Soil Constituents and Formation

Soil is a dynamic medium made up of minerals, organic matter, water, air and living creatures including bacteria and earthworms. It was formed and is forever changing due to five major physical factors; the parent material, time, the climate, the organisms present and the topography. The way in which we manage soil is another major factor influencing the character of the soil. Note that the use of the term mineral describes the occurrence of crystals of quartz, mica, etc., and is not used to indicate plant nutrient elements.

Soil is not uniform on the surface, and it is not uniform with depth. It has layers or horizons. If you dig a pit to about 2 metres deep, you can observe these soil horizons. There is topsoil, or A horizon, where all the organic matter accumulates and is consequently the most fertile horizon. Some soils do not have an A horizon because it has been eroded away, for example, after a wind erosion event. Needless to say, these soils are often very infertile. Below the A horizon is the B horizon, or the subsoil. This is less fertile than the A horizon and usually has more clay, iron and aluminium compounds, as water has leached the finer particles down the profile. Below the subsoil is the parent material, the rock from which the soil has formed. Physical, chemical and biological weathering of this rock eventually leads to the formation of soil. Valley floor and plateau soils tend to be deeper than those on the slopes due to erosion removing the soil from the slopes. There is more time for soils to form in valleys and plateaux. Organisms add nutrients to the soil, aid in improving soil structure and help stabilise the soil against erosion, once again changing the soil.

Excerpt, with permission, from: Hunt, N. and Gilkes, R. (1992). Farm Monitoring Handbook. University of Western Australia. Nedlands; WA. 3.