Vegetation and soil

soil erosionSoil types and distribution

The soil is a natural body of weathered minerals and organic matter differentiated into horizons of variable depth which differ from the material above and below in terms of their physical make-up, chemical composition and biological character. Soil types are a product of the dominant soil-forming processes operating in a given environment. Climate is the key control over soil type at a global scale (although factors such as drainage, parent material and relief can sometimes dominate at the local scale to produce intrazonal soils).

For instance, if precipitation levels exceed potential rates of evapo-transpiration then soil minerals will be dissolved by infiltrating water and carried away in solution via through-flow. The resulting acidified soils found in such a climatic regime are called pedalfers. They develop wherever downward and lateral movements of soil water encourage the processes of leaching, eluviation and occasionally podsolisation (an extreme form of leaching). The UK is a region where the dominant zonal soil type is a type of pedalfer known as a brown earth.

In some areas of the UK, impermeable rock restricts groundwater flow and promotes water-logging. This is especially common in areas of high altitude and/or low gradient in north-western Scotland where evaporation rates are lower and rainfall higher than in southern England. Anaerobic (oxygen-depleted) conditions result, allowing gley soils to develop. This is an example of intrazonal soil formation - podzol development in excessively leached free-draining areas is another.

How might climate change influence distribution patterns of UK soil types?

The brown earth can be expected to remain the domniant zonal soil type for most of the British Isles, barring an extremely high-impact global warming scenario. However, extensive changes can still be anticipated in the pattern of intrazonal soils for the UK if temperatures rise by just 1-2C (a low-impact warming scenario) and rainfall patterns continue their present trend towards greater seasonal variability.

Some gley soils, especially in areas of southern England, might come to resemble brown earths if evaporation rates rise and levels of rainfall-fed infiltration fall during summer months. This seems likely if, as is predicted by some scientists, summers continue to get drier and winter rainfall becomes concentrated in more sporadic heavy downfalls which generate large amounts of surface run-off (thereby limiting the proportion of precipitation that effectively enters the soil). In some areas, seasonal gleying may become the norm, resulting in mottled soil with both blue-grey and reddish-brown colouration on account of both ferric and ferrous iron being present.

Warming could also reduce the extent of podzolic soils in lower altitude areas, given that higher evaporation rates would reduce levels of leaching, eluviation and podzolisation. However, higher rainfall in the north of the UK and Wales (some estimates suggest a 10% annual increase in these areas by 2050) might offset any such soil water losses, thereby preventing too much change.

In a more extreme global warming scenario of 2-3C of more, it is possible that parts of south eastern England might become dominated by pedocal soils, rather than pedalfer soils. Pedocal soils result wherever precipitation levels are lower than evapo-transpiration rates. Under such conditions, soil water can be drawn surface-wards by capillary action, enriching the upper horizons of the soil with previously dissolved minerals. Pedocal soils include the chernozem and chestnut brown soils which currently dominate in drier continental areas of Europe and Asia found at similar latitudes to the UK. High-impact climate change scenarios describe an environment in southern parts of the UK where capillary action would certainly be the dominant process throughout summer months, with only mild winter leaching to offset this.

Student Practice Question:

Describe and explain the pattern of soil types in an area you have studied.

A UK-focused answer might try to apply climate change as a factor that could account for a changing pattern in future years. A good response to this question might suggest that the current soil distribution pattern is becoming more dynamic and will change over time, with pedocal soils becoming more common in the driest areas.