare soil Soil degradaCon in 15% of world´s total area Impacts lead to reducCon in yields Land conservaCon & rehabilitaCon essenCal for sustainable agriculture development Economic impact of degraded soil more severe in agriculture-­‐dependant countries (LEDCs) Soil conservaCon measures Managing soil degrada3on: Organic farming •  Afforesta3on •  Pasture extension •  Bening crop produc3on • 
Need for policy makers & public to reduce pressure on soil resource Methods to reduce/prevent soil erosion: • 
Mechanical (wind breaks) •  Vegeta3on cover •  Soil husbandry Mechanical methods to reduce water flow: • 
•  Bunding •  Terracing Contour ploughing Cropping & soil husbandry methods against water & wind damage: • 
•  Maintaining a crop cover for as long as possible Keeping in place stubble and rot structure afer harvest •  Plan3ng a grass crop Management of salt affected soils: • 
Flushing soil with water and leaching salt away •  Applica3on of chemicals Reduc3on in evapora3on losses to reduce upward movement of water in soil Soil conservaCon measures Summary of soil conserva1on methods: •  Strip & alley cropping (agroforestry) •  Rota3on farming •  Contour planning •  Agroforestry •  Adjus3ng stocking levels (less ca?le) •  Mulching •  Cover crops •  Terraces, banks & ditches Contour farming Strip farming Case study: Dust Bowl Facts • 
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Great plains region devastated by drought in the 1930s Li?le rainfall, light soil,high winds Soil lacked strong grass root system Dust chocked ca?le 60% of popula3on lef Land overexploited Natural grass cover turned to wheat crops 1935 rehabilita3on: –  Plant trees & grass to anchor the soil –  Plow 6 terrace in contour pa?erns to hold rainwater –  Leave por3ons of farmland to lie fallow each year to regenerate soil •  By 1941 much of the land was rehabilitated •  Same mistakes during WWII