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. 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 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- , 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" () 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 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. of enzyme molecules involved in cer- Among the tree crops, copper con- tain reactions. For example, potatoes centration in tissues of citrus has per- grown in the Netherlands on soil that haps been the most extensively studied is low in available potassium but has in various parts of the world. If copper adequate copper tend to blacken in in citrus leaves falls below about 4 storage because of the oxidation of p.p.m. (parts per million) in dry mat- tyrosine (and related phenolic com- ter, severe copper deficiency symp- pounds) by the enzyme tyrosinase to toms are almost certain to be observed. the black pigment, melanin. In the range of about 4 to 5 p.p.m., On the other hand, potatoes grown mild to moderate deficiency symptoms on soil low in potash and low in copper may occur. Copper deficiency rarely have a high content of tyrosine (as do occurs when the copper concentration potatoes grown in low-potash and in leaves is 6 p.p.m. or more. A copper high-copper soil) but have much less concentration above 16 p.p.m. in nor- tendency to produce blackening of mal citrus foliage is uncommon, unless cut surfaces. Potatoes grown on low- contamination from foliage sprays is copper soil exhibit less than one-tenth responsible. of the tyrosinase activity of potatoes About the same relationship be- grown on normal copper soil. The tween copper concentration in leaves black pigment, melanin, is probably and incidence of deficiency symptoms like that produced in the spores of has been noted in a large number of COPPER AND SOIL FERTILITY 131 Other tree crops and many vegetable soil extractants (acid or salt solutions) and agronomic crops. and the copper content of plants grow- Analyses of fibrous roots of citrus ing thereon or to response to copper indicate that copper concentration in fertilization. Such relationships are them may be about 5 to lo times the rule of thumb, and their use to deter- concentration found in the leaves of mine needs often is limited to a small the same trees, but we are not sure how range of soil types and plant species. the concentration of copper in roots of Probably the most reliable—but not plants is related to copper deficiency. infallible—index of the copper status Small grains that suffer severe cop- of soil with respect to a particular crop per deficiency (severe stunting, necrosis is the copper content of the foliage of of foliage, distorted heads) may con- the crop growing on it. tain a normal concentration of copper in the aboveground tissues. IN FLORIDA, I and my coworkers F. Steenbjerg found that in Denmark found that copper cxtractabJe by strong fertilization of such plants with a small acid digestion of soil correlated reason- amount of copper sulfate actually re- ably well with the copper content of duced copper concentration in tissues the tissues of plants grown on it. No while at the same time it increased doubt "wet ashing" soil with strong growth and yield considerably. Heavy oxidizing chemicals liberates all the fertilization with copper restored nor- copper associated with the organic mal yield and growth and increased matter fraction of the soil, and subse- copper concentration in the tissues. quent extraction with more dilute acid This type of irregular relationship of displaces that held by exchange or concentration in tissues, incidence of other reactions on the surface of the deficiency symptoms, and response to clay, but leaves intact most of the copper fertilization, however, is the copper within the crystal lattice of exception rather than the rule. clay particles. Oxidation of Florida soils (containing 95 to 98 percent of THE COPPER CONTENT of soils, accord- sand) with concentrated nitric and ing to limited analyses, varies quite sulfuric acids and digestion of the resi- widely. due with dilute sulfuric acid brought Most mineral soils that have a tex- more than 90 percent of the total cop- ture between loam and clay have a per content of the soil into solution, total content of native copper of 10 to A biological method of estimating 200 p.p.m.