As gardeners, we often think of the soil as little more than something that holds up plants to which we sometimes need to add water and a little fertilizer. We forget its complexity and the universes within universes that it contains.
Mycorrhizae, the associations between plant roots and fungi, is one of those universes. When the ancestors of vascular plants first emerged from primordial seas, ancient fungi helped facilitate this transition. Today, almost all terrestrial plants participate in these associations. These associations are “mutualistic” or “symbioses” because in most cases, there is reciprocal dependence of both plants and fungi that results in gain and loss. For the gardener, there may be value in cultivating mycorrhizae.
There are several accepted types of mycorrhizae. The highly specialized orchid mycorrhizae make possible orchid seed germination and “feed” the young plant; the 16,000 or so orchid species are strictly dependant (obligately symbiotic) on these fungi in nature. Most of these orchid fungi are unidentified and seem to limit the distribution of orchids. Ericaceous plants such as cranberry, blueberry, azalea, and rhododendron form a fungal association that allows them to thrive in infertile, acidic soils; as a result of these associations, plants in this family survive in the subarctic, at treeline on mountains, and in the relatively harsh environment of the chaparral. One odd group of plants has no chlorophyll and cannot photosynthesize and transform solar energy into fixed carbon (sugars). Their survival is by means of an association with a fungus that acts as a bridge between the nonphotosynthesizing plant and another (photosynthesizing) plant that provides both the fungal symbiont and the dependant plant life-sustaining sugars.
The two most common mycorrhizal types that we know most about are endomycorrhizae and ectomycorrhizae, so called because of the physical characteristics of plant fungal colonization. They are quite distinct groups of fungi.
Although several different types of mycorrhizae can be found in a single habitat, some generalization can be made about mycorrhizal type and habitat. Endomycorrhizal associations tend to be found in habitats such as the prairies of the U.S. Midwest that are characterized by great plant species richness; this group dominates also tropical, deserts, and understory plants. Nutrient use in these habitats is high and phosphorus particularly may limit plant growth. The fungi colonizing these plants grow intracellularly (hence “endo-”) to form what under a microscope appear as tiny tree-like structures called “arbuscules”. The fungal hyphae (“roots”) extend into the surrounding soil to capture nutrients, minerals, and water. The fungus in essence increases the surface area of the plant root and amplifies the plant’s ability to absorb nutrients and water.
The boreal forests of the northern hemisphere are composed of nearly monospecific stands of firs, pines, spruces, hemlocks, and larches. In this sort of habitat, the plant–fungus associations are mostly ectomycorrhizal. Fungi grow between root tip cortical cells and form a dense mat that covers the root tip and extend into the soil where inorganic nutrients (such as nitrogen, phosphorus, and potassium) and water are absorbed and translocated to the host plant. The mushrooms and puffballs seen in forests are the fruiting bodies of these fungi.
There are as many as 5000 species of these ectomycorrhizal fungi. In the relatively plant-species poor habitats where they are found, they are often obligately dependant on a specific host plant. In contrast, endomycorrhizal fungi that grow in habitats with great plant diversity, tend to have broad host ranges, meaning that they can colonize many different kinds of plants, sometimes simultaneously, and they are able to survive independently of a host plant. About 80% of all mycorrhizal associations are endomycorrhizal. There are only 150 known species in this group; it belongs to a single fungal order, Glomales.
In nature, these fungal associations provide plants a competitive edge and are probably essential to the survival of most plants, but a well-fertilized and watered garden may be more or less free of mycorrhizal fungi. Mycorrhizal associations are a two-way street. The fungus facilitates plant absorption of nutrients and water, but in return, the plant translocates sugars and, under drought, water to its fungal partner. What’s translocated to the fungus may be as much as 30% of the plant’s net fixed carbon. In a well-managed garden, the partnership may not be worth the cost to the plant.
There are other benefits to the plant and the gardener, though. Mycorrhizal fungi improve soil physical structure and composition. Individual hypha is only about 0.0002 inches in diameter but can extend across several acres. While hyphae may account for only a few percent of the total soil mass, there can be several miles of hyphae in a single ounce of soil. Hyphae increase soil porosity, which promotes air and water conductivity within soil. Fungi also enrich soil by converting plant sugars into fungal storage carbohydrates that are deposited in soil.
Mycorrhizal fungi interact with other soil organisms, including pathogens, and may reduce the incidence and severity of root diseases. In the ectomycorrhizal associations, the fungal mat covering root tips may act as a mechanical barrier to root pathogens. But these fungi may also produce antibiotic compounds that inhibit root pathogens, compete effectively with pathogens for the same limited resources, and induce in the plant a generalized defense response.
Legume plants (peas, beans, lupine) fix atmospheric nitrogen when colonized by rhizobia bacteria. The interaction between mycorrhizal fungi and rhizobia seems to promote the colonization of legumes by both fungus and bacterium, resulting in greater plant nitrogen and phosphorus content when present in combination than when either is alone used to inoculate the legume.
The concept of the sustainable garden is in the news. Sustainable gardens can take many forms and might be organic or might focus on planting local, native plants only. But whatever, improving plant growth, reducing or eliminating fertilizer and water additions, and suppressing root pathogens by cultivating mycorrhizal fungi is in accordance with this idea.
Mycorrhizae clearly have many potential advantages for gardeners, but the unwelcome news is that experimental work with mycorrhizal fungal inoculation is equivocal. While there has been certain promise in forestry and agriculture, in general, results are often unpredictable or unsustainable. There are a number of products available, but to my knowledge their efficacy is unproven.
Currently, the best advice for cultivating garden mycorrhizae may be to follow a few cultural guidelines. Flooded soil, chemical fertilizers, liming, systemic fungicides, and fallow fields suppress soil fungal flora. Some cultural practices promote and help to maintain mycorrhizal fungi. Crop rotation, mulching and minimal soil disturbance (minimum tillage), and use of cover crops are among these. For an overwintering garden, mulching is an excellent practice. Shredded hardwood or conifer bark make great mulches, are readily available in most parts of the country, and will support fungal growth.