What Exactly is a Biological Soil Crust?

Biological soil crusts are thin layers of microbes that colonize bare patches of soil. Biological soil crusts help build soil nutrients and improve soil structure.

Soil Crusts are Tiny Communities that Can Help You Build Healthy Soil

Did you ever walk out in the desert following a rainstorm?  If so, you may have noticed a thin black or green layer  on the surface of the soil.   Of course. you may have noticed a sprinkling of dry, black powder on the soil surface on a sunny day too, but when this film is dry, it is easy to mistake for a layer of fine, black sand.  When the ground is wet, even the most causal observer can be made aware that this layer is alive.

As the rain comes down, you may notice that the soil does not wash away or erode easily where this layer is present.   If you are observant, you may also notice that the layer turns green soon after it became moist.  The green color lasts for about  as long as the soil remains moist.  In dry weather, it blackens as photosynthetic cells become dormant.  This is because this film you are looking at, this cryptobiotic crust, is alive?

Cryptobiotic crusts contain cyanobacteria that harvest nitrogen from the atmosphere and turning it into ammonia (ie: fertilizer).

Cryptobiotic Crusts Create Nutrient Cycles that Help Soils Support Life.

Cryptobiotic crusts are cyanobacterial, algal, and fungal mats that set on the surface of every healthy soil.  These complex communities  include numerous species of fungi, cyanobacteria, and algae.  Together, they create the factories that build nutrients in our soil.   These microscopic communities catalyze the capture of atmospheric carbon and nitrogen.  The green algae, including cyanobacteria, turn carbon dioxide to sugars.  They also turn inert atmospheric nitrogen, (dinitrogen), to ammonia, that can be used to make amino acids.  You know, those molecules you pay top dollar for in the nutrition store.  These sugar and amino acid molecules become building blocks for complex metabolic pathways that produce other organic acids, polysaccharides, nucleotides, proteins, lipids, and other biomolecules we tend to think of as food.  These foods act as fertilizers that support the growth plants and beneficial microbes in your soil.

Now all of us know that you won’t stay healthy long without minerals, and this is where the fungi found in soil crusts become important.  As nature’s decomposers, well fed fungi are experts at decomposing even the most recalcitrant sources of nutrients.   Fungi can literally dissolve rocks, remove the minerals from them, and transport those minerals to the microalgae in their crust community.  By keeping the microalgae happy, the fungus guarantees itself a steady supply of sugars and amino acids.  By keeping the fungus happy, the microalgae guarantees a steady supply of minerals.  Like any partnership, the association requires sharing and free trade.

Crust Filaments Hold Soil in Place

Everyone living in the Southwest deserts hates being outdoors in a windstorm.  Dust can literally be carried from miles away.  It scrapes against you, gets in your eyes, and covers your vehicles with just enough silt and clay to make a fine layer of mud when the rain finally hits. But have you ever noticed that the dust you despise always seems to come from building sites, plowed fields, or dirt lots that receive a lot of traffic?   If you walk out on the mesas, where the crust is unbroken and minimal disturbance has taken place, you will see a lot of bare ground, but there is not a lot of dust blowing!  What’s holding the soil in place is the crust!

Crusts hold soil particles in place because most microbes secrete sticky, glue-like substances that help them adhere to surfaces.  As crust communities grow, this glue begins holding soil particles together in aggregates that improve soil structure.  Larger particles allow more air and water to penetrate the surface.   Many species of cyanobacteria and fungi in soil crusts are filamentous.  That means they grow by connecting cells in long strands, or filaments.  As millions of these microbes interact, filaments branch out, become intertwined, and create thick mats that bind soil particles and hold them in place.  This is why soils with healthy crust communities resist wind and water erosion.

Soil crusts can retain or repel water

Let’s talk a little more about that sticky substance bacteria make.  It is basically a mix of complex polysaccharides that can confer mixed blessings to your soil.

On the positive side, polysaccharides are used by many bacteria and fungi to create matrices on their cell surface that bind water, keeping it close to them.  These polysaccharides act like natural hydrogels, holding 10, 100, 0r even 1000 times their weight in water!  This is water that remains in your soil, not only for the benefit of the crust microbes, but also to the benefit of every plant and soil critter in your field or garden.  Combine this improved water holding capacity with the improved porosity created by soil aggregates, and you begin to recognize that soil crusts can help make rain or irrigation more available to your plants for a longer period of time in your soil.

But there is a dark side to soil crusts too, and we see that often in dryer regions of the Chihuahuan desert.  Very fine crusts like the ones shown here simply aren’t developed enough to significantly impact soil structure.  While they protect against erosion, which is critical on these thin soils, they also tend to form smooth surfaces that can actually repel water, forcing it to run off.

Water in a desert is such a priceless commodity that I’ve heard more than one soil expert recommend getting rid of soil crust communities to improve water infiltration.  However, a preponderance of data demonstrating benefits for reducing erosion, supporting seed germination, and improving soil nutrient cycling suggests eradication would be a mistake.  Especially since we know that more developed crusts actually improve water infiltration.

Don’t Fight It, Feed It!

Alternatives to crust eradication would be to ignore the crust altogether and take steps that improve overall soil water holding capacity and microbial health.  This can be done by applying natural mulches like straw or wood chips directly to the soil surface, and/or by maintaining vegetative cover. Mulches would themselves retain water and build soil moisture.  Mulches and live vegetation would also provide more substrate and more surface area on which microbes can thrive, and would cycle more nutrients into the soil.

Now, keep in mind that in the presence of mulches and vegetation, you might not recognize visible soil crust layers.  This does not mean you have destroyed them.   Remember that crust microbes are a community, and when you introduce big new players, like plants, to the community, the roles of each microbe will change. Microalgae, the most visible components, will have to compete with plants for sunlight and other resources.  This means they may not reproduce as often, they may not produce as much chlorophyll, and they may appear less green, and more hidden then they were when they were the only show on stage.  But the industrious among them will find ways to thrive.  Some will colonize pore spaces in the mulch.  Others will grab on to germinating seedlings, and take up residence on a nice, cool, green leaf. As for the fungi, they will go to work decomposing the mulch, or transporting minerals to plants.

Keeping Valuable Soil Crust Microbes Healthy

The same basic principles for maintaining healthy soil microbial communities apply to keeping soil crusts healthy.  These include:

  • minimizing traffic that can damage the crust
  • minimizing the kind of tillage that overturns soils and buries rusts.
  • limit the use of agrochemicals
  • provide habitat for crust microbes by adding organic mulches or vegetation

 

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