Soil is produced by the decay of organic material and the weathering of rock. Distinct layers are seen in healthy soils, with the topsoil (A horizon) being the most fertile for plant growth. Desertification will reduce or remove the O and A horizons and produce drier B and C horizons.

Native prairie grasses have deep roots (up to 5 meters [16 feet] long), which allow them to access deep water supplies and to weather droughts. The native grass roots also hold the soil in place much better than do shallowrooted annual crops like wheat.

JIM RICHARDSON/National Geographic Creative