What is Soil Made Of? At its broadest, the term soil can refer to any kind of loose sediment. In geology, soil is the end result of rock weathering. And in soil science, it is weathered rock combined with decayed organic material that can support plant growth. Soil is made of sand, silt, clay, and humus (decayed organic material). How much of each is in the soil depends on the type of rock it weathered from, how long weathering has been occurring, the local climate (wet or dry, hot or cold), the abundance of plant life to decay into organics, and the ecosystem of living things that decomposes the dead plants.