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
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
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
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
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
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!).