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Copyright © 2002 Regents of the University of Minnesota. All rights reserved. Zinc (Zn) is an essential nutrient required in some fertilizer programs for crop production in Minnesota. While some soils are capable of supplying adequate amounts for crop production, addition of zinc fertilizers is needed for others. In Minnesota, Zn may be needed in fertilizer programs for production of corn, sweet corn, and edible beans. Several research projects have focused on the use of this nutrient, and much of the following information is based on the results of that research. The Role of Zinc in the PlantThe specific role of Zn in growth and development of plants is not known. This nutrient is an important component of various enzymes that are responsible for driving many metabolic reactions in all crops. Growth and development would stop if specific enzymes were not present in plant tissue. Zinc, however, is needed in very small amounts. Plant uptake of this nutrient is calculated in terms of ounces per acre instead of pounds per acre. Therefore, Zn is classified as a micronutrient. Deficiency SymptomsPlants fail to develop normally when they are deficient in Zn and certain characteristic deficiency symptoms will appear. With corn, these symptoms usually appear in the first two or three weeks of the growing season. If the deficiency of Zn is severe, these symptoms may last throughout the entire season. A deficiency of Zn in corn is characterized by the development of broad bands of striped tissue on each side of the midrib of the leaf. These stripes begin on the part of the leaf closest to the stalk and appear first on the upper part of the plant (see Figure 1). A Zn deficient corn plant also appears to be stunted. The lack of normal elongation in a corn plant is shown in Figure 2. Figure 1. This young corn plant shows typical zinc deficiency symptoms. Note the broad white stripes on both sides of the midrib of the leaf.Figure 2. Zinc deficiency creates shortened internodes on the corn stalk. A normal plant (bottom) is shown in contrast to the zinc-deficient plant. Zinc deficiency in edible beans first appears as a yellowing of the lower leaves. As the season progresses, this yellowing develops into a bronze or brown color. The leaves have a rusty appearance. For this crop, however, care must be taken to avoid confusing sunburned leaves with Zn deficiency. For both corn and edible beans, suspected deficiency symptoms should be confirmed with plant analysis. Soil Conditions and the Need for Zinc FertilizersResearch at the University of Minnesota as well as other universities has identified soil conditions where a response to Zn fertilizers might be expected. These conditions are:
The P-induced Zn deficiency might be a concern when high rates of manure are applied to crop land. The manure, however, also contains Zn that can be used for crop growth. Therefore, P supplied from manure should not create a Zn deficiency for crop production in Minnesota.
Table 1. The effect of high rates
Predicting the Need for ZincThe need for Zn in a fertilizer program can be determined through soil tests or plant analysis. Plant analysis can confirm a suspected Zn deficiency. Plant analysis, however, should be used in combination with soil testing before arriving at firm recommendations for using Zn in a fertilizer program. A guide to the relative levels of Zn in the tissue of several important agronomic crops is provided in Table 2. The Zn concentration changes with stage of growth for the various crops. It's important that crops be sampled at the growth stage listed if interpretation of plant analysis information is to be accurate.
Table 2. Relative levels of Zn concentration in plant tissue
When a soil test indicates the need for Zn, small amounts are needed in a fertilizer program to provide for optimum yield (Table 3). The Zn status of Minnesota soils is easily measured by routine soil testing. The DTPA procedure is used by major soil testing laboratories and is a reliable indicator of the need for Zn in the fertilizer program. The interpretations of this test, along with corresponding fertilizer recommendations, are summarized in Table 4.
Table 3. The effect of rate of Zn
Table 4. Zinc recommendations for
The addition of Zn to a starter fertilizer is the most economical approach to Zn fertilization. This method provides the nutrient the year it is needed. This is especially important when corn and edible beans are rotated with other crops. If use of a starter fertilizer is not an option, the Zn fertilizer should be broadcast and incorporated before planting either corn or edible beans. Sources of ZincSeveral sources can supply Zn when needed. Zinc sulfate is usually used to supply the needed amount of Zn when dry fertilizer materials are used. This material can be either broadcast and incorporated before planting, or used in a starter fertilizer. It blends well with other dry fertilizer materials. Approximately 3 lb. of the zinc sulfate material will supply 1 lb. Zn per acre. A zinc-ammonia complex (10% Zn) can be used to supply Zn when fluid fertilizers are used. This material mixes easily with other fluid fertilizers. Zinc oxide can correct a Zn deficiency but is slowly soluble and not effective in a granular form. To effectively correct a Zn deficiency, zinc oxide must be finely ground. Spreading any finely ground material is a problem in Minnesota because of the wind. So use of finely ground zinc oxide is limited to situations where suspension fertilizers are used. Foliar applications of Zn have not been consistently effective in correcting deficiencies of this nutrient. This method of application should be used on a trial basis only. For foliar applications, powdered zinc sulfate can be dissolved in water and applied to the leaf tissue. The amount dissolved should supply 0.5 to 1.0 lb. Zn per acre when a rate of 20 gallons of water per acre is used. A zinc chelate can also be mixed with water. The amount of chelate mixed with water should supply 0.15 lb. Zn per acre when water is sprayed at a rate of 20 gallons per acre. Research has shown that all sources of Zn (except granular zinc oxide) have an equal effect on crop production (Table 5). The yields presented in Table 5 are averages for four rates of applied Zn (0.1, 0.3, 1.0, 3.0 lb. Zn/A). Consider cost before choosing a source of Zn for the fertilizer program.
Table 5. The effect of Zn source
SummaryZinc is needed in small amounts for crop production in Minnesota and is, therefore, classified as a micronutrient. Field corn, sweet corn, and edible beans are Minnesota crops that respond to the use of this nutrient. A soil test is the best management practice for predicting the need for adding Zn to a fertilizer program. This nutrient is most effective if applied in a starter fertilizer. Several sources of zinc can be used with both liquid and dry fertilizers to optimize production of corn and edible beans when this nutrient is needed. George Rehm, Extension Soil Scientist
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