Field calibration of a soil sulfate test on sward lucerne on the eastern Darling Downs, Queensland

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Abstract
The relation between phosphate-extractable sulfate and the response of lucerne to applied sulfur was examined on 20 established lucerne (Medicago sativa) swards on the black earths of the Darling Downs. Two rates of S were used (0.100 kg ha-1) and soil depths to 90 cm were considered. Six sites responded at the first cut and relative yield of lucerne was correlated with soil sulfate. The best fit of the data was obtained using a square root quadratic model and sulfate in the 0-80 cm zone. The critical soil level was 3.5 ppm. Of 17 attributes examined, only chloride concentration (0-60 cm) and conductivity of the soil water suspension (0-60 cm) significantly improved the variance in relative yield explained, but this was considered to be fortuitous. The relative importance of sub-soil sulfate (40-80 cm) to lucerne nutrition on these soils is shown.