CORRECTION OF IRON CHLOROSIS IN CITRUS WITH CHELATED IRON

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1 20 FLORIDA STATE HORTICULTURAL SOCIETY, 1952 Thank you. Do processed citrus juices and frozen concentrates produced from parathion sprayed fruit become contaminated with parathion Beckenbach? Beckenbach: Well, a lot of very careful analytical work has been done on that problem and it just isn't any problem. There are a few cases where bare traces of parathion were found in such concentrated juices, but in most cases there was no detectable amount at all in the juices by the time they were processed. What are the problems encountered by vegetable farmers in the effective use of fungi cides, Beckenbach? Beckenbach: Yesterday many of you attended the vege table session where you saw the work that had been done in developing equipment that could put on fungicides and insecticides, but main ly fungicides, where it needs to be put on. On fighting the inroads of fungus diseases, as you well know, you have to apply your fungicides to all parts of the plant and as new growth develops on the plant you have to keep that growth protected. Of course, you can do that with a hand sprayer on a small scale. But the commercial grower of today who grows crops in vast acreages has to have the equipment to apply these products effici ently, effectively and economically. And that is a problem. As you saw yesterday, there is still a great deal of work being done to de velop equipment that can do that job. Time is drawing close here so I believe we'll make this the last question. Are there any quick field testing methods available to farmers to check the amount of spray residue on their crops? Taylor? Taylor: I would have to say no, there is not. In our inspection work and some few tests that the field inspectors can make, those are large ly qualitative tests rather than quantitative tests. What the grower would want to know is not whether there was a residue on his crop or not, because he knows that. He put it there. But he wants to know, he would have to know how much? and that would have to be done at the laboratory. I don't think the grower will ever be able to make a quan titative determination. Well, ladies and gentlemen, we thank you for your attention and I'll now turn the meeting back to President Holland. CORRECTION OF IRON CHLOROSIS IN CITRUS WITH CHELATED IRON CD. Leonard and Ivan Stewart Florida Citrus Experiment Station Lake Alfred Iron chlorosis is a serious nutritional prob lem in fruit trees and many other plants throughout the world. It is considered to be Florida Agricultural Experiment Station, Journal Series No the most difficult of all known mineral de ficiencies to correct. This deficiency is widely distributed in the Central Florida citrus-grow ing area on both acid soils and calcareous soils. It is now the most extensive nutritional deficiency in Florida citrus. The use of a chelating agent to supply iron to plants in the field is presented here as the first really successful means of correcting iron

2 LEONARD AND STEWART: IRON CHLOROSIS 21 chlorosis. This represents a new approach to minor element fertilization problems. It shows promise as a method of supplying plants with readily available forms of nutrient elements which up to now have been deficient or un available in the soil under many field condi tions. Symptoms of Iron Chlorosis The symptoms of iron chlorosis in citrus are easily distinguished from those of other nu trient deficiencies. The veins of the leaves stand out prominently because they remain dark green, whereas the interveinal areas range from light green in mild chlorosis to yellow or ivory color in severe deficiency (Fig. 1). In badly affected trees, there is heavy defoliation resulting in dieback of many limbs. Bloom and fruit set are relatively light, production of fruit is often greatly reduced, and much of the fruit fails to color properly on chlorotic trees. Severely affected trees eventually die, but may be taken out before this stage is reached, because of poor production. Cause of Iron Chlorosis The possible causes of iron chlorosis or "lime-induced" chlorosis on calcareous soils have been discussed extensively in the litera ture and they are still controversial (3, 5). Generally it is associated with high lime and alkaline soil reaction. Iron chlorosis of citrus is most extensive in Florida on acid soils, where most of the citrus is grown. The total iron content of these soils is low, ranging