Long‐Term Evaluation of Phosphorus and Zinc Interactions in Corn

Abstract

High rates of P fertilization or high levels of plant‐available P in soils have been frequently reported to induce Zn deficiency in corn ( Zea mays L.). The objective of this study was to evaluate the long‐term effects of annual applications of P and Zn fertilizers on corn yields. A long‐term trial was established in 1965 on a Webster (fine‐loamy, mixed, mesic Typic Haplaquoll)‐Canisteo (fine‐loamy, mixed, calcareous, mesic Typic Haplaquoll) soil complex, which tested very low in available P (6‐ppm, Bray‐1 method). Corn was grown continuously until 1990. Treatments were annual applications of 0, 20, 40, or 80 lb P/acre (triple superphosphate) and 0 or 10 lb Zn/acre (zinc sulfate) broadcast and incorporated into the soil. The fertilization treatments resulted in wide ranges of soil‐test values for P and Zn. Phosphorus fertilization increased corn yields in every year of the study. Zinc fertilization resulted in a statistically significant but small (2 bu/acre) average yield increase over the 26 yr. This yield increase would not offset the cost of the Zn fertilizer applied. Analysis of variance and observation of yield responses indicated that corn responses to Zn application were similar at all levels of applied P in most years. The conclusion of this study was that continuous applications of P fertilization at very high rates did not induce greater Zn deficiencies in corn than those observed at low rates of fertilization. Rates of P fertilization that increase soil‐test values above values deemed optimum for corn production would result in diminished profits for producers cropping similar soils but would not increase needs for Zn fertilization. Research Question High levels of plant‐available P in soils have been reported to induce Zn deficiencies in corn. Although most soils of the Corn Belt originally tested low in P, expectations for increased profits has prompted many crop producers to build‐up available P to levels higher than needed to maximize profits from fertilization. Therefore, the question arises as to whether these high P levels might induce Zn deficiencies, which further reduce profitability. The objective of this study was to evaluate the long‐term effects of annual application of P and Zn fertilizers on corn yields. Literature Summary Numerous reports suggest that high rates of P fertilization or high levels of available P may induce Zn deficiency symptoms in various crops. High P availability may depress Zn concentration in plant tissue, may increase physiological requirements for Zn, or may result in accumulation of toxic levels of P in the leaves when Zn levels are low. Corn is highly sensitive to Zn supply compared with other field crops, and Zn deficiencies are more likely to occur in calcareous soils. Reports on the effects of P and Zn on yields of corn, however, have shown inconsistent interactions. Research on P and Zn interactions deserves continued attention in the Corn Belt because fertilization practices have increased available P to very high levels in many soils. Study Description A long‐term trial was established in 1965 on an area of Webster and Canisteo soils in north‐central Iowa. The soils tested very low in P and were marginally adequate in Zn. Corn was grown continuously until 1990, and fertilization treatments were the combinations of 0, 20, 40, or 80 lb P/acre and 0 or 10 lb Zdacre. Granulated triple superphosphate and zinc sulfate were broadcast and incorporated into the soil in spring before planting. Soil samples were collected in 1965, in 1977, and in 1991 following termination of the study, and were analyzed for available P and Zn. Corn yields were adjusted to 15.5% moisture content. Applied Questions Did fertilization increase the availability of P and Zn in the soil? The P treatments resulted in a wide range of soil‐test P values across plots and years. Application of the highest P rate increaed soil‐test values to levels several times higher than levels needed to maximize corn yields. Zinc fertilization increased the initially marginal levels of available Zn to very high levels. Did fertilization increase corn yields? Phosphorus fertilization increased yields in every year of the study. Zinc fertilization increased yields in 5 of the 26 yr. Over all years, Zn fertilization increased yields only by 2 bu/acre. Did high levels of P fertilization induce Zn deficiencies? The observed responses to Zn fertilizer were similar at all levels of applied P. Even the abnormally high annual rate of 80 lb P/acre did not induce Zn deficiency in this soil, which had levels of available Zn that were only marginally adequate. Recommendation Excessive rates of P fertilization for corn in this soil would reduce profits for producers but would not increase need for Zn fertilizers.

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