FERTILIZER PHOSPHORUS RECOMMENDATIONS FOR WINTER-WHEAT IN TERMS OF METHOD OF PHOSPHORUS APPLICATION, SOIL-PH, AND YIELD GOAL

The objective of this study was to predict the most profitable amount of P fertilizer applied to winter wheat in terms of grain yield increase as influenced by soil test P, method of P application, and projected yield level. Winter wheat (Triticum aestivum) was grown in 22 field experiments between 1978 and 1982 with treatments consisting of six P rates applied with the seed and broadcast. The soils were Keith-Holdrege and Uly-Colby silt loam (fine-silty, mixed mesic Typic Argiustolls). Yield increase to applied P was related by multiple regression to rate and method of P application, soil P level, soil pH, and grain yield level (R2 = 0.76, SE = 0.20 Mg ha−1). This function estimates maximum profit at specific fertilizer and grain prices. The regression function provides a mathematical solution for incorporating yield goal into fertilizer P recommendations. The model shows that P requirements for winter wheat increase as the yield level increases. Most importantly, the model indicates that it is economically prohibitive to follow the traditional recommendations of doubling the rate of broadcast P compared to seed applied P for winter wheat. When grain yields are increased with applied P, seed applied P results in much higher grain yield increase per kilogram of applied P than broadcast P. This results in being able to pay for more P when seed applied than broadcast. At medium and low levels of soil P, seed application resulted in 2 to 4 times more profit from fertilizer P application than broadcast P. © 1989 Soil Science Society of America.