Report on the Agricultural Experiment Stations, 1953 Page: 60
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60 REPORT ON EXPERIMENT STATIONS, 1953
It has long been realized that applications of inorganic iron salts
to the soil are not always effective in preventing iron chlorosis of
plants. Foliar sprays likewise are sometimes not effective.
Although the methods of placing iron salts in holes and of injecting
iron solutions into trees have produced satisfactory results, they are
not practical for large-scale use. At the Florida station it was found
that chelating agents, which form stable complexes with iron, were
very effective when applied to the soil. A single application of chelated
iron (ferric sodium ethylenediamine tetraacetate, known as
EDTA), made during the dormant season, resulted in complete correction
of iron chlorosis within 6 weeks. New leaves were green, and
appeared 2 or 3 weeks earlier in the spring than on untreated chlorotic
trees. Response was obtained from as little as 6 grams of chelated
iron applied to the soil around deficient orange trees, and more than
twice as much iron was found in the leaves of these trees than in
those of untreated ones.
Large soil applications of ferrous sulfate resulted in only slightly
more iron in the leaves than where no iron was used. The Florida
scientists believe that this is because the iron in inorganic salts, such
as ferrous sulfate, is precipitated as the very insoluble hydrated ferric
oxide, and thus becomes unavailable to plants. Iron applied in the
chelated form is not free to react in the soil, and thus is not rendered
unavailable by chemical precipitation but remains in a form which
plant roots can utilize.
The North Carolina station found that certain metal ions have
the ability to activate the enzyme system which brings about the
reduction of nitrates in plants. It was established that this nitrate
reductase enzyme (from soybean) can be inactivated by the metal
chelating agent EDTA, and then reactivated by certain metal ions.
The most effective activators were manganese, iron, zinc, and magnesium,
listed in order of decreasing activating ability.
Inorganic iron salts did not give satisfactory control of chlorosis
of the ornamental plant Dracaena sanderiana, and export of this plant
from Puerto Rico was threatened. Experiment station researchers
found, however, that good control could be obtained with iron EDTA,
either when applied as a foliar spray or when added to the nutrient
solution in the greenhouse. Sprayed plants recovered from severe
chlorosis in 2 to 3 weeks, but recovery was slower for those receiving
the EDTA in the nutrient solution. After 2 months there was no
difference between methods of application, and all plants were suitable
for export. Plants treated with ferrous sulfate showed improvement
but never reached export quality. The new growth from
these plants was suitable, however, for export.
Spray applications of iron EDTA at the California station gave
good control of iron chlorosis of pears growing on calcareous soils.
Both surface and subsurface soil applications of this material, in
amounts up to 4 kilograms per tree, have given no response to date
under field conditions. It was found also that caution must be exercised
when using iron EDTA as a spray on different plants because
the range between ineffectiveness and toxicity is quite narrow.
Utah station scientists found that bicarbonate and calcium ions
were the only ones present in large quantities in soils where iron
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United States. Office of Experiment Stations. Report on the Agricultural Experiment Stations, 1953, book, 1953; Washington D.C.. (digital.library.unt.edu/ark:/67531/metadc5989/m1/62/: accessed November 18, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.