—usually 25 to 60 p.p.m. availability of copper in soils has been Very sandy soils, such as are common widely used and appears to be fairly in the Atlantic Coastal Plains, contain reliable. A specially prepared copper- I to 30 p.p.m. of native total copper, free nutrient solution is inoculated with most between 3 and 15 p.p.m. with spores of the common bread mold, Organic soils contain about the same Aspergilliis niger, a small amount of the range of native total copper as mineral soil to be assayed is added, and the soils. As a rule, the subsoil (B horizon) culture is incubated for several days. may contain somewhat less copper The spore color developed is compared than the topsoil (A horizon). In some with a series of standards to estimate soils, the total copper does not vary micrograms of available copper. Black significantly with depth. In others, spores indicate adequate available cop- the subsoil may contain significantly per, and progressive shades of lighter more copper than the topsoil. color indicate increasing degrees of No good chemical method has been copper deficiency. developed for assaying the amount of copper in soil available to plants. THE REACTIONS of copper with soil Rough correlations have been found are not so clearly understood as are between the copper removed by several those of other essential elements. 132 YEARBOOK OF AGRICULTURE 1957

Cu in Dried Vegctat¡on-p. p, m, Because of these reactions of copper 8,0 with soil, a high proportion of copper added to soil as a fertilizer or fungicide residue is fixed rather permanently in the top few inches of the soil, unless the soil is very acid or cultivated deeply. This tight fixation may be due to the formation of complexes and chelates of copper with ligno-proteins and other húmate compounds in soil organic matter and to a lesser extent to some analogous complexes with clay. Thus organic matter, the kind and amount of clay minerals, and acidity arc all major factors afi'ecting the avail- lÖ 20 30 40 50 ability of copper in soils. Cu Added to Soil---Pounds per Acre, 6 Incties /. The relationship oj copper content oj pastures COPPER FERTILIZATION of most soils and forage crops to the application oj copper sulfate deficient in copper usually produces a to soils in Sweden, satisfactory crop response. On mineral soils, a single application Some copper may be held by the of 5 to 25 pounds of copper sulfate colloidal fraction of the soil in much (bluestone) an acre usually is enough the same way as base elements, such as to correct the deficiency with pasture calcium or magnesium. A major part and field crops. of copper, however, evidently is fixed For organic soils, 50 to 200 pounds of with a degree of security such that it copper sulfate an acre may be required cannot be displaced readily by other for normal yields of vegetable and common soil cations (such as calcium, field crops. Other soluble salts, such as magnesium, or potassium) in the ordi- copper chloride or nitrate, have been nary range of soil acidity. used with results equal to copper sul- fate. Insoluble oxides of copper and FOR EXAMPLE, scientists in Florida various copper ores also have been found that a normal Nad (common used. They may be quite efí'ective if the table salt) solution at pH 6.0 would dis- rates are adjusted for copper content place less than 5 percent of copper and the materials are finely divided. added to sandy soil at the rate of 100 In the sandy soils used for citrus or- pounds of copper to the acre and that chards in Florida, annual applications most such soils could fix about three of 15 to 50 pounds of copper sulfate an times as much added copper as am- acre formerly were recommended for monia ion. A high proportion of copper the control of ammoniation (copper de- added to hydrogen or calcium-satu- ficiency) of citrus. In time, however, rated clay was held in nonexchange- this practice led to toxic accumulations. able form; that is, it could not be "ex- It is now recommended that a total changed" or displaced and leached of 50 to 100 pounds an acre of copper out readily by solutions of other cations sulfate be added over a period of years commonly found in soils and fertilizers. to newly planted citrus orchards on Soil holds copper most securely in the virgin soil prone to be copper deficient. range from pH 7 to 8, appreciably less It is applied as an ingredient of mixed securely at pH 6, and progressively less fertilizer (usually about 30 pounds of securely as the soil becomes more acid. copper sulfate or the equivalent per In addition, the kind of minerals pres- ton) in several applications a year for 5 ent in the clay fraction may influence to 6 years before the trees begin heavy the reaction of copper with soils. bearing. No further copper is recom- COPPER AND SOIL FERTILITY 133 Cii In Dried Veiretation p. p, m. required to remove i pound of copper 8.0- from an acre of citrus in the fruit.

