PHOSPHORUS SOIL TEST CALIBRATION FOR LOWLAND RICE ON AN INCEPTISOL

Efficient use of an adequate level of P in crop production on high-P-f ixing acid soils is essential, due to high crop response to P Fertiliz ation and the high cost of P fertilizers. Soil-test-based P fertilizat ion recommendations are not available for lowland rice (Oryza sativa L .) grown on Inceptisols. The objective of this study was to calibrate P soil testing for the response of lowland rice (cv. Javae) to added P . Two field experiments were conducted in two years at two locations i n central Brazil on Haplaquept Inceptisols. In the first year, the bro adcast P levels used in both experiments were 0, 87, 175, 262, 350, an d 437 kg P ha(-1), applied as triple superphosphate. In the second yea r, each residual broadcast P level from the previous year received ban d application of 0, 22, 44, and 66 kg P ha(-1). Soil test levels of Me hlich-1 extractable P were categorized, based on relative grain yield from the first year, as very low (0-2.6 mg P kg(-1) soil), low (2.6-8. 8), medium (8.8-13.0), or high (>13.0). The amount of broadcast P need ed to build up soil P test concentrations at very low, low, medium, an d high soil test categories was 80, 220, 292, and >444 kg P ha(-1), re spectively. The rice crop produced 0 to 70%, 70 to 95%, 95 to 100%, an d 100% of relative yield at the very low, low medium, and high P level s, respectively. The amount of banded Fertilizer P required to obtain maximum yield at both locations was 66 kg P ha(-1) for the very low an d low soil P levels, 44 kg P ha(-1) for the medium soil P level, and 2 2 kg P ha(-1) for the high soil P level.