The effect of gypsum on the properties and crop productivity of 6 contrasting vertisols of the Gwydir Valley, New South Wales was investigated in 1978 and 1979. These soils are often used for dryland wheat production, although crop growth is generally restricted by their structural instability. In 2 of the soils used in our study, the surface aggregates were sodic and dispersive (poor soils), 2 were relatively stable when wetted (good soils), whilst the other 2 soils had surface aggregates that were intermediate in behaviour (intermediate soils). The effects of added gypsum at 4 rates (0, 2.5, 5.0 and 7.5 t ha-1) on soil water profiles, soil properties and the growth of wheat were monitored over a 2 year period. Dryland wheat grain yields were increased by as much as 230% following the application of gypsum. Benefits were most pronounced on clays with sodic topsoils, a high water-holding capacity and adequate nutrition; plant response to gypsum on nearby soils with non-dispersive surfaces was less pronounced. Yield increases were associated with better seedling establishment, greater tiller production, increased grain weight, and lower incidence of 'Crown Rot' disease. Plant response to gypsum was related to improved water penetration into the soil, allowing greater storage of water in the subsoil, rather than loss via evaporation and possibly runoff. Increases as high as 137% in the soil water storage to a depth of 1.2 m were observed. Crop performance was also strongly influenced by rainfall, time of sowing and weed control. Where nitrogen and, to a lesser extent, phosphorus, were deficient in gypsum-treated soil, they had to be added before the extra soil water could be utilised effectively by wheat. On the lighter textured clays, gypsum appeared to aggravate nitrogen deficiency, apparently because of enhanced leaching.
