Physical crusts form when organic matter is depleted from the surface layer, soil aggregates become weak, and raindrops disperse the soil into individual particles that clog soil pores, seal the surface, and form a layer that is dense when dry. A physical crust consisting of numerous thin bands can form when sediment from erosion is carried downslope and buries the soil surface. Physical crusts are more common on silty, clayey, and loamy soils and are relatively thin or weakly expressed, if present at all, on sandy soils. Soils with a high content of sodium disperse readily in water and are more susceptible to crust formation than other soils.
To examine a crust, lift the soil surface with a knife tip and look for cohesive layers or thin bands parallel to the soil surface. These layers have no apparent binding by visible strands of organic material, such as cyanobacteria. Fragments of physical crusts disperse or “melt” when placed in water. A vesicular crust is a type of physical crust with many small, unconnected air pockets or spaces similar to those in a sponge.
A biological crust occupies a large amount of the surface of calcareous and gypsiferous soils. Soil texture, moisture, temperature, season of precipitation, and history of disturbance largely determine the dominant organisms in the crust. For example, moss tends to be dominant in the Columbia Basin, whereas cyanobacteria and lichen are dominant in the Mojave, Sonoran, and Chihuahuan Deserts.
Prevent crust formation by keeping organic matter on your garden. Adding a healthy layer of organic matter to your garden from time to time will help prevent soil erosion.