By Sam Knapp
Mycorrhizal fungi are receiving lots of attention in the farming community. More and more companies are selling pre-packaged mycorrhizal inoculum for gardeners and farmers. Increased yields, reduced transplant shock, reduced need for fertilizers, improved drought tolerance, and reduced soil erosion are just a few of the benefits to soil and crop health that companies promise.
Although these benefits are possible, they are not guaranteed and depend heavily upon your farming practices and your farm’s unique conditions. Understanding the ecology and biology on our farms helps us make informed decisions that impact plant health, soil health, and our bottom line. This article will cover some of the basic biology and ecology of mycorrhizal fungi, as well as practical guidelines for promoting mycorrhizas on your farm to maximize benefits to crop health. By the time you’re finished reading, you should know some basics about managing mycorrhizas on your farm.
Mycorrhizal fungi
Broadly speaking, mycorrhizas are groups of fungi that coevolved with plants to live symbiotically within root tissues or cells. They depend on their plant hosts to survive and last only weeks to months without living root systems to colonize. Mycorrhizal fungi have suites of enzymes that allow them to decompose complex organic matter (e.g. cellulose, lignin, proteins) and release nutrients that are otherwise unavailable to plants. Networks of tiny filaments called hyphae (plural) allow mycorrhizas to transport water and nutrients back to their plant hosts in exchange for carbohydrates from photosynthesis.
Mycorrhizal fungi are categorized into four main groups depending on the types of plants they colonize and the structures they form. Orchid mycorrhizas associate with orchids and help them mine nutrients from woody and mineral substrates.
Ericoid mycorrhizas associate with plants in the family Ericaceae which includes the genera Vaccinium (blueberries, cranberries, etc.) and Rhododendron (azaleas, Labrador tea, etc.) among others. Their hyphae form coiling structures inside the plant’s cell walls to facilitate nutrient exchange with host-plant roots.
Ectomycorrhizas associate with most coniferous trees, some deciduous trees (including birches, oaks, aspen, and elms), and a few other woody plants. Their hyphae do not penetrate root cell walls but instead form dense nets of hyphae in the spaces between root cells.
Lastly, arbuscular mycorrhizal fungi associate with the vast majority of flowering and non-flowering vascular plants, including most annual crops, deciduous trees, grasses, and ferns. Their name derives from fanning hyphal structures called arbuscules which penetrate root cell walls to facilitate nutrient exchange. Arbuscular mycorrhizas are sometimes referred to as vesicular mycorrhizas for tiny hyphal storage structures (vesicles) or endomycorrhizas for their growth habit inside root cell walls.
Mycorrhizas also differ by the types of enzymes they produce for decomposing organic matter and the nutrients they supply their hosts. Ericoid mycorrhizas possess the most potent set of enzymes for decomposition. Because ericaceous plants commonly grow in very acidic soils with lots of organic matter (i.e. wetlands), ericoid mycorrhizas have evolved to degrade tough organic molecules like tannins, lignins, and chitins.
Ectomycorrhizas also tend to live in acidic soils, sometimes with saturated conditions and/or coniferous litter. The main ecological function of both ericoid and ectomycorrhizas is to provide their plant hosts with nitrogen. Bacterial nitrogen mineralization—the process by which bacteria degrade organic matter and release nitrogen in soluble, plant-available forms—is slow in acidic soils (pH < 5), and plant litter in these ecosystems tends to have more carbon than nitrogen.
Ericoid mycorrhizas and ectomycorrhizas help their hosts overcome environmental nitrogen limitations so long as they receive steady supplies of carbohydrates from the roots of their host plant. In contrast, most arbuscular mycorrhizal plants live in soils closer to neutral pH (between 5 and 8) where bacteria are more active and plant litters contain higher ratios of nitrogen to carbon. In these conditions, phosphorus rather than nitrogen is the limiting nutrient.
Unlike nitrogen, phosphorus is not readily available in the atmosphere and has few input sources in natural systems. Phosphorus is most available to plants when soil pH is between 6 and 7.5 but otherwise tends to bind with clay and metal ions. Arbuscular mycorrhizas help their plant hosts acquire phosphorus (and nitrogen) ionically bound to metals or locked into organic matter, but they lack the potent enzymes necessary to break down tanins, lignins, cellulose, and other complex organic molecules.
Patterns of mycorrhizal symbiosis
There are a couple general patterns of mycorrhizal symbioses in natural and/or agricultural ecosystems. First, plants rarely associate with more than one type of mycorrhiza (i.e. ectomycorrhizas, arbuscular, etc.). Aspen trees are an example of plants that can associate with both ectomycorrhizas and arbuscular mycorrhizas, but this ability is exceedingly rare in agricultural crops.
Most agriculturally important plants associate with arbuscular mycorrhizas only. As a side note, it’s common to find mycorrhizal fungi living within non-host plants. The relationships range from commensal—no net benefit to the fungi or the host—to parasitic—the fungi benefit at the expense of the host. In these instances, fungi do not form mycorrhizal structures that facilitate nutrient exchange with their hosts and live only as endophytes—a general term for fungi that live within plant tissue.
