Availability of Desorbed Phosphorus and Internal Phosphorus Requirement by Sorghum in an Alfisol

Response of sorghum to P was strongly influenced by soil P status as well as applied P level. The response pattern was similar at all the three physiological stages of crop growth sampled, viz. boot leaf initiation, 50 per cent flowering and maturity, and could be described by quadratic equations involving two variables i.e., soil P and fertilizer P. The fertilizer P requirement for optimum dry matter production decreased with increase in P status of soils. A balance sheet of P showed that P uptake by sorghum matched the calculated availability of applied P at low P availability conditions. However, at higher P status, P uptake by sorghum was less than that of the estimated available amount. Phosphorus concentration in the third leaf was higher at the boot leaf stage (BLS) compared with 50 per cent flowering and at maturity. The relationship of P concentration with dry matter yield as revealed by a quadratic equation was linear at BLS and inverse linear at 50 per cent flowering and at maturity indicating a decrease in P concentration in plant tissue with the advancement of maturity. The internal P demand of sorghum in soils with varying contents of Olsen-P can be computed from the equations reported in this paper.