Organic matter provides the greatest number of binding sites because it has an extremely large surface area and is very reactive chemically. Organic matter content in soil depends on climate, vegetation, position in the landscape, soil texture, and farming practices. Abundant rainfall, combined with lush natural vegetation, gives rise to soils with high organic matter contents. Desert soils have very low organic matter contents. Grassland vegetation generally produces more organic matter deeper in the soil than forest vegetation.
Organic matter decomposes more slowly in wet soils.
As a result, poorly drained soils in low-lying areas tend to have more organic matter than better drained soils higher in the landscape. Sandy and gravelly soils tend to be droughty soils that support less vegetation. The difference is particularly marked where rainfall is limiting for plant growth. Farming practices that return crop residues and animal wastes to soils help maintain soil organic matter content. Practices that harvest or destroy residues tend to reduce soil organic matter.