How Do Fungi Enable Plants to Obtain Nitrogen?

About Symbiosis

• Fungi and plants share a system of interdependence because fungi, being heterotropic organisms, easily absorb elements like phosphorus and nitrogen, while autotropic plants make their own food in the form of carbohydrates through the photosynthesis process. When a fungus attaches to a plant, the plant's surface area open to nutrient and water absorption increases greatly. Conversely, the plant gives the fungus carbohydrates to use as energy. Two types of mycorrhizal fungi exist in associations with plants: ectotropic, or extracellular; and endotropic, or intracellular. The former encircles the root cells but does not penetrate them, while the latter actually enters the cells of a plant root.

Benefits to the Soil

• Perhaps most importantly, mycorrhizae offer host plants protection against certain pathogens lurking in the soil. The fungi can also bind soil particles together into clumps, both through physically wrapping the particles and by secreting a sticky substance, called glomalin. Air pockets created by the clumping hold air, water or nitrogen between the stable aggregates, where plant roots need them. As a plus, the clumping fights erosion, keeping organic matter and nutrients in the garden, rather than running off into the surrounding area.

When Fungi Fail to Help

• If the soil is already high in nitrogen, or phosphorus or other nutrients, plants can scavenge enough by themselves and do not seek fungi to colonize their roots. One plant group, Brassicaceae, the mustard family, never form mycorrhizal associations under any circumstances. Studies of vegetable gardens show that leeks benefit greatly from symbiosis with fungi, but tomatoes and peppers only rely on the fungi in years when they are low on nutrients or water. Radishes, being Brassicas, do not ever enter mycorrhizal alliances.

Encouraging Mycorrhizae

• To harness this gift of the natural environment, take steps to keep the soil's population of mycorrhizal fungi healthy. Minimize digging, especially rototilling, so as not to disturb the fungi and nutrients in the soil. Avoid the use of synthetic fungicides, as well as leaf blowers, which can remove organic material. Growing a diverse mix of plants in your garden for as much of the year as possible provides the fungi active plant roots to develop around. Particularly, an overwintering cover crop offers host roots for mycorrhizae. so they can proliferate in preparation for the spring.