from 400 to 2000 pounds per acre. This would be sufficient iron for citrus trees if it were in available form. It has been shown (2, 4, 7) that ex cesses of certain heavy metals, such as cop per, manganese and zinc, will induce iron chlorosis in many different kinds of plants, including citrus. Excessive phosphate may, under certain conditions, cause iron deficiency in citrus and other plants (1, 2). The lib eral use of copper, phosphate, and possibly of manganese in citrus fertilizers for many years appears to be related to the recent rapid in crease of iron chlorosis in Florida. Iron chlorosis of citrus is caused by a de ficiency of available iron in the tree, even though it may be induced by excesses of cer tain heavy metals. It has been found to occur more extensively in locations where the soil ph is below 5.5. The regular application of lime or dolomite to maintain the soil ph at about 5.5 to 6.0 tends to prevent iron chlo rosis, probably by rendering the heavy metals less soluble in the soil. Liming has been found to be much more effective in prevent ing iron chlorosis than in correcting this dis order. Experimental It was reported in a previous publication (8) that iron chlorosis in citrus trees failed to re spond to a large variety of treatments, includ ing heavy liming of acid soils, applications of up to lbs. of ferrous sulfate per tree, use of sulfur or aluminum sulfate with ferrous sulfate, injection of solid ferric citrate into the trunks of trees, ferrous sulfate foliage sprays, and others. More recently soil applications of iron with sodium tripolyphosphate (Na5P3O10), tetrasodium pyrophosphate (NaJP2O7), and so dium hexametaphosphate (NaJPaOas) have failed after several months to correct iron chlorosis in citrus on both acid soils and cal careous soils when applied in amounts as high as 20 pounds per tree. These materials will chelate iron, and are widely used in water softening. They are different from the orthophosphates, which occur in superphosphate fer tilizers. Certain chelating agents have long been used in nutrient solutions for sand- and waterculture work, including iron citrate and iron tartrate, but these also proved unsuitable for use in the soil. Evidently a different complexing or chelating compound, which would hold iron in the soil in a form available to the trees, was needed. Salts of ethylenediamine tetraacetic acid (EDTA) proved suitable for this purpose. As reported previously (8), various amounts of iron were chelated with EDTA and applied in solution to acid soils around chlorotic cittrus trees. EDTA is a chelating compound which is widely used in industry (6), but this was the first time it had been applied to plants growing in the field. Ten or more grams of chelated iron per tree, applied to the soil, brought about greening of chlorotic trees within six weeks after application. The treated trees presented a distinct contrast with nearby untreated chlorotic trees, which re mained yellow. New flushes of growth have invariably followed applications of chelated iron to soil around chlorotic trees.

3 22 FLORIDA STATE HORTICULTURAL SOCIETY, 1952 Twenty grams of chelated iron per tree, when applied to the soil as uniformly as possi ble under chlorotic trees over the area of a circle with radius of from 8 to 10 feet from the trunk, consistently resulted in complete greening of all the chlorotic leaves on the tree. In many instances, 10 grams of che lated iron per tree produced rapid greening of chlorotic trees; in other cases, this amount resulted in a slightly slower recovery frorr chlorosis than the 20 gram application. Dry iron EDTA chelates containing from 8 to 13% iron are now being manufactured com mercially. These materials have been applied to several thousand acres of citrus and these groves are being observed. Properties of Metal Chelates. When ferrous sulfate is dissolved in water, the molecule breaks down to form two ions, the ferrous ion (divalent iron) with two positive charges, and the sulfate ion with two negative charges. Most organic compounds, however, ionize very little or not at all in aqueous solution. Since ions must be present in solution for chemical reactions to take place, the removal of ions from solution can be used to prevent undesirable reactions from occurring in the solution or in the soil. Certain organic com pounds can react with metallic ions in such a way as to form a new compound in which the metal is held inside the organic compound by strong chemical