Split Nitrogen Application Improves Yield and Partial Factor Productivity of Nitrogen in Rice (cv. RD49) Under Low-Fertility Sandy Loam Soil

Nitrogen (N) management is critical for improving rice productivity, but increasing N inputs may reduce N-use productivity when grain production does not increase proportionally. This study evaluated the effects of urea rate and application method on grain weight and partial factor productivity of N (PFPN) of rice (Oryza sativa L. cv. RD49) grown in low-fertility sandy loam soil under controlled greenhouse pot conditions. Three urea rates (0.53, 1.07, and 1.60 g pot−1) and two application methods—basal application (BD) and basal plus topdressing (BTD)—were evaluated across two growing seasons. Grain weight was significantly affected by urea rate, application method, and their interaction. The lowest urea rate (N1; 0.53 g pot−1) maintained grain weight comparable to the highest rate (N3; 1.60 g pot−1) while producing the highest harvest index and substantially greater PFPN. Grain weight exhibited a significant quadratic response to urea rate, whereas PFPN, calculated on the basis of total fertilizer N applied, showed significant linear and quadratic responses and declined markedly with increasing urea rate. BTD resulted in greater mean grain weight and PFPN than BD. These findings indicate that the lowest tested urea rate provided a favorable balance between grain production and fertilizer-N input under the conditions evaluated, while the greater grain weight and PFPN under BTD highlighted the importance of coordinating N rate and application timing. These findings provide a basis for targeted field validation of N rate and application timing for RD49 rice grown in low-fertility, coarse-textured soils.