Leaf phosphorus and sorghum yield under rainfed cropping of a Vertisol

Little attention has been devoted to the calibration of plant P tests for s orghum, especially under rainfed cropping although information is needed fo r developing an efficient P management strategy for increasing crop product ivity. A field experiment was conducted for three years (1987-1989) to stud y the response of sorghum to fertilizer P (0, 10, 20 and 40 kg P ha(-1)) on a Vertisol, low in extractable P, at the ICRISAT Center, Patancheru (near Hyderabad), India. One sorghum crop was grown each year during the rainy se ason (June-September). Leaf tissue samples consisting of newest, fully-deve loped leaf, were collected at 50% flowering stage of the crop, for establis hing relationship between leaf P concentration and grain yield. During the three years,sorghum grain yield and leaf P concentration increased in respo nse to P application up to 40 kg P ha(-1) and the leaf P concentration was linearly related to grain yield (r(2) varied from 0.724 to 0.993). The crit ical leaf P concentration at 90% of the maximum grain yield was found to be about 0.25% P. Phosphorus content in the grain was not significantly corre lated to yield.