bonds. The resulting metal "chelate" compound dissociates or ion izes very little, and the bound metal remains as a part of the chemically inactive molecule. EDTA is a strong chelating compound, and when it reacts with ferric iron (the oxidized state of the element) the resulting iron chelate is very stable under acid conditions. This means that the chelate will dissociate or ionize very little and therefore will release very few of the chemically-inactive chelated iron atoms into solution in the form of chemically-active ions. The chelated iron cannot, therefore, react with phosphates or other materials in the soil which would render it insoluble or other wise unavailable to plants. The iron EDTA chelate is very soluble in water. The stability of the iron EDTA chelate is measured and expressed numerically by its "stability constant." Since ferric iron is tight ly bound by EDTA, this chelate has the very high stability constant of 10 (10 raised to the th power). All of the EDTA chelates with other heavy metals have much lower stability constants than iron, (6) a few of them being: Copper, ]0i8.3. zinc, loic.i; and manganese, These values show that the ferric iron EDTA chelate is about 107 or 10 million times more stable than the copper EDTA che late, and a billion times more stable than the zinc EDTA chelate. On a practical basis, this means that essentially all of the iron EDTA chelate applied to the soil will remain in that form, since other metals in acid soils can not displace iron from the chelate. In calcareous soils, hydroxyl ions compete strongly for the iron in the iron EDTA chelate, resulting in the precipitation of a considerable amount of the previously chelated iron as the very in soluble ferric hydroxide. This limits the use fulness of iron EDTA chelates on calcareous soils. It is not yet clear just how the iron enters the tree from the chelate. The iron may be slowly released from the chelate through an exchange reaction at the root surface as the solution bathes the roots, and then taken up by the roots before it is tied up in the soil in some unavailable form. The other possi bility is that the entire iron chelate compound may be taken up by the roots. This problem is being studied with isotope procedures. Iron Content of Citrus Leaves Chlorotic citrus leaves, after being washed with a detergent were found to contain from 20 to 55 ppm. total iron (oven-dry basis) the lower values being for severely chlorotic (yel low) leaves. Table 1 shows the mineral con tent of leaves from chlorotic grapefruit trees after receiving various amounts of chelated iron applied to the soil. All trees except the untreated ones became green. All treatments increased total iron in the leaves, although this did not increase uniformly with amounts ap plied above 20 grams of iron per tree. There was no significant change in the leaf content of N, P, K, Ca, or Mn. High rates of application, up to 400 grams of chelated iron per tree, have produced no toxicity symptoms in citrus, and the iron con tent of the leaves is generally no higher than when 10 or 20 grams of chelated iron is ap plied per tree. The effect of ferrous sulfate on the iron content of chlorotic orange leaves is shown in Table 2. The trees were severely chlorotic

4 LEONARD AND STEWART: IRON CHLOROSIS 23 (yellow) before treatment. In contrast to the large amount of iron as ferrous sulfate which was required to green-up the trees, as little as 20 grams of iron in the iron EDTA chelate produced greening of all leaves, in cluding later flushes of growth. This work has not been underway long enough to know how long a single soil appli cation of chelated iron will maintain a citrus tree free of iron chlorosis. The first trees were treated more than a year ago and are still com pletely green and growing vigorously. The effect of chelated iron treatments on fruit quality is being studied on the current crop. 100 grams of chelated iron per tree, but larger amounts up to 300 grams were re quired in some cases. These amounts are too large to be practical for grove use, be cause of the cost of the chelate. A search is now underway for chelating agents which are more efficient suppliers of iron when used on calcareous soils. Chelated Iron Applied as a Foliage Spray Iron EDTA foliage sprays have corrected iron chlorosis in some trees growing on cal careous soils, but have failed in other in- Fig. 1. Varying degrees of iron chlorosis in orange leaves. The leaf in the upper left-hand cor ner came from a severely chlorotic tree after Use of Chelated Iron on Calcareous Soils In work done to date, iron EDTA chelates have not proved satisfactory for correcting lime-induced chlorosis on calcareous soils. Greening of chlorotic trees on alkaline soils has been accomplished with soil applications of it was treated with 20 grams of chelated iron. stances. Where sprays have been ef fective, they caused the entire leaf to become green rather than developing green spots such as those produced by ferrous sulfate sprays. Iron EDTA sprays tend to burn the fruit and may burn the leaves sufficiently to cause some

