The effect of gypsum and irrigation frequency on irrigated wite clover pastures grown on three soils of the Riverine Plain of south-eastern Australia

Abstract

Gypsum was applied at 10-1 5 t/ha to white clover (Trifolium repens L.) pastures grown on three Riverine Plain soils, viz Billabong clay, Conargo sandy loam and Purdanima sandy loam that had coarse textured surface horizons of < 2.5,15 and 30 cm depth, respectively, underlain by clay subsoils. Gypsum treatment doubled growth rates where the depth of the coarse textured horizon was 15 cm or less. This was attributed to increased water entry during irrigation and higher subsoil moisture contents resulting from increased hydraulic conductivity after gypsum treatment, which were only effective where the subsoils occurred at less than 30 cm, the effective depth of water extraction by the plants. Optimum irrigation intervals for the white clover pastures were unaffected by gypsum treatment on Billabong clay and Purdanima sandy loam, where the corresponding evaporation intervals from a United States water basin class A pan were 38 mm and 51 mm, respectively. On Conargo sandy loam with 15 cm surface horizon, gypsum treatment increased the optimum irrigation interval from 37 mm to 73 mm. Gypsum treatment also halved the amount of water used to produce a unit of dry matter on Billabong clay. A critical exchangeable sodium percentage value of around 6 for the clay subsoils is indicated; above this gypsum response can be expected. For irrigated white clover pastures, subsoils need to be < 30 cm below the surface and have clay contents < 25% before significant yield increases to gypsum can be expected.