Exploring the Yield Response of Paddy ( Oryza sativa L.) under Varying Levels of Soil Nitrogen, Phosphorus and Potassium

Assessment of crop responses under varying levels of soil available nitrogen (N), phosphorus (P) and potassium (K) is important to develop site-specific nutrient management (SSNM) strategies. This study was conducted to investigate the spatial variability of soil N, P and K levels in paddy grown soils in Anuradhapura, Kurunegala and Polonnaruwa districts and corresponding yield response for nutrient application. Seventy-one paddy fields were selected and four plots (3×6 m) were established at each site comprised of a treatment applied with ample amount of N, P and K and corresponding omission treatments. At harvesting, total yield of each plot was recorded. Considerable heterogeneity of soil N (12.5-43.9 mg/kg), P (2.7-34.2 mg/kg) and K (14.6-221.9 mg/kg) was observed in paddy grown soils of the three districts. A significantly higher average yield of N applied treatment (5.8 Mg/ha) than that of N omitted treatment (3.9 Mg/ha) indicated a yield response for N application. However, nonsignificant differences of average yields of P (5.7 Mg/ha) and K (5.6 Mg/ha) omitted and ample treatments (5.8 Mg/ha) revealed a poor yield response for the application of these nutrients. Stepwise multiple regression analysis revealed that neither the initial soil P and K levels nor the other studied factors (growing season, irrigation scheme, soil texture, pH, EC, available Fe and Mg) have an impact on yield responses observed for P and K applications. Our results indicated large spatial heterogeneity in soil P and K levels in paddy grown soils. Absence of a significant response for the applied P and K and a considerable variability of yield responses highlighted the need of further studies on P and K reserves of paddy growing soils and factors controlling the variability in yield response. Further, site-specific application of N is shown as a priority to improve the productivity of paddy lands.