Assessing the fertilizer phosphorus requirement of grain sorghum

Assessing the fertilizer phosphorus (P) requirements (FPR) of crops is an i mportant component of research for efficient and rational use of fertilizer s. Soil and plant tests are employed to determine the FPR. Two methods were evaluated for determining the FPR of sorghum grown on a Vertisol under rai nfed conditions. The first method was based on P on applied P uptake, and g rain yield relationships for grain sorghum. In this method, P uptake at a g iven yield was determined from the relationship between total P uptake and grain yield. The amount of fertilizer P applied for the given P uptake and grain yield was then, determined from the relationship between P applied an d P uptake. In the second method, FPR was determined using the equation: FP R=(Up-U0)/PRF, where Up is P uptake at a given yield, U0 is P uptake from u nfertilized soil, and PRF is the recovery of applied P. The parameters, U0, Up, and PRF were determined in a field experiment with sorghum grown on a Vertisol under rainfed conditions. There was a good agreement between the o bserved value of FPR and predicted values determined by the two methods. Th ese results suggest that the simple models based on P uptake can be utilize d for determining the fertilizer requirements of crops.