RELATIONSHIP BETWEEN EXTRACTED PHOSPHORUS AND SORGHUM YIELD IN A VERTISOL AND AN ALFISOL UNDER RAIN-FED CROPPING

Little attention has been devoted to calibrating the soil tests for P in the field for crops grown under rainfed conditions in different soi l types. Field experiments were conducted during the 1990 rainy season (June-September) at the ICRISAT Center, Patancheru (near Hyderabad), India on nearby Vertisol and Alfisol sites having a range in extractab le P, for establishing relationships between extractable P and sorghum yield. In the Vertisol, 90% relative grain yield of sorghum was obtai ned at 2.8 mg kg(-1) Olsen extractable P while in the Alfisol, 90% rel ative grain yield was achieved at 5.0 mg P kg(-1) soil. These results suggest that a single critical limit of available P does not hold true for grain sorghum in the two soil types under similar agroclimatic co nditions and that the critical limit is lower for the clayey Vertisol than the sandy Alfisol.