20 FLORIDA STATE HORTICULTURAL SOCIETY, 1952

Dr. Bellows:

Thank you. Do processed citrus juices and

frozen concentrates produced from parathion

sprayed fruit become contaminated with

parathion Dr. Beckenbach?

Dr. 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.

Dr. Bellows:

What are the problems encountered by

vegetable farmers in the effective use of fungi

cides, Dr. Beckenbach?

Dr. 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.

Dr. Bellows:

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? Dr. Taylor?

Dr. 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.

Dr. Bellows:

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. 122.

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

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 25 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 ortho-

phosphates, which occur in superphosphate fer

tilizers.

Certain chelating agents have long been

used in nutrient solutions for sand- and water-

culture work, including iron citrate and iron

tartrate, but these also proved unsuitable for

use in the soil. Evidently a different complex-

ing or chelating compound, which would hold

iron in the soil in a form available to the trees,

was needed. Salts of ethylenediamine tetra-

acetic 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 cit-

trus 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.

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 1025 (10 raised

to the 25th 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,

1013.4 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

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

24 FLORIDA STATE HORTICULTURAL SOCIETY, 1952

leaf drop. Further studies will be required

to determine the advisability of using che-

lated 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.

Summary

A chelate of iron with ethylenediamine tetra-

acetic 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 Spring 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

None

12

15

20

30

40

60

120

N

%

3.11

3.U

3.OS

2.89

2.86

2.98

2.99

2.88

Comoe >8ition

P K

% %

.204

.200

.163

.185

.177

.204

.189

.167

3.29

2.98

3.38

2.98

3.06

2.71

3.33

2.75

of LeaTOB

Ca

%

1.88

1.88

2.06

1.83

1.75

1.96

1.71

1.71

Mn

ppm.

25

21

25

22

35

18

25

22

Fe

55«

60

80

HO

115

150

120

110

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.

Total Iron Content of Severely Chlorotic Valencia Orange Leaves

as Affected by Applications of Chelated and Unchelated Iron on

Acid SoU.

Grams Iron

Applied

par Tree

None

None

9,000

33,000

20

Condition of Tree

Severely Chlorotic

Deep Green, Healthy

Most of Leaves Became

Severely Chlorotic

All Leaves Became Green

All Leaves Became Green

All Leaves Became Green

All Leaves Became Green

Material Applied

llone

Hone

100 lbs.FeSQ4.7H2O

. . «

200 lbs.FeS04 (Ann.)

1/3 lb. Fo-EDTA

Flush of

Growth

Spring

Summer

Spring

Summer

Spring

Summor

Spring

Summer

Spring

Summer

Iron in

Leaves

vm.*

25 30

62

124

43

24

82

51

100

95

*Moderately chlorotic leaves. All other trees, chlorotic at the

start of the exoerlatent, greened up after treatment.

•Oven-dry basis.

Polyphosphates, pyrophosphates, and hexa-

metaphosphates, 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:

431-444. 1952.

2. 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: 196-

200. 1939.

3. Chapman, H. D., Brown, S. M., and Rayner, D. S. Nutrient deficiencies of citrus—symptoms, cause

and control. Citrus Leaves 25 (3): 1-12. 1945.

4. Colehour, J. K. Unpublished data, Florida Citrus Experiment Station. 1948.

5. 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: 11-21. 1949.

6. Martell, A. E. and Calvin, M. Chemistry of the metal chelate compounds. Prentice-Hall, Inc.,

New York. 1952.

7. 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): 25-27. 1952.

8. Stewart, Ivan, and Leonard, C. D. Iron chlorosis —its possible causes and control. Citrus Mag. 14

(10) : 22-25. 1952.