I've noticed a huge surge in promotion for a product called "Myke" in my area, which is an innoculant of mycorrhizal fungi. Recent gardening books recommend it, blogs praise it, and I know of several garden centres that increase their usual one-year warranty to five years if you buy this stuff and use it when you plant. Since I always feel sceptical of miracle products, I wanted to find out if this is a genuinely effective product or just a money-maker. And, as always, the answer is probably somewhere in between.
The first question to answer is what, exactly, are mycorrhizal fungi? Simply put, they are a type of fungus that colonizes plant roots. The fungus and the roots form a symbiotic relationship with each other that is mutually beneficial. In exchange for nutrients from the plant, the fungi help plants absorb more moisture, phosphorus, nitrogen, and micronutrients from the soil, protect them from some diseases and even block the over-absorption of some heavy metals. There are many different kinds of these fungi, with probably more than 2000 species that have relationships with up to 90% of plant life.
The evidence is pretty clear that mycorrhizal fungi are beneficial to plant growth, and studies have shown that plants grown in sterile soils show better root growth when the appropriate mycorrhizal fungi are able to colonize the plants' roots. However, most commercial products are intended to be used in the garden, which is very far from a sterile environment. In fact, there are probably all sorts of these beneficial microorganisms already present in your soil. There are some situations, however, that might reduce their presence. These include fallow or monoculture situations, in which the appropriate host plants might not be present, recent fires, over-tilling, over-fertilization (especially with phosphorus), water-logged soils, or the application of systemic fungicides. After disturbance, mycorrhizal fungi will recolonize soils, but some types may move slowly and take many years to return. Many urban soils suffer from one or more of these problems, so many homeowners might feel that adding mycorrhizae could be beneficial to them.
Because there are so many different types of these fungi, commercial products contain a blend, in the hopes that one of the types included will be appropriate for the plant to which it is applied. However, the appropriate fungi may not be present in the blend, or the fungi added could actually be out-competed by native mycorrhizal organisms already present in the soil. The soil itself could also be inhospitable to the introduced species, having the wrong temperature, pH, or moisture level. Gardeners who use these products should avoid using it throughout soil, and ensure that it directly contacts plants' roots, where it has the best chance of survival.
On the topic of whether adding products containing mycorrhizal fungi when planting, the Ohio State University Extension tells us that "the available evidence is very inconsistent". These products will probably not harm plants, but whether or not it will help them seems to be a bit of a gamble. However, you can encourage the presence of beneficial fungi by nurturing healthy soils, and especially by avoiding the heavy use of phosphorus fertilizers. The old advice to add bone meal when planting to stimulate root growth may actually be harmful, as the excess phosphorus could inhibit the colonization of mycorrhizal fungi on the plant roots, actually slowing root growth.
Hopefully future research into this area will give us better answers about the effectiveness of these products; in the meantime, consumers should take marketers' claims with the proverbial grain of salt. For more reading on this topic, check out the fact sheets available from the University of Alaska Fairbanks and Ohio State University.