By Dorivar Ruiz-Diaz, Kansas State University Nutrient Management Specialist

In Kansas, zinc is a micronutrient that can be deficient in corn and grain sorghum production. Wheat has seldom responded to Zn fertilization in research trials, and the K-State Soil Test Lab does not recommend Zn for wheat at any Zn soil test level. Similarly, no Zn recommendation is made for alfalfa, cool-season grasses, or sunflowers.

Corn exhibits Zn deficiency at a relatively early growth stage as stunted plants (shortened internodes) with a broad, chlorotic strip between the midrib and leaf margin on fully expanded leaves. There may be some reddening associated with the chlorotic tissue, depending on the corn hybrid.

The most severe deficiencies typically have been where topsoil has been removed, either by erosion or mechanically in building terraces or land leveling for floor irrigation. Sandy soils are more likely to be low in available Zn than medium- and fine-textured soils. Starter phosphorus fertilizer on soils that are marginal to deficient in zinc has increased the severity of Zn deficiency. This interaction is considered to be physiological at the plant level. When soil Zn levels are already low, increased P concentrations may exacerbate Zn deficiencies.

If corn yields in a research test or producer field are significantly lower where starter P fertilizer has been applied than where no starter has been applied, then you can suspect that the soil is low in zinc and needs to have some zinc applied with the starter. Here are a couple examples from past K-State research by former researchers Larry Murphy and Ray Lamond:

Effect of Phosphorus and Zinc as Starter Fertilizers on Corn Yields on Low-Zinc Soils
P2O5 in starter     Zn in starter     Corn Yield     P (%) in leaf     Zn (ppm) in leaf
(lbs/acre)               (lbs/acre)          (bu/acre)         tssue              tissue
0                            0                        101               0.14               12
80                          0                          73               0.73               10
0                          10                        102               0.16               24
80                        10                        162               0.41               17

Phosphorus and Zinc Effects on Corn Yields on Low-Zinc Soils
P2O5 (lbs/acre)     ZN (lbs/acre)      Application Method      Corn yield (bu/acre)
0                                0                             --                            107
40                              0                        Broadcast                    121
                                                            Starter                           93
0                               10                       Broadcast                    121
                                                            Starter                         115
40                             10                       Broadcast                    139
                                                            Starter                         140

In both cases, corn yields were significantly reduced when P was applied as a starter, if no Zn was included in the starter. Large amounts of starter-applied P can actually enhance Zn deficiency if the soil is low in zinc and no zinc fertilizer is applied.

Zinc is an essential nutrient for plant growth, involved in protein synthesis and necessary for growth regulation and enzyme systems. Plants absorb Zn as inorganic cations, which are held on exchange sites on clay and organic matter. Zinc is found in mineral complexes and in organic matter, with the organic fraction being the dominant source in most Kansas soils.

How can producers know whether to apply Zn for their corn, grain sorghum, or soybeans? A soil test is the best way to assess Zn needs. Similar to other micronutrients, Zn is needed in low amounts for optimum yields, and for Kansas soils the critical soil test level is 1 ppm. Zinc is an immobile nutrient that can be tested using the same sample as used for the routine soil test (0 to 6 inch depth). Fertilizer Zn application is not recommended if soil test levels are above 1 ppm.

Care should be used in collecting the sample to avoid potential contamination by not using a collection bucket made of rubber or galvanized steel. Both contain Zn. A plastic collection bucket and a stainless steel or chrome-plated soil probe is recommended.

Zinc can be banded as a starter or broadcast and incorporated with equal response. Banded Zn rates can be lower than broadcast rates because of greater efficiency. However, residual Zn levels will be lower from banded than broadcast applications. On newly terraced fields, a broadcast application of Zn fertilizer and/or manure to the new terrace channels should be considered. A broadcast rate of 5 to 10 pounds per acre of Zn will substantially increase Zn soil test levels to the point where additional Zn application will not be needed for several years. However, producers should continue to test their soils and monitor Zn levels.

Several sources of Zn fertilizer are available. Zinc sulfate and liquid Zn products are the most common sources. Liquid Zn products include chelates, lignin sulfonates/polyflavonoids, and Znammonium complexes. Zinc chelate (EDTA) banded as a starter is more efficient than other sources, and Zn rates can be reduced by half or three-quarters compared to the full recommended rates with similar performance.

If banded, an application rate of 0.5 lb zinc/acre of inorganic zinc is generally sufficient, although this will need to be done annually on low-testing soils. If broadcast, higher rates will be needed. When broadcasting, inorganic zinc will be more economical than chelates.

Zinc oxide is not recommended because of its very low solubility in all soils, especially in neutral to alkaline pH soils. Zinc oxy-sulfate products are available, in which Zn oxide is partially acidified with sulfuric acid. At least 50 percent water solubility of Zn in the Zn oxysulfate products is needed. Manure also is an excellent source of Zn.

Application of Zn when soil levels are deficient (less than 1 ppm) can be a high-return investment. However, before applying Zn, producers should determine whether it is needed by soil testing.