Response of upland rice ‘Nerica-4 ’to rates of N and P fertilizers in south west Ethiopia

Decline in soil fertility is a major constraint in Abol Woreda of Gambella region of SW Ethiopia due to the continuous cropping, removal of crop residues after harvest, poor application of organic manures to soils, etc. The scientific information available concerning the response of upland ‘Nerica-4’ rice variety to rates of N and P fertilizers in Gambella for its optimum productivity is limited. Therefore, a field experiment was carried out during 2014 cropping season at Abol Woreda, Gambella, South West Ethiopia to investigate optimum rates of N and P fertilizers for highest yield of rice (Oryzasativa L.). The treatments consisted of factorial combinations of four rates of N (0, 46, 92, and 138 kg N ha−1) and P (0, 23, 46, and 69 kg P ha−1) was laid out in a randomized complete block design (RCBD) with three replications. The effects of nitrogen were significantly (P = 0.05) different for harvest index, 1000-grain weight, plant height, panicle length, grain yield, straw yield, number of filled spikelets and unfilled spikelets, productive tillers, number of tillers and dry grain weight. On the other hand, many growth, yield, and yield components did not differ significantly (P > 0.05) due to application of P except number of unfilled spikelets, straw yield, and grain yield. Conversely, the interaction of N and P was significant (P = 0.01) for all yield and yield components except straw yield, harvest index and panicle length. Application of N and P significantly (P = 0.01) increased grain yield of rice up to the level of 92 kg N and 69 kg P ha−1. Highest grain yield (6.05 t ha−1) was obtained with the combined application of 92 kg N ha−1 and 69 kg P ha−1, and the yield advantage over the control was 113.7% (3.22 t ha−1). The economic analysis revealed that 92 kg N ha−1 with all rates of P (0, 23 and 46 kg) gave highest net benefits. Grain yield was significantly and positively correlated with straw yield (r = 0.66**), productive tillers (r = 0.423**), plant height (r = 0.632**), grain weight (r = 0.54**), panicle length (r = 0.453**), number of spikelets panicle−1 (r = 0.441**) and number of filled grain (r = 0.362**). This indicates that N and P increase grain yield of rice by positively affecting the important yield components. Therefore, it is inferred that farmers in the region can apply 92 kg N ha−1 with no phosphorus application to improve the grain yield of rain-fed Nerica-4 rice as well as for highest economic benefit.