The size of the rock can range from gravel, to sand, silt, and clay. The organic matter, in variable stages of decomposition, is called humus. B. Soil profiles are defined by the composition of soil from the surface downward. Topsoil has the most humus and is the most vulnerable to weathering. Loam topsoils have the best mix of sand, silt, and clay for agriculture.
Vegetation is scarce but there is some diversity.
B. More than a third of the world's total land area is arid or semiarid due to drought and overgrazing, which can lead to desertification. Dry Shrublands, Dry Woodlands, and Grasslands A. Dry shrublands and dry woodlands occur in western or southern coastal regions of the continents between 30o and 40o north and south latitudes. D. Grasslands extend across the interior of continents in the zones between deserts and temperate forests. Shortgrass prairie of the American Midwest is typified by short, drought-resistant grasses that have been replaced by grains that require irrigation. Tallgrass prairie was originally found in the American West where water was more plentiful.