Page 37 - referenceguide6th_web

PROBLEM SOILS

Problem Soils: Iron Chlorosis compared to a 4 bushel crop without a cover crop.

Iron deficiency chlorosis (IDC) is an issue that plagues Sorghum provides a bigger challenge; however, it is possible
to use different cover crops to solve the IDC issue. The covers
many farmers in regions where the soil contains high
amounts of calcium carbonate, particularly over much of prior to sorghum should be a legume like vetch, sweetclover,
or spring field peas. The plant proteins from the legumes will
the western Great Plains. The excess calcium carbonate release slowly, and since the legume also fixed additional ni-
converts iron from the highly available Fe (2+) into un-
available Fe (3+). Soybeans and sorghum are two crops trogen and decays more rapidly, it will be able to add substan-
tial amounts of biologically produced, slow release nitrogen
that are particularly affected by this malady. Many reme-
dies have been tried to help with IDC, but at best they tend to reduce the amount of fertilizer nitrogen needed.
to give temporary relief of symptoms. Another new tool to prevent IDC is inoculating with mycor-
rhizal fungi, a beneficial fungi that colonize plant roots and
then extends root-like structures up to two feet past the root
zone that absorb water and nutrients and bring it back to the
plants. This is particularly helpful in obtaining immobile nu-
trients like phosphorus, iron, and zinc, and also function to
make the plant more drought tolerant. Spores for mycorrhi-
zal fungi are now available from Green Cover Seed. Manure
and compost can also be great aids as they contain nitrogen
in organic forms that release slowly just like cover crop resi-
There may be relief on the horizon. We know that IDC occurs dues. They also contain plant available iron that are not read-
on soils with excess calcium carbonate, but it has been a mys-
tery why it occurred on some soils but not on others with ily tied up by calcium carbonate and contain organic acids
that make the iron already present in the soil more available.
equal calcium carbonate, or why it occurred on one soil in
some years but not in other years. It seems that IDC occurs Since IDC usually occurs in spots in fields, these areas should
be targeted for manure or compost applications.
where there is a combination of excess calcium carbonate and
free nitrate in the soil. Yes, that is the same nitrate that plants
use as a source of nitrogen fertilizer. Understanding why this
occurs requires an understanding of the chemical interac-
tions that takes place in the soil immediately surrounding the
roots of a plant, called the rhizosphere.
Plants must remain electrically neutral; if they take up a nu-
trient with a positive charge, they must give off a positively
charged ion to maintain neutrality. When plants take up a
negative charge, they must give off a negatively charged ion
to counteract this charge. This negatively charged ion is bi-
carbonate, which will convert iron into unavailable forms. Sorghum roots on the right side were treated with MycoApply mycorrhizal fungi.
Since nitrate has a negative charge and is taken up in large Left side is untreated. The greater root mass of the treated plant help plants avoid
the symptoms of iron chlorosis
quantities when available in soil, it results in a large amount Finally, don’t abandon the old remedies for IDC such as
of bicarbonate in the rhizosphere, and unavailable iron.
resistant varieties, in-furrow iron, and foliar iron. Though
To work around this phenomenon, it is necessary to first they are seldom sufficient by themselves, in combination
reduce the amount of free nitrate in the soil, and then pro- with cover crops, mycorrhizal fungi, and manure they can
vide nitrogen in another form. In the case of soybeans, pro- provide a synergistic effect that can recover most of the
viding another source of nitrogen is easy, since soybeans are yield potential lost to IDC.
a legume. It is only necessary to use a grass cover crop like The bottom line is that there are effective actions we can
rye or oats to soak up the nitrate and convert it into pro- take to manage IDC, and we no longer have to accept mis-
tein in the residue. This protein will later decay and slowly
release the nitrogen back. One research trial showed a 40 erable performance of sorghum and soybeans on those
areas of our fields susceptible to this condition.
bushel soybean crop where an oat cover crop was used,

37