The spatial pattern of sodium in soils was investigated in a catenary sequence in central Rhodesia. Sodic horizons occurred predominantly in mid-slope and are related to (a) fluctuations in water table, (b) poor lateral drainage and a perched water table and (c) the weathering rate of feldspars. Observations suggest that a subsurface dam wall is responsible for lack of groundwater drainage and that this impediment to lateral drainage is caused primarily by excess sodium and the dispersion of clays. Near-surface fluctuations of the water table encourage weathering and increase the sodium concentrations. The existence of a perched water table and readily dispersibte soil may have important implications in erosion, explaining why sodium-rich soils are often associated with severe erosional phenomena especially in tropical and subtropical environments.