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Land Management & Natural
Hazards Unit |
SOIL |
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Soil
Atlas of |
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What is soil?
There is an old Chinese proverb that states: "Man…despite his artistic pretensions and many accomplishments, owes his existence to a thin layer of topsoil …and the fact that it rains". When different people refer to 'The Soil' they usually have rather different ideas of what this means. To the gardener or farmer, soil is the upper few centimetres of ground that is cultivated and nurtured to produce crops. To the engineer, it is the 'overburden' or unwanted loose material at the ground surface that needs to be removed to provide a more stable foundation upon which to work. To the geologists it is the loose 'unconsolidated' material overlying the rocks they study. However, to the vast majority of the human race living in cities and towns, soil is simply the 'dirt' or 'dust' to be cleaned from their hands or the vegetables that they buy to eat. Many large supermarket retailers in the western world now do this before putting them on display in order to make such produce 'more attractive and presentable'.
In fact, soil is all of these different things. Soil is the living, breathing skin of our planet and it is affected by, and is the result of, the many and varied interactions that occur between the atmosphere, as governed by climate and weather patterns, the biosphere, that is the local vegetation and animal activities including those of man, the geosphere, the rocks and sediments that form the upper few metres of the Earth's solid crust. Those of us who study soil have a definition for it. We say 'soil is any loose material at the surface of the Earth that is capable of supporting life' and these life-supporting functions have been understood for a very long time. What is soil made of? All of us have come into contact with soil at some time in our lives and most are familiar with such terms as clay, sand or peat. In reality, soil consists of a complex mixture of mineral and organic particles that represent the products of weathering and biochemical processes that break down the local rocks and sediments into individual grains of increasingly smaller sizes and also break down the dead vegetation and organisms that fall on or remain within it. When we handle the soil, the fact that it usually stains and moistens our fingers, shows that it also holds different amounts of water and chemicals and the amounts of these that can be held by the soil are determined by the size and origin of the mineral and organic particles present. The two other final components that make up the soil are the organisms, both plants and animals, that live (and die) within it and the air that enables them to live there. |
The soil in profile Soil is the product of various environmental weathering processes that operate on geological materials on the Earth's surface over a period of time. If we dig down into the soil to about 1 or 2 metres depth and look at the vertical section revealed, we notice a number of roughly horizontal layers that look slightly different. These layers are the result of the local environmental weathering processes and they have colour, physical structure and chemical characteristics that differ significantly from those of the underlying rocks and sediments. Soil scientists call the layers 'soil horizons' and, as a means of shorthand and easy communication, assign letters of the alphabet to distinguish the different types. Know Your A, B, C! When a soil pit is dug and the vertical profile of the soil examined, normally, an uppermost layer that is darker than those beneath can be recognised. This is the 'topsoil' or 'A horizon' which contains most of the organic material within the soil; hence its darker colour. It is the engine room of the soil where most of its biological and chemical activity occurs. If the topsoil layer is removed by erosion or human activity, most of the soil's ecological potential goes with it. Although the topsoil layer will regenerate over time, if left undisturbed, it may take hundreds of years for its full original potential to be restored. Below the dark-coloured topsoil are one or more brighter coloured layers; the 'subsoil' or 'B horizons'. These layers contain much less organic material (making them different in colour) but are still exploited by plant roots and soil animals that use the water, air and nutrients stored in them. They are usually brown or reddish in colour because they contain iron oxides weathered from clay minerals in the soil. These photographs show how soil profiles can differ quite radically in their appearance depending on their position in the landscape (JH). The profile on the right, a Cambisol under woodland, shows a classic A-B-C sequence of soil horizons, with colour differences reflecting the relative distributions of organic matter and iron oxide produced by the weathering of minerals in the soil (JH). In contrast the profile on the left, a Stagnic Luvisol under pasture, shows an more complex sequence of A-E-B horizons. Colour differences again reflect changes in the distribution of organic matter and iron oxide minerals, with the paler coloured E horizon containing less clay or iron oxide than the A or B horizons. However, superimposed on these basic colour differences is a 'mottling' effect caused by periodic waterlogging of the soil as a result of the impermeable clayey nature of the B and C horizons (JH).
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In many European soil types, between the dark coloured 'A' and brighter 'B' horizons, is a pale coloured layer. This horizon has a smaller content of very fine material such as clay, organic matter, nutrients and chemicals such as iron than either the overlying A or underlying B, hence its paler colour. Such layers, from which some soil components have been 'leached' out, are known as 'eluvial' or 'E' horizons and usually represent the most impoverished parts of the soil profile with respect to biological activity and nutrient availability. Towards the base of the subsoil, the soil structure gradually dies out as the factors affecting its development decrease in influence. Eventually a layer is reached where the influence of environmental weathering processes is minimal, either because there is hard rock present or because there has not been enough time for the processes to have a significant impact or because the layer is too far from the land surface to be affected. This layer is called the soil 'substrate' or 'C horizon' or 'soil parent material' and has either no structural development or shows joints and bedded layers characteristic of rock formations.
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