Profit-Maximizing Critical Values of Soil-Test Potassium for Corn

Abstract

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 (Zeu 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. Research Question Soil testing is widely used to guide K fertilization for corn, and critical values of soil tests are used to indicate when fertilization is, or is not, needed. Failure to identify a critical value leaves uncertainty concerning when fertilization is needed. Although fertilizers are applied to increase profits, the methods used to identify critical values have focused more on yield responses to fertilization than on profits from fertilization. Here we show how to select critical values to maximize profits from fertilization. Literature Summary Critical values for soil tests have been most often identified by analysis of relationships between soil-test values and relative yields (a measure of yield response to fertilization). Models usually are fit to these relationships and then solved for soil-test values associated with desired yield levels (expressed as percentages of the highest yields attained by adding fertilizer). Recent studies involving soil tests for P in corn, however, revealed marked disagreements between models commonly used to identify critical values. These disagreements make it difficult to determine which critical values should be used to guide fertilization. Study Description Twenty-eight response trials with corn having treatments of 0, 50, 100, and 150 lb K/acre were conducted in farmers' fields. Plant-available K was evaluated by five soil tests. Yield responses were expressed as relative yields by representing yields of nonfertilized plots as percentages of the mean yields of the two highest-yielding treatments for each site. Net returns to fertilization were estimated by subtracting assumed costs of fertilization from the assumed value of additional grain produced on fertilized plots compared with nonfertilized plots. Critical values of soil-test K (STK) were identified by analysis of yield responses or analysis of critical-value profitability (CVP) indices. Analyses of yield responses involved fitting various models to relationships between relative yields and STK values. The CVP indices were based on net returns to fertilization calculated for scenarios in which alternative STK values were used to guide fertilization over 28 fields corresponding to the 28 trials in the study. The STK values that produced maximum CVP indices were denoted as profit-maximizing critical values. Applied Questions What are the advantages of using net returns to identify critical values? The results show that analysis of net returns has several advantages over analysis, of yield responses. These include (i) critical values directly reflect the economic rationale for fertilization; (ii) the costs of using inappropriate critical values can be easily evaluated; (iii) uncertainty caused by disagreements among models is avoided because relationships between CRP indices and soil-test values show clear maxima; and (iv) soil tests can be easily compared for their ability to generate profits when used to guide fertilization. What were the best soil-test methods and the profit-maximizing critical values? Maximum profitability indices for the ammonium acetate, Mehlich-3, nitric acid, and rate of K release soil-test methods were similar. Profit-maximizing critical values for these methods 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 method was not appropriate for the soils in this study. Recommendation Critical values for soil tests should be identified by considering net returns rather than yield responses to fertilization.