As TO ANIMALS, it is generally recog- nized that the blood of those that sufifer from copper deficiency is low in hemo- globin—that is, they have a type of anemia. Copper is not a part of the hemoglobin molecule, but it seems to be essential for the formation of hemo- globin. Other symptoms of copper de- ficiency in animals are retardation of growth, failure to fatten, coarsening and depigmcntation of hair, poor re- production, diarrhea, abnormalities of 0 10 20 30 40 50 60 70 80 90 bone formation, nervous disorders, and Cu ¡n Dry Soil—Pounds per Aero, 6 Inchtis general weakness. Incipient copper de- 2. The relationship oj copper content of pastures ficiency may first show up as a reduc- and forage crops to the copper content {perchloric tion in reproductive efficiency. acid extractable) oj soils in Sweden. THAT TOXIC LEVELS of copper in the mended until indications of deficiency soil or nutrient solution can cause re- are again apparent. duced growth, chlorosis of the foliage, On the muck soils of Michigan, and abnormal, stunted root develop- farmers are advised to apply to respon- ment of such plants as corn, beans, and sive crops initial applications of loo squash was demonstrated in 1917 by pounds an acre of copper sulfate and Dr. R. H. Forbes, of the University of continue smaller annual applications California. until a total of about 250 pounds an His analyses indicated that copper- acre has been applied. This provides stunted plants contained somewhat enough residual available copper for more copper in the foliage and much many years. more copper in the roots than healthy Copper-deficient plants usually re- plants. He concluded that the abnor- spond quickly and satisfactorily to foli- mally high concentration of copper in age sprays of such copper compounds injured roots was combined largely as Bordeaux mixture. Foliage sprays are with proteins and localized mainly often valuable emergency treatments inside the root in the promeristem and when symptoms of copper deficiency central stele and not in the outer epi- are first observed. Soil applications dermal or cortical tissues. usually are the most practical way of Later studies have disclosed that supplying the copper requirements of toxic amounts of copper in the soil or plants unless copper sprays are required nutrient medium may reduce growth, routinely for the control of disease. depress the iron concentration in It was found in nearly all instances leaves, and cause symptoms of iron studied that single applications of cop- chlorosis. Copper toxicity also may per to the soil produce strong residual interfere with the uptake of certain effects, which may persist for many other heavy and years. This is because copper is held and otherwise derange the normal tightly by the soil, it is not subject to process of nutrient accumulation by leaching out of the main root zone, and roots. This is associated with stunting, the amount removed when a crop is reduced branching, and thickening and harvested is small compared to the abnormal dark coloration of rootlets. amounts usually applied. For example, Somewhat similar toxic efí'ccts can about 30 years of heavy cropping are be produced by other heavy metals, 134 YEARBOOK OF AGRICULTURE 1957 such as , cobalt, zinc, and man- ganese. 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 Bor- deaux fungicides or from copper sul- fate 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 j. The tip of a corn root that is exposed to a toxic in tree vigor and yield, severe chlorosis concentration of copper. The dark part indicates that of foliage, and dieback of the twigs. copper is localized in the inner promeristem and cen- Similarly, copper toxicity symptoms tral stele portion oj the root, of several crops have been reported in some old vegetable fields having a orchard topsoils containing about 5 large amount (more than 400 p.p.m.) p.p.m. or less of total copper indicate of copper in the topsoil, the result of that the addition of 10 to 25 p.p.m. of many years of frequent spraying of copper may benefit citrus seedlings, celery with Bordeaux mixture to con- and additions of 50 to 200 p.p.m. may trol fungus diseases. reduce growth and cause chlorosis. The The chlorotic foliage of affected cit- degree of toxicity obtained is controlled rus trees in Florida has an abnormally largely by the exchange capacity (re- low iron content, and the sparse, dark, lated primarily to the organic matter stubby fibrous roots in the topsoil have content) and degree of acidity of these an exceedingly high copper content. very sandy soils. Such copper toxicity symptoms of We have evidence also that a high citrus in most instances are associated phosphate content of soils reduces cop- with an acid soil condition (pH 4.0 to per toxicity, as judged by total growth 5.5) produced by application of acid- of seedlings in pots. Paradoxically, high forming