Soil sulfur status, sulfur fertiliser responses and rainfall sulfur accessions at trial sites in the National Reactive Phosphate Rock Project

The sulfur (S) status of the National Reactive Phosphate Rock Project trial sites was determined using 2 chemical extractants KCl-40 (0.25 mol KCl/L heated to 40 degrees C) and MCP (0.01 mol mono-calcium phosphate/l). There were marked differences between sites in KCl-40 extractable S down the soil profile. Soil profiles were classified into 4 groups according to the shape of the extractable S profile. KCl-40 was better able to distinguish between responsive (significant at P = 0.10 only) and unresponsive sites than MCP. Mean S accession in rainfall over the 3 years ranged from 0.40 kg/ha.year at Jericho (site T17) in the centre of Tasmania to 20.33 kg/ha.year at Smithton (site T16) on the north-west coast of Tasmania. The proportion of S-responsive sites increased from 26% in the first year to 43, 42 and 48% in years 2, 3 and 4 respectively. Such a large proportion of S-responsive sites limits the use of direct application of reactive phosphate rock fertiliser.