Effect of NPK on growth and nitrogen fixation of Sesbania rostrata as a green manure for lowland rice (Oryza sativa L.)

The stem-nodulating tropical legume Sesbania rostrata is a promising green manure species for low input rice-farming systems in lowland areas. However, its success as biofertilizer depends on its biomass production and N₂ fixation. Nutrient imbalances and soils low in available nutrients can considerably affect biofertilizer production. Use of mineral N, P, and K fertilizers in growing S. rostrata as biofertilizer for lowland rice was therefore evaluated in pot experiments, and in the fields in Central Luzon, Philippines. Two soils low in Olsen P (3-7.3 mg kg⁻¹) and exchangeable K (0.05-0.08 meq 100 g⁻¹) were used. Increasing amounts of N (0, 10, 20, 30, and 40 mg kg⁻¹), P (0, 50, and 100 mg kg⁻¹), and K (0, 100, 200, and 300 mg kg⁻¹) were applied to S. rostrata grown in the greenhouse, whereas small amounts of N, P, and K fertilizers (30, 15, and 33 kg ha⁻¹, respectively) were applied in the field. Mineral N application depressed nodulation and N₂ fixation in roots. It however, stimulated nodulation and N₂ fixation in stems. Applying 30 kg N ha⁻¹ as urea increased total N accumulation by 5. rostrata and yield of the subsequent rice crop (IR64). Applied P and K both stimulated growth, nodulation, and N₂ fixation of 5. rostrata. Nitrogen accumulation in P-and K-fertilized S. rostrata was about 40% higher than that in nonfertilized green manure. Thus integration of mineral N, P, and K fertilizers in a green manure-based rice-farming system can considerably improve biofertilizer production and increase rice grain yield.