128 YEARBOOK OF AGRICULTURE 1957 content of the soil is high. In these re- gions, the boron content of water used for irrigation is sometimes great enough to cause injury to crops. Copper and Carl S. Scofield and L. V. Wilcox, of the Department of Agriculture, found that 0.5 p.p.m. oí boron in irrigation Soil Fertility water injured some crops, A boron con- tent of I.o p.p.m. caused injury to Walter Reuther most crops, even those with high boron requirements. The boron that occurs in injurious Soils high in organic matter concentrations in irrigation water may and weathered, sandy soils are be derived from the solution of exposed outcrops of soluble boron minerals, likely to be deficient in copper. from underground waters, or directly A great deficiency may cause from volcanic gases dissolved in per- colating waters. The areas in which serious stunting of growth and boron toxicity may occur are not large; visible symptoms of disease in nevertheless, the injury to crops in some of these areas is serious. These plants, but moderate deficiency areas are located primarily in southern may merely reduce yields. California, western Nevada, and parts of Arizona. Copper once was regarded as a plant Certain control measures are avail- poison, as indeed it may be when too able for eliminating or preventing the much of it is used on soil. As a matter accumulation of toxic concentrations of fact, a 5-percent solution of copper of boron in soils. Thorough leaching of sulfate was one of the first spray formu- contaminated soils is usually recom- lations used for the chemical control of mended when possible. Mixing irriga- weeds. Concentrations of o.i percent tion water high in boron content with to 0.2 percent of copper in the form of water low in boron content is recom- water suspensions of insoluble hydrox- mended as a method of utilizing all the ides, carbonates, or oxides are an effec- water available for irrigation without tive fungicide. Research workers noted building up toxic concentrations. long ago that solutions containing as Borax, or other boron fertilizers, little as I part per million of soluble therefore must not be used indiscrimi- copper killed algae and fungus spores. nately. The effect may be disastrous. Bordeaux mixture, the first widely Fortunately, the effect in humid re- used fungicide applied to foliage by gions is temporary; the boron soon spraying, is prepared by dissolving 5 to leaches from the soil. In the absence 10 pounds of copper sulfate in 100 gal- of oífícial recommendations, farmers lons of water and adding approxi- should use borax at the rate of only a mately an equal weight of lime (cal- few pounds to the acre and only on small cium hydroxide) or soda ash (sodium areas until experience shows the need carbonate). for boron and the proper amount to use. Many researchers before 1927 ob- Boron fertilization is becoming a nec- served that Bordeaux sprays sometimes essary and accepted practice in many had stimulating effects on vigor and areas of the United States, just as plas- yield that were not associated with the tic is becoming more widely used in control of fungus diseases. Other re- the kitchen. But just as the housewife search workers noted that minute knows that plastic has its limitations, amounts of copper were distributed the farmer knows that the use of boron through all plant tissues. Some thought has its limitations. that the stimulating effects of small COPPER AND SOIL FERTILITY 129 amounts of copper on plants and fungi twigs and defoliation on them, the for- were due to some indirect action of mation of multiple buds in the leaf copper or to "an irritation response." axils and shortening of internodes, a Others thought that copper might be dying back of the twigs, and a charac- an essential element in the metabolism teristic staining of the fruit because of of plants and animals. the formation of gum-soaked areas in The first credible evidence that cop- the rind. Affected fruits frequently per was an essential element in the split open and drop before they attain nutrition of lower plants was provided full size. In Florida, where it was first by H. Bortels, a German scientist, in noted and described, the disease was 1927. He showed that a deficiency of known as exanthema, dieback, or copper in the culture medium of the ammoniation. The last term indicates common bread mold, Aspergillus niger, that growers recognized that it was reduced growth by 50 percent and associated with heavy applications of changed the color of the spores from "ammonia" (nitrogen) fertilizers. black to brown. The addition of minute Copper deficiencies of other tree amounts of copper to the cultures pro- crops have been reported in other duced normal growth and black spores. sections of the United States and the Confirmation of his results was soon