PASTURE YIELD RESPONSES TO SULFUR FERTILIZERS IN RELATION TO NUTRIENTRATIOS IN WHITE CLOVER

The agronomic performance of triple superphosphate (TSP), superphospha te (SSP), 15% (15SS), 22% (22SS), 33% (33SS), 36% (36SS) and 50%(50SS) S superphosphate, TSP/S, diammonium phosphate (DAP)/S, RPR (North Car olina phosphate rock/S), plus no S or P was measured over three years on a ryegrass/white clover sward near Dipton (mean annual rainfall 102 0 mm) in Central Southland, New Zealand. These fertilisers were applie d in November 1994, and in September 1996, SSP, TSP, TSP/S, DAP/S, and RPR/S were re-applied. There was a large pasture yield response to al l treatments compared with no S or P, but no significant difference in pasture yield between SSP, 33SS, 36SS, 50SS, and DAP/S. Superphosphat e was higher yielding than TSP, 15SS, 22SS, TSP/S, and RPR/S. The rate of S-o applied initially was strongly related (r(2) = 0.97) to total pasture yield. There were no significant differences in the proportion of S-o oxidised 30 months after application between treatments. In th e first two years, clover S/P ratios were above the optimum range for balanced nutrition (0.7-0.8) for all fertilisers except 15SS, TSP, TSP /S, DAP/S, and RPR/S. Triple superphosphate, 15SS, and 22SS had clover S/P below the optimum range in the third year. Clover 100 S/N ratios for 33SS, 36SS, and 50SS were in the optimum range (4.8-5.0 for 98% RY ) for most of the trial and increased into that range for SSP, TSP/S, DAP/S, and RPR/S by the end of the third year. Most other treatments h ad clover S/N ratios below the optimum range. In this Southland enviro nment there was no pasture yield response from applying extra S-o over and above an average of 22 kg sulphate S ha(-1) yr(-1) (SSP). A lower average rate of 5 kg sulphate S ha(-1) yr(-1) plus 15 kg S-o ha(-1) y r(-1) (TSP/S, RPR/S), resulted in a reduction in pasture yield compare d with SSP. To achieve similar pasture yield to SSP over three years, an initial application of at least 33 kg sulphate S ha(-1) and 77 kg S -o ha(-1) (33SS) was required.