Biological Soil Crust Activity

Biological Soil Crust

Don’t Bust the Crust! It’s Alive!

The plant communities of Arches National Park are dependent upon microbial communities called biological soil crusts or cryptobiotic crusts. These communities – made up of cyanobacteria, green algae, fungi, lichens, mosses, and other tiny organisms – create a matted crust atop the sand that retains moisture, creates nutrients, and provides grasses and shrubs a foothold in the sand.

Once damaged, crusts may take many years to grow back. Areas that have been stripped of crusts are vulnerable to erosion, flooding, dust storms, loss of organic materials, and invasion by non-native weeds that thrive on disturbed soil.

Take a Closer Look

Looking through an electron microscope, one can see the sheaths and filaments that make up the crust. Additional scientific analysis reveals how crusts function.

A black & white microscope image of many light-colored strands within a light-colored sheath. Small particles are stuck to these strands and sheath.
Cyanobacteria magnified 950 times
View of Cyanobacteria – If You Were Smaller Than An Ant!

USGS Canyonlands Research Station


Nitrogen Fixation

All plants need nitrogen to live. Air is 79% nitrogen, but most plants cannot utilize it. Cyanobacteria and lichens convert the nitrogen into a usable form. Without nitrogen most plants in the area could not survive. Without the plants there would be fewer animals.

Erosion Control

Cyanobacteria “holds the place in place.” The process begins when wet filaments start to grow. As they dry, the filaments form sheaths that stick to sand grains. Over time, these sheaths weave an intricate web of subsurface particles that reduce soil erosion.

Black and white microscope image of many round lumpy particles being held together by thin white strands.
Biological Soil Crust magnified 90 times showing the bacteria sticking to grains of sand
and "holding the place in place".

USGS Canyonlands Research Station


Water Infiltration

Without crust, porous desert soils retain little water. Sheaths swell like sponges, absorbing and storing the desert’s limited precipitation. Water infiltration rates are significantly lower in disturbed areas than in pristine areas, resulting in fewer seedlings and greater erosion.

Mature Crust

  • After many years, with the addition of fungi, mosses and lichens, crusts become fully functional.
  • Cyanobacteria decreases erosion.
  • Lichens improve nitrogen fixation.
  • Mosses improve water absorption.

Immature Crust

  • When crust forms, you cannot see it with the naked eye because it lacks color and texture.
  • Crust takes at least five years, usually more, to develop color or its characteristic bumpy texture.
  • Because people do not recognize this initial stage of growth, they often trample the crust.
  • The filaments and sheaths of the cyanobacteria bind the soil together.
  • At this stage, no fungi, mosses or lichens are present.

What Can I Do?

Help protect the crust and the plants and animals that depend on it. While exploring Arches National Park and other desert environments, stay on established trails, or walk in dry washes or on bare rock. You can also help us by telling others. Spread the word!

A field of lumpy black dirt with a worn footpath going through it. The path is loose sand and shows footprints.
Mature Biological Soil Crust
The pathway through the crust can take hundreds of years to recover.

USGS Canyonlands Research Station


In this activity there are 10 questions about biological soil crust. Choose from the words on the lower left of each question to fill in the blanks. Click on the word that you think is the correct answer.

Go to Question 1.

Last updated: June 11, 2021

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