The effect of irrigated crop production on the properties of a sodic vertisol

Abstract

Irrigated cotton yields in the lower Macquarie Valley, New South Wales, declined within a decade of the inception of irrigation, particularly on grey clays. Degradation of subsoil structure was thought to be involved. Two adjacent sites, one used for irrigated cotton production since 1968 ('cultivated'), the other for native pasture and grazing ('uncultivated'), were compared. The soil at each site, a sodic grey cracking clay (Vertisol), was described and sampled for a range of physical and chemical analyses. In the cultivated soil there was increased surface sodicity, greater dispersion of soil aggregates at 0 - 0-3 m, decreased organic carbon levels to 0 - 3 m and increased subsoil compaction. Also, the cultivated soil was observed to have more massive structure, with fewer macropores, between 0.2 and 0.7 m than the uncultivated soil. Probable reasons for this degradation of chemical and physical fertility following 15 years of intensive irrigated farming are discussed.