Improve your soil, increase your yields, and reduce your expenses with AM fungi

By David Douds, USDA ARS-ERRC Researcher
Christine Ziegler, Editor

Posted June 24, 2004: Good crop yields depend, in part, on good soil nutrient management. Most farmers rely on nutrient inputs to manage soil nutrients. However, many organic and sustainable farmers also work to enhance natural biological processes in the soil in order to provide the nutrients crops need to thrive. Many of these biological processes are powered by mutually beneficial relationships (symbioses) that develop between plants and bacteria (such as nitrogen fixing bacteria) or beneficial soil fungi.

One of the most important of these symbioses develops between plant roots and fungi, producing structures called mycorrhizas. The word “mycorrhiza” translates literally from Greek to mean “fungus-root”. There are several types of mycorrhizal fungi, but the most important mycorrhizas in agriculture and horticulture are arbuscular mycorrhizas. Arbuscular mycorrhizal (AM) fungi are so called because they produce microscopic tree-like structures (“arbuscules”) within cells of the plant’s root. Remarkably, AM fungi are found naturally in most soils around the world, and chances are that you can probably find some right now in the soil under your feet.

AM fungi deliver their benefits by functioning, in effect, as extensions of a plant’s root system

AM Fungi Improve Crop Growth And Yield

AM fungi offer four important benefits to crop plants. They help to:

increase a plant’s uptake of nutrients,
increase the plant’s disease resistance,
enhance the plant’s ability to grow under drought conditions, and
improve soil structure around the root zone.

By providing these four benefits to your crop plants, AM fungi help you to:

decrease your input costs,
increase your crop yields, and
improve environmental conditions around your farm.

After colonizing a root, the fungi develop thread-like structures called “hyphae” that extend into the soil. These filamentous structures explore a far greater volume of soil than root hairs can, coming into contact with nutrients such as phosphorus, copper, and zinc that do not move easily through the soil solution. The fungal hyphae then transport the nutrients back to the root where the nutrients are released into the root cells. This increase in nutrition contributes to the plant’s ability to resist disease and avoid water stress.

As AM fungi grow through the soil, they also modify the balance of microbes in the soil. AM fungi appear to selectively enhance populations of soil bacteria that inhibit the growth of plant pathogens. This action reduces disease pressure on your crops.

AM fungal hyphae also stabilize soil particles by physically “wrapping” the particles into small clusters or clumps (aggregates), and by releasing a glue-like substance called glomalin that binds the soil particles together. Soil aggregates increase the number of empty spaces (pores) in the soil’s structure. These pores, in turn, allow the soil to hold more air (needed for root and microbial activity), and improve the soil’s ability to absorb and retain water during periods of heavy rain or snow melt. In these ways, soil aggregates promote better plant growth and reduce soil erosion.

As part of their symbiosis with plants, AM fungi depend on plant roots to supply the sugars the fungi need to grow and reproduce. Clearly, plants and fungi benefit from their symbiotic relationship, and you can benefit from it, too, by supporting AM fungus populations in your soil with fungus-friendly farm management practices.

Seven steps to cultivate your soils’ native AM fungus populations

AM fungi are affected by a number of standard agricultural practices. Here is a list of suggested practices that you can use to support and multiply the native populations of AM fungi in your soil:

