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.