Ever wonder what holds the soil together?
It’s not Velcro, tape, or oil: it’s glomalin, a sticky glycoprotein that glues sand, silt, clay, and organic matter together, to create soil aggregates. This creates what farmers and gardeners call “tilth,” which is a feeling of smooth soil granules that flow through your fingers. It’s also naturally brown, and when removed, the soil is left as a mineral grey color.
Not only is glomalin an important soil glue, but it stores a whopping one third of the world’s soil carbon–which is really important for stabilizing our climate. And glomalin was only discovered in 1996 by scientist Sara F. Wright. Yet the relationship between mycorrhizal fungi and plant roots is possibly the oldest and most abundant plant-microbe association on earth!
Glomalin is produced exclusively by arbuscular mycorrhizal fungi (AM fungi). AM fungi infect the roots of plants, but help them survive by transporting nutrients and water back to roots through the fine filaments called hyphae that extend out into the soil.
The glomalin is excreted by the fungi to form a protective barrier that contains nutrients and water flow and provides rigidity to the hyphae to span air spaces between soil particles in search of nutrients and water. There can be hundreds of miles of hyphae in just a pound of soil. As feeder roots expand, and become permanent roots, the hyphae move down to the new feeder roots. The hyphae on the permanent roots now stop transporting nutrients, and the protective glomalin sloughs off into the soil. While the hyphae only live days the weeks, the glomalin can last as a glue for more than 40 years.
Don’t add phosphorus!
First, mycorrhizal fungi are very sensitive to phosphorus, and thrive in low phosphorus soils. Despite this, they are the most critical microorganism for converting the phosphorus already in soil to a form plants can absorb. So be sure to not add any synthetic phosphorus to your soils.
In fact, synthetic nitrogen and phosphorus fertilizers are known to cause AM fungi to stop working, or kill them outright.
Next, only aerate lawns when necessary, as any tillage that cuts up the soil also breaks apart strands of living hyphae in the soil. Compacted soils reduce hyphal growth, though, so check if your soils are compacted and aerate and add compost if needed. Yard clippings, and composted yard clippings can encourage mycorrhizal fungi, which in turn convert nitrogen and phosphorus for plant growth, and produce glomalin, and in turn create a great soil.
Still want more? You can learn a bit more science behind glomalin here.
Written by Mary Travaglini, Montgomery County, MD Department of Environmental Protection
Photos by the U.S. Forest Service and Dr. Sara Wright of the USDA.