The nitrogen balance and the availability of residual nitrogen of green manures in relation to their chemical composition were studied in pot and incubation experiments.Azovla,milk-vetch and common water hyacinth were used as green manures and labelled by the fertilization of (¹⁵NH₄)₂SO₄.Result obtained are summarized as follows:1.Although plant materials having lower C/N ratio usually give more available nitrogen and the C/N ratio of azolla was lower than that of milk-vetch,yet azolla contained high lignin about 20%,and consequently the percentage recovery of mitrogen by rice plant from azolIa was much lower than that from milk-vetch.