Effect of P and K on Yield and Chemical Composition of Forage Sorghum1

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Abstract
Even though P and K are both essential nutrients, the response of agronomic crops to P/K interaction under field conditions has not been adequately documented. This research was conducted to examine the influence of a high and low level of P over a wide range of K levels and vice versa on yield and chemical composition of forage sorghum (Sorghum bicolor L.). In Exp. I, K was applied at rates of 0, 56, 112, and 244 kg/ha and split with P application of 0 and 116 kg/ha. For Exp. II, P was applied at rates of 0,29,58, and 116 kg/ha. For Exp. II, P was applied at rates of 0, 29, 58, and 116 kg/ha and was split with K applications of 0 and 224 kg/ha. These studies were conducted from 1978 thru 1980 on a Davidson clay (Rhodic Paleudult: clayey, kaolinitic, thermic) soil. Potassium application (Exp. I) increased yields only in years with less than optimum moisture conditions and then only when P was applied. When no P was applied, yields decreased with increased K application. Phosphorus (Exp. II) increased yields for each of the 3 years studied. In years with less than optimum moisture conditions, application of K increased yields at the higher P levels. Tissue K concentrations increased with K application while Ca and Mg concentrations decreased with increased K application. However, when P was applied Ca and Mg concentration increased. In the absence of applied P, the decreased in Ca and Mg concentration was related to competition with K for uptake. This competition may be responsible for reduced yields when K application was increased in the absence of applied P. When P was applied (in the presence or absence of applied K), Ca and Mg concentration increased in the tissue. A linear relationship between yield and Mg concentration in the tissue was present.

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