soil

Common types of soil profile.

Soil developed on a bed of chalk. Soils develop as a result of weathering of rocks, breakdown of vegetation, and the effect of water. Chalk is an unusual rock. When it is weathered the chalk is changed to calcium bicarbonate. This is soluble and removed by running water. As a result, chalk normally does not have much material on the top of it. Chalk soils consist of a thin top soil composed of decayed vegetation.

Loose covering of broken rocky material and decaying organic matter overlying the bedrock of the Earth's surface. It is composed of minerals (formed from physical weathering and chemical weathering of rocks), organic matter (called humus) derived from decomposed plants and organisms, living organisms, air, and water. Soils differ according to climate, parent material, rainfall, relief of the bedrock, and the proportion of organic material. The study of soils is pedology.

Soils influence the type of agriculture employed in a particular region – light well-drained soils favour arable farming, whereas heavy clay soils give rise to lush pasture land. Plant roots take in nutrients (in the form of ions) dissolved in the water in soil. The main elements that plants need to absorb through their roots are nitrogen, phosphorus, and potassium.

Soil is formed by the weathering of rocks. Physical weathering breaks the rock into small pieces. Chemical weathering releases the minerals that plants need for growth.

A soil can be described in terms of its soil profile, a vertical cross-section from ground-level to the bedrock on which the soils sits. The profile is divided into layers called horizons. The A horizon, or topsoil, is the uppermost layer, consisting primarily of humus and living organisms and some mineral material. Most soluble material has been leached from this layer or washed down to the B horizon. The B horizon, or subsoil, is the layer where most of the nutrients accumulate and is enriched in clay minerals. The C horizon is the layer of weathered parent material at the base of the soil.

Two common soils are the podzol and the chernozem soil. The podzol is common in coniferous forest regions where precipitation exceeds evaporation. The A horizon consists of a very thin litter of organic material producing a poor humus. Coniferous tree leaves (needles) take a long time to decompose. The relatively heavy precipitation causes leaching of minerals, as nutrients are washed downwards.

Chernozem soils are found in grassland regions, where evaporation exceeds precipitation. The A horizon is rich in humus due to decomposition of a thick litter of dead grass at the surface. Minerals and moisture migrate upwards due to evaporation, leaving the B and A horizons enriched.

The organic content of soil is widely variable, ranging from zero in some desert soils to almost 100% in peats.