Tuesday, January 4, 2011

The Soil and Soil profile

Plants and hence all life depend on the soil. Rock breaks up chemically and physically to form different soil types. The physical break up produces sand and silt, but chemical decomposition is more complex. Chalk and lime stone dissolve away leaving little residue and therefore are overlain by thin soils. Silicates on the other hand slowly react with water to form clay minerals. What happens then depends on the flow of water through the soil. If, as in temperate climates, rainfall continues through most of the year, chemicals such as iron hydroxide will tend to wash out of the top 30 cm or so, leaving a pale Grey earth, or podzol, and re depositing the iron underneath as a darker layer that can develop into an impervious hard-pan, often eliminated by ploughing. In the tropics, high rainfall during the wet season mobilizes the iron but evaporation concentrates it near the surface. During the dry season plants draw water from further down, bringing iron and aluminum hydroxide to the surface and producing red late rite. If this continues, the late rite becomes hard and impervious, making the soil infertile.
Another important soil type is the black chernosem of the steppes. During the dry summers grass draws up calcareous solutions which make the humus black and insoluble so that iron hydroxides are not leached out. Intensive agriculture can eradicate such soil profiles but the physical nature of the soil is still important. If it is sandy, it is light and easily drained but holds little organic matter and nutrients soon wash away. Clay, by contrast, is heavy and waterlogged when wet, and hard when dry, but does retain nutrients. Vegetable matter can form soil too, such as peat, which retains water well. The most fertile soils are mixture of sand and clay- loam- with plenty of organic matter. Soils are valuable and vulnerable. If exposed to acidic pollution they can release aluminum salts that poison plants and water. If vegetation s is removed they get washed away altogether.
 The uppermost layer, or topsoil, is rich in organic matter, but short of minerals. This layer is penetrated by roots and has its own established ecosystem. Beneath is the subsoil, rich in minerals but short of organic materials. Underneath is layer of weathered rock and still deeper is the unweathered bedrock.