Soils need all kinds of fungi, not just mycorrhizal types. Here are some reasons why.

Updated: Dec 19, 2019

Soil fungal hyphae, aerobic, beneficial fungi.
Free-living fungal hyphae in compost. 400x tot. mag.

Pick a fungal-inoculum product off the shelf. Unless it’s the work of a boutique or specialized producer (like this one), the product you’ve selected contains only mycorrhizal fungi -- anywhere between one and a dozen-or-so strains. And maybe a Trichoderma species or two.

What do these fungal types have in common? They are all plant symbionts, depending for survival on plant roots into which they can grow. These are the fungi that have evolved over billions of years to consume the carbon secreted by plants, meanwhile digesting from the surrounding mineral and organic matter an array of soluble nutrients to feed back to their partner plants.

The entire inoculum market is composed of only a few-dozen of the hundreds of species known to exist in healthy soil. To rely solely on these products for microbial inoculation, therefore, is to ignore the vast benefits unlocked by our understanding of microbial ecology. In better understanding how the microbial world works together, and therefore how our farm fields work (or should work), we can’t leave out the many thousands of species of free-living fungi estimated to exist out there. They all play some role in their own microbial ecosystems, and therefore play direct and indirect roles in plant health.

The following are some good reasons for including free-living fungal types in biological remediation.

Bacterial-Fungal Competition

To reproduce the organism communities that thrive in our own regions, we sample local soils and develop those microbes on a diet of diverse organic materials (basically, composting). Quick bacterial development, and the resulting heat, are important to the process, and newly ‘cooked’ material will almost always be dominated by bacteria.

Soil Fungi, testate amoeba, bacteria in compost sample.
Free-living fungal hyphae (diag. and center-left), testate amoeba (center). Compost sample. 400x tot. mag.

Bacterial communities are great, but we need fungal development too. Bacteria and fungi are nature’s two decomposers, the only organisms that make the enzymes necessary for breaking organic and mineral materials down to their constituent compounds. As such, there is well-documented competition between the two for resources. Given enough time, fungi will overcome bacterial dominance, aided by bacteria-feeding microbes like protozoa and nematodes.

What can we do to give fungal development a boost? Simple: inoculate clean composted material with free-living, saprotrophic fungi. For this application, we can’t use plant-symbiont types because our stored inoculum material lacks the plant roots they require. Useful species include some commonly known by their mushrooms: turkey tail (Trametes versicolor), oyster mushroom (Pleurotus ostreatus), king stropharia/garden giant (Stropharia rugosoannulata). These species are all free-living saprotrophs that produce enzymes capable of breaking down lignin, and so they decompose woody material.

Soil Aggregation

By increasing the variety and numbers of fungi in our soils, we are increasing aggregation by means of both chemistry and simple mechanics. Fungi produce sticky compounds that help them adhere to and digest the materials on which they grow. These glues, along with the hyphal growth habit of encircling bits of soil and binding them together, play huge roles in building individual soil aggregates, and therefore structure.

Clay Flocculation

Also related to soil structure, this mechanism applies in dramatic fashion to heavy-clay soils. Bacteria-sized clay platelets can stack like pancakes, leaving no space for moisture and oxygen flow, and producing compaction. Among mineral elements, the strongest cations (those carrying a positive electrical charge) are calcium and magnesium; in the right proportions, these elements bind with clay particles (negatively charged), causing them to attract and repel one another in ways that counteract the tendency to stack.

In the Montmorillonite clays common to North America, we need far more calcium than magnesium (about a 7-to-1 ratio) to get the best clay flocculation. In conventional farming practice, this balance is sought through repeated applications of calcium-heavy compounds like lyme and gypsum; meanwhile, plowing and other destructive practices continually work against farmers.

The best way to keep heavy clays flocculated, therefore, is to develop fungi, along with plenty of the other microbes that compose the soil food web. Fungi consume huge amounts of calcium, necessary in building their ever-growing hyphal networks. Nematodes and microarthropods graze on those hyphae, then secrete excess calcium in soluble form, ready to bind to the negative charges of all those clay platelets.

Nutrient Cycling

And that leads into the broader factor of soil nutrient cycling. Fungi digest not only calcium, but also plenty of other nutrients from the mineral and organic matter in our soils. The more diverse our fungi, the more we'll develop communities of grazers that feed on those miles and miles (literally) of microscopic hyphae, leaving behind an array of waste nutrients in water-soluble form. Those nutrients feed both plants and other microbes -- and on and on the cycle goes.