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.
- Increased nutrient uptake.
- Increased disease resistance
- Enhanced water relations
- 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
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:
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.
- 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.
- 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.
- 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.
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