world. Usually shoots die back and provided by other scientists, who dis- the foliage may show marginal or covered that copper is essential for the spotted necrosis and chlorosis. Multiple normal growth of a wide variety of bud formation, resetting (shortening fungi and yeasts and also of green of shoot internodes), malformation of plants and animals. R. V. Allison and leaves, and an excessive gumming also his associates in 1927 showed that the may occur. almost complete failure of many crops A disease of grains called white tip, to grow on the peat soils of the Florida yellow tip, or reclamation disease has Everglades could be cured by fertiliza- been reported in various parts of the tion with copper. They postulated that world. It responds to copper fertiliza- the disorder was due to a lack of suffi- tion. It is characterized by a necrosis cient copper for normal plant growth. of the tips of older leaves and a mar- ginal chlorosis of the tips of younger THE SYMPTOMS of copper deficiency leaves, which may remain unrolled in green plants vary considerably with and tend to wilt readily. The heads species and perhaps other complicat- may be dwarfed and distorted. Grain ing factors. No general description of production may be reduced more than visual copper deficiency symptoms can the vegetative growth. therefore be made. Symptoms of copper deficiency have Crops showing a moderate response been described for sugarcane, a num- in vigor and yield to applications of ber of vegetable crops, peanuts, and copper to soil may not exhibit striking other plants. symptoms of disease other than lack of normal vigor. Usually some parts of THE FUNCTIONS OF COPPER in the fields or orchards in which moderate mineral nutrition of plants appear to responses to copper are obtained have be numerous, varied, and complex. In a few plants that show acute patho- fact, none of the essential nutrient ele- logical symptoms of copper deficiency. ments has a single, simple job in the Symptoms of copper deficiency of economy of plant growth and devel- citrus were among the first to be recog- opment. Copper is no exception, al- nized as such in the field and are fairly though evidence concerning many of typical of symptoms on other tree its functions is quite meager. crops. Primary symptoms of the dis- Copper seems to be concentrated ease are gum pockets under the bark, more in the rootlets of plants than in stained spots on the bark of terminal leaves or other tissues—it may there- 40Gi57°—57 10 130 YEARBOOK OF AGRICULTURE 1957 fore have an important function in root Aspergillus niger and inhibited in pro- metabolism. duction in spores of that organism Analyses of the tissues in a copper- when grown when copper is deficient. deficient plant indicate it to be ab- Plants sufí'ering from copper defi- normally high in proteins and amino ciency are low in ascorbic acid oxidase acids, although similar effects have activity. Other enzymes that appear to been noted with several other deficien- involve copper are cytochrome G and cies of essential plant nutrients. lacease. Heavy fertilization with nitrogen Because of its inherent physical and tends to increase the severity of patho- chemical properties, copper forms a logical symptoms of copper deficiency. vast array of compounds with pro- Plants supplied with ammonium nitro- teins, amino acids, and other organic gen in culture solutions respond fav- compounds that commonly occur in orably to higher levels of copper than the juices of plants and animals. do plants supplied only with nitrate Two groups of such copper com- nitrogen, an indication that copper is pounds, known as complexes and che- related somehow to utilization of lates, are probably of particular signif- ammonium nitrogen by plants. All the icance in the special functions that evidence suggests that copper is im- copper performs in the life processes portant in the utilization of proteins of plants and animals. In complexes, in the growth processes of plants. because of its special properties, cop- The rate of photosynthesis of leaves per is held securely by a number of on copper-deficient plants is abnor- single chemical bonds to other atoms mally low. The concentration of cop- in molecules of proteins, amino acids, per in chloroplasts (minute corpuscle- and related species of compounds. like bodies in plant cells in which the Chelates are similar to complexes, ex- green pigment chlorophyll is concen- cept that copper is held with tre- trated) is larger than in the leaf as mendous security by extremely strong a whole. We have evidence that cop- multiple chemical bonds. per is involved in oxidation-reduction reactions in plants. THE COPPER CONTENT of tissues of Copper probably functions as an plants sufí'ering from copper deficiency enzyme activator or as an integral part is abnormally low.