In the late 19th century, the german forester A.B. Frank described the relationship between fungi, specifically truffles, and the trees that they associate with. The work was undertaken in the hopes that truffles could be cultivated and the Germans could live happily ever after, knee- deep in truffles. Although things did not work out this way as so often happens, the story has come full circle, back to food.
Most seed –bearing plants in the wild, and quite a few of the sporulating type, satisfy at least some of their nutrient needs through mycorrhizal relationships. The “fungus root” is, for many genera, absolutely essential to normal growth and development. All of which comes as no surprise to OMS members. Our hobby depends largely on the fact that trees depend on fungus, and that fungus produces large, beautiful and delicious mushrooms for us to admire, collect and eat. Tuberales, Boletus, Cantherellus, Tricholoma, Russulas and Lactarius are all woodland mushrooms that exist primarily for their association with the “higher” plant life (i.e. trees). As it turns out, our vegetable plants also rely on fungal relationships to thrive.
These fungi live their entire lives underground and do not produce large fruit bodies themselves. They cannot even live without a plant host, yet they are not parasitic. They are the Arbuscular Mycorrhizae and they can go a long way towards maximizing your organic garden’s production of strong roots and lush foliage.
Arbucular mycorrhizae are endomycorrhizae meaning that they interface inside of the plants root inside its cells in contrast with the ectomycorrhizae, also known as sheathing mycorrhiza, which commonly associate with trees and do their business through the cell walls. Arbuscular approximately means “little tree”, so named because of the form the fungus takes inside the root cell approximates a silver maple.
The relationship is, as the kids say, complicated. At it’s most basic however the plant gives up a portion of photosynthesized carbohydrates in return for nitrogen, phosphorous, certain trace minerals, protection from certain toxic substances such as some heavy metals, and more water than it would be able to absorb otherwise, which renders the plant more drought resistant. The fungus is able to provide these benefits by increasing the surface area and reach of the plant’s root system and, by virtue of its unique fungal metabolism, make phosphorous, normally a “pathchy”, insoluble and inaccessible nutrient, available to the plant. Mycorrhizal fungus has also been found to be the source of glomalin, an as yet little understood exudate that holds soil together and therefore contributes to its tilth, an inexact description of a soil’s texture.
Mycorrhizal inoculation is possibly best utilized by the home gardener in an “organic” system. Studies have shown that “slow release” of nitrogen and modest application of rock phosphorous is preferable for cultivation of relationships between plants and fungi, presumably because the plant is “encouraged” to form the relationship in order to get what it needs. In field- trials of habitat restoration performed by Tim Meikle and Michael Amaranthus this type of fertilization regime boosted colonization rates from a 0- 20% range to a 16- 20% range. Outplanting success, measured by seedling survival rates was increased from a maximum of 60% for seedlings under “traditional fertigation” to a minimum of 65% in the heavily inoculated seedlings under the “alternative fertilization” regime. Compost and seed meal are examples of amendments which release nitrogen slowly as they degrade. Conventional fertilizers generally provide nutrients in a highly soluble form that plants can readily utilize without a mycorrhizal intermediary.
Commercial mycorrhizal innoculant comes in various forms and preparations. Straight spore preparations are the most common and most highly recommended as they are highly shelf-stable and most versatile. A powder form can be dusted on seeds when starting, mixed with water for a drench, used as a side dressing, mixed into potting soil or starting mix, used as a root dip, or mixed into soil when transplanting or outplanting. Soils or composts containing mycelia or spores are not recommended, not only for their high cost but also their lack of versatility and possible problems concerning the viability of the inoculant.
Perrenial plants, once inoculated, will continue to be colonized by the fungus season after season. An annual garden bed that is only lightly or not at all tilled will possibly continue to harbor spores and mycelium year after year although heavier colonization rates could probably be achieved by freshly inoculating each seasons annual vegetables. A long established, organically maintained and untilled or lightly tilled garden bed will likely already have a diverse and healthy population of mycorrhizal fungus and benefits from additional inputs may be less dramatic. However inoculation is not a binary relationship, there is a matter of degree of root colonization, which I suspect is especially true when short- season annual crops are concerned.
Although the research on arbuscular mycorrhizal fungus is less than absolutely definitive and the science is still relatively young, most of the available evidence points toward a clear- cut plant benefit from these relationships. Soil degradation through physical disturbance, contamination, erosion, over- fertilization and imprudent application of agricultural chemicals is a reality not only in agricultural settings but in our home gardens as well. Many sites could probably benefit from a deliberate bolstering of the below- ground fungal populations.
These are my peat and fiber pots where I have inoculated the seeds with Fungi Perfecti’s “Mycogrow for vegetables” the white strands are mycelia, the body of the fungus.