Zinc and Soil Fertility

Lloyd F. Seatz and J. J. Jurinak.

Zinc is essential for plants and animals. Many of our major farm regions contain areas that do not have adequate quantities of available zinc to support normal plant growth.

Zinc deficiencies in the field were first observed in Florida in 1927 on crops growing in peat soils. Zinc was used in the 1930's to cure nutritional difficulties in citrus in Florida and California. Deficiencies on other tree crops, such as pecans and tung, were encountered later in Florida and along the gulf coast. Corn and other crops on the Coastal Plain of the Eastern States have since been found to suffer from too little zinc.

Deficiencies also are common on calcareous (limestone) and noncalcareous soils throughout the West, particularly for cherries, apples, peaches, pears, apricots, citrus, and walnuts. Zinc deficiency in field crops in the West has increased since 1950. Regions affected include the Columbia Basin in Washington and the Sacramento Valley in California. The highly phosphatic soils of central Tennessee and Kentucky when heavily limed have also produced zinc-deficient crops, particularly corn and Sudangrass.

Several reasons can be offered.

Increased fertilization has produced larger yields, which have tended to remove larger amounts of available zinc from the soil than formerly. Other soils, naturally low in zinc, may develop zinc deficiencies when they are brought under cultivation. The Columbia Basin in Washington is an example of this type of area.

Two FACTORS GOVERN the ability of a soil to provide enough to a growing plant--the total supply of zinc in the soil and its availability to the plant. Zinc-deficiency disease therefore may be caused by a naturally low zinc fertility level or by fixation processes, which retain, or fix, zinc in a form that the plant cannot utilize.

Most soils in the United States usually have more than enough zinc for the requirements of normal plant growth, but crops may have zinc-deficiency disease. Thus the major problem is one of availability.

Occasionally the total supply of zinc in the soil as in regions that support an intensive cropping program and that have strongly weathered and coarse-textured soils of low inherent fertility may be the limiting factor.

Chemical tests are used to determine the ability of the soil to supply zinc to plants. Zinc is extracted from the soil with reagents and the results are compared with crop response or deficiency. Acidified potassium chloride solution, 0.1 normal hydrochloric acid, 0.04 normal acetic acid, and acidified ammonium acetate have been used as the extracting agents, but the most promising for both acid and calcareous soils is dithizone.

This method was introduced in 1951 by Ellsworth Shaw and L. A. Dean, of the Department of Agriculture. When dithizone-extractable zinc from a soil sample is less than 0.4 to 0.5 parts per million, symptoms of zinc deficiency can be expected from crops growing in that soil.

Biological methods based on the sensitivity of the fungus Aspergillus niger to zinc have been used with some success. The bioassay technique of determining the ability of soil to supply zinc takes about 8 days for incubation and analysis. The actual working time to analyze one sample is less than with chemical methods.

The concentration of zinc is highest in the surface soil and declines with depth. The reason lies in the uptake of zinc from the subsoil by plants and its translocation to the leaves. When the leaves fall and decay, zinc is released from the plant tissues and is fixed in the surface soil. The continuation of this fixation process near the surface depletes the subsoil of zinc and increases the possibility of deficiency to deep-rooted perennials, as fruit trees.

MANY SOIL FACTORS are associated with the deficiency of zinc in plants. One of the first factors to be correlated with deficiency was soil reaction, or soil pH. It was observed that less zinc was taken up after lime was put on an acid soil. The reduced availability of zinc, as the pH of the soil is increased, is generally attributed to the formation of insoluble zinc hydroxide, although undoubtedly other factors related to the adsorptive and exchange properties of the soil exert some influence on zinc availability as the pH of the system is altered by liming.

It is commonly thought that zinc availability is at a minimum in the soil PH range of about 5.5 to 7.0. Zinc is readily available at lower pH values. As the reaction rises above pH 7, the situation becomes more complex.

Evidence from experiments suggests that the positively charged zinc ion may be converted to a negatively charged zincate complex. The conversion would tend to reduce further the solubility of zinc in alkaline soils, where the predominance of calcium ion would favor the formation of very insoluble calcium zincate. The effect of possible zincate formation on zinc availability has not been established. The data, however, indicate that care must be exercised when attempting to define the chemical nature of the zinc ion in alkaline soils.

