One of the most valuable organisms your soil can have is a fungus known as mycorrhizae, which means “fungus root” in Greek. The full name of the fungus will really make an impression at your next cocktail party: vesicular-arbuscular mycorrhiza (VAM), a fancy name for a fungus that every gardener should know intimately.
Mycorrhizae are fungi that establish a graceful, symbiotic relationship with the roots of most plants. They invade the roots of vegetables, flowers, shrubs, and trees; connect them, one to the other; and then send out their filaments, called hyphae, as much as 200 times farther into the soil than the roots they colonize. Mycorrhiza has the ability to better mine this wider area for water and nutrients, especially phosphorus, which it transmits back to the roots. The plant pays for this service with the glucose the fungus needs.
This extended feeding area makes mycorrhizae-associated plants just plain healthier. They have better root formation as well as fewer root diseases and other soil pest problems. They also require less moisture and fertilizer, while showing an increased tolerance to salt.
The key to this wonderful, subterranean environment is the undisturbed labyrinth of hyphae. Tilling and even simple hoeing can rip apart this delicate lace, which can then take months or even years to reestablish. When tilling is a yearly event, the fungus will never get a foothold. Too much synthetic fertilizer can also damage the fungus, as can pesticides and, of course, fungicides. Solarizing your soil (covering it with clear plastic to let the sun’s heat kill weeds and insects) will also destroy mycorrhizae.
Soils high in organic matter are where mycorrhizae flourish, so adding compost is one of the best ways to encourage the establishment and growth of the fungi. Rather than digging in organic matter, which will destroy the fungal net, lay the compost on top of the soil and let it naturally decay into the soil.
Mycorrhizal inoculants are available from several companies, but adding these to your soil is usually not necessary unless the soil microbe population has been damaged. In addition, many strains of VAM are plant specific, and what you buy may not be the right one. Often, the products will contain a mix of many different fungi in the hope that one will match your plant.
Get our latest tips, how-to articles, and instructional videos sent to your inbox.