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“even the best
mycorrhizal fungi work
better when combined
with phosphorus
solubilizing bacteria.”
and extend the root zone by sending out threadlike hypha into the
surrounding soil. In exchange for
the plant’s sugars, the fungi unlock
unavailable phosphorus from the
soil and feed it to the plant. But
even the best mycorrhizal fungi work
better when combined with phosphorus solubilizing bacteria. The
bacteria hitch a ride on the fungi as
it penetrates the soil, and swim to
places in the soil solution that the
fungi can’t reach. As the bacteria
reproduce, they exude organic acids
and enzymes that unlock phosphorus from soil particles and organic
matter and feed it to the fungi. The
phosphorus solubilizing bacteria
also colonize the developing root
hairs to directly feed the plant. So
when choosing inoculants to use as
a “starter blend”, make sure that it is
dominated by beneficial bacteria.
In addition to mineralization, some
plant-growth-promoting rhizobacteria actually make rooting hormones
directly on the surface of the roots.
For example, as plants grow they
exude amino acids from root cells.
One of the amino acids is called tryptophan. Beneficial microorganisms
such as Bacillus subtilis and Bacillus
firmus feed on the tryptophan and
change it into a powerful auxin called
IAA (indole acetic acid). The auxin
stimulates new root growth. Other
microorganisms on the dream team
are especially adept at producing
growth hormones called cytokinins.
Cytokinins stimulate cell division. So
the combination of auxins and cytokinins work together to stimulate more
lateral root growth and more root
mass. The results? Better root strike,
more efficient uptake of water and
minerals, and a healthier and more
stress resistant plant.
Speaking of stress, did you know
that some beneficial microorganisms
can actually rescue plants from stress?
During times of heat stress, drought
stress or UV stress plants produce
ethylene gas. The ethylene signals the
plants to stop producing new root and
shoot growth and the plant temporarily shuts down. But some microorganisms produce an enzyme called ACC
deaminase that blocks the production
of ethylene and rescues normal plant
growth. The plant recovers more
quickly from stress and continues to
grow and reproduce instead of going
into shock. Therefore, any good
“maintenance blend” of microbial
inoculants should contain a healthy
dose of ACC+ bacteria.
Some microorganisms have been
isolated from disease suppressive soils
and provide an extra level of protection from pathogens. For example,
Bacillus subtilis GB03 is registered as a
bio-pesticide and helps protect plants
against root rot. Other beneficial
microorganisms such
as actinomycetes and Pseudomonas
fluorescens provide protection against
a wide range of pathogens, including
infectious bacteria, fungi and even
viruses. But one of the best defensive
players on the microbial dream team
is trichoderma sp. Trichoderma is
actually a beneficial fungus, but it parasitizes and feeds on other fungi! It
also helps feed the plant and activates
the plant’s natural immune system.
Generally speaking, it’s much
better to inoculate plants with
multiple forms of plant-growthpromoting rhizobacteria than it is to
use a single strain. For example, P.
fluorescens has been isolated from
disease-suppressive soils, but when
used alone can decrease yields. It’s
also been found that some yeasts
(Saccharomyces cerevisiae) don’t play
well with our friendly trichodermas.
So choose carefully when selecting a
polymicrobial blend, and know what
you’re looking for. If you’re using
water soluble nutrients and choose
a blend that’s loaded with endo- and
ecto-mycorrhizal fungi, you are wasting your money! By using the perfect
blend of growth promoting and disease suppressing microbes, you
can have the best of both worlds!
It’s also important to choose a
blend that’s formulated specifically
for the correct stage of plant growth.
A good “starter blend” should be
loaded with phosphorus solubilizers
combined with a healthy complement of growth hormone producers. On the other hand, a good
“bloom” or “maintenance blend”
should be better at stress protection and growth promotion. Some
microbes even produce a bouquet
of volatile organic compounds
that help the plant produce more
blooms and bigger blooms. Volatile
organic compounds produced by
specific bacillus sp. can activate up
to 600 different growth regulating
genes in the plant! By switching on
the genes that tell the plant to keep
manufacturing and storing sugars,
it is possible to keep your favorite
plants biologically active all the way
to the day of harvest.
In short, microorganisms are the
biostimulant factories, with different
microorganisms performing different roles at different times. The
right team of microbes can help your
plants reach their true genetic potential. So get to know the key players in
the root zone. The more you know,
the better you’ll grow!