Soil horizons

The soil profile indicates the changes in ped structure and chemical composition with depth. With depth, there are changes in physical, chemical and biological properties. These changes are caused by several factors. One most noticeable change in the top soil is the amount of litter, its particle size and the extent of litter decomposition. The uppermost layer consists of least decomposed litter, which has fallen to earth more recently. With depth, the litter is increasingly decomposed, degraded and smaller in size. Also with depth, the amount of total organic matter, of root colonization and the extent of fungal hyphae network change. The effect of organic coagulants from SOM, from exu-dates and from other soil organisms is to increase the natural clumping size of peds. Growth of fine roots, fungal hyphae and Actinobacteria filaments through peds forms a meshwork which further holds peds together. Thus the soil layer most active with organisms and labile litter can have large ped sizes. Therefore, with increasing depth, soils display a gradation of chemical content and physical structure. Together with changes in the amount of water retained, soil air composition, soil water acidity and redox potential, these gradients along the soil depth profiles (over several centimetres) strongly affect species composition. The thickness of each layer along the profile varies for each field, forest or region, depending on biogeography and the ecosystem. Some of the soil characteristics that can be seen in the profile are shown in Figs 2.8 and 2.9. These gradients cause changes in the structure, texture and colour of the soil along the profile. Soil scientists have devised a terminology to describe the soil profile layers, based on appearance and composition. The layers along the profile are called horizons (Table 2.1). There are three main horizons, the top organic horizon (A); followed by the inter-

AL Ao hyphae Ah Bh B

Fig. 2.8. Soil profile and horizons illustrating organic matter distribution in arbitrary soils. (A) Temperate forest site with fresh litter deposit, partly decomposed litter continuing into a densely meshed hyphal net, with organic accumulation below. (B) Forest site with mostly decomposed litter, with a thin mat of hyphal net structures, above the humic layers. (C) An agricultural field with no litter accumulation, but with some humic organic matter. (D) An agricultural field under conservation practice, showing some surface litter, with a hyphal mesh structuring a decomposed litter layer and build-up of organic matter below it.

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