NITROGEN AND PHOSPHORUS SUFFICIENCY LEVELS ASSESSMENT FOR IRRIGATED LOWLAND RICE GROWTH AND YIELD IN CYUNUZI, EASTERN RWANDA

The present study was undertaken in Cyunuzi (Eastern Rwanda) paddy field using “upland 26’’ variety with the objective of assessing the sufficiency levels of nitrogen and phosphorus in order to develop tools for fertilizer recommendation for optimal rice growth and yields. The study was carried out as a three replicate 4x3factorial experiment laid in a Randomized Complete Block Design. The treatments included four nitrogen (urea) levels ( 0, 40, 80 and 120kg N/ha), three phosphorus (single super phosphate) levels( 0, 34 and 70kg P2O5/ha) and their combinations. All experimental plots received potassium fertilizer (KCl) at the rate of 34kg K2O/ha. The soil in experimental plots was analyzed for total N, available P (Bray I), exchangeable K, organic C, CEC, pH and bulk density prior to treatments application and for pH, total N and available P at the harvest. The crop (Oryza sativa) leaves were analyzed for total nitrogen and phosphorus at the flowering stage. Plant height, tiller number, leaf area and the above-ground biomass were measured as growth parameters whereas the yield components including the number of panicles per plant, total grain per panicle, filled grain per panicle, empty grain per panicle, weight of 1000 grains, the yield per plant and the total yield per plots were determined at harvest. The results showed that applied phosphorus had no significant effect on grain yield (P<0.05) and did not affect any yield component. Nitrogen application significantly improved the crop growth and yield by increasing plant height, leaf area, tillering and panicle numbers. Observed differences in grain yield among nitrogen rates were significant (P < 0.01). Tillering activity was the growth parameter that affected most significantly the grain yield, therefore the tiller number was used in estimating the soil nutrients sufficiency levels along with the yield functions. The levels of soil N in the experimental area were estimated to reach only 96, 99 and 97% of the amount required for maximum vegetative growth of the crop, respectively, under 0, 34 and 70kg P2O5/ha. For maximum grain yield, the soil nitrogen sufficiency levels reached 98, 99 and 98% under 0, 34 and 70kgP2O5/ha, respectively. It is recommended that nitrogen and phosphorus should be applied at rates of 109kg N/ha and 34kg P2O5/ha but this should be periodically reviewed depending on the actual soil P availability.