Michigan State University Extension
Soils & Soil Management - Fertilizer - 05209706
07/10/97
Secondary and Micronutrients for Vegetables and Field Crops Extension Bulletin E-486, Revised August 1994
By M.L. Vitosh, D.D. Warncke and R.E. Lucas Department of Crop and Soil Sciences Michigan State University Extension
Zinc is essential for plant growth because it controls the synthesis of indoleacetic acid, which dramatically regulates plant growth. Zinc is also active in many enzymatic reactions and is necessary for chlorophyll synthesis and carbohydrate formation. Because zinc is not readily translocated within the plant, deficiency symptoms first appear on younger leaves. Research shows a need for zinc in many areas where dry edible beans are grown. Corn, onions, soybeans and barley have also shown benefits from zinc applications at some locations. Several other states report that Sudangrass, sorghum, tomatoes and potatoes have been responsive.
Soils associated with zinc deficiency are usually neutral to alkaline in reaction. The more alkaline the soil, the greater the need for zinc. Deficiency is particularly noticeable on crops growing where calcareous subsoils have been exposed by land leveling or erosion, or where subsoil is mixed with topsoil, such as after tiling and spoil-bank leveling. Lake bed soils and organic peats show the greatest zinc deficiencies in Michigan.
Observations and field tests show that dry edible beans following sugar beets often need zinc. The large quantities of phosphorus fertilizer used for sugar beets and the high zinc requirement of dry edible beans are believed to cause the problem. A recent reduction in phosphorus use on sugar beets and the long-term use of zinc fertilizers have reduced the incidence of zinc deficiency.
Zinc deficiency varies from year to year. Wet, cool, cloudy weather during the early growth season increases the deficiency. Zinc deficiency in corn is occasionally noted in June, but the deficiency disappears after the soils dry out and warm up. Crops on poorly drained organic soils show a deficiency probably because of restricted root growth. Field and vegetable crops often show differences in response to zinc fertilizer. The relative crop response to fertilizer zinc is given in (Vis. 6). Dry edible beans, corn, onions, sorghum, snap beans, spinach and sweet corn are the most responsive crops.
High soil phosphorus levels have been known to induce
zinc deficiency, especially in responsive crops (Vis. 20)
For years, the cause of this interaction was
suspected to be the formation of an insoluble zinc
phosphate, which reduced the concentration of zinc in the
soil solution to deficiency levels. Zinc phosphate has
since been shown to be soluble in soil and is an
acceptable source of zinc when finely ground. High levels
of phosphorus in plants have been shown to restrict zinc
movement within the plant, resulting in accumulation in
the roots and deficiency in the tops. Therefore, large
applications of phosphorus fertilizer may contribute to
zinc deficiency in zinc-responsive crops.
Zinc Deficiency Symptoms
Zinc-deficient dry edible beans first become light green. When the deficiency is severe, the area between the leaf veins becomes pale green and then yellow near the tips and outer edges. In early stages of deficiency, the leaves are deformed, dwarfed and crumpled. In later stages, they look as if they have been killed by sunscald (Vis. 18), (Vis. 19), (Vis. 21). On zinc-deficient plants, the terminal blossoms set pods that drop off, delaying maturity.
Zinc deficiency in corn appears as a yellow striping of the leaves. Areas of the leaf near the stalk may develop a general white to yellow discoloration. In severe deficiency, the plants have shortened internodes and the lower leaves show a reddish or yellowish streak about one-third of the way from the leaf margin (Vis. 16), (Vis. 17). Plants growing in dark sandy or organic soils usually show brown or purple nodal tissues when the stalk is split. This is particularly noticeable in the lower nodes.
Deficiency in onions shows up as stunting, with marked twisting and bending of yellow-striped tops (Vis. 22). In potatoes, early symptoms are similar to leaf roll. The plants are generally more rigid than normal, with smaller than normal leaves and shorter upper internodes.
