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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.



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A physical crust is a thin layer with reduced porosity and increased density at the surface of the soil. A biological crust is a living community of lichen, cyanobacteria, algae, and moss growing on the soil surface and binding it together. A chemical crust or precipitate is white or pale colored and forms in soils with a high content of salts. Both chemical and biological crusts can form on and extend into a physical crust. This information sheet deals only with physical and biological crusts.

Physical crusts generally indicate that the amount of organic matter in the soil has decreased and/or erosion has occurred. They have low aggregate stability, disperse readily when wet, and are easily reformed by raindrop impact or flowing water. They seal the soil surface, reduce the rate of water infiltration, and can increase runoff. Physical crusts generally have a very low content of organic matter and support little soil biological activity. The dense nature of the crusts can impede seedling emergence. Water that ponds in flat, crusted areas is likely to evaporate, reducing the amount of water available to plants. Physical crusts generally help to control wind erosion, but they do not protect the soil from water erosion.

Biological crusts stabilize the soil surface, protecting it from erosion. Depending on soil characteristics, biological crusts may increase or reduce the rate of water infiltration. By increasing surface roughness, they reduce runoff, thus increasing infiltration and the amount of water stored for plant use. Some organisms in biological crusts can increase the amount of nitrogen and other nutrients in the soil. In semiarid ecosystems biological crusts can provide a significant amount of nitrogen for plant growth. The germination of plants may be enhanced or inhibited, depending on the nature of the biological crust and the plant species. In general, the relative importance of biological crusts increases as annual precipitation and the potential plant cover decrease.

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