Source: George Rehm, University of Minnesota Extension

For many years, zinc (Zn) has been recognized as an essential micronutrient needed in small amounts for optimum yield.  In Minnesota, this micronutrient, is required in fertilizer programs for certain production situations for optimum yield of corn and edible beans. Application of Zn in a fertilizer program has not been shown to increase yield of other agronomic crops in this state. Research has also shown that small amounts or low rates of application can produce dramatic increases in corn yield. The data in Table 1 are one example of large increases produced by the application of low rates.


Table 1.  Effect of rate of zinc applied in a band at planting on corn yield.


                      Zinc Rate (lb./acre)                                        Yield (bu./acre)


                           0.0                                                                      62


                           0.1                                                                     131


                           0.3                                                                     137


                           1.0                                                                     140


                           3.0                                                                     142


DTPA extractable Zn = 0.3 ppm (low): Zn applied with 8-20-0 suspension in a band at planting.


We should not expect yield increases similar to  those in Table 1 every time that Zn is applied in a fertilizer program.  The soil test for Zn at this experimental site was very low. The Zn was applied in a band near the seed as part of a suspension fertilizer.


In early June of this year, several corn fields in this Cannon Falls neighborhood showed severe Zn deficiency symptoms. Why this year and not in other years? Since very little is known about the chemistry of Zn in soils and physiological reactions of this essential nutrient in plants, this is a difficult question to answer.


Considering soil pH, Zn is usually more available as soil pH moves to the acid side of 7.0. Thus Zn fertilization is usually more of a concern in the western 2/3 of the state. Zinc availability has also been linked to soil organic matter content.  The soil test for Zn usually increases as the soil organic matter content increases. So, Zn deficiency symptoms in corn will usually appear first on eroded portions of the landscape where the organic matter content is low.


Also, the solubility or availability of Zn in soil is affected by soil temperature. The solubility of this micronutrient decreases as soil temperature drops. This relationship could be the explanation for the increased incidence of Zn deficiency symptoms in corn this year. Soil temperatures were below normal in early June and were combined with higher-than -normal soil moisture. This combination probably had a negative effect on root growth and development. With reduced root growth and a reduction in solubility of Zn is soil, there was probably a reduction in Zn uptake by the corn plant and, thus, a greater incidence of Zn deficiency symptoms. As would be expected, Zn deficiency symptoms in this neighborhood were most severe on soil with the lowest organic martter content.


The role of Zn in plant nutrition and plant physiology has not been well defined. This micronutrient is essential for some enzymatic reactions. However, in contrast to nitrogen and sulfur, the exact function of this micronutrient in plants is not known.


In Minnesota management of Zn in a fertilizer program is not complicated. As stated previously, there are only two crops (corn, edible beans) that respond to the addition of Zn in a fertilizer program. So, use in a band when the responsive crop is planted makes sense when the results of the analysis of a soil sample show a need for this micronutrient. The soil test for Zn used by most soil testing laborfatories is accurate and dependable. Suggestions for rates of Zn to use are summarized in Table 2. Application in a band placed near the seed at planting is preferred.


Table 2.  Zinc suggestions for fertilizing corn or edible beans.


                                                                                                        Zinc to Apply


                                                                                                      ___________


              Zinc Soil Test*                                                     Band       or Broadcast


                    ppm                                                                          lb. Zn/acre


          0.0 to 0.25                                                                    2                    10


         0.26 to 0.50                                                                   2                    10


         0.51 to 0.75                                                                   1                      5


         0.76 to 1.00                                                                   0                      0


       1.01+                                                                               0                      0


* extracted by the DTPA analytical procedure


Foliar application of Zn is a possibility; but, this method of application should only be used when Zn deficiency symptoms appear. In order for a foliar application of Zn to be effective, the Zn must get into the leaf tissue. The chelated Zn products are best for foliar application. Supplying Zn as zinc sulfate for this management practice would not be effective. Rates of Zn to use for a foliar application have not been defined by research. A plan to supply about 0.1 lb. actual Zn per acre as a foliar application would be appropriate.


Some growers in the neighborhood this year applied Zn as a foliar treatment when the Zn deficiency symptoms appeared. As I watched the fields after the foliar application, the deficiency symptoms appeared to disappear. I don't know if the early deficiency will reduce yields; check strips were not left in the treated fields. In other fields, the early deficiency symptoms disappeared without the foliar application as temperatures warmed and corn continued to grow. Again, I don't know if there will be a reduction in yield.


When needed, there are several sources of Zn that can be used in a fertilizer program. For growers who use dry fertilizers, zinc sulfate is the product of choice. It is relatively inexpensive and easily blends with other dry fertiilizer materials. There are several sources of Zn that can be blended in fluid fertilizers. All are equally effective. In my opinion, choice of a fluid Zn product should be dictated by least cost.