Mycorrhizae are fungi that form symbiotic relationships with nearly 90% of plant species on Earth. They serve multiple roles from assisting in phosphorus and water uptake to increasing resistance to pathogens. A nice overview of mycorrhizae in agriculture by R.M. Muchovej of the University of Florida, including great photos of different kinds of mycorrhizae can be found here.
Sir Albert Howard, one of the pioneers of “organic” agriculture, argued that mycorrhizae were a critical component of plant nutrition– bridging the gap between plants and “humus.” Some of his hypotheses about how mycorrhizae work were incorrect, but his attention to the role of soil biology in plant nutrition is now getting some vindication. Mycorrhizae are indeed incredibly important to plant nutrition and growth for many crops.
Cover crops can influence the populations of mycorrhizae by providing a host plant during times that would otherwise be bare fallow. Even weeds can serve an important ecological role in hosting mycorrhizae.
One of the few families of plants that does not form mycorrhizal associations is the Brassica family. When Charlie White was a graduate student at the University of Maryland, he asked the question of whether Brassica cover crops (esp. forage radish) actually hurt mycorrhizal populations because in addition to being non-hosts for mycorrhizae, Brassicas are known to have anti-fungal compounds. His results showed that although forage radish does not host mycorrhizae, it does not negatively impact mycorrhizal populations. Read his full research article.
Inoculation to Increase Mycorrhizae Populations?
Although there are many commercial products claiming to boost mycorrhizal populations in soil, there is little evidence of the efficacy of these products because until recently they have been relatively untested.
Our understanding of cover crop effects on mycorrhizae and mycorrhizae effects on cash crops will undoubtedly continue to grow as scientists and farmers continue to explore these essential relationships.