Optimum incorporate age of S. rostrata and A. afraspera to release NH4+-N in saline soil.   Somsri Arunin, Manop Tuntatameya Yuttachai Anulaxtipun and Phirach Pongwichian   ABSTRACT        The study on optimum incorporation age of Sesbania rostrata and Aeschynomene afraspera was conducted at Ban Thabo, Tawatburi district, Roi-et province during 1990-1993. Two species of green manure were planted and incorporated at 30, 45, 60, 75 and 90 days after sowing as green manure for rice. Chemical fertilizer as urea was used to compare with green manure. The experimental design was 2x5 factorial with 2 checks under randomized complete block design with 3 replications. It was found that biomass and N-accumulation of S. rostrata and A. afraspera were increased with increasing age of incorporation. These two legumes gave high N-content of 2.326 and 2.275 % but they have low C/N ratio of 13.7 and 12.7, respectively. S. rostrata showed higher biomass and N-accumulation than A. afraspera. After incorporation and used as green manure for rice, the result showed that rice yield increased with increasing age of legumes. However, the effect of legumes species was not found. The difference between utilization of green manure and chemical fertilizer was also not found. And rice yields varied with amount and distribution of rainfall. The results on N-mineralization showed that the released pattern of N from both species were similar at all ages of incorporation during 2-28 days after incorporation. S. rostrata at 90 days old gave the highest soil NH4+-N both in the field of open and close system and laboratory of 28.7, 66.7 and 24.0 mgN/kg soil at 28, 14 and 3 days after incorporation, respectively. Similarly, A. afraspera at 75 days old gave the highest soil NH4+-N of 22.8, 37.1 and 29.0 mgN/kg soil at 28, 14 and 3 days after incorporation in all systems.         Keywodrs : Sesbania rostrata, Aeschynomene afraspera, optimum incorporation age, N-mineralization