Responses to sulphate and elemental S in six provinces in China and the utility of mono-calcium phosphate extractable S in predicting soil S status

Reductions in atmospheric inputs of S in rainfall, and the world's reliance on non-, or low-S containing TSP, DAP, MAP and urea has resulted in a widening gap in the S input/output balance, and an increasing incidence of S deficiency in agriculture. A technology to incorporate elemental S, with or without sulphate S, into ammonium phosphate fertilizers has been developed and patented by Shell. The fertilizers produced by this process are generically called "Sulphur Enhanced Fertilizer" (SEF or Thiogro). A wide range of experimental N, P, S formulations have been made, with elemental S concentrations up to 15% or greater. Agronomic testing of several of these products has been undertaken in a wide range of crops in temperate and tropical environments in China, generating a weighted mean crop yield increase to Thiogro of 13%, compared to the zero S control, in S responsive sites. Soil analyses from 82 of the 101 field trials were used to establish the utility of the mono-calcium phosphate extract to estimate the potential S response at each site. The accepted critical soil S test value for the MCP extract is 10 mg/kg. Predictability of an S response was low (47% correct) at low soil test values and acceptable (72%correct) on non-responsive sites. These data indicate that MCP extraction is not a good way to identify potentially S responsive sites.