Zinc Fertilizer Carriers
Several zinc compounds can be used to correct a deficiency. Zinc sulfate, zinc oxide, zinc chloride, zinc sulfide and zinc carbonate are common inorganic salts. Organic compounds such as zinc chelates (zinc EDTA and zinc NTA) are about five times more effective than inorganic salts with equivalent amounts of zinc. Organic carriers, however, have a lower zinc concentration, ranging from 9 to 14 percent.
The zinc concentration of zinc sulfate ranges from 25 to 36 percent, and that of zinc oxide, 70 to 80 percent. In field tests, granular zinc oxide was not as effective as the powdered formulation. The test also showed that mixing the zinc carrier with the fertilizer was more effective than incorporating the carrier in the granule.
Rates and Methods of Applying Zinc Fertilizer
To be effective, soil-applied zinc must be applied near the seed at planting time. Mixing zinc with a phosphate fertilizer, such as 6-24-24, is acceptable.
Seed treatment with zinc oxide is not recommended. Tests have shown that 1 pound of zinc per acre from zinc oxide applied on bean seed reduced emergence and yields. Sidedress applications of zinc after the crop has emerged have not been very effective. If a zinc deficiency problem is diagnosed after emergence, spray the foliage with 0.5 to 1 pound of zinc per acre. This amount can be found in 1.5 to 3 pounds of zinc sulfate. The solution should not exceed 5 pounds of the salt per 100 gallons of water. Response to spray applications is usually obvious within 10 days. It may be apparent in five days if the treatment is applied when the plants are growing vigorously. For plants with waxy leaves, such as onions, a wetting agent in the water may be needed to obtain good foliage cover.
Spraying crops such as corn, onions and potatoes has had mixed results. The reason for poor results may be inadequate zinc being translocated into the roots. If foliage sprays are used, they should be applied when plants are small to obtain best results.
Some fungicides that contain zinc can be used as foliar treatments. These fungicides may help correct zinc deficiency; however, they should not be relied on entirely because the amount of zinc applied in fungicides is very small.
Zinc Carryover
Residual carryover of available zinc varies from slight to moderate, increasing as soils become less alkaline. On highly responsive soils, zinc broadcast at rates above 25 pounds per acre showed good carryover for seven years after application. When zinc is banded at the rate of 3 to 4 pounds per acre, yearly applications are needed. After adding a total of 25 pounds of zinc per acre through smaller annual applications, growers can often reduce the rate of zinc application or eliminate application altogether. Growers should use the zinc soil test to decide if continued use of zinc fertilizer is necessary.
Soil and Plant Tissue Tests for Zinc
Plant tissue tests can help diagnose a need for zinc. Tissues containing less than 20 ppm of zinc are often deficient. Values of 30 to 100 ppm are normal; values over 300 ppm may be considered excessive or toxic. Zinc response and suggested rates of banded zinc for soil test levels can be found in MSU Extension Bulletins E-550A and E-55OB. Recommendations are based on soil pH and available zinc level.
Zinc Toxicity
Excessive soil zinc levels may occur on extremely acid soils (< pH 5.0) or in areas where zinc-enriched municipal sewage sludge or industrial waste has been added to cropland as a soil amendment. Though instances of plant zinc toxicity are rare in Michigan, the crop and variety being grown are critical.
High levels of available soil zinc that result in 100 to 300 ppm zinc in crown leaf tissue seldom result in zinc toxicity in corn, which is very zinc tolerant. However, if the soil levels result in 40 to 50 ppm or more of zinc in the leaf tissue of some varieties of dry edible beans, toxicity may occur because dry edible beans are a zinc- sensitive crop. Soybeans and most small grains fall somewhere between corn and dry edible beans in zinc tolerance. Vegetable crops are generally sensitive to high zinc levels, while grasses usually tolerate high levels of available soil zinc.
A general guide for zinc concentration in mature leaf tissue is as follows: deficient less than 20 ppm; sufficient 25 to 150 ppm; excessive or toxic 300 ppm or more. Because plant tolerance to zinc toxicity varies greatly, specific soil extractable levels, which might indicate toxicity, have not been established.
