Sulfur mineralisation in a coarse-textured soil after different sulfate fertilisation histories, and yield responses of wheat and lupin

The effect of annual applications of gypsum at rates of 0, 5, 15, and 30 kg S/ha on grain yields of lupin, wheat, and canola were studied between 1992 and 1995, and the residual value of these sulfate (SO 4 2– ) inputs was examined in 1996 and 1997. The top 0.1 m of soil was collected in 1994, 1996, and 1997 to determine KCl-40 extractable S. Extensive soil sampling was undertaken in 1996 and 1997 to determine quantities of SO 4 2– in soil layers to 0.5 m, and to measure rates of net SO 4 2– mineralisation. Application of gypsum resulted in significantly higher grain yields in 2 out of 3 wheat crops, and in canola, but SO 4 2– fertiliser use did not increase lupin yield. Wheat yields were also significantly higher in treatments with a prior history of gypsum use but lupin grain yields were not enhanced by previous gypsum applications. The quantities of SO 4 2– in the top 0.2 m of soil decreased sharply in the first year after SO 4 2– fertilisers were withheld with further declines in SO 4 2– to low levels across all treatments in the second year gypsum was not applied. A buildup of SO 4 2– was evident in soil between 0.3 and 0.5 m. However, S budgets for the top 0.5 m suggested that little SO 4 2– leached below 0.5 m in the years studied. Previous sulfur fertilisation did not significantly affect the total growing season net SO 4 2– mineralisation, which was 2.2–8.0 kg S/ha. Mineralisation of organic S over a summer fallow period (0–5.8 kg S/ha) and the total S net mineralised over 17 months (7–17 kg S/ha) was significantly affected by previous S fertiliser history. Net SO 4 2– mineralisation was not correlated with the labile organic S fraction extracted using the KCl-40 soil test.