Test pit

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Soil colours and hydrology[1][edit]

  • 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 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 smell 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. 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.

  1. Interpreting Soil Colour, Agriculture Victoria http://vro.agriculture.vic.gov.au/dpi/vro/vrosite.nsf/pages/soilhealth_interpret_colour Accessed 30 April 2019