DON’T Fertilize With Phosphorus- Years of phosphorus (P) fertilization have led to very high levels of P in many soils. Plants that are able to absorb sufficient P through their roots alone will prevent the establishment and growth of AM fungi in their roots by restricting the flow of plant sugars to the fungi. Over time, this action reduces the population of the beneficial fungi in the soil as a whole. In soils that do not receive P fertilizer, natural AM fungus populations help crop plants extract all the P they need from sources already existing in the soil, making P fertilization unnecessary. Eliminating P fertilizer inputs can save money, abate the environmental impacts of P mining and refining, and reduce runoff of P into surface waters.
DON’T Till More Than Necessary- AM fungal hyphae have two functions: 1) they act as the nutrient absorbing organs of the fungi, and 2) they colonize new plant roots. Tillage destroys the network of hyphae in the soil and disrupts both of these functions. To illustrate this point, research has shown that seedlings grown under no-till management become colonized by AM fungi more rapidly and absorb more P than seedlings grown in tilled soils.
DON’T Let Fields Lie Fallow In Winter- When fields lie bare in the winter, AM fungi can find no living plant roots to host them and provide sugars during mild fall and spring weather. Without enough host roots, the AM fungi populations decline and are less able to colonize the next crop.
DO Plant A Winter Cover Crop- Winter cover crops provide important host roots for AM fungi (as discussed in point 3) and are proven to help prevent soil erosion and nutrient loss. Most cover crop roots boost AM fungi populations in the soil and increase the chance that the following crop will be colonized and improved through the fungal association.
DO Develop A Diverse Crop Rotation- Continuous crop monocultures tend to propagate specific fungus species that grow best with that crop, creating less diverse AM fungus communities in the soil. In fact, the fungus species that propagate especially well with a certain crop may actually contribute to the yield decline seen in continuous monocultures. Research has shown that crops grow best when colonized by many different AM fungus species. Therefore, growing a variety of crops in rotation is the best way to develop continuous, diverse populations of AM fungi species.
DO Grow Crops That Support AM Fungi- Certain crop plants do not allow AM fungi to colonize their roots. These crop plants include members of the Brassica family (such as rapeseed, cabbage, cauliflower, broccoli, etc.), spinach, sugar beet, and lupine. If you grow these crops, crop rotation is vital to maintain the presence of AM fungi in your soil because repeated plantings of these crops will depress AM fungi populations. In order to sustain AM fungi populations while growing these crops, you can intersperse the crops with cover crops or companion plants that are more hospitable to the fungi.
DO Relax A Bit About Weed Control- Weeds can act as kind of a “mini crop rotation” within your fields. Since the diversity of the AM fungus community usually depends on the diversity of plants that grow in your soil, strict and complete weed control decreases the diversity of AM fungi. From this perspective, the best approach is to manage weed pressure so that it hovers below levels that reduce crop yields, but is large and diverse enough to support many kinds of AM fungi. Though this approach is unorthodox to modern agriculture, it can save both time and money by reducing input application and/or tillage, and it allows you to reap more potential plant health and productivity benefits from your AM fungus population.

Yes, Your Soil Probably Does Contain AM Fungi!

Research has demonstrated that even soils that have been intensively managed for long periods of time contain populations of AM fungi, and these populations will increase when treated with the fungus-friendly practices listed above. Experiments at The Rodale Institute® and other locations have shown that a single winter cover crop or one no-till cycle can increase the number of AM fungi that colonize the next crop. In fact, eight years after transitioning from conventional to organic management in the Rodale Institute’s Farming Systems Trial®, AM fungus spores had become more numerous in organic plots than in the conventionally farmed plots. (data were not gathered for earlier years in the transition).

Currently there are no “do-it-yourself test kits” available to estimate the amount of AM fungi present in your soil. This is due to the fact that laboratory equipment is required to isolate and count AM fungus spores, and propagule assays require four weeks of growth testing in a greenhouse. Laboratories listed in the resources section (at the end of this fact sheet) will measure AM fungus colonization of root samples. However, you can safely assume that your soil does contain AM fungi, and that these fungus populations will increase in response to the practices listed above.

You can also buy AM fungi to add to your soil. AM fungus inocula can be purchased for a variety of applications (see source list below). One of the best ways to use these inocula is to mix them with the potting media in which you grow your vegetable seedlings. This practice allows the AM fungi to precolonize your seedlings prior to outplanting, improving their growth from the beginning and establishing fungi populations in your field soil. Methods of “on-farm” inocula production have been developed in tropical countries, and researchers from the USDA are currently field testing an inoculum production method applicable to more temperate climates. To learn more about these developments, visit www.newfarm.org.

Using the information above, you can take immediate steps to cultivate your soils’ native AM fungi populations. AM fungi can improve your crops’ growth and yields, simply and inexpensively (essentially for free!).

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