Page 70 - Soil and water contamination, 2nd edition

Environmental compartments 57

Therefore, soil erosion typically occurs in hilly or mountainous regions. Nevertheless, in flat
lowland areas with clay soils, the infiltration capacity may be so low that substantial overland
flow and thereby soil erosion occur (see Bleuten, 1990).
Since many pollutants enter the soil at or near the soil surface (e.g. application of
fertilisers or pesticides , atmospheric deposition ), the topsoil layers that are susceptible to
soil erosion also often contain the largest concentration of contaminants. In this manner,
sediment-associated pollutants are transported laterally across the landscape and accumulate
in valley bottoms or enter the river network. Sections 12.5 and 12.6 will further elucidate the
processes of soil erosion .

3.2.5 Physico-chemical conditions in soil
The physical and chemical conditions occurring in soils are closely interlinked with the
processes responsible for soil formation and vary as much as there are soil types over the
world. One of the most important factors controlling physical and chemical processes in
soil is the particle size distribution of soil material (soil texture ). Together with the organic
matter content , the soil texture determines the soil’s capacity to hold water. Fine materials
have a greater water-holding capacity than coarse materials. Soil texture also plays a

Figure 3.3 Texture classes adopted by the US Department of Agriculture (USDA, 1999).

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