Zinc is an extremely important micronutrient that has many roles in plant health and deficiencies are widespread, even if unknown to the grower. Recommendations for zinc levels in soils are dependent on crop, soil type, pH and other nutrient status and can range depending on which institution is offering the recommendation. Generally speaking, below 1ppm on your soil test indicates that you should apply some type of zinc fertilizer. However, growers should pay attention to their soil tests and site-specific factors, because while 1ppm of zinc in one soil type may be sufficient, 4ppm in another soil with zinc antagonists may be a better target.
Deficiency symptoms are generally seen in new growth, early in the life cycle of the plant and result in stunted growth, shortened, sometimes split internodes and discoloration of new leaves—the color of which can vary depending on plant species. Internally, zinc deficiency can result in reduced water uptake, phytohormone (hormones that regulate plant growth) activity and uptake of other nutrients. In corn, zinc deficiency results in a broad band of bleached tissue on either side of the midrib, beginning at the base of the leaf and generally staying in the lower half of the leaf. Severe zinc deficiency may result in new leaves that are nearly white, a phenomenon called 'white bud.'
Zinc availability is very sensitive to pH, and is therefore reduced by over-liming or by other agents causing high pH. However, rates and acidifying forms of N commonly used in agriculture generally alter the pH enough in the rhizosphere to enhance zinc uptake. Zinc is also well known to interact with P; where zinc is deficient, P uptake is increased in certain plants and vice versa. Zinc deficiency is also more common on cool and wet soils with low organic matter.
Specifically, we are interested in the interaction of zinc and calcium, a topic on which there exists little information. Feedback from growers indicates that when zinc levels are not sufficient, they don't see a good response from our products containing calcium. Why this happens, we are not exactly sure, however, we theorize that perhaps the limiting factor is zinc, rather than the calcium, which results in no visible effects from the application. One thing we do know is that alkaline earth cations, specifically calcium, can inhibit zinc uptake. This may have something to do with the fact that a large amount of basic cations in soil generally result in higher pH values, which is known to inhibit zinc uptake. One way to combat this problem is to apply some slightly acidifying N fertilizer that will cause a temporary shift in pH—favorable to zinc uptake—in the rhizosphere to combat the inhibitory effects from calcium. The take-home message is that if calcium-based products are needed in your system, it is prudent to pay attention to your zinc levels and adjust with a zinc fertilizer, or another method to ensure your plants are getting the requisite amount of zinc.