Any experienced grower can recognize great soil. It has a loose structure something like potting soil, a darker color that indicates good amounts of humic matter, and an "earthy" sort of aroma. These type of soils are often found in forests or undisturbed grasslands where years of decomposed leaf litter or annuals have been taken down into the soil by earthworms and other living organisms.
Mycorrhizal fungi are major contributors to this type of excellent soil, primarily through the effects of their hyphae (tiny root-threads) which fill every square inch of soil surrounding their host plants and form an inter-linked underground food-gathering web. While each individual root-thread is microscopic in size, many trillions of them can combine to have powerful effects on their soil environment. Just a gram of healthy soil can contain miles of hyphae.
For heavy clay soils, these hyphae penetrate between the densely stacked clay platelets and aggregate them into small "chunks," allowing air to enter and water to drain. This opening-up aeration of the root zone is vitally important to plants, large and small. Plant roots in clay without mycorrhizal fungi get no oxygen and are either in an "underwater" situation or in brick-hard material if it dries out.
On the other hand, sandy soils have plenty of air in the root zone, but lack organic matter and need more frequent watering. For these soils, the mycorrhizal hyphae bind together the grains of sand and form an underground "biomass" to hold water and nutrients. Desert growers from Egypt to Arizona rely on mycorrhizal fungi.
The specific material that makes great soils is called glomalin, as named by USDA-ARS scientist Sara E. Wright who discovered that mycorrhizal hyphae secrete a sticky protein that coats soil particles to form aggregates. You could think of glomalin as a sort of super glue that creates wonderful soil texture from either clay or sand. Humus is typically credited with improving soil, but it is more likely the glomalin that actually does the job. As the hyphae age and die off, they also add organic matter and nitrogen to the soil.
Personally, I deal with clay soil by adding large amounts of clean sharp sand, and when I say large amounts I mean aiming for roughly a 50-50 blend in the top 6-8 inches. This is the one and only time that I suggest using a large rototiller on garden soil. The combination of sand, some compost or ground-up leaves, dry organic fertilizer, and mycorrhizal inoculant blended into heavy clay results in a wonderfully aerated and productive soil for years to come. Late fall or early winter is the perfect time to do this one-time soil-improvement project. (And, no, sand and clay do not make concrete - no matter what you may have heard.)
Obviously, this technique will not be practical for large farms, but it makes great sense for home gardeners or market growers who want top performance from their plants. Get the glomalin busy!
Good growing, my friends,
Don Chapman, BioOrganics