Copper and Soil Fertility

Walter Reuther.

Soils high in organic matter and weathered, sandy soils are likely to be deficient in copper. A great deficiency may cause serious stunting of growth and visible symptoms of disease in plants, but moderate deficiency may merely reduce yields.

Copper once was regarded as a plant poison, as indeed it may be when too much of it is used on soil. As a matter of fact, a 5-percent solution of copper sulfate was one of the first spray formulations used for the chemical control of weeds. Concentrations of 0.1 percent to 0.2 percent of copper in the form of water suspensions of insoluble hydroxides, carbonates, or oxides are an effective fungicide. Research workers noted long ago that solutions containing as little as I part per million of soluble copper killed algae and fungus spores.

Bordeaux mixture, the first widely used fungicide applied to foliage by spraying, is prepared by dissolving 5 to 10 pounds of copper sulfate in Ion gallons of water and adding approximately an equal weight of lime (calcium hydroxide) or soda ash (sodium carbonate).

Many researchers before 1927 observed that Bordeaux sprays sometimes had stimulating effects on vigor and yield that were not associated with the control of fungus diseases. Other research workers noted that minute amounts of copper were distributed through all plant tissues. Some thought that the stimulating effects of small amounts of copper on plants and fungi were due to some indirect action of copper or to "an irritation response." Others thought that copper might be an essential element in the metabolism of plants and animals.

The first credible evidence that copper was an essential element in the nutrition of lower plants was provided by H. Bortels, a German scientist, in 1927. He showed that a deficiency of copper in the culture medium of the common bread mold, Aspergillus niger, reduced growth by 50 percent and changed the color of the spores from black to brown. The addition of minute amounts of copper to the cultures produced normal growth and black spores.

Confirmation of his results was soon provided by other scientists, who discovered that copper is essential for the normal growth of a wide variety of fungi and yeasts and also of green plants and animals. R. V. Allison and his associates in 1927 showed that the almost complete failure of many crops to grow on the peat soils of the Florida Everglades could be cured by fertilization with copper. They postulated that the disorder was due to a lack of sufficient copper for normal plant growth.

THE SYMPTOMS of copper deficiency in green plants vary considerably with species and perhaps other complicating factors. No general description of visual copper deficiency symptoms can therefore be made.

Crops showing a moderate response in vigor and yield to applications of copper to soil may not exhibit striking symptoms of disease other than lack of normal vigor. Usually some parts of fields or orchards in which moderate responses to copper are obtained have a few plants that show acute pathological symptoms of copper deficiency.

Symptoms of copper deficiency of citrus were among the first to be recognized as such in the field and are fairly typical of symptoms on other tree crops. Primary symptoms of the disease are gum pockets under the bark, stained spots on the bark of terminal twigs and defoliation on them, the formation of multiple buds in the leaf axils and shortening of internodes, a dying back of the twigs, and a characteristic staining of the fruit because of the formation of gum-soaked areas in the rind. Affected fruits frequently split open and drop before they attain full size. In Florida, where it was first noted and described, the disease was known as exanthema, dieback, or ammoniation. The last term indicates that growers recognized that it was associated with heavy applications of "ammonia" (nitrogen) fertilizers.

Copper deficiencies of other tree crops have been reported in other sections of the United States and the world. Usually shoots die back and the foliage may show marginal or spotted necrosis and chlorosis. Multiple bud formation, rosetting (shortening of shoot internodes), malformation of leaves, and an excessive gumming also may occur.

A disease of grains called white tip, yellow tip, or reclamation disease has been reported in various parts of the world. It responds to copper fertilization. It is characterized by a necrosis of the tips of older leaves and a marginal chlorosis of the tips of younger leaves, which may remain unrolled and tend to wilt readily. The heads may be dwarfed and distorted. Grain production may be reduced more than the vegetative growth.

Symptoms of copper deficiency have been described for sugarcane, a number of vegetable crops, peanuts, and other plants.

THE FUNCTIONS OF COPPER in the mineral nutrition of plants appear to be numerous, varied, and complex. In fact, none of the essential nutrient elements has a single, simple job in the economy of plant growth and development. Copper is no exception, although evidence concerning many of its functions is quite meager.

Copper seems to be concentrated more in the rootlets of plants than in leaves or other tissues it may therefore have an important function in root metabolism.

