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Zinc (Zn), a micronutrient essential for crop growth, is supplied by the soil to support optimum crop production in most of Minnesota. However, there are production situations where this micronutrient, when added to a fertilizer program will produce profitable increases in crop yield.

Zinc fertilization is not complicated. Of the major crops grown in this state, corn and dry edible beans are known to be responsive to zinc fertilization. Various research trials have been conducted to evaluate the effect of zinc fertilization on a variety of crops. Except for the two just mentioned, none have responded positively to the application of zinc in a fertilizer program.

Analysis of a soil sample for zinc is the best management tool available to accurately predict the need. The soil test for zinc that is used by soil testing laboratories is built on a solid research base. It is accurate and reliable. It can be used with confidence. Suggestions for the amount of Zn to apply vary with the amount of Zn measured. These suggestions are summarized in Table 1.

Table 1. Zinc suggestions for corn and dry edible bean production and the relation to the amount of Zn extracted from the soil.

* Zn extracted by the DTPA procedure

The suggestions listed in Table 1 are appropriate for both corn and dry edible bean production. Lower rates are needed for correction of a Zn deficiency if this micronutrient is applied in a band at planting rather than broadcast and incorporated prior to planting.

When the soil test for Zn is low, small amounts applied in a band can have a positive substantial effect on crop yield (Table 2). Compared to broadcasting, there is a higher probability that roots will come in contact with this nutrient if the zinc fertilizer is applied in a band near the seed at planting. The cost of the banded zinc is also substantially less.

Table 2. Response of irrigated corn to Zn fertilization.

* soil test for Zn = low

The magnitude of the yield increase shown in Table 2 is not typical of most situations. These results, however, show how important a micronutrient can be when it is deficient in the soil. In Minnesota, it is more realistic to expect increases in corn yields of 10 to 15 bu. per acre provided the soil test for Zn is in the low range.

There are several sources that can be used to supply Zn in a fertilizer program. These sources have been compared using suspension fertilization applied in a band (Table 3). The zinc rate for all sources was constant at 0.5 lb. Zn per acre. The soil test for Zn was low at both research sites.

Table 3. Yield of irrigated corn as affected by the source of Zn applied in a band near the seed at planting.

In this research, all sources of Zn had an equal effect on corn yield. There are some perceptions that one source of Zn is more effective than another. However, research has shown that, when applied in a band near the seed, all sources have an equal effect on yield.

Zinc sulfate blends easily with other dry fertilizers. Therefore, this source would be a good first choice when the producer is using dry fertilizers in a band at planting. The solubility of zinc oxide decreases as soil pH increase. Therefore, this material would not be a good choice for mixing with dry fertilizers.

The chelated Zn sources and the Zn complex can be mixed easily with fluid fertilizers such as 10-34-0. Usually there is a substantial difference in cost of these two sources. In addition, there may be free ammonia in the complexes. Therefore, there is a risk in placing fluids containing a zinc complex in contact with the seed at planting. The amount of Zn that can be applied in this way is not known. This is especially true with dry edible beans. It would be wise to plan on placing any fluid fertilizer containing a zinc complex near the seed so that there is some soil between seed and fertilizer.

Zinc is important for crop production in Minnesota. When needed in a fertilizer program, its use can have a positive economical impact on production. Crop producers are urged to test soil for this micronutrient to see if they have been missing some profit potential.