Fertilizer doesn't feed plants. The sun feeds plants. And seventeen chemical elements--14 derived from the soil and 3 from air and water--help plants use the sun's light to build carbohydrates to use as food. Yet people still feed their plants with high-nitrogen fertilizer to make them fat and glossy. The results are often increased insect and disease attack, water pollution, reduced drought tolerance, and in the case of mature trees, even decline and death.
What if you could reduce fertilizer use and instead offer your garden's plants a way to help themselves? That is what mycorrhizal inoculation does. In the past two years, products containing mycorrhizal fungi have begun to appear in garden centers, home improvement stores, and mail order catalogs.
Don't let the big words worry you. They mean simply to place spores of special fungi in the root zones of new and established trees, shrubs, annuals, and perennials. Dr. A. B. Frank, a German scientist, coined the term "mycorrhizae" in the late 19th Century from the Latin word myco (fungus) and Greek word rhizae (root). The fungi colonize plant roots and, in effect, quickly create masses of new "fungus roots" that dramatically increase the plant's ability to absorb water and nutrients and fight off disease. Fine strands called hyphae penetrate soil particles that plant roots can't reach. At the same time, the fungus feeds on sugars from the plant.
Surprisingly, the vast majority of land plants naturally form symbiotic relationships with mycorrhizal fungi. Pull up a wild seedling most anywhere, and looking closely enough you will likely find mycorrhizae. The only places not rich in mycorrhizae are those disturbed by human activity. In fact, mycorrhizal inoculation was first developed during the 1970s to help reforest spoiled lands such as strip mining sites. As it turns out, mycorrhizal inoculation is equally valuable for many urban and suburban soils, where drought and compaction are problems.
Recent studies show that the addition of mycorrhizal fungi measurably helps young and established trees, shrubs, annuals, and perennials growing in soils typical of roadsides and garden edges. My experience with Urban Arborists in New York City, suggests that--particularly with mature oaks in park and roadside settings where slightly acid pH is maintained but where soil compaction is extreme--placing mycorrhizal inoculants in auger holes can prolong the life and improve the looks of the trees. Less dieback seems to occur on branches, and the trees leaf out vigorously and appear better able to withstand drought.
The fungi are not harmful to plants or animals, and typically you only have to inoculate once, at planting time, by mixing the fungal inoculant into the planting hole. For dosage, follow product recommendations.
Two main types of mycorrhizal fungi are available: ectomycorrhizal and endomycorrhizal. (Mycorrhizae is the symbiotic association of the fungi and plant roots.)
Ectomycorrhizae form mostly outside a plant's roots. They colonize the roots of many common trees and shrubs including alder, arborvitae, aspen, basswood, beech, birch, chestnut, chinquapin, eucalyptus, fir, hazelnut, hemlock, hickory, larch, oak, pecan, pine, poplar, spruce, and willow. If they appear aboveground at all, it is usually in the form of small puffballs or mushrooms with dusty brown spores. But if you carefully dig into the root zone, you will likely find masses of white or orangish "roots" growing out of the plant's roots. These are the ectomycorrhizae, and in exchange for the plant's feeding them with sun-derived carbohydrates, these fungal roots exponentially increase the plant's ability to draw what it needs from the soil. Indeed, it is now thought that without mycorrhizae, many species of trees and shrubs might never have survived at all.
Endomycorrhizae form mostly inside a plant's roots. This is the most widespread type of mycorrhizae. The list of plants they live with is long, reaching from bamboo to camellia, and from apple trees to fescue grass. Their hyphae do grow extensively out of the roots. For the most part, endomycorrhizae are invisible to the naked eye, though if you managed to remove the soil from around the roots of plants host to these fungi, you could see them with a microscope. (They would look similar to the fine roots on the plant.)
Vesicular-arbuscular mycorrhizae (VAM), a subgroup, occurs naturally in 90 percent of all vascular plants, including agricultural crops, grasses, desert plants, flowers, citrus, and shrubs. Other subgroups are more specialized and grow only with orchids, for example, where they are necessary for orchid seeds to germinate. Still others associate only with ericaceous plants (heaths and heathers).
Mycorrhizae provide the most benefit to long-lived trees, shrubs, and perennials planted in less intensively maintained soils. In heavily fertilized perennial or vegetable beds, mycorrhizae are of least benefit. Nitrogen and phosphorus fertilizers in particular inhibit their growth.
Adding VAM fungi to soil has been shown to improve growth of some slower-growing annuals and vegetables. The fungi penetrate the plant' s root cells, resulting in bushier plants with higher yields of fruits or vegetables.
For established trees, shrubs, and perennials, lightly cultivate mycorrhizal inoculant into soil within the drip line. Alternatively, dig holes around the drip line and backfill with inoculant, or inject it into the root zone.
Mycorrhizae are not a miracle cure -- any more than nuking plants with nitrogen is nourishing them -- but using these fungal roots is an important step toward healthy plants that live off the soil rather than doses of fertilizer.
* There are 150 different species of vascular-arbuscular endomycorrhizal (VAM) fungi and hundreds if not thousands of ectomycorrhizal fungi. Only three ericoid mycorrhizal fungi are known.
* Mycorrhizal fungi are more specific to the soil than to plants. If your soil is poor quality -- dry and compact, with little or no topsoil -- adding mycorrhizal fungi will likely enhance your plants' ability to find and extract nutrients.
* Mycorrhizal plants are much better at extracting phosphorus from soils where the nutrient is unavailable due to pH.
* Food crops known to respond to endomycorrhizae include apple, artichoke, celery, corn, eggplant, fig, garlic, grapes, onion, pistachio, potato, soybean, squash, strawberry, and tomato.
* Older varieties of plants often respond more favorably to mycorrhizae than newer hybrid varieties.
* To determine if mycorrhizal fungi are present in your soil, send a soil sample to a lab for biological testing and analysis. The test results will tell you how to amend your soil for optimum plant health.
Arborist William Bryant Logan lives in New York and is the author of The Tool Book (Workman, 1997; $40).
Article published on June 23, 2008.