Analyses of the tissues in a copper-deficient plant indicate it to be abnormally high in proteins and amino acids, although similar effects have been noted with several other deficiencies of essential plant nutrients.

Heavy fertilization with nitrogen tends to increase the severity of pathological symptoms of copper deficiency. Plants supplied with ammonium nitrogen in culture solutions respond favorably to higher levels of copper than do plants supplied only with nitrate nitrogen, an indication that copper is related somehow to utilization of ammonium nitrogen by plants. All the evidence suggests that copper is important in the utilization of proteins in the growth processes of plants.

The rate of photosynthesis of leaves on copper-deficient plants is abnormally low. The concentration of copper in chloroplasts (minute corpuscle-like bodies in plant cells in which the green pigment chlorophyll is concentrated) is larger than in the leaf as a whole. We have evidence that copper is involved in oxidation-reduction reactions in plants.

Copper probably functions as an enzyme activator or as an integral part of enzyme molecules involved in certain reactions. For example, potatoes grown in the Netherlands on soil that is low in available potassium but has adequate copper tend to blacken in storage because of the oxidation of tyrosine (and related phenolic compounds) by the enzyme tyrosinase to the black pigment, melanin.

On the other hand, potatoes grown on soil low in potash and low in copper have a high content of tyrosine (as do potatoes grown in low-potash and high-copper soil) but have much less tendency to produce blackening of cut surfaces. Potatoes grown on low-copper soil exhibit less than one-tenth of the tyrosinase activity of potatoes grown on normal copper soil. The black pigment, melanin, is probably like that produced in the spores of Aspergillus niger and inhibited in production in spores of that organism when grown when copper is deficient.

Plants suffering from copper deficiency are low in ascorbic acid oxidase activity. Other enzymes that appear to involve copper are cytochrome C and laccase.

Because of its inherent physical and chemical properties, copper forms a vast array of compounds with proteins, amino acids, and other organic compounds that commonly occur in the juices of plants and animals.

Two groups of such copper compounds, known as complexes and chelates, are probably of particular significance in the special functions that copper performs in the life processes of plants and animals. In complexes, because of its special properties, copper is held securely by a number of single chemical bonds to other atoms in molecules of proteins, amino acids, and related species of compounds. Chelates are similar to complexes, except that copper is held with tremendous security by extremely strong multiple chemical bonds.

THE COPPER CONTENT of tissues of plants suffering from copper deficiency is abnormally low.

Among the tree crops, copper concentration in tissues of citrus has perhaps been the most extensively studied in various parts of the world. If copper in citrus leaves falls below about 4 p.p.m. (parts per million) in dry matter, severe copper deficiency symptoms are almost certain to be observed. In the range of about 4 to 5 p.p.m., mild to moderate deficiency symptoms may occur. Copper deficiency rarely occurs when the copper concentration in leaves is 6 p.p.m. or more. A copper concentration above 16 p.p.m. in normal citrus foliage is uncommon, unless contamination from foliage sprays is responsible.

About the same relationship between copper concentration in leaves and incidence of deficiency symptoms has been noted in a large number of other tree crops and many vegetable and agronomic crops.

Analyses of fibrous roots of citrus indicate that copper concentration in them may be about 5 to 10 times the concentration found in the leaves of the same trees, but we are not sure how the concentration of copper in roots of plants is related to copper deficiency.

Small grains that suffer severe copper deficiency (severe stunting, necrosis of foliage, distorted heads) may contain a normal concentration of copper in the aboveground tissues.

F. Steenbjerg found that in Denmark fertilization of such plants with a small amount of copper sulfate actually reduced copper concentration in tissues while at the same time it increased growth and yield considerably. Heavy fertilization with copper restored normal yield and growth and increased copper concentration in the tissues. This type of irregular relationship of concentration in tissues, incidence of deficiency symptoms, and response to copper fertilization, however, is the exception rather than the rule.

THE COPPER CONTENT of soils, according to limited analyses, varies quite widely.

Most mineral soils that have a texture between loam and clay have a total content of native copper of 10 to 200 p.p.m. usually 25 to 60 p.p.m.

Very sandy soils, such as are common in the Atlantic Coastal Plains, contain 1 to 30 P.P.m. of native total copper, with most between 3 and 15 p.p.m.