5 24 FLORIDA STATE HORTICULTURAL SOCIETY, 1952 leaf drop. Further studies will be required to determine the advisability of using chelated iron sprays in groves. Leaching of Chelated Iron Iron EDTA chelate is very soluble in water. Field trials have shown that it is rapidly leached from the acid, sandy soils of Central Florida. An application of 6.5 pounds of che lated iron per acre (which is double the rate used for citrus) was all removed from the sur face foot of soil by leaching within one month after application, by about two inches of rain. This may reduce the value of soil applications made during periods of heavy rains. At this time, foliage sprays of iron EDTA chelate do not look promising for chlorotic citrus trees growing on calcareous soils, but will require further study. Iron chlorosis in citrus failed to respond to many different treatments designed to increase the uptake of iron by the trees, including some treatments that have been successful with other crops. Grams Applied par Tree Iron Total Iron Content of Severely Chlorotic Valencia Orange Leaves as Affected by Applications of Chelated and Unchelated Iron on Acid SoU. Condition of Tree Severely Chlorotic Deep Green, Healthy Material Applied llone Hone Flush Growth of Iron Leaves vm.* in Summary 9,000 Most of Leaves Became Severely Chlorotic 100 lbs.fesq4.7h2o.. «Summor A chelate of iron with ethylenediamine tetraacetic acid (EDTA) is presented as the first successful corrective for iron chlorosis in citrus in the field. When applied to acid soils at the rate of only 20 grams of iron per tree, the leaves on chlorotic citrus trees became green within six weeks. From 100 to 300 grams of iron in this chelate per tree was re quired to correct chlorosis on calcareous soils. Chlorotic trees which became green after treatment with the iron EDTA chelate showed a marked increase in total iron in the leaves, as compared with chlorotic leaves from un treated trees. There was no significant change in the leaf content of nitrogen, phosphorus, potassium, calcium, or manganese. Table 1 Mineral Composition of Pluah Leavoa from Moderately Chlorotic Grapefruit Trees Treated with Varying Anounto of Iron Chelated with EDTA Applied to Acid, Sandy Soil Grams Chalated Iron Applied per Tree N % U 3.OS Comoe >8ition P K % % of LeaTOB Ca % Mn ppm. *Moderately chlorotic leaves. All other trees, chlorotic at the start of the exoerlatent, greened up after treatment Fe 55«60 80 HO , Oven-dry basis. 200 lbs.fes04 (Ann.) 1/3 lb. Fo-EDTA Polyphosphates, pyrophosphates, and hexametaphosphates, which will chelate iron, have up to now failed to correct the chlorosis. Work is continuing in an effort to find more satisfactory chelating agents for other nutrient elements and for iron in calcareous soils. LITERATURE CITED 1. Biddulph, O., and Woodbridge, C. G. The uptake of phosphorus by bean plants with particular ref erence to the effects of iron. Plant Physiol. 27: Chapman, H. D., Liebig, G. F., Jr., and Vanselow, A. P. Some nutritional relations as revealed by a study of mineral deficiency and excess symp toms of citrus. Soil Sci. Amer. Soc. Proc. 4: Chapman, H. D., Brown, S. M., and Rayner, D. S. Nutrient deficiencies of citrus symptoms, cause and control. Citrus Leaves (3): Colehour, J. K. Unpublished data, Florida Citrus Experiment Station Guest, P. L., and Chapman, H. D. Investigations on the use of iron sprays, dusts, and soil appli cations to control iron chlorosis of citrus. Proc. Amer. Soc. Hort. Sci. 54: Martell, A. E. and Calvin, M. Chemistry of the metal chelate compounds. Prentice-Hall, Inc., New York Reuther, Walter and Smith, Paul F. The effect of copper on growth of citrus seedlings and its possible relation to acid-soil chlorosis in Florida citrus groves. Citrus Mag. 14 (11): Stewart, Ivan, and Leonard, C. D. Iron chlorosis its possible causes and control. Citrus Mag. 14 (10) :