Effect of crop duration and soil type on the ability of soil sulfur tests to predict plant response to sulfur

Soil testing for sulfur (S) is becoming increasingly important as soils become more S-deficient due to lower S inputs in fertiliser and higher offtake in product. Because of the differing rates of S supply required by a range of crops, the relative importance of the soil sulfate and organic S pools could be expected to vary between crops. A glasshouse experiment was conducted using an Aquic Haplustalf of granitic origin and an Ultic Haplustalf of basaltic origin collected from unfertilised pasture sites on the Northern Tablelands of New South Wales from Uralla and Walcha, respectively. The soil was labelled with S-35. The experiment consisted of 2 rates of S fertiliser (0 and 17.7 mg/kg soil, equivalent to 0 and 30 kg S/ha), 3 crop species (radish, corn, ryegrass), and 2 soils (granite and basalt). S was extracted using 2 extracts, mono-calcium phosphate (MCP) and 0.25 M KCl heated at 40 degrees C (KCl-40), termed primary extracts, and the components of S in the extracts were determined. The relationship between the amount of S removed from the extract (S before planting - S after cropping) by the 2 test extractants in the - S treatment and plant S uptake was also determined. A higher S concentration was found in the KCl-40 extract than in the MCP extract in the granite soil. Conversely, the MCP extract had a higher S concentration in the basalt soil because of the relative size of the adsorbed S and ester-S pools in this soil. More than 33% of the S-35 was recovered in the extracted S pool, and <11% from the organic S pool in both extractants and in both soils. Data showed a lower specific activity (SA) of the S in the MCP extract as measured by ICP than in KCl-40 in both soils and all crops. The specific activity ratio of the extractants, as measured by ICP, was closest to 1.00 for the KCl-40 extract in both soils for each crop. The removal of S from the MCP extractant during cropping was lower than plant uptake in all treatments except ryegrass in the basalt soil. The KCl-40 extract tended to underestimate S supply at low uptake and overestimate at higher uptake. The results reported in this experiment showed that the KCl-40 extract generally performed well for all 3 crops and on both soils. The data showed that the KCl-40 extract removed S from pools similar to those reached by the crops.