Chelates are among the newest chemical compounds for preventing and curing certain plant deficiency diseases.
Sequestrene of Iron, also known as Sequestrene NaFe, the first of several iron chelates on the market, has proved to be more efficient in preventing and curing chlorosis (leaf yellowing) caused by a deficiency or unavailability of iron than such chemicals as iron sulfate and iron ammonium citrate.
Soon, other chelates containing metals like zinc, manganese, and copper will be available to help prevent or cure diseases caused by deficiencies of those elements.
Chelating Agents and their Functions
What are chelates, and how do they act? The word chelate is derived from a Greek word meaning “claw.” Chelating agents behave as though they have two claws, which are very efficient in quickly grasping and readily releasing metallic ions.
Many naturally occurring acids, including citric, malic, lactic, and tartaric, also have the power to grasp and release metals. Still, they are not as efficient as ethylene-diamine tetraacetic acid, EDTA for short, which is the basic ingredient in Sequestrene of Iron and other iron chelates.
The iron in chelates is strongly held in soluble form against the action of phosphates and other chemicals in the soil. Because it remains in solution rather than dissociated form, the plants readily absorb the iron.
Then, too, the iron in chelated form is needed in much smaller amounts than chemicals formerly used. For example, a grant of chelated iron applied to the soil around an iron-deficient tree is as efficient as a pound of iron sulfate.
History of Chelates
In 1951, Louis Jacobson was the first to report that iron EDTA was a satisfactory source of iron for growing plants in solution cultures.
The real impetus to the chelate investigations, however, was provided by Doctors Stewart and Leonard at the Florida Citrus Experiment Station.
They discovered that iron chelates would remain soluble in the soil until citrus trees could absorb them, and thus, they could restore severely chlorotic trees to full vigor. The yields of such trees increased five to ten times.
Chlorotic citrus trees in Florida have remained green for two years after applying A pound of Sequestrene of Iron.
The results were as good as those achieved with three or four annual treatments with iron sulfate.
Conditions Favoring Chlorosis
Oddly enough, the occurrence of iron chlorosis does not necessarily mean a shortage of iron in the soil.
It may occur in soils containing sufficient amounts of iron for plant growth. Still, the iron may be either unavailable to the plant or the plant may be unable to utilize it effectively.
Chlorosis due to iron deficiency is usually found under two widely different sets of environmental conditions: in crops growing in calcareous (containing calcium minerals) soils and in acid soils.
How to Use Chelates
Iron chelates are applied in either of two ways: in powdered form applied directly to the soil or dissolved in water and sprayed directly on the leaves.
For best results with soil applications, the iron chelate should be distributed uniformly over the soil surface beneath the plant and then thoroughly watered to a depth of at least 6” inches.
To assure an even distribution, the proper amount of chelated iron powder should be mixed with fertilizer or some inert material such as sand, or the required amount should be dissolved in water and sprayed or sprinkled over the soil.
Leaf Sprays
Leaf sprays should be applied as thorough cover sprays; sufficient liquid should be applied to thoroughly wet all leaf surfaces.
On outdoor plants, such sprays are best applied during the middle third of the growing season when there is little chance of harming the crop. Foliage sprays should be confined to crops, not in blossoms or fruit.
One brand of iron chelate, Iron Tetrine, used at the rate of one pound in 100 gallons of water per acre of crop as a foliar spray, was shown to be beneficial to celery, corn, cabbage, beans, watermelon, cantaloupe, eggplant, turnips, broccoli, peas, cauliflower, cucumber, pepper, tomato, squash and radishes.
The same concentration and rate of application were also found to be beneficial to the following ornamentals: gardenias, chrysanthemums, roses, rhododendrons, gladiolus, azaleas, and snapdragons.
Some of the newer hydrangea varieties appear more susceptible to iron chlorosis than the older kinds.
The green color of some chlorotic hydrangeas was fully restored in some cases within five days by saturating the soil with a solution of Sequestrene of Iron. In others, the normal color was restored after two or three weekly applications.
The chart accompanying this article details the amount of Sequestrene Iron to use in the soil or in foliage sprays to cure chlorosis in some ornamental trees and shrubs.
Several Brands Available
In addition to Sequestrene of Iron (NaFe) and Iron Tetrine, iron chelates are also available under the proprietary name Versen-Ol. There will be others.
All Yellowing Is Not Due to Iron Deficiency
Gardeners should not jump to the conclusion that a deficiency of iron causes all yellowing of leaves.
Chlorotic leaves may also result from:
- Fungus
- Virus
- Insect, or mite attack
- Low temperatures
- Toxic materials in the air or soil
- Excessive soil moisture
- Surpluses of metals such as copper, manganese, and zinc
- A lack of available iron
Correct diagnosis is therefore important to secure the preventive or curative results desired with the metallic chelates.
44659 by P. P. Pirone