COMPARISON OF FIELD CALIBRATIONS FOR MEHLICH-3-P AND K WITH BRAY-KURTZ-P1 AND AMMONIUM ACETATE K FOR CORN

Because of both economic and environmental concerns, soil testing is becoming an increasingly important component of crop-production systems. This increasing demand for soil testing will necessitate more efficient methods for routine soil testing. The Mehlich-3 soil test has been proposed as an extractant that can be used for simultaneous extraction of several nutrients including both P and K. Before this extractant can be used in place of the current extractants, it must be evaluated in field calibration studies. This study was designed to compare the ability of the Mehlich-3 extractant to predict corn (Zea mays L.) yield response to P and K with the Bray-Kurtz P1 extractant for P and the neutral, 1N ammonium acetate extractant for K. A field study was conducted at 67 locations across Pennsylvania over 4 yr. A factorial arrangement of P (0 and 88 kg ha-1) and K (0 and 168 kg ha-1) was applied at each location. Corn-grain yield was measured as the response parameter. The Mehlich-3 extractant was highly correlated with the Bray-Kurtz P1 extractant (r=0.99) and neutral, 1N ammonium acetate K extractant (r=0.96). The calibrations for the Mehlich-3 P and Bray-Kurtz P1 were similar. While there was a large amount of scatter in both data sets, a Cate-Nelson partitioning of the data resulted in critical levels of 43 and 45 kg P ha-1 for the Bray-Kurtz P1 and Mehlich-3 P extractants, respectively. The calibrations for the Mehlich 3 and the neutral, 1N ammonium acetate extractants were also similar. Again there was considerable scatter in the data. The Cate-Nelson critical levels for K were 0.20 and 0.24 cmol kg-1 for the Mehlich 3 and neutral, 1N ammonium acetate extractants, respectively. On this basis it appears that the Mehlich 3 is at least as good as the currently used extractants for P and K and could readily replace them in Pennsylvania. At the same time the large amount of scatter indicates the need for a more mechanistic approach to soil testing. © 1990, Taylor & Francis Group, LLC. All rights reserved.