Organic matter and soil fertility changes under irrigated pastures were followed for 5 years at Deniliquin, N.S.W. The effects of three annual pastures and of three perennial pastures were studied. Four years after their establishment an organic matter layer (mat) had formed under all pastures, and after its ofrmation there was no further accumulation of organic carbon in the 0 - 3 in. soil horizon. The mean annual increase in organic carbon was 625 lb/acre under annual pastures and 1146 lb/acre under perennial pastures. The carbon/nitrogen ratios of both soil and mats, and the relationships of both organic carbon and nitrogen to the cation exchange capacity of the mats, were similarly affected by the annual and perennial pastures. The heterogeneous nature of the mats obscured any differences in their carbon/nitrogen ratios, which ranged from 12:8 to 22:0. The cation exchange capacity of the 0-3 in. soil horizon remained unchanged. The cation exchange capacity of the organic matter of the mats was approximately 100 m-equiv./100 g. After mat formation the underlying soil had a pH of approximately 6.0 under clovers and 6.5 under ryegrasses. The pH values of the mats ranged from 5.0 to 6.6.
