Overstory #8 - Mycorrhizae--Essential Partners in Plant Health
People have taken plants out of their natural environments to cultivate them in agricultural systems such as plantations, orchards and farms. However, we have not changed the plant's basic needs. Plants co-evolved with soil life over hundreds of millions of years. Many plant and tree species adapted to depend on some kinds of soil microlife. Soil fungi known as mycorrhizal fungi make vital contributions to plant and soil health. Mycorrhizae simply means, "root fungus," and refers to fungi which live in close association with the root systems of plants, extending out from the plant's own roots. By nurturing and, where necessary, re-introducing mycorrhizal species, we can cultivate an essential ecological connection that will benefit our crops, our environment, and our own bottom line in agriculture or forestry.
Mycorrhizal fungi have evolved in association with plants, acting to greatly increase the ability of plants to take up water and certain nutrients, while often protecting associated plants from pests and diseases. They live in and around the root zone of plants, extending far out from the plant's roots with their own network of thread-like filaments known as hyphae. This greatly extends the effective surface area of the plants roots. The association with mycorrhizal fungi greatly enhances plant health in most species, and has even become essential for the survival of certain types of plants. Avocados, bamboos, bananas, cassava, coconuts, coffee, mahogany, mangoes, palms, papayas, soybeans, and sweet potatoes are just a few examples of plants that benefit from mycorrhizae.
One challenge in restoring degraded lands is that many important soil organisms including mycorrhizal fungi have died off or are dormant. Mycorrhizal fungi are particularly vulnerable because they can multiply only while living on the root system of a host plant, and so they cannot survive long in bare-soil conditions. Nor can they thrive in conditions where soluble fertilizers have been used continually for many years.
Poor mycorrhizal activity is a problem many people would like to remedy in forestry, organic agriculture, and land reclamation. There is currently a great interest in restoring these degraded lands by replanting a diverse range of vegetation. In order to have healthy plants, we must also make sure that essential soil microorganisms are present. This can be accomplished in part by nurturing and re-introducing mycorrhizal fungi.
How Mycorrhizal Fungi Benefit Plants
Better uptake of nutrients: With the help of mycorrhizal fungi, a plant can take up many times more nutrients, particularly phosphorous, than would be possible in the absence of the fungi. When a dependent plant lacks mycorrhizae, growers often have to load the soil with high levels of soluble nutrients. This heavy feeding is expensive, and further damages the health of the soil and water.
Soil improvement: Mycorrhizae enhance the soil by by improving the structure of soil. This helps to increase water holding capacity, and traps nutrients that otherwise could be leached by rains.
Faster rehabilitation of degraded sites: Because they enhance the plant's ability to take up nutrients and water, mycorrhizal fungi can help plants compensate for low nutrient availability, poor soil structure, low water holding capacity often prevalent on harsh sites.
Healthier plants, less disease and fewer pests: Most experts in integrated pest management say that plant health is the most important aspect of pest management--healthy plants have much fewer pest problems. Better nutrition and water uptake through mycorrhizae helps plants stay healthy.
Biocontrol of certain pathogenic organisms: By infecting the root system of a plant, mycorrhizae can interfere with pathogenic organisms, effectively protecting the host plant from diseases.
Tolerance for problem soils: Mycorrhizal fungi may also help regulate the uptake of soil toxins, allowing plants to better tolerate salty or problem soil conditions.
Strategies for Improving Mycorrhizae Activity on Your Site
- Use green manures and mulch. Using plenty of organic matter will foster beneficial soil microorganisms.
- Refrain from using soluble chemical fertilizers, especially those which have high levels of phosphorous. Use organic fertilizers when possible.
- If the soils are degraded, consider adding a small amount of soil from a nearby healthy forest area to each planting hole to "inoculate" the soil with healthy microlife.
- For badly degraded sites, plants can be inoculated with commercially-available mycorrhizae prior to planting.
David M. Sylvia, Mycorrhizal Symbioses, a very easy to read introduction to mycorrhizae.
A Practical Introduction to Mycorrhiza
Working with Mycorrhizas in Forestry and Agriculture - Introduction to the structure, development and function of mycorrhizas.
Special thanks to Dr. Mitiku Habte at the University of Hawaii for his feedback in writing this article.
About the Authors
Kim M. Wilkinson is the Education Director for Permanent Agriculture Resources and editor of The Overstory. She has B.A. degrees in Anthropology and Ecology from Emory University.
Craig R. Elevitch is an agroforestry specialist with more than ten years of public and private sector experience in tropical agroforest and forest management. He has a M.S. degree in Electrical Engineering (Dynamical Systems) from Cornell University.
Related Editions to The Overstory
- The Overstory #78--Reforestation of Degraded Lands
- The Overstory #70--Rhizosphere
- The Overstory #61--Effects of Trees on Soils
- The Overstory #42--Improved Fallow
- The Overstory #33--Mushrooms in Agroforestry
- The Overstory #28--Microlife
- The Overstory #20--Five Fertility Principles