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

Abstract

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 (SO42–) 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 SO42– in soil layers to 0.5 m, and to measure rates of net SO42– mineralisation. Application of gypsum resulted in significantly higher grain yields in 2 out of 3 wheat crops, and in canola, but SO42– 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 SO42– in the top 0.2 m of soil decreased sharply in the first year after SO42– fertilisers were withheld with further declines in SO42– to low levels across all treatments in the second year gypsum was not applied. A buildup of SO42– was evident in soil between 0.3 and 0.5 m. However, S budgets for the top 0.5 m suggested that little SO42– leached below 0.5 m in the years studied. Previous sulfur fertilisation did not significantly affect the total growing season net SO42– 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 SO42– mineralisation was not correlated with the labile organic S fraction extracted using the KCl-40 soil test.