PHOSPHORUS AVAILABILITY TO IRRIGATED LOWLAND RICE AS AFFECTED BY SOURCES, APPLICATION LEVEL AND GREEN MANURE

To evaluate alternative fertilizer phosphorus (P) sources in lowland r ice, two field experiments were conducted under irrigated conditions i n Quezon Province, Philippines during 1990-1991 crop year. In another field experiment fertilizer P recycling through a green manure crop ap plied in the succeeding rice, was studied. Addition of fertilizer P in creased grain yield by 1.5-2.0 t/ha (46%) in 1990 wet season (WS) and by 1.6-2.1 t/ha (56%) in 1991 dry season (DS). However, fertilizer P s ource and application level did not effect grain yield significantly. Results indicated that the less water-soluble and less expensive parti ally acidulated phosphate rock (PAPR), phosphate rock (PR) and less re active PR were as effective as the more soluble but more expensive tri ple superphosphate (TSP). The relative effectiveness (RE) of local gua no was significantly lower than that of other sources of fertilizer P. Fertilizer P applied to a pre-rice Sesbania rostrata green manure inc reased rice grain yield by 1.5-1.9 t/ha during 1991 DS. Further, S. ro strata fertilized with Morocco phosphate rock (MPR) gave significantly higher rice grain yield than did rice fertilized with MPR applied alo ne. In the P source experiments Olsen method and Pi correlated better with growth attributes than Bray 2 P. Phoshorus uptake did not differ significantly among P sources and levels. Results suggest that P uptak e was improved with green manuring. Correlation analyses revealed a cl ose correlation between P uptake and dry matter yield and P uptake and grain yield.