Comparison of two procedures to evaluate phosphate-fertilizing field trials

Growth response of agricultural crops to different input levels (e.g., fertilizer rates) can be described by the law of diminishing increments, which has been formulated mathematically amongst others by Mitscherlich (1928) and von Boguslawski and Schneider (1962, 1963, 1964). In the present paper, the economically optimal phosphate-fertilizer requirements in 43 long-term phosphate (P)-fertilization experiments were calculated using equations on the basis of the yield functions of Mitscherlich or von Boguslawski and Schneider, respectively. For three field trials with linear or disproportionately high yield responses on P fertilization, none of the two procedures could be used. The same held for four trials with maximum yields already appearing at the first fertilization level. Similar P-fertilization optima were calculated for 36 trials following the law of diminishing increments, resulting in a highly significant correlation (R-2 = 0.946) between both procedures. The correlation coefficients between the phosphate balance at the calculated optimum profitability and CAL-P content in the soils at trial start were R-2 = 0.70 (Mitscherlich, 1928) and R-2 = 0.65 (von Boguslawski and Schneider, 1962, 1963, 1964), respectively. An optimal soil P content of 8-10 mg CAL-P (100 g soil)(-1) was deduced. Both methods are applicable to calculate the optimum phosphate-fertilizer requirement if yield effects due to P fertilization follow the law of diminishing increments and the increments of fertilization levels in each trial are established in a way that diminishing yield increments can be expected.