Particle sizes are divided into three sizes, with the largest being sand, medium sized particles are silt, and the smallest are clay. In fact, clay soil particles are so small the soil is dense enough to use for ceramic pottery. The vast majority of soils are combinations of sand, silt and clay. Soil is not dead like rock but is alive, with millions of microorganisms thriving in the spaces between particles, carrying on vital processes that influence the availability of nutrients for plants. Their main role is to break down organic matter and turn it into humus, which then boosts nutrient content and overall fertility.
Soils also contain varying amounts of organic matter.
This is the remnants of dead plants which eventually break down into humus a form most beneficial to living plants. Ideal agricultural soils contain a good deal of organic matter in various stages of decomposition. An example of this role of organic matter is found on the Great Plains of the American midwest. This land was once prairie and covered with a thick mat of sod with roots that could extend many feet into the earth. Every year these grasses shed leaves, stems, and bits of roots, all sources of organic matter, into the surrounding soil where they gradually decomposed.