What is soil, anyway?

Most soils evolve slowly over centuries through the weathering of underlying rock and the decomposition of plants. Others are formed from deposits laid down by rivers and seas (alluvial soils) or by wind (loess soils).

Soils have 4 basic components:

• Mineral particles: sand, silt, and clay

• Organic matter

• Water

• Air

A sample of typical topsoil contains about 50 percent pore space filled with varying proportions of air and water, depending on the soil's current moisture content. The other 50 percent of the volume is made up of mineral particles (sand, silt, clay) and organic matter; most mineral soils range from 2-6 percent organic matter by weight in the topsoil. Organic soils like peats are formed in marshes, bogs, and swamps, and contain 30-100 percent organic matter.