Soil is more than that brown mud the dog tracks into the house after a rainstorm; it is an intricate ensemble of living microorganisms, humus (partially and completely decayed organic matter) and inorganic particles worn down from parent rocks. The process from rock to soil is a slow one. An average inch of topsoil, richest of the soil layers in organic matter and the creatures that decompose such material, takes a thousand years or more to form.
As any home gardener knows, soils vary widely in fertility, mineral content, physical structure and the way they react to wind and water. Some soils drain slowly, making them poor choices for unsurfaced roads or septic systems. Others are highly erodible, and the smallest disturbance can lead to a gully or streambank washout. The type and depth of soil play a major role in determining what kind of plants grow in an area. The plant community in turn affects what species of fish and other animals, both domestic and wild, can survive there.
Erosion is a natural process. It shapes our hillsides, valleys, rivers and streams; it creates fertile floodplains and it helps distribute nutrients throughout the watershed. Erosion provides necessary sediments to creeks and rivers and allows them to create a rich variety of habitats such as spawning gravels, deep pools and sandbars where new vegetation can take hold. Erosion in upper watersheds is needed to form our coastal beaches.
In stable watersheds, the rate of erosion is slow and in balance with natural restorative cycles. But in many watersheds, human use of the land has accelerated the rate of change beyond nature’s short-term healing capabilities—in some places even beyond long-term recovery. The desertification process occurring in many arid and sub-arid regions is a dramatic example of how human-induced changes in vegetation and soil can lead to wide-scale ecologic and economic collapse.