Biological soil crusts, also called cryptogamic crusts or cryptobiotic crusts, are complex associations of cyanobacteria, green and brown algae, lichens, mosses, microfungi, and other kinds of bacteria that grow in and on the soil in the open spaces between plants.

These organisms pioneer barren, open areas where nothing else can grow. The cyanobacteria and microfungi weave a network of filaments through the top few inches of soil. The filaments exude a mucilaginous sheath that binds loose sand and soil particles together, thereby stabilizing and protecting the soil surface from wind and water erosion.

In addition to stabilizing the soil, the microbial community is important in the development of desert vegetation communities. As the microbes grow and die over time, they add organic matter to the soil, which aids in water retention and provides nutrients for desert plants. The cyanobacteria also convert atmospheric nitrogen to a form plants can use. The addition of nitrogen is important in desert ecosystems because nitrogen levels typically are low and often restrict the growth of plants.

Over time, the bacterial community grows and produces a roughened soil surface with little humps and peaks. These raised sites provide places where bits of lichens and mosses can grow. The peaks grow at a rate of about 1/4-inch per 100 years, so after a few centuries, the little peaks can grow to over 1-inch high. Some years ago, I found an old fire pit with charcoal in southeastern Utah with 1-1/4 inch high peaks growing inside the fire ring; the Park Rangers estimated the fire pit to have been at least 500-years old.

Because of their importance in stabilizing the soil and in aiding the growth of plants, we should avoid walking on or otherwise damaging (biking, driving, pitching tents) biological soil crusts. Mature crusts can be recognized as being darker and rougher than the surrounding bare soil, so when you see biological crusts, stay on trails, walk in washes, walk on bare rock, or just walk around them to the extent that we can. If a group of hikers must walk across crusted surfaces, spread apart so as not to create a rut where wind and water erosion can further damage the biological community.