Dr. Ted's Chunky Style Myco-bits

The benefits of mycorrhizal inoculation

Ted St. John, Ph.D.


The main benefit is improved uptake of soil phosphorus. Because of better phosphorus nutrition, mycorrhizal plants can grow much more quickly than non-mycorrhizal plants. The experiments that show this growth response are done in controlled conditions, and it is unusual for a user in the field to see responses of the kind that are often shown in scientific (or advertising) photos. The user may see a gain of a few percent up to double or triple, depending on plant species, soil factors, fertilization, and whether they already may have native mycorrhizal fungi.

Mycorrhizas appear to have only a minimal effect on uptake of nitrogen, although ECM and ericoids may have some effect. Mycorrhizas do not fix nitrogen, but some people may confuse this symbiosis with the N-fixing symbiosis between legumes and bacteria of the genus Rhizobium. There are sometimes big effects of mycorrhizas on certain micronutrients, especially zinc and copper.

Mycorrhizal plants are often more drought tolerant. This is a tricky point, since big mycorrhizal plants in pots use water much more quickly than little non-mycorrhizal plants in pots. However, it appears that it can be a very real effect in the ground. This is probably an indirect effect of phosphorus nutrition; plants fertilized with phosphorus show the same improvement. However, in natural conditions, mycorrhizal plants are certainly better suited to face dry conditions than non-mycorrhizal plants.

Mycorrhizal plants are more resistant to many root diseases. The scientific results do not all agree, and tests with certain pathogens have shown the reverse. However, in nature it appears that a range of beneficial organisms really do fight disease, and those good guys are more abundant if the plants are mycorrhizal. This explains most of the inconsistent findings, and we can now say with some confidence that mycorrhizal inoculation is an important part of a holistic disease-fighting program.

In restoration, reclamation, and erosion control, a very important ingredient is the network of mycorrhizal fungi in the soil. The network builds soil structure, which helps hold the soil together. It also allows survival of many kinds of seedlings that would otherwise never get big enough to be evident on the job site. Because some species in the seed mix show up only when inoculated, inoculation in effect increases plant diversity.

The soil network, in combination with healthy mycorrhizal host plants, is very important in resisting weed invasion. There have now been a good number of field projects that have successfully fought off weeds, where other methods that did not involve inoculation have consistently led to nightmare weed infestations. Inoculation is often not enough in itself, but must work with rapidly growing native plants and in many cases, some means to temporarily immobilize nitrate, such as a layer of straw or wood chips.