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Soil
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Plant growth in natural and agricultural Systems
Is a medium that contains water and nutrients Anchorage for roots What are the uses of soil?p.129 Soil Fertility: is the result of the characteristics of and the processes operating in the soil enabling plants to absorb nutrients and water
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Soils Soil Characteristics Soil Processes Soil Types
Mineral Constituents Particle Size Humus Content Soil Characteristics Soil Nutrients Soil Acidity Leaching Soil Processes Podzolisation Gleying Brown Earths Soil Types Podsols Gleyed Soil Rendzina Soil & Human Activity Soil Degradation Soil Improvement
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(unaltered/insoluble)
Soil Characteristics 1. Mineral Constituents Weathering Physical Weathering Chemical Weathering Primary minerals e.g. quartz (unaltered/insoluble) Secondary minerals e.g. clay & seisquioxides (Fe, Al Oxides) (altered/soluble)
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Soil Characteristics 2. Particle Size a). Soil Texture
- degree of coarseness or fineness of mineral matter usually expressed as % of clay, silt or sand. - soil texture controls: size and spacing of soil pores water content water flow aeration
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Soil Characteristics 2. Particle Size a). Soil Texture
- degree of coarseness or fineness of mineral matter usually expressed as % of clay, silt or sand. - soil texture controls: size and spacing of soil pores water content water flow aeration Clay soils root penetration Sandy soils Nutients (CEC)
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Soil Characteristics 2. Particle Size b). Soil Structure
- aggregation of individual particles into peds. - different soil structures include: crumb granular platy blocky prismatic columnar
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Crumb Prismatic (More Pointed) Columnar (More Angular) Blocky Platy Granular
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A = Columnar B = Prismatic C = Blocky D = Crumb E = Platy F = Granular
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Soil Characteristics 3. Humus Leaf litter Organic matter
Fermentation Organic matter Humification 2. Discrete humus Humus 1. Intimate humus Mor humus Mull humus Slow decomposition of humus in upland areas Coniferous Forests Moorlands Crumbly black nutrient rich layer Under grass vegetation Biologically Active. E.g. temperate Deciduous Woodland Moder humus Transitional
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Soil Characteristics 4. Soil Nutrients Cation exchange 5. Soil Acidity
Cations exchanged on clay – humus particle (absorbed) and H ions released from vegetation From weathering of rock / organic matter in humus Can be moved between the soil particle (clay / humus) or soil solution and the roots and the particle or soil solution 5. Soil Acidity Hydrogen cations increase soil acidity and weathering of soil! NB – Nutrient uptake is by active transport and water by osmosis Cation Exchange Capacity (CEC) Low in sand, higher in clays!
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Soil Components Mineral skeleton Air Water Living Organisms
(texture & Structure) Air Water Living Organisms Organic Matter
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Responsible for decomposition and nutrient recycling and aid drainage
Soil Components Mineral skeleton (texture & Structure) Air Water Responsible for decomposition and nutrient recycling and aid drainage Living Organisms Breaks down dead material to form humus which is a food source for soil organisms and contains humic acid & lignin. Humus retains water, binds soil together, reduces erosion, increases drainage and acts as a thermal insulator. Organic Matter
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Living organisms in soil
Function Plants Detritivores e.g. worms, millipeeds, woodlice, dung beetles Decomposers e.g. Bacteria / Fungi N Cycle Bacteria Mycorrhizal fungi Roots bind soil, provide leaf litter for humus and nutrients. Roots weathers regolith. Break up dead organic matter, releasing nutrients and allowing decomposers to complete decomposition. Voids left by OM’s break-up aids drainage Digest D.O.M. left by detritivores Involved in fixation, nitrification and denitrification Symbiotic relationship exits: Fungal network in soil provides plants with nutrients and releases sugars for uptake by plants
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= Removal of soluble material in soln
Soil Processes 1. Leaching = Removal of soluble material in soln Precipitation > Evapotranspiration = Soil Moisture Surplus Good Drainage Oxygen Carbonic Acid Organic Acids e.g. Humic Acid Pass through the surface vegetation Chemical Weathering Breakdown of Clays Ca + Mg Elluviated from A Horizon Increasingly Acid Illuviation in B horizon
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Vegetation often coniferous
Soil Processes 2. Podsolisation = Intense leaching Precipitation substantially > Evapotranspiration = Soil Moisture Surplus Good Drainage Soils well drained and sandy Vegetation often coniferous or moors Leaching Dissolves bases Dissolves sesiqoxides e.g. Ca, Na, Mg, K, SiO2 e.g. Fe, Al PODZOL A Horizon - Bleached B Horizon – Red brown sesiqoxides Al Fe Illuviated (inwashed)
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Terms Sesiqoxides = 2 metal ions joined to 3 oxygen atoms. Usually oxides of iron and aluminium, formed from the weathering process.
