The colour of a soil is usually the first thing people notice of the soil. Colour can be a useful indicator of some general properties of the soil, as well as the biological and chemical processes occurring.
What Determines Soil Colour?
Mineral matter – rocks are broken down to form soils, and sometimes these rocks give their colour to the soil. More usually the colour of the soil results from compounds such as iron.
Organic matter – humus, the final stage of organic matter breakdown is black. Throughout the stages of organic matter breakdown the colour imparted to the soil varies from browns to black. Sodium content influences the depth of colour of organic matter and therefore the soil. Sodium causes the organic matter (humus) to disperse more readily and spread over the soil particles, making the soil look darker (blacker).
Iron – Red, yellow, grey and bluish-grey colours result from iron in various forms. Under average conditions of air and moisture, iron forms a yellow oxide imparting a yellow colour to the soil. Where soils are well draining or under dry conditions, iron forms red oxides imparting a red colour to the soil. Yet in waterlogged soil, with a lack of air, iron forms in a reduced state giving the soil grey/green/bluish-grey colours.
Water – soil colour darkens as the soil changes from dry to moist. But longer term colour changes are linked to water relations as well. Careful observation of colour can help to identify problems of waterlogging or leaching. Poorly drained soils are often dominated by blue-grey colours often with yellow mottling. Well drained soils will usually have bright and uniform colours.
Relating Soil Colours to Soil Hydrology
Bright colours and strong reds indicate that the soil, or horizon, is well drained, or at least rarely suffers from prolonged saturation.
Dull colours: yellows, and greys, often found together in mottled horizons indicate that the soil is not well drained and does suffer from prolonged saturation.
Blue-grey and blue-green colours are a certain indication that the soil is saturated for most of the year. The colours are due to iron (normally red as and oxide) being present in a reduced form (the opposite of being oxidised) and may be combined with sulphur, as a sulphide. Hence, such soils can often give off a bit of a niff like bad eggs.
Precipitation of iron compounds, usually orange or dull red, sometimes in association with manganese (black), is an indication of oxidation occurring in a generally waterlogged environment. Iron goes into solution in water that is low in oxygen, it can then be transported through the landscape until it reaches a more aerated zone when it precipitates. This iron movement process: solution, transport, precipitation/enrichment, is extremely common in soils and takes many different forms. It can occur as a groundwater discharge process, as a vertical leaching process of the upper part of the soil profile, or as a lateral (throughflow) process within the upper part of the soil profile. Concentrations of iron may occur as pans, as buckshot or ironstone and as laterite. The mottling of soils mentioned above occurs because of a more diffuse, small scale (a few centimetres) redistribution of iron within the material of the soil profile and the formation of iron hydroxides (yellow).
White or grey horizons between the topsoil and a clay subsoil can result from a long process of leaching. These horizons are washed out and usually have less clay than the darker topsoil and are often sandy loams with a very high proportion of fine sand. Sand in these horizons is often over 70% of the total mineral matter. The leaching occurs laterally, i.e. instead of water moving downwards through the soil it follows a path parallel to the ground surface and the upper surface of the subsoil. Such horizons are called A2 horizons, if they are particularly pale they are referred to as bleached A2 horizons. They invariably become saturated rapidly in winter and contribute to waterlogging. They are very rarely saline being so highly leached.
White colours in the subsoil are often due to the presence of calcium carbonate. This can be tested for using a little hydrochloric acid. If carbonate is present a few drops of acid will cause the soil to fizz and bubble as carbon dioxide is formed by the reaction of acid and carbonate. The depth from the ground surface to such a layer is often a good indication of the amount of leaching that has occurred throughout the formation of the soil. For instance, the yellow, Gellibrand marl, clay in the Heytesbury has calcium carbonate occurring once a depth of 0.8 to 1.2 metre is reached. Originally, the material throughout the profile was calcareous but time and rain has caused the upper 0.8 metre to be completely leached of free calcium carbonate.