Adsorption studies in comparatively simple systems indicate that the clay fraction of the soil exerts a strong attractive force for zinc ions. A part of the attraction is related to the similarity in size and charge between magnesium and zinc ions. The similarity allows zinc ion to react with the clay mineral lattice, where it may substitute or possibly exchange for magnesium in the clay mineral, thus making it relatively unavailable. Inferences based on the knowledge of the electronic structure, small size, and charge of zinc ion allows one to predict that zinc will be tenaciously adsorbed by clay and other soil minerals. And indeed, at low concentrations, the zinc ion manifests itself in such a fashion.

The influence of excess soil phosphate on the availability of zinc has not been clearly defined. Excess phosphate has been associated with low availability of zinc in the major fruit regions of the West. The indiscriminate use of phosphate fertilizers in orchard management is not recommended there. Many soils of Kentucky and Tennessee that support crops suffering from zinc deficiency have a high content of native phosphate. These examples support the contention that zinc interacts with the phosphate radical to form an insoluble zinc phosphate complex, and so lowers its availability. Soils of the citrus regions of Florida, however, have not shown an increase in soil fixation of zinc within the limits of ordinary Phosphate applications.

Attempts were made at Washington State Agricultural Experiment Station in 1953-1954 to induce zinc deficiency in field corn and beans by a heavy application of phosphate fertilizer. The phosphate treatments did not alter their uptake of zinc.

The detrimental effects of phosphate fertilization on zinc uptake may be due to the depressing action of the calcium ion in superphosphate. No conclusive evidence has shown this to be the case, but the apparent contradiction of various workers as to the effect of phosphate on zinc nutrition supports the view that the interaction is not a simple zinc-phosphate relation but that other factors must exert an influence.

Zinc ion has also been shown to be strongly adsorbed on commonly occurring lime minerals. The minerals are calcite (calcium carbonate), dolomite (calcium-magnesium carbonate), and calcian-magnesite (magnesium carbonate with calcium impurity).

The lime minerals containing magnesium carbonate exhibited a greater adsorptive capacity for zinc ion because of the compatability of zinc ion with the magnesium carbonate crystal lattice. Thus, if the available zinc present in the soil is near the critical level, the presence of appreciable amounts of lime minerals may present an additional hazard to proper zinc nutrition of plants. Zinc deficiency could well be worsened in soils of semiarid and and regions where these minerals are widely distributed, and zones of lime accumulation can be found frequently within the root zone of crops. The practice of land leveling for irrigation may increase the importance of zinc adsorption on lime minerals by bringing the zone of lime accumulation nearer the surface.

The effect of liming with calcitic and dolomitic limestone on acid soils thus lowers zinc availability by increasing soil pH; it may also increase temporarily the adsorptive capacity of the soil for zinc. The ability of the lime minerals to increase the adsorptive capacity of any given soil, whether acid or alkaline, for zinc ions would probably be important only in soils of coarse texture where clay is not the dominating fraction of the soil.

Deficiency of zinc in California has been associated with soils of a high content of organic matter. Symptoms have been observed in spots that formerly were corrals or barnyards. In soils of average organic matter content, however, the destruction of the organic fraction has little influence on the amount of zinc extracted from the soil an indication that the organic fraction in most soils does not affect greatly the availability of zinc. In organic soils, or soils where an appreciable portion of the adsorptive capacity derives from the organic complex, however, the organic fraction would have a more vital part in making zinc available.

ZINC DEFICIENCY in plants causes several abnormalities in structure.

The palisade cells of leaves from most affected plants are larger and are transversely divided, rather than columnar, as in normal leaves. Zinc deficiency therefore may lead to cell enlargement rather than to cell differentiation. Other abnormalities may be a reduction in the number of chloroplasts, the absence of starch grains, the presence of oil droplets in the chloroplasts, the presence of calcium oxalate crystals, and the accumulation of phenolic materials in the leaves. These changes in chemical composition indicate that zinc is related to normal metabolism of carbon within plants.

The roots of zinc-deficient plants are also abnormal. Tomato roots may have a series of swellings with whorls of root hairs near the root tip. Secondary roots may develop later in them. Cell structure is also deranged; cells even in the meristematic, or actively growing, region may be enlarged, and an irregular arrangement with many air spaces may occur among the cells. Older tissue becomes necrotic and has flaky masses of exfoliated cells. The metabolic products of the root cells also are abnormal. Tannin, fats, and calcium oxalate crystals are present in abnormally large amounts. Starch is absent.

Zinc is also related to seed production in several plants. A threshold value of zinc has been established for peas and beans below which the plants produce only small, seedless pods. When plants are supplied zinc in concentrations above this value, the pods are larger and contain seeds.