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Soil Processes 3. Gleying Poor Drainage Restricted System! =
Impermeable underlying rock Gentle Slopes Water logging E.g. Moor land Plateaux Reduction Fe3+ Fe2+ Ferrous (Grey Blue) Ferric (Red ish) Red Mottles Pockets of air re-oxygenise it
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Strong leaching & podsolisation
Soil Types Wet cold climate 1. Podsol Worms absent& limited bacterial activity Coniferous forest & heathland vegetation Litter breakdown slow, plant material unchanged for many months Rainfall much greater than evaporation Acidic mor humus (discrete) formed - pH 4 Strong leaching & podsolisation Acidic water dissolves Fe & Al and carries it out of A horizon Fe & Al re-deposited in B horizon
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Podsol Litter of needles, cones & twigs
Large fermentation layer of partly decayed organic matter Discrete black, peaty mor humus layer up to pH 4 Light ash-grey layer of sand & clay Thin, hard iron pan layer Dark red-brown layer of iron oxide (pH 5) with aluminium oxide below Highly weathered bedrock forming a regolith layer
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Soil Types Mild wet climate 2. Brown earth Worms & bacteria active
Deciduous forest (e.g. oak trees give off more Ca & K) Leaf litter rapidly decomposed Rainfall exceeds evaporation Less acidic mull humus (pH 5.7) merges with A horizon (intimate humus) to give brown colour Mildly acidic leaching occurs Clays & soluble elements (Ca, Mg, Na K) leached down from A horizon along with some Fe & Al Soluble elements & some Fe & Al re-deposited in B horizon
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Brown Earth Litter several cms thick under deciduous woodland
Small fermentation layer of partly decayed organic matter Intimate dark brown mild mull humus layer pH 5.7. Merges with A horizon beneath and fermentation layer above Brown A horizon with presence of humus, becoming paler with depth with pH about 6.5 Small amounts of Fe and Al re-deposited in B horizon gives it a red colour Weathered bedrock forming a regolith layer
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Soil Types 3. Gleyed Soil Water movement through soil restricted
High groundwater Impermeable layer in soil Saturated soil Groundwater gley Pore spaces filled with water & exclude air Surface-water gley (often associated with valley floors) (often associated with development of an iron pan) Reduction of iron occurs from its red ferric form to its grey-blue ferrous form New form of iron gives soil its characteristic grey-blue colour
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H layer 0cm A horizon C horizon 120cm
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Soil Types 4. Rendzina Chalk or limestone bed rock
Grasses or beech woodlands Leaf litter rich in Ca, K, Na & Mg which are alkaline Calcium saturated clays & blocky structure Plenty of bacterial activity Thin, intimate less acidic mull humus formed - pH 7 to 8 Little leaching of minerals No B horizon
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Soil Structure (crumb, blocky, platy)
Aeration Thermal Capacity Soil Structure (crumb, blocky, platy) Effect of Soil Properties on Fertility and Productivity Waugh p.260 and core text Water Drainage, infiltration and retention pH Soil texture(sand, silt, clay)
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Soil & Human Activity Human activity can either: damage (DEGRADE) or
improve (UPGRADE) soils by altering the characteristics of soils and influencing soil processes. minerals leaching texture podsolisation structure gleying humus (organic content) calcification nutrients acidity
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Soil & Human Activity Soil Degradation Harvesting of crops
nutrients & organic matter Deforestation soil structure destroyed & nutrients lost Removal of vegetation Overgrazing Ploughing structure (drainage) & nutrients Monoculture nutrients Heavy machinery & overstocking structure & drainage
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Soil & Human Activity Soil Improvement Fertiliser (NPK or FYM)
Nutrients, nutrient retention & organic matter Cover crops nutrients & soil stability Growing vegetation Afforestation Improved farming methods crop rotation nutrients replenished shelter belts less soil erosion field drainage aeration & drainage liming acidity, soil structure & nutrients mulching organic content & nutrient retention ploughing & contour ploughing structure, drainage, aeration.
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