fertilizers and large residues phosphorus may increase the incidence of sulfur used to control pests. of chlorosis symptoms on such soils. If the copper level in the soil is not Field studies of acid, sandy soils in too high, normal vigor of aiïected trees citrus orchards in Florida indicate that can be restored by applying one-fourth slight toxic effects in the trees may oc- to one-half pound of chelated iron (iron cur when the copper level in the soil ethylenediamine tetraacetate, or Fe- reaches about 1.6 milliequivalents of EDTA) per tree and sufficient lime or copper per milliequivalent of exchange soda ash to raise the pH of the topsoil to capacity in 100 grams of dry soil. At about pH 7. The iron chelate quickly twice this concentration, mild to severe corrects the chlorosis by supplying iron toxic effects are likely to be observed. to the top—presumably mainly through In other words, the lower the clay and the healthy roots in the subsoil, which organic matter content of a soil, the are not affected by copper toxicity. lower is the amount of added copper The heavy liming usually reduces the required to produce toxicity. availability of copper enough to per- Scientists in France found that cop- mit the gradual restoration of normal per accumulated over many years in rooting in the topsoil. acid soils from residues of Bordeaux, Studies with pots of virgin Florida sprays in vineyards may be toxic to a MANGANESE AND SOIL FERTILITY 135 variety of crops. Pot tests indicated that a small amount of manganese sulfate addition of about 200 p.p.m. of copper in Bordeaux sprays. from copper sulfate produced a toxic Cereal crops grown on manganese- reaction in soils low in copper, but deficient soils often fail to yield enough otherwise comparable to Alsatian vine- to replant the field. The cereals need yard soils, which were found to con- less manganese than alfalfa does. Pep- tain as much as 400 p.p.m. of copper. permint, spearmint, and rhubarb grow Among the crops tested, vines were normally on a soil in Michigan that most resistant to copper toxicity; clo- had too little manganese for onions, ver and alfalfa were most sensitive. potatoes, alfalfa, oats, and beans. Symptoms of copper were Manganese-deficient soils have been found in southern France in spinach found in many parts of America. and gladiolus grown in a field once oc- Chemical analyses have disclosed cupied by a peach orchard that had that the manganese content of plants been sprayed heavily for many years of the same crop vary greatly. with Bordeaux mixture. Only the parts Manganese with the aid of iron as- of the field having quite acid soil (pH sists in the synthesis of chlorophyll, 4.5 to 4.7) were affected; the copper since all chlorophyll tissues have the content of that soil was 98 to 130 p.p.m. highest concentrations of manganese. Future research in disease control In research with field peas in Michi- and soil fertility with copper-contain- gan, I found that manganese controls ing compounds should evaluate the the state of oxidation of several oxida- residual effects, because nearly all the tion-reduction systems in the plant. copper applied to crops will normally Conditions existed in manganese-defi- be fixed in the few inches near the sur- cient plants that favored the oxidation face of the soil. of iron, ascorbic acid, and glutathione to their respective oxidized forms. In a normal plant these constituents occur in their reduced form. The staining of iron in plant tissue Manganese and showed the deposition of ferric iron, an insoluble form of iron, in the veins of pea leaves of a manganese-deficient Soil Fertility plant, whereas the normal plant gave no test for ferric iron in the veins of the G. Donald Sherman leaves. The normal leaf had an even distribution of ferrous iron, an avail- Plants require tiny amounts of able and active form of iron, through- out the leaf tissue; the manganese-de- manganese to grow and mature ficient leaf showed only small amounts properly. Otherwise they fail of ferrous iron in tissue near the veins. Recalling the pattern of the chlorosis as completely as if they lacked (yellowing) developed on pea leaves, the major elements. the results showed an almost complete absence of iron in the chlorotic, yellow Paul M. Harmer, in research work parts of the leaf. The manganese-defi- at Michigan Agricultural Experiment cient leaves contained more iron, which Station, reported a i oo-bushel increase supports the function of manganese in an acre in yields of potatoes after the the movement of iron in plants. manganese-deficient soil in which they were growing received a liberal appli- SYMPTOMS of a deficiency of manga- cation of manganese sulfate. He ob- nese have been established for most of tained an increase of 561 bushels of the agronomic and horticultural crops potatoes an acre after he had applied by growing the plants in purified cul-