Influence of lime and phosphorus on soybean (Glycine max (L.) Merrill) yield, leaf and seed composition on a paleudult soil

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Abstract
This research was undertaken on a paleudult soil in southern Brazil, 30° south latitude, to quantify lime and P effect upon soybean (Glycine max (L.) Merrill). A lime x P factorial experience with lime treatments of 0, 0.5, 1, and 2 times SMP interpretation to pH 6.5, and 0, 44, 88, 132, and 176 kg P/ha with 3 replications were installed. The experiment was conducted for 2 years (1973–74, 1974–75), with leaf‐N, P, and K; yield; seed‐N, P, and K; Bray P2 (0.03N NH4F + 0.1N HC1) avail‐able‐P and soil pH measurements completed each year. Data was evaluated with linear, quadratic, logarithmic, polynomial, segmented line, and multiple regression using the coefficient of determination as goodness of fit. The best model fit between P treatment and Bray P2 available‐P was a quadratic equation; the model between relative yield and Bray P2‐P with 54% of the relative yield attributed to Bray P2 available‐P, a sigmented line. This model indicated point of maximum yield (91% relative yield) was obtained at 7.4 ppm‐P, with no increase in relative yield with increasing levels of soil available‐P. To calculate the P fertilizer necessary to increase available soil‐P to the level of maximum yield of equation Yp = [1639(7.4 ‐ xs)]1/2, where Yp = kg P/ha fertilizer needed; and xs = initial Bray P2 soil available‐P in ppm's. The lime effect upon soil pH was best described as a linear relationship. Yield increase with lime at this site was not significant at the 5% level. The leaf‐N, P, and K increased significantly with soil available‐P levels. A second degree polynomial with logarithmic function best defined these relationships. The calculated DRIS indices and sum proved useful to evaluate the plant‐N, P, and K balance of each treatment. Only seed‐P level was directly related to soil available‐P. Both seed‐N and seed‐K were highly correlated with indirect effects of soil available‐P levels. Results from this study suggest the segmented line model would best interpret soybean yield response to Bray P2 available‐P for this soil. To obtain maximum yield using this model rather than the second degree polynomial would require less fertilizer P. Foliar analyses interpretation confirmed adequate plant‐P level would be supplied for maximum yield at this level of fertilization.