Healthy soil is alive. 

It is a living, thriving cesspool of microorganisms feasting on decaying organic material. And that’s just the way plants like it. 

Many of these microorganisms reside in the rhizosphere, a micro-biome directly surrounding a plant’s root system. These soil microbes (e.g. bacteria, fungi, algae, etc.,) host functions beneficial to both the plant and the soil. They are known to recycle nutrients, fight disease, fixate nitrogen from the air and increase drought resistance.   

Of all the microbial life found in soil, there is one fungus in particular that has been getting a lot of attention: arbuscular mycorrhizal fungi (also called AMF). And rightfully so, these little wonders work miracles. 

These fungi create spore colonies which grow to encompass plant root systems in an ancient symbiotic relationship between plant and fungus. In fact, this relationship is so old that evolutionary botanist claim that it was mycorrhizal fungi which allowed water-based plants to first move to dry land by unlocking access to soil nutrients.  

Ancient history aside, these fungi continue to provide multiple functional benefits to their soil microbiome.

As the fungi colonizes a root system, tiny fungal filaments spread out into the soil and function as a nutrient highway, drawing in nitrogen and phosphorus from beyond the natural reach of the root system and exchanging these nutrients (i.e. N and P) for carbohydrates produced in the plant. This exchange is a critical part of the carbon cycle, allowing carbon, originally taken from the air by the plant during photosynthesis, to be recycled back into the soil.  

This is mycorrhizal fungi as a nutrient re-distribution network; working to maximize fertilizer usage by allowing plants access to previously untapped nutrients, thus making fertilizer use more efficient. 

Mycorrhizal fungi networks also work to increase the drought resistance of plants in two ways. First, as the fungal network surrounds the root system like a cotton ball, functioning as a secondary root system, the fungi draws water towards plant from beyond the root’s natural reach. Additionally, the tiny fungal threads that create this nutrient highway are able to reach water trapped between tiny clay soil particles that thicker plant roots can’t tap into, unlocking access to additional water that would otherwise be wasted.    

Yet these marvelous little fungi have still more to offer than just increasing the efficiency of the root system. Being in a symbiotic relationship, the fungi does it’s best to protect the plant. Mycorrhizal fungi are known to kill harmful nematodes that attack the plant root and some researchers argue that the fungi increase disease resistance.     

Beyond the benefits to the plant, mycorrhizal fungi are a key component of soil health. The fungal networks excrete a glue-like substance which helps build soil aggregates. These soil structures texture the soil like cottage cheese and are vital for increasing water infiltration and retention in the soil. The structures also aerate the soil, creating tiny air pockets which allow root systems easy pathways to penetrate deeper into the soil, further bolstering the drought resistance.

Mycorrhizal fungi already exist in the soil, but the health and efficacy of the fungi can be greatly affected by how the land is tended. 

Healthy fungi are not difficult to encourage; preferably, they like to be left alone. Heavy tillage and rototilling are known to break up fungi networks and soil aggregates that take time to rebuild. The less the soil is disturbed the happier the fungi are. It’s also possible to overdo a good thing; an overabundance of phosphorus in the soil (usually from too much fertilizer) can overfeed the fungi, hampering nutrient redistribution. And, obviously, mycorrhizal fungi don’t like fungicides—it straight up kills them.  

If disturbed, these fungi networks will return, eventually. Time is usually sufficient, but if heavy tillage and fungicide usage have depleted the natural presence of the microbes, the fungi may be reintroduced into the soil by using a store-bought mycorrhizal inoculate during seeding or by mixing fungi-laden soil into a compost or soil mixture which can then be spread onto the crop or garden.

As research advances, science continues to discover more and more about the intricate and complex co-dependent root-fungi relationship. While new secrets are still being unlocked, one thing is for certain, whether planting a garden or growing a crop, these friendly little fungi are key drivers of a healthy soil ecosystem.   

Refs:

Bucking, Heike., Kafle, Arjun,. (2015) Role of Arbuscular Mycorrhizal Fungi in the Nitrogen Uptake of Plants: Current Knowledge and Research Gaps. Agronomy, 5, 587-612, dio:10.3390/agronomy5040587. Accessed on May 10/2018 

Pozo, Maria, J., Azcon-Aguilar, Concepcion., (2007) Unraveling mycorrhiza-induced resistance. Current Opinion in Plant Biology. 10(4), 393-398, Dio:10.1016/j.pbi.2007.05.004. Accessed on May 10/2018

Van der Heijden, Marcel., Martin, Francis., Selosse, Marc-Andre., Sanders, Ian R., (2015) Mycorrhizal ecology and evolution: the past, the present, and the future. New Phytologist, 204(4), 1406-1423. dio.org/10.1111/nph.13288. accessed on May 10/2018

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