THAT TOXIC LEVELS of copper in the soil or nutrient solution can cause reduced growth, chlorosis of the foliage, and abnormal, stunted root development of such plants as corn, beans, and squash was demonstrated in 1917 by Dr. R. H. Forbes, of the University of California.
His analyses indicated that copper-stunted plants contained somewhat more copper in the foliage and much more copper in the roots than healthy plants. He concluded that the abnormally high concentration of copper in injured roots was combined largely with proteins and localized mainly inside the root in the promeristem and central stele and not in the outer epidermal or cortical tissues.
Later studies have disclosed that toxic amounts of copper in the soil or nutrient medium may reduce growth, depress the iron concentration in leaves, and cause symptoms of iron chlorosis. Copper toxicity also may interfere with the uptake of certain other heavy metals and phosphorus and otherwise derange the normal process of nutrient accumulation by roots. This is associated with stunting, reduced branching, and thickening and abnormal dark coloration of rootlets.
Somewhat similar toxic effects can be produced by other heavy metals, such as nickel, cobalt, zinc, and manganese. Nickel is appreciably more toxic than copper. Cobalt is slightly less toxic. Zinc and manganese are about one-tenth and one-thirtieth as toxic, respectively.
Instances of toxic concentration of copper developing in agricultural soils as a result of accumulation in many years of residual copper from the Bordeaux fungicides or from copper sulfate fertilization have been reported in Florida and France.
Toxic effects of high copper in citrus orchard soils in Florida are manifested in severe cases by a marked reduction in tree vigor and yield, severe chlorosis of foliage, and dieback of the twigs. Similarly, copper toxicity symptoms of several crops have been reported in some old vegetable fields having a large amount (more than 400 p.p.m.)
of copper in the topsoil, the result of many years of frequent spraying of celery with Bordeaux mixture to control fungus diseases.
The chlorotic foliage of affected citrus trees in Florida has an abnormally low iron content, and the sparse, dark, stubby fibrous roots in the topsoil have an exceedingly high copper content. Such copper toxicity symptoms of citrus in most instances are associated with an acid soil condition (pH 4.0 to 5.5) produced by application of acid-forming fertilizers and large residues of sulfur used to control pests.
If the copper level in the soil is not too high, normal vigor of affected trees can be restored by applying one-fourth to one-half pound of chelated iron (iron ethylenediamine tetraacetate, or Fe-EDTA) per tree and sufficient lime or soda ash to raise the pH of the topsoil to about pH 7. The iron chelate quickly corrects the chlorosis by supplying iron to the top presumably mainly through the healthy roots in the subsoil, which are not affected by copper toxicity. The heavy liming usually reduces the availability of copper enough to permit the gradual restoration of normal rooting in the topsoil.
3. The tip of a corn root that is exposed to a toxic concentration of copper. The dark part indicates that copper is localized in the inner promeristem and central stele portion of the root.
Studies with pots of virgin Florida orchard topsoils containing about 5 p.p.m. or less of total copper indicate that the addition of 10 to 25 p.p.m. of copper may benefit citrus seedlings, and additions of 50 to 200 P.P.M. may reduce growth and cause chlorosis. The degree of toxicity obtained is controlled largely by the exchange capacity (related primarily to the organic matter content) and degree of acidity of these very sandy soils.
We have evidence also that a high phosphate content of soils reduces copper toxicity, as judged by total growth of seedlings in pots. Paradoxically, high phosphorus may increase the incidence of chlorosis symptoms on such soils.
Field studies of acid, sandy soils in citrus orchards in Florida indicate that slight toxic effects in the trees may occur when the copper level in the soil reaches about 1.6 milliequivalents of copper per milliequivalent of exchange capacity in too grams of dry soil. At twice this concentration, mild to severe toxic effects are likely to be observed. In other words, the lower the clay and organic matter content of a soil, the lower is the amount of added copper required to produce toxicity.
Scientists in France found that copper accumulated over many years in acid soils from residues of Bordeaux sprays in vineyards may be toxic to a variety of crops. Pot tests indicated that addition of about 200 p.p.m. of copper from copper sulfate produced a toxic reaction in soils low in copper, but otherwise comparable to Alsatian vineyard soils, which were found to contain as much as 400 p.p.m. of copper. Among the crops tested, vines were most resistant to copper toxicity; clover and alfalfa were most sensitive.
Symptoms of copper poisoning were found in southern France in spinach and gladiolus grown in a field once occupied by a peach orchard that had been sprayed heavily for many years with Bordeaux mixture. Only the parts of the field having quite acid soil (pH 4.5 to 4.7) were affected; the copper content of that soil was 98 to 130 p.p.m.
Future research in disease control and soil fertility with copper-containing compounds should evaluate the residual effects, because nearly all the copper applied to crops will normally be fixed in the few inches near the surface of the soil.