The processes of soil formation C R O P P T Alistair McNaught 2001 The processes of soil formation C R O The processes of soil formation can most easily be seen where a soil is known to have been built up by gradual deposition. Studland beach in South Dorset is a good example of this. A series of sand dunes have been built up by coastal deposition, progressively creating new land near the shore. The further inland you go from the shore, the older the dunes. This gives the ideal opportunity to study the processes of soil formation. P P OLD YOUNG T Alistair McNaught 2001 The processes of soil formation C R
The soil is a mineral soil consisting of mainly sand grains. Very few plants present. Specialist plants colonise the soil. As they die they add organic matter to the soil. As organic matter levels rise, other species colonise and a diverse vegetation community begins. The soil is now a complete mix of organic and mineral matter. The high levels of organic matter allow good nutrient and moisture stores - encouraging plants. O 100 P P T Organic 75 content (%)
50 25 0 0 50 100 150 200 250 Time (years) Alistair McNaught 2001 The processes of soil formation C limate R Climate influences soil formation by controlling the rate and type of weathering. Polar regions - frost shattering dominates. Permafrost inhibits soil drainage. Soils mainly mineral with little release of nutrients by chemical weathering. Any organic matter builds up on surface as peaty deposits. O P P
T Temperate regions (humid) - high rainfalls and moderate climate gives mix of physical and chemical weathering. Excess rainfall over evaporation may result in soils being leached as nutrients wash to lower layers Tropical wet and dry - alternate mobilisation of chemicals (wet season leaching) and reprecipitation (dry season capillary action). May result in lateritisation as solutes precipitate near surface. Soils can be fairly fertile if well managed but easily degraded by overuse. Temperate regions (continental) - low rainfalls and high summer temperatures encourage nutrients to remain near surface. Leaching balanced by capillary action returning dissolved nutrients to surface layers. Can result in salinisation if irrigated carelessly. Tropical humid - high rates of
chemical weathering and leaching. Soils very poor in nutrients unless maintained by lush vegetation cover. Agricultural potential generally very low. Alistair McNaught 2001 The processes of soil formation C Relief Relief influences the thickness of the soil (steep slopes wash away soil more rapidly) and the processes at work. Sunny slopes have more wetting and drying cycles, more biological weathering and more rapid recycling of organic matter. North South O P Sunny Shady Thin soil P T Thick soil Alistair McNaught 2001 The processes of soil formation
C R O rganisms P P T Organisms can have both direct and indirect effects. Animals: Burrowers aid the breakup of rock and mixing of organic and mineral soil components. Insects, arthropods (eg spiders) and worms can form huge populations in the soil matrix and their activities help (i) aerate and mix soils (ii) decompose and recycle nutrients Plants: The nature of vegetation exerts a strong influence on soil character. Deep rooted plants aid biological weathering. The nature of the plant influences soil processes - for example stability - plants with well developed root systems (eg marram grass on dunes) stabilise soils and speed their development. nutrients - nitrogen fixing plants (eg legumes) increase soil nutrient content decomposition - some plant products (eg pine needles) decompose to form acidic residues which influence soil nutrient avalability. Alistair McNaught 2001 The processes of soil formation C R O
We use the term parent material in preference to bedrock because many soils are developed on sands and clay or recent glacial deposits. The parent material will influence the mineral and nutrient content of the soil (eg chalk creates alkaline soils, sand creates acidic soils) Parent material P T the drainage/waterlogging characteristics of the soil (which will depend on the permeability of the parent material) the rate of development of the soil (resistant rocks will weather to soil more slowly than loosely consolidated sands). Thin soil, low in nutrients and Forms deep heavy soil with poor acidic in character. Poorly drained drainage. Difficult to work for giving rise to bogs on lowland. arable agriculture but good for dairying, providing lush grass all Example - Dartmoor year. Eg Oxford Vale Granite Clay Thin soil because main weathering is by carbonation, leaving few residues for soil formation! Easy to work and freely draining but high pH makes some nutrients unavailable.
Chalk Alistair McNaught 2001 The processes of soil formation C R O P Process T The processes taking place depend on the interaction of all the other factors. Some of the key soil forming processes are summarised here: Mechanical processes Chemical processes Organic sorting - redistribution of organic material by soil organisms Humification - decomposition of organic matter into humus eluviation - washing of soil particles out of a soil horizon to a lower level chelation - release of organic acids which leach minerals out of the soil illuviation - soil particles washing ito a soil horizon from a higher level leaching - loss of soluble soil nutrients by downward
movement of soil water podsolisation - usually related to chelation - an extreme form of leaching which leaves a bleached upper (A) horizon. Humid temperate regions. gleying - seasonal or permanent waterlogging of the soil gives the horizons a mottled blue/grey appearance calcification - the concentration of calcium near the surface as a result of capillary action in hot seasons of continental temperate regions. ferralitisation - the formation of clay, iron and aluminium complexes due to extremes of chemical weathering in tropical regions. salinisation - deposition of salts in the upper layers due to extreme capillary action in areas with groundwater near surface. Semi arid regions. Alistair McNaught 2001 The processes of soil formation C R O Soils are a dynamic matrix with living and non-living components. They can grow, mature and decline - like any living organism. The Studland dune example showed the impact of time in maturing a soil. Once the natural vegetation is cleared for agriculture the soil quality can decline catastrophically if not managed well. Natural vegetation P Agriculture P Time Organic
content Good management Poor management Time Alistair McNaught 2001 The processes of soil formation C This is the final slide: R The slideshow covered how a soil forms and the many different influences on a soil. These can be summarised with the mnemonic O CROPPT - because most soils are croppt by farmers! CROPPT stands for Climate - Relief - Organisms - Parent material - Process - Time P P T Alistair McNaught 2001
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