Second, mycorrhizas can often colonize a wide variety of hosts. For example, roughly 240 species of arbuscular mycorrhizal fungi are known to colonize over 200,000 species of plants. An arbuscular mycorrhizal fungus that colonizes maple trees might also colonize grasses, sunflowers, or ferns if given the chance. In other words, they’re not choosy.
Third, mycorrhizas can form networks of hyphae that enable nutrient and water exchange between plants, even of differing species. These “mycorrhizal networks” only form between fungal individuals of the same species, but they can facilitate sharing of carbohydrates, macronutrients, micronutrients, and water between plant hosts connected to the network.
Lastly, each species of mycorrhizal fungus has unique abilities to decompose substances, mobilize nutrients and water, and reproduce. Therefore, diverse mycorrhizal communities can decompose larger varieties of materials and cope with a larger range of conditions than communities with only a handful of species.
Are mycorrhizal fungi beneficial to plants on my farm?
The easy answer is yes, but it depends. It’s widely accepted that mycorrhizas improve the ability of plants to take in water, macronutrients like N and P, and micronutrients (Zn, Cu, Fe, K, Ca, and more). They also release proteins (e.g. glomalin) and carbohydrates that improve soil structure and support other soil organisms, which improves a soil’s ability to retain water and nutrients.
Mycorrhizal colonization has been found to suppress some fungal pathogens in agricultural systems including white rot, fusarium root rot, and verticillium wilt, although the degree of suppression is very context dependent. Studies have even shown that mycorrhizal colonization can improve plant growth rates. However, not all plants form symbioses with mycorrhizas.
Notably in agricultural systems, plants in the families Brassicaceae (mustards, horseradish, broccoli, kale, etc.) and Chenopodiaceae (beets, spinach, pigweeds, etc.) are non-mycorrhizal and do not benefit from mycorrhizas. Furthermore, crops that do form mycorrhizal symbioses don’t benefit equally from colonization. Generally, plants with thick, poorly branched root systems with few root hairs benefit more than those with dense, finely branched roots.
For this reason, crops like alliums, potatoes, and peppers benefit more than cereals and other dense-rooted crops. Lastly, unless you’re growing tree nuts or Vaccinium fruit, only arbuscular mycorrhizas will give your crops the benefits of mycorrhizal symbiosis. For example, inoculating your compost pile with mycorrhizas from birch or pine roots will not benefit your crops because those trees only associate with ectomycorrhizas.
How do I promote mycorrhizal growth on my farm?
Broadly speaking, you want to make decisions that maximize the abundance and diversity of arbuscular mycorrhizas for the crops that benefit most. Mycologists coined the term “inoculum potential” to describe the amount of mycorrhizal hyphae and spores available to inoculate plant roots. Colonization occurs when existing hyphal networks, hyphal fragments, or germinated spores or sclerotia—compact packets of hyphae and carbohydrates for asexual reproduction—penetrate active root tips. There are several management-related factors that can influence the mycorrhizal inoculum potential in your soils.
Ensuring that arbuscular mycorrhizal host plants are present may be the most important thing you can do to increase or maintain inoculum potential. Although some arbuscular mycorrhizas can survive unassisted more than six months, they cannot substantially grow or reproduce without hosts. Root colonization allows mycorrhizas to produce spores and increases the overwintering survival of hyphae. Avoid fallow periods whenever possible and ensure mycorrhizal cash crops or cover crops are present. If you’re growing non-mycorrhizal crops, interplanting with mycorrhizal cover crops can help to maintain fungal communities.
Tillage is another factor that can greatly influence the inoculum potential in your soil. Physical soil disturbances like plowing, rotary tilling, and general cultivation break apart hyphal networks into fragments, and the magnitude of fragmentation depends on the severity of the disturbance. Leftover spores and broken hyphae can colonize new plants, but the benefits of mycorrhizal colonization will be delayed until the hyphal networks can regrow.
Additionally, spores and hyphae can be displaced to deeper soil layers where root contact is less likely to occur. Limiting tillage will improve the chance that intact mycorrhizal networks are available to colonize your plants. Existing networks can be especially helpful for young seedlings, enabling them to immediately receive nutrients beyond the reach of their roots without the “start-up costs” of carbohydrates necessary to establish their own mycorrhizas.
Several experiments have shown that soils in reduced tillage systems have higher inoculum potential and fungal diversity than conventionally tilled soils. Mycorrhizas will perform best in no-till systems, but if you must till, limiting your tillage depth will improve the inoculum potential. One study found that spore density and richness were only slightly lower at 4–8 inches depth compared to 0–4 inches depth in both grassland and farmed soils.
Occasional but shallow soil disturbance should not drastically decrease the species richness of mycorrhizas in your soil, but you’ll still want to avoid tilling to the depth of future rooting zones to preserve mycorrhizal networks.
