Vegetation v

The development of vegetation is clearly influenced by parent material, relief and climate. However, the development of vegetation also feeds back positively to modulate these factors. For example, soils will tend to form where plants help stabilize the substrate, preventing erosion by surface water or wind. At the same time plant roots may help break up parent material, while evapotranspiration will influence microclimate. Vegetation is the main contribution of organic matter to soil and although most soils contain less than 5% organic matter (by weight), it is an extremely important component. The type of vegetation developed controls the nature of the soil organic matter (SOM), comprising a complex mixture of various biopolymers (Box 4.10) including cellulose and lignin (Table 4.6). SOM, often called humus, is composed of the elements carbon, hydrogen, oxygen, phosphorus, nitrogen and sulphur bonded together to form huge macromolecules. The presence of SOM influences soil structure and thereby its water-holding capacity, keeping mineral surfaces and soilwater in close contact.

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