Two of the most
important relationships between microorganisms and domesticated plants are the
relationships between the nitrogen-fixing bacteria called Rhizobia and the
mycorrhizal fungi called VAM fungi with vascular plants. The relationship
between Rhizobia and the so called nitrogen fixing plants such as the mesquite
and acacia can be appreciated mostly from the research done on Rhizobia and
leguminous crop plants like the peas and beans.
If a soil contains the Rhizobial bacteria, then legumes planted into
that soil can benefit from this bacteria’s ability to take inert nitrogen gas
from the air and oxidize it, or fix it, into nitrate, a form of nitrogen usable
by the plant. Soil devoid of Rhizobial bacteria need to be inoculated with a
“starter” population so that legumes planted in it can take full advantage of
this symbiotic relationship and the “free” nitrogen it provides to the plant.
Of course in return the bacteria in this relationship receive nutrients and
other benefits from the plant.
But the discovery of
mycorrhizal fungi is relatively new to researchers when compared to the time
when nitrogen fixing bacteria were discovered and explored. Unlike Rhizobial
bacteria which live in close association with legumes, VAM type of fungi infect
almost all higher plants. Also unlike Rhizobia, VAM fungi penetrate the living
cells of plants and form the organs called vesicles and arbuscles in the root.
By doing so, the fungi link the plant to the soil, assisting in the transport
of mineral nutrients, particularly phosphorus, to the plant. In return, carbon
compounds from the plant are released into the soil where soil microorganisms
rely on these compounds as an energy source. The plants in both cases become
home to these invading microorganisms and a relatively safe and necessary place
for them to live.
It is a widely held belief
among researchers that VAM mycorrhizae can play an important role in poor or
eroded soils. Researchers believe that VAM fungi can play a major role in
increased agricultural production particularly when soil nutrients have been
depleted. For this reason we might be able to imply their importance when establishing
landscape plants in poor soils. The known roles of VAM fungi are the increased
absorption of minerals, stimulation of growth, increased production,
enhancement of resistance to environmental stresses and soil diseases.
Although VAM fungi stimulate
phosphorus uptake of plants from the soil, over application of phosphorus to
the soil actually inhibits VAM growth. Concentrations of phosphate phosphorus
over 50 ppm in the soil have been reported to severely inhibit VAM
colonization. However, the application of what we would consider normally
insoluble phosphates, such as rock phosphate and bone meal, are effective to
maintain VAM fungi. VAM fungi aid in making the application of this type of
phosphorus, normally mostly unavailable to plants, available to the plant as a
slow release source. An additional advantage of insoluble phosphate with VAM
then is the long- term availability of phosphorus, compared to more soluble
phosphates.
Besides increasing the
availability of phosphorus to plants, VAM fungi-infected plants have high a
potential for resistance to soil and environmental stresses. According to
fossil records, VAM fungi infected roots of Early Devonian land plants over 400
million years ago. The Early Devonian period is recognized as a time when plants
invaded the land from the oceans. Researchers have speculated that VAM mycorrhizae
may have played an essential role in assisting ocean inhabiting plants in their
advance toward terrestrial living by helping them to adapt to a new stressful
and continuously changing environment.
Poor soils include soils
naturally devoid of organic matter either naturally or through erosion, and
those intensively cultivated or subjected to excessive use of fertilizers and
agrochemicals. In the past to maintain economic levels of productivity, farmers
were forced to increase the use of inputs such as agrochemicals. With the use
of VAM mycorrhizae, agricultural production inputs such as fertilizers and
agrochemicals could be reduced. In landscapes, decreased use of chemicals such
as fertilizers and pesticides would probably be standard practice. Instead,
more concentration on enhancing the soil environment to promote soil
microorganisms would be recommended.
In general, the population
of VAM spores in poor soils, such as the soils damaged by severe dryness or
erosion, is very low. VAM has been known to develop well in close association
with grasses. Grasses are very effective for the revival of soil with low a
density of VAM fungi. VAM develop well in orchards where grass is used for a
sod. However, some growers believe that a clean culture, bare soil, is best for
high quality. This is true among growers of landscape plants as well. Thus our
soil management system must be re-evaluated in light of this information.
When concentrating on
enhancing the soil environment for VAM development it was mentioned above that
the amount of agrochemicals and applied chemical fertilizers be reduced. Other
agrochemical inputs would have to be addressed as well. The application of several
kinds of fungicides, such as copper fungicides, iprodione, iprodione-like
compounds, benomyl and benomyl-like compounds, have been shown to severely
inhibit VAM growth. The application of herbicides may also inhibit the
intensity of VAM infection by decreasing the population of specific so called
weeds which strengthen the activity of VAM fungi.
Although several kinds of
VAM fungi have been known to have the resistance to adverse pH conditions, most
of the VAM fungi are vulnerable to high pH soils, preferring pH ranges less
than 7.5. The growth of VAM fungi also is poor in anaerobic soils. Although VAM
fungi infect the roots of aerenchima-developing plants such as mangrove and
paddy rice plants the percentage of VAM infection is generally low
The application of uncomposted
organic matter severely inhibits VAM development. Applications of composted
organic matter, however, stimulates VAM formation and the growth of plants. One
of the VAM stimulators is low concentrations of ethylene. Ethylene evolves from
composting organic matter and, at very low concentrations, may be effective for
VAM growth. About 0.5 ppm ethylene gas stimulates VAM hyphal growth and VAM
development in association with plants. But ethylene at concentrations above
0.5 ppm can depress hyphal growth and mycorrhizal formation. In soils with poor
aeration where uncomposted organic matter has been applied, ethylene is often
detected at concentrations high enough to inhibit VAM growth.
Activated charcoal incorporated
into soils has been shown to be very effective in VAM development. But excess
applications of charcoal may inhibit plant growth. The reason for this
inhibition is not known but is thought to be associated with changes in the
soil chemistry. Therefore, an appropriate amount of charcoal to be applied is
less than eight tons per acre which works out to be somewhere around one third
of a pound per thousand square feet. Zeolite is a mineral soil amendment which
shows some promise in assisting with the development of mycorrhizae without the
problems of that are associated with charcoal. It absorbs antagonists which
inhibit VAM growth without the problems associated with charcoal.
