Beneficial soil fungi

By James Hoorman, Hoorman Soil Health Services and Dale Strichler, Green Cove

Mycorrhizal fungi (MF) are one of the most beneficial organisms on the planet. These fungi colonize plant roots, acting as root extenders to aid roots. MF are more efficient than roots and MF benefits are numerous.

 Increased root mass. Plants allocate a certain amount of energy to the root system. In the absence of MF, plants must build root hairs, which requires a lot of energy. Plants colonized with MF do not produce root hairs and instead use a much smaller amount of energy to allow MF to perform the job of absorbing water and nutrients. The hyphae (root-like structure) of MF is about 1/16th the diameter of a root hair (1/10 the diameter of human hair), and it takes about 1/256th the energy investment per mm of length to build than a root hair. With this energy savings, MF colonized plants tend to build much better root systems.

Improved drought tolerance. One of the primary functions of a root system is water uptake and a colonized root will do so much better than an uncolonized roots.  The MF hyphae extends up to 18 inches past the root zone, while root hairs only extend a few millimeters. Also, the narrow diameter of the hyphae means they can grow into crevices much too small for root hairs to reach.  These two differences combined together extract 1-2 weeks of extra water from a dry soil before moisture stress kicks in.

Faster establishment. MF hyphae can grow much faster than roots, and thus enable seedlings to access water and fertility easier than uncolonized plants. This is particularly important for small seeded, slow establishing plants like most forage grasses and legumes, and perennial crops like fruit trees for which a faster start can mean fruit production a year earlier than it would occur otherwise. Sometimes in no-till crops in early spring, the corn and soybeans may seem a little slower to establish, even a less dark green.  This occurs when MF are getting established on crop roots.  The effect is short term, and crops respond better later in the growing season when nutrients and water are needed in great amounts. 

 Reduction in root disease and nematodes. MF coat the roots with a protective layer of a fungal tissue called chitin (the same compound that makes up crab shells). Nematodes and disease organisms that attack plant roots lack the enzymes to penetrate this layer, thus MF confer a moderate level of protection against these pests.

Increased nutrient uptake. Along with increased water uptake, MF colonized plants also exhibit superior uptake of mineral nutrients. This difference is most pronounced for nutrients that do not move well within soils, such as phosphorus, iron, and zinc. MF plants take up 6x more phosphorus, 25% higher calcium, 38% more potassium, 30% higher magnesium, 3.5x higher zinc, 2x more copper, 28% higher manganese, and 80% more iron than uncolonized plants without MF. Plants, animals, and humans who consume these extra nutrients are healthier.  

Improved soil structure. The hyphae of MF give off an exudate called glomalin, that is the most powerful and persistent soil aggregating agent found in nature. Glomalin is only produced by MF fungi. Soils growing MF colonized plants will usually exhibit a rapid improvement in soil structure and soil will appear a couple shades darker. Glomalin is a reddish-brown color and helps soils aggregate and store more carbon, nutrients, and water. 

Reduction in compaction. The hyphae can exert a much higher pressure per square inch than roots to penetrate dense soil layers, and the finer diameter means it is also easier for the hyphae to penetrate these layers. Once a hardpan is penetrated by hyphae, the glomalin takes over, and creates a lubricated path for plant roots to follow. MF improve soil aggregation, increase water infiltration, improve drainage, and increase water holding capacity.

Reduced weed growth. Many weed species (pigweeds, smartweed, wild mustards, lambsquarters) are not colonized and do not benefit from MF. Inoculating the crop with MF makes the crop root much more competitive for water and nutrients than the weed roots. North Dakota State University research shows a 54% reduction in weed biomass from inoculating a sunflower crop with MF.

Overall improved soil biology. Most beneficial soil microbes live in the rhizosphere, the thin layer of soil surrounding the plant roots. The MF hyphae increase surface area by a thousand times, allowing beneficial soil microbes a place to colonize and live.  The MF feed the soil microbes sugars obtained from the plant.   Everyone benefits, including humans from this beneficial relationship.  Colonization with beneficial MF is an important step in improving soil biology. 

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