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 during 1989 and 1990. Treatments were 0, 50, 100, and 150 lb K/acre. Plant-available K was measured by ammonium acetate, Mehlich-3, K saturation, nitric acid, and rates of release soil tests. Analyses of yield responses involved fitting various models to relationships between relative yields and soil-test K (STK) values. Profitability indices were calculated for multifield scenarios in which alternative STK values were evaluated for their abilities to generate net returns when used to guide 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 inappropriate critical values can be easily evaluated; and (iii) soil tests can 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 the 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 the soils in this study.