Direct and residual phosphorus effects on soil test values and their relationships with grain yield and phosphorus uptake of upland rice on an ultisol

Abstract

Establishing relationships between extractable phosphorus (P) in the soil and grain yield and P uptake in long-term field experiments are crucial for evaluating the efficacy of soil tests for determining the P requirements of crops. A field experiment was conducted for six years (1993–1998) to evaluate the response of four improved upland rice cultivars to direct and residual P. Fertilizer P was applied at 0, 45, 90, 135, and 180 kg P ha− 1 once in 1993 and the crops were grown during 1993–1998 without any fresh application of fertilizer P. The relationship between Bray 1 P and fertilizer P added in the initial P buildup phase were described by a linear regression equation (R=0.985). The prediction of decline in Bray 1 P with time, during the residual P phase was affected by P rate and well described by linear regression equations. The relationships between soil P test and P uptake were closer than between soil P and grain yield. The relationships between Bray 1 P and grain yield or total P uptake of the crop were significant in 1994 and 1995 seasons but not in the 1996 and 1998 seasons when the Bray 1 P values in all P treatments had fallen to low values. Results suggest that Bray 1 P indicated the availability of P to rice crops and thus can be used for determining the long-term fertilizer P requirements of upland rice in the Ultisol.