Did you know the largest terrestrial organism is a fungus? A single honey mushroom (Armillaria ostoya), known as the “humongous fungus,” occupies an area of 2,385 acres in Malheur National Forest in Oregon. Why does this matter? Certain fungi can form beneficial relationships with plant roots, effectively increasing the area of soil that roots can access. The fungi living in our gardens and fields don’t spread quite that large, but they can still provide our plants with significant benefits.
Mycorrhizal fungi are soil microorganisms that form beneficial relationships with most plants. There are two types of mycorrhizal fungi: ecto and arbuscular. Ectomycorrhizal fungi form relationships by infecting the outer layer of plant roots, and are found mainly in woody plants like pine trees. They form relationships with 5%-10% of plant species. Meanwhile, arbuscular mycorrhizal fungi (AMF) form relationships with about 80% of plant species. AMF are different in that they penetrate inside root cells and form arbuscules. Arbuscules are where nutrient exchange occurs between the plant and the fungi.
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When it comes to our home gardens, we are more concerned with the AMF, as they are the ones that form relationships with our vegetables and ornamental plants. Our plants are constrained to their root systems and can only uptake the necessary nutrients needed for growth if those nutrients are in close contact with the roots. This is where AMF comes to the rescue. The hyphae (“roots” of fungi) of AMF can be five to 100 times thinner than the thinnest plant roots and can travel up to 100 times further than plant roots, at a much faster rate. That means AMF can scavenge a much greater area for nutrients and can fit into the small spaces between soil particles much easier than a plant. The AMF then trade essential nutrients they’ve acquired with the host plant in exchange for sugars. Plants fix carbon from the air through photosynthesis to create sugars, so it is a great deal for the plant. AMF are obligate biotrophic organisms, which means they are completely reliant on the host plants for food in the form of sugars. In addition to providing nutrients to the host plants, AMF has been known to reduce weed pressure, provide plants with resistance to some pathogens and pests, and reduce plant stress from drought. If you’re asking me, the plants are getting the better end of the deal, but I guess if you were completely reliant on a plant you, too, would provide as many benefits as possible.
So now that we know how important AMF are for optimum plant growth, what practices can we use to take advantage of the benefits that AMF provide to our plants? Gardeners should reduce or eliminate disturbances (tilling, digging, etc.) to the soil. By disturbing the soil, the delicate hyphae networks of AMF are broken, and the effective range of the AMF is significantly reduced. If disturbances occur every year, each year the AMF will have to form new networks. Secondly, don’t over-fertilize your garden. If plants have access to an excess supply of nutrients, that will eliminate the need for nutrients provided by the AMF. Gardeners can lower their fertilizer costs and at the same reduce their potential contribution to nutrient runoff. It is a wise idea to test your soil before applying fertilizer. Lastly, only apply fungicides when necessary. Fungicides kill, well, fungi, which happen to be the “F” in AMF. Repeated fungicide application can significantly reduce AMF populations.
Some common garden vegetables that don’t form relationships with AMF are beets, brussels sprouts, cabbage, kale and spinach. If for some reason the only plants you grow are the ones I just listed, it doesn’t mean you can ignore AMF. AMF can provide benefits to your lawn and other plants around your garden. Fungal hyphae also play an important role in the formation of soil structure. Soil structure is a critical but often overlooked element of successful gardening, but that’s an article for another day.
Arbuscular mycorrhizal fungi are incredible organisms that play an important role in plant health. By taking advantage of the AMF already living in our soil, gardeners can reduce their inputs and have a more naturally productive garden.
Frank Clarke is an agricultural environmental sustainability educator for Cornell Cooperative Extension of Cayuga County. He can be reached at email@example.com or (315) 255-1183.