FACT SHEET: Mycorrhizae

The blow-by-blow on beneficial root fungi, mycorrhizas--rocket boosters for your plants.

September 29, 2003: The maintenance of viable yields in organic of low-input agriculture can be dependent upon nutrient management. Organic agriculture enhances and utilizes natural processes. Among the natural biological processes that contribute to soil nutrient management are symbioses between plants and bacteria (as in nitrogen fixation) or fungi (as in mycorrhizas).

The most important symbiosis between plants and fungi is the mycorrhiza, literally translated as a “fungus-root.” There are several types of mycorrhizas, but the most important in agriculture and horticulture are the endomycorrhizae, or arbuscular mycorrhizae. Arbuscular mycorrhizal [AM] fungi are native to most soils, and are so called because they produce microscopic tree-like structures (“arbuscules”) within cells of the root. (See photo.)

How do AM fungi benefit crop growth?

Having its roots colonized by AM fungi benefits a plant in a number of ways.

  1. Increased nutrient uptake.
  2. Increased disease resistance
  3. Enhanced water relations
  4. Increased soil aggregation

1+2+3+4 = decreased input costs + increased yields + environmental benefits

AM fungi function, in effect, as extensions of a plant’s root system. In addition to growing within the root, much of the body of the fungus, called “hyphae,” is in the soil. These filamentous structures of the fungus are more effective than root hairs at exploring the soil for nutrients such as phosphorus, copper, and zinc, which do not move through the soil solution. The fungus picks up these nutrients and brings them back to the root where they are released from the arbuscules. This enhanced nutrition contributes to increased resistance to pathogens and water stress.

The mycorrhizal fungus hyphae in the soil function in other ways to benefit plant growth and the environment in general. One way is in their interaction with other organisms. AM fungus hyphae have been shown to select for bacteria that are antagonistic to plant pathogens. Another thing the hyphae do is stabilize soil particles into aggregates, both by enmeshing them and releasing a glue-like substance called glomalin, which holds them together.

What is the impact of your farm management upon the AM fungi indigenous to your soils?

AM fungi are affected by a number of standard agricultural practices. Here are the negatives and positives of a few common practices from the soil biology point of view:

  1. Fertilization- Years of P fertilization can lead to very high soil P levels. Plants that are able to absorb sufficient P via their roots alone in high nutrient soils inhibit the spread of colonization by the fungus. This reduces the flow of sugars to the fungus which lessens the amount of the fungus in the soil. Low or no P fertilization is necessary in such soils.
  2. Winter Fallow- Another practice that negatively impacts AM fungi is over winter bare fallow. This removes potential host roots from which the fungi can receive sugar during mild fall and spring weather, thereby decreasing viability and the ability of the fungi to colonize the next crop. An over winter cover crop may not only be useful for nutrient management, but can serve as another host plant for the mycorrhizal fungi and will boost the amount of AM fungi in the soil.
  3. Crop Rotation- Continuous monocultures have been shown to select for less beneficial AM fungi. The reason for this is that the AM fungus species that proliferate with a particular crop may not be the best ones for stimulating the growth of that crop. These mycorrhizal fungi may even contribute to the yield decline seen in continuous monoculture. Therefore, it is best to grow a variety of crops in rotation. Further, some plants do not become colonized by AM fungi and therefore will depress populations of these fungi. Among these crop plants are members of the mustard family (rapeseed, cabbage, cauliflower, etc.), spinach, and lupine.
  4. Weed Control- Weeds can act as kind of an instantaneous crop rotation. Since the diversity of the AM fungus community can be proportional to the associated plant community, strict and complete weed control decreases the diversity and efficacy of the indigenous community of AM fungi. The best thing, from the soil biology perspective, is to manage weed pressure to keep it just below levels that would impact yields.
  5. Tillage- The mycorrhizal fungus hyphae in the soil act both as the nutrient absorbing organ of the mycorrhiza and as the way in which new roots are colonized. Tillage disrupts both of these functions. On the other hand, seedlings grown in no-tilled soils become colonized by AM fungi more rapidly and have greater phosphorus status than those grown in tilled soils.