SOIL PROFILE Soils differ widely in their properties because of geologic and climatic variation. However, they have a unique structural characteristic: a vertical sequence of produced by the combined actions of percolating waters and living organisms. These layers are called and form the layers horizons soil profile. Soil horizons Soil profiles can differ from one location to another, but they are usually made up of: – also called ground, is of organic material at various stages of decomposition formed by the decay of living organisms, the humus; O Horizon made up – commonly called topsoil, consists largely of minerals (sand, and ) and appreciable amounts of organic matter, and provides plants with nutrients needed for good growth; A Horizon silt, clay – this light coloured, horizon contains less organic matter than the A horizon and is present only in forested areas; E horizon 1 leached – called subsoil, consists of minerals washed down from the layers above; B Horizon – consists mostly of big rocks; C Horizon weathered – made up of a mass of partially weathered or at the base of the soil profile. R Horizon parent rock, unweathered, By examining a soil profile, it is possible to evaluate its fertility. As the soil weathers and/or organic matter decomposes, the profile of the soil changes. For instance, a highly weathered, infertile soil usually contains a light-coloured layer in the subsurface soil from which nutrients have On the other hand, a highly fertile soil often has a dark, deep surface layer that contains high amounts of organic matter. leached away. MORE “E” is short for eluviated because the development of this horizon is favoured by high rainfall and sandy parent material that encourage an extensive water percolation. 1 Soil types Soil can be categorised into different types based on the dominating size of the particles within it. However, most soils are composed of a combination of minerals and organic matter. Mineral particles are classified according to their size: 2 is the largest soil particle; it allows water to drain right through and does not have the ability to hold many nutrients; Sand is the intermediate soil particle, commonly found in waterways, and Silt is part of a good soil mix with moderate drainage and nutrients; Silt wetlands floodplains. is the smallest soil particle; it has a very high water-holding capacity and is a very good nutrient holder; Clay soil contains a balance of all three soil materials (sand, silt and clay) plus organic matter and it is the ideal farming soil. Loam MORE Depending on the amount of mineral present, soils can be heavy or light. A heavy soil contains a higher proportion of clay and can be very difficult to work because it tends to be either very wet and sticky or very dry and hard. Light soils are easy to work but lose water and nutrients. Both types need to be enriched by the addition of organic matter. 2