A SOIL SULFUR TEST FOR PASTURES AND CROPS

Soil testing for S has generally been unsuccessful when using extractants that remove only sulfate from the soil. An assessment of a range of extractants to predict S status was undertaken on soil samples taken from 18 field trials in northern N.S.W. The extractants were water, 0.01 M monocalcium phosphate (MCP) and 0.5 M NaHCO3, 0.25 M KCl heated for 3 h at 100, 80, 40, or 25-degrees-C. The highest correlation between soil S test level and % maximum yield was found in the 40-degrees-C KCl extractant (r2 = 0.73). This compares with an r2 value of 0.47 for the widely used MCP extractant. A study using a soil from a pot experiment where rice was grown showed that the KCl extract removed more S from the HI reducible (ester sulfate) fraction than did MCP. This S fraction is believed to be important in supplying S to plants. A comparison of the specific radioactivity of soil extractants and rice plants confirmed that the KCl 40-degrees-C extract removes S from similar soil pools as do plants. The procedure is recommended for wider evaluation.