PROFIT-MAXIMIZING CRITICAL-VALUES OF SOIL-TEST POTASSIUM FOR CORN

Identification of critical values for soil tests is a fundamental step in using soil testing to guide fertilization. Critical values usually are selected to attain a desired percentage of maximum yield. Here we demonstrate how critical values can be selected to maximize profits. Twenty-eight response trials with corn (Zea mays L.) were conducted du ring 1989 and 1990. Treatments were 0, 50, 100, and 150 lb K/acre. Pla nt-available K was measured by ammonium acetate, Mehlich-3, K saturati on, nitric acid, and rates of release soil tests. Analyses of yield re sponses involved fitting various models to relationships between relat ive yields and soil-test K (STK) values. Profitability indices were ca lculated for multifield scenarios in which alternative STK values were evaluated for their abilities to generate net returns when used to gu ide fertilization. The results showed several advantages of using net returns instead of yield levels: (i) critical values directly reflect the economic rationale for fertilization; (ii) costs of using inapprop riate critical values can be easily evaluated; and (iii) soil tests ca n be compared easily for their abilities to generate profits when used to guide fertilization. Profit-maximizing critical values identified for the soil tests were 112 ppm for the ammonium acetate, 70 ppm for t he Mehlich-3, 831 ppm for the nitric acid, and 0.90 lb/acre/d for the rate of release. The K saturation soil test was not appropriate for th e soils in this study.