The types and amounts of fertilizers you use will also affect future mycorrhizal inoculum potential in your soil. Plants grown in soils with high levels of soluble nitrogen and phosphorus do not need mycorrhizal symbioses to acquire the nutrients needed for growth. Experiments show that soils with excessive and easily available phosphorus or nitrogen have low mycorrhizal colonization rates and reduced mycorrhizal abundance and diversity through time.
A host of studies shows that organically farmed soils have higher mycorrhizal abundance and diversity compared to their conventionally farmed counterparts, and differences in fertilization methods are thought to be the main cause. Most common organic fertilizers such as animal manure, compost, green manures, and rock phosphate do not detrimentally affect arbuscular mycorrhizal populations; however, excessive amounts of phosphorus can harm mycorrhizal communities regardless of the source.
Lastly, using fungicides will devastate mycorrhizal communities in your soil. This includes organically certified products containing copper. Most fungicides, depending on the concentration used, will decrease mycorrhizal populations by several orders of magnitude, and nearly all affect mycorrhizal enzyme activity in soil and roots. They’re best avoided if you’re interested in preserving healthy and diverse mycorrhizal populations.
Do I need to purchase mycorrhizal inoculum for my soil?
Sources of mycorrhizal inoculum (spores, sclerotia, and hyphae) are common throughout the environment. Even if you’ve never added inoculum to your fields, you would likely find dozens of arbuscular mycorrhizal species present. Because of the abundance of indigenous sources of mycorrhizas (i.e. those local to your farm), following the guidelines above should be sufficient for promoting healthy fungal populations on your farm.
That said, there may be situations when using commercially available inoculums might be preferable. If your soils have received excessive amounts of soluble fertilizers over multiple years, been fumigated with fungicides or steam, or been fallow for more than a season, populations of indigenous mycorrhizas may be low. Natural fungal populations will rebound given time and host plants to colonize, but you may want to inoculate with external sources if you need mycorrhizal benefits immediately.
Other instances might include urban farms isolated from natural sources of mycorrhizal inoculum or container gardens with pre-sterilized soil medium. Most products contain spores from a handful of species to get you started. You can also create your own inoculum at home by giving some of your indigenous mycorrhizas ideal conditions to grow and produce spores.
This usually involves creating a soil mixture with some native soil, growing annual mycorrhizal plants in containers with the soil mixture, and allowing the plants to winter kill. The mycorrhizas grow, reproduce, and produce overwintering spores in the soil mixture, which can be used to inoculate transplants or spread onto your garden next season.
Clever crop rotations can also increase amounts of mycorrhizal inoculum in your soil for the crops that benefit most. Leguminous cover crops (clovers, vetches, peas, etc.) are strongly arbuscular mycorrhizal and can “prime” soils with abundant and species-rich fungal communities.
For example, clover would be a better choice than buckwheat (non-mycorrhizal) to precede alliums because (1) abundant populations of mycorrhizas often form to support the nutrient needs of nitrogen-fixing plants, and (2) alliums lack the extensive root systems of other crops and benefit more from increased nutrient and water gathering capacity that comes with mycorrhizal symbiosis. From this perspective, following legumes with brassicas, beets, or chard may be a wasteful use of a resource. You may want to save abundant mycorrhizal resources for host crops without extensive root systems.
Mycorrhizas are an important part of the ecosystem on your farm. By promoting mycorrhizal growth, you can take advantage of freely available resources and improve your crop and soil health. If you’re interested in more information, see the suggested readings below and watch for new information from the farming and scientific communities. Numerous research projects into resource sharing through mycorrhizal networks, species specific interactions between fungi and plants, competitive exclusion of fungal pathogens, and more are currently happening.
Suggested further reading:
Gosling, P., Hodge, A., Goodlass, G., & Bending, G. D. (2006). Arbuscular mycorrhizal fungi and organic farming. Agriculture, ecosystems & environment, 113(1-4), 17-35.
An accessible scientific review of the affects of arbuscular mycorrhizas on plant and soil health, as well as how conventional and organic farm practices affect arbuscular mycorrhizal populations. A free PDF is available from researchgate.net.
“How to Inoculate Arbuscular Mycorrhizal Fungi on the Farm Part 1,” The Rodale Institute. https://tinyurl.com/yypfoqg6
Part 1 of a two-part series by the Rodale Institute on making your own mycorrhizal inoculum for your farm.
Rillig, M. C. et al. (2019). Why farmers should manage the arbuscular mycorrhizal symbiosis: A response to Ryan & Graham (2018)’Little evidence that farmers should consider abundance or diversity of arbuscular mycorrhizal fungi when managing crops’. The New phytologist.
A group of scientists wrote this letter in response to another group who published an article devaluing the importance of managing for arbuscular mycorrhizas in agricultural ecosystems. They make a compelling argument as to why mycorrhizas can benefit yields and ecosystem health. A free PDF is available from wiley.com.
Sam Knapp is a graduate student in Applied Ecology at Michigan Technological University and farms 1/2 acre of vegetables, specializing in winter-storage crops, in the Upper Peninsula of Michigan.
