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Weathering and soil formation

Soil profile with weathering parent materials. Weathering is the name given to the process by which rocks are broken down to form soils. Rocks and geological sediments are the main parent materials of soils (the materials from which soils have formed). There is a very wide variety of rocks in the world, some acidic, some alkaline, some coarse-textured like sands, and some fine-textured and clayey. It is from the rocks and sediments that soils inherit their particular texture. When you see rocks in the landscape it is easy to appreciate how long the process of breaking down rocks to form soil takes. In fact, it can take over 500 years to form just one centimetre of soil from some of the harder rocks. Fortunately, in some respects at least, huge amounts of rocks were broken down during the Ice Age over 10,000 years ago and converted into clays, sands or gravels, from which state it was easier to form soils.

There are three main types of weathering; physical, chemical and biological. Physical weathering is the influence of processes such as freezing and thawing, wetting and drying, and shrinking and swelling on rocks and other sediments, leading to their breakdown into finer and finer particles. Chemical weathering is the decomposition of rocks through a series of chemical processes such as acidification, dissolution and oxidation. Some minerals, while stable within solid rock, become less stable on being more exposed to the atmosphere and so begin to alter in the rocks near the surface, destabilising the rocks. Biological weathering is the effect of living organisms on the break down of rock. This involves, for example, the effects of plant roots and soil organisms. Respiration of carbon dioxide by plant roots can lead to the formation of carbonic acid which can chemically attack rocks and sediments and help to turn them into soils. There are a whole range of weathering processes at work near the surface of the soil, acting together to break down rocks and minerals to form soil. These weathering processes have given rise to most of the world's soils.

A key concept to understand is how erosion, and thus soil formation, is a continual process. As rocks and sediments are eroded away, so more of the solid rock beneath becomes vulnerable in turn to weathering and breakdown. The natural processes of nature, in the form of wind, rain, snow and ice, start to have their effect on these rocks and sediments as they 'come within their range'. Once the process starts, then other physical, chemical and biological processes also start to contribute to the breakdown of the rocks, leading to the formation of the precious soil. Most of the tiny particles making up our soils will have started as solid rock. Little or nothing will grow directly in rock; before plant life can flourish the rock first needs to be broken down to form soil. It is true to say that weathering and the formation of soil provide an excellent example of the wonders of nature.