Desert soil is generally composed of sand, gravel and small rock, with very little humus, the organic material found in soil. The aridity of the desert plays a major determining factor in the amount of humus in the soil.
The U.S. Soil Taxonomy lists two soil orders that are generally found in deserts throughout the world: aridsol and entisol. The former has a minimal humus content, but displays differentiation of an upper layer. This upper layer tends to contain significant salt and mineral deposits. By contrast, the entisol order features no differentiated layers. Lacking in nearly any humus content, the quartz sand soil of this order is often found in large desert and dune regions.