Decomposing organic matter in alkaline soils often intensifies the chlorosis in fruit trees. Peach trees irrigated with water containing 267 parts per million (p.p.m.) of bicarbonate became stunted and chlorotic. When water from another source containing 76 p.p.m. of bicarbonate was used, the trees began to grow, and the new growth was green. Apparently there is no satisfactory general explanation for the relationship of bicarbonates to iron chlorosis.

The relative amounts and ratios of calcium, potassium, phosphorus, and nitrogen in plants are altered with the development of iron chlorosis. chlorotic leaves have a higher ratio of potassium to calcium and phosphorus to iron and contain more nitrogen than green leaves. These conditions may be a result of chlorosis rather than a cause.

The concentration of certain heavy metals copper, zinc, cobalt, manganese have a direct bearing on iron chlorosis. No general statement can be made about the harmful concentration of any one of them. One plant species may tolerate more of a certain minor element than another. Sorghum and wheat, for example, made good growth at a copper concentration that caused chlorosis in a variety of soybean.

Iron chlorosis in acid, sandy soils in Florida is thought to be caused by an accumulation of copper applied to the soil and plants as fertilizer and sprays. These soils now contain about 700 pounds of copper and manganese an acre. Similar virgin soils contain 5 to 10 pounds of copper and 30 to 40 pounds of manganese an acre. Many orange groves have become unthrifty and chlorotic in recent years.

SEVERAL METHODS have been devised to overcome a lack of iron: Controlling the soil moisture by irrigation, ample drainage, some cover crops, and shallow cultivation; grafting and budding susceptible varieties on chlorosis-resistant rootstocks; adding soil amendments that contain soluble iron; adding amendments that make the soil iron more available; spraying the plants with solutions of iron salts; and injecting iron salts into the trunks and limbs of trees.

The treatment of soils with acidifying materials, such as sulfur or ammonium sulfate, to make iron more available, has been more successful on acid soils than on calcareous soils.

Soluble inorganic iron salts applied to alkaline, calcareous soils are absorbed by some plants but not by others. Because iron is not readily translocated in many plants, spraying with soluble iron salts is not always Satisfactory results can be obtained by injecting iron into trees, but growers have not liked the method because the individual treatments take so much time. Holes are drilled in the base of the tree a few inches apart and gelatin capsules filled with iron salt (ferrous citrate, ferric citrate, or ferric phosphate) are placed in the holes. The holes are then sealed with grafting wax or emulsified asphalt.

The selection of plants that are not susceptible to iron chlorosis and the use of iron chelates have been two of the most promising ways of dealing with chlorosis.

Where the proper and unsusceptible plant material has been used in fruit culture, the results have been good. For example, Concord and many other American grapes (Vitis labrusca) are susceptible to chlorosis, but many European varieties (V. vinifera) have a high degree of resistance to chlorosis. These resistant varieties have been used as rootstocks for Concord grapes grown on alkaline, calcareous soils.

For 5 years the Concord graft on Malaga, Muscat, Rose of Peru, and Tokay rootstocks were vigorous, productive, and practically free from chlorosis. On the other hand, the self-rooted Concord plants were chlorotic from the beginning, and 98 percent of them were dead at the end of 5 years. Grafting V. labrusca varieties on resistant V. vinifera rootstocks thus appears to offer a method of controlling chlorosis in grapes.

We know of no satisfactory rootstock for the peach.

IRON CHELATES are reagents, such as citric acid, which bind the iron ion through two or more positions within their structures. The iron ion is held in such a way that it cannot free itself to form another compound when treated with such common precipitating agents as phosphate or hydroxide. Some of the synthesized chelates that combine with iron are very soluble, yet the iron is retained in a soluble complex form available to plants as a nutrient. Very likely many organic compounds formed from soil organic matter chelate, or complex, metal ions, such as Fe, Cu, and Mn. Many of these organic compounds may be readily destroyed by micro-organisms.

The characteristics of a satisfactory chelating agent for soil applications are: The chelated metal ion is not easily replaced by other metals; the metal-ion complex is stable against hydrolysis in all kinds of soil; the chelating agent is not decomposed by soil micro-organisms; the chelate is water soluble and not easily fixed in the soil colloidal fraction; the metal ion is available to the plant at the root surface and after it enters the plant; the chelating agent is not toxic to plants; and the chelating agent is available to the grower.

Ivan Stewart and C. D. Leonard, of the Florida Citrus Experiment Station at Lake Alfred, corrected iron chlorosis in citrus trees by adding iron chelate, Fe-EDTA (iron-ethylenediamine tetraacetic acid) to an acid, sandy soil. Their investigations have stimulated research on the practical importance of metal chelates for the correction of iron deficiency in many countries.

The chelating agents that have been used in soils to test their effectiveness in correcting iron chlorosis are ethylenediamine tetraacetic acid (EDTA), hydroxethyl ethylenediamine-tricetic acid (HEEDTA), diethylenetriamine pentaacetic acid (DTPA), cyclohexane trans 1,2-diaminotetraacetic acid (CDTA), and an aromatic aminopolycarboxylic acid (APCA). The relative effectiveness of these chelates in making iron available at pH 7, in decreasing order, is: CDTA, APCA, DTPA, HEEDTA, EDTA.

Fe-APCA has been the most satisfactory chelate used thus far on alkaline calcareous soils. Ten pounds to the acre corrected chlorosis in greenhouse studies with soybeans. As much as 1,000 pounds an acre was not toxic. Fe-DTPA has been the next most satisfactory chelate. Fe-HEEDTA and Fe-EDTA have been good sources of iron on acid soils, but were ineffective on alkaline calcareous soils. The higher cost of producing Fe-APCA limited its practical use in 1957.

Less expensive chelates having some of the characteristics of APCA were being produced and tested in the field. APCA was the least toxic of the chelates tested. Several companies produce Fe-DTPA and Fe-EDTA.

It is important to understand the metabolic fate of chelates in plants, since the compounds, if they persist in plant fluids, may affect the mineral metabolism of the plant and other growth processes. The effect of very small applications of APCA on the mineral metabolism of soybeans suggests that this compound may have a favorable physiological effect besides its capacity to chelate iron in the soil.