The effect of gypsum on cation exchange in two red duplex soils

Gypsum at rates of up to 15 t ha-1 was applied to the surface 10 cm of two red duplex soils in northern Victoria. At both sites, approximately five years after application, the following changes in soil properties were established: 1. Only when 15 t ha-1 of gypsum was applied was exchangeable sodium in the upper 25 cm significantly decreased. 2. Exchangeable magnesium was not decreased at depths greater than 15 cm. After five years only the 15 t ha-1 treatments still contained some undissolved gypsum in the A horizon. Of the gypsum that had dissolved, 53% had been involved in exchange. This represented an efficiency of exchange of 26-34%, based on the ratio of sodium replaced to calcium dissolved. At both sites over this five-year period, the mean solubility of gypsum from the 15 t ha-1 treatment was 120-130 mm of rainfall per t of gypsum.