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 soil s become more S-deficient due to lower S inputs in fertiliser and high er offtake in product. Because of the differing rates of S supply requ ired by a range of crops, the relative importance of the soil sulfate and organic S pools could be expected to vary between crops. A glassho use experiment was conducted using an Aquic Haplustalf of granitic ori gin and an Ultic Haplustalf of basaltic origin collected from unfertil ised 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, ryegr ass), and 2 soils (granite and basalt). S was extracted using 2 extrac ts, 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 ext racts were determined. The relationship between the amount of S remove d from the extract (S before planting - S after cropping) by the 2 tes t extractants in the - S treatment and plant S uptake was also determi ned. 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 siz e of the adsorbed S and ester-S pools in this soil. More than 33% of t he S-35 was recovered in the extracted S pool, and <11% from the organ ic S pool in both extractants and in both soils. Data showed a lower s pecific activity (SA) of the S in the MCP extract as measured by ICP t han 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 K Cl-40 extract in both soils for each crop. The removal of S from the M CP extractant during cropping was lower than plant uptake in all treat ments except ryegrass in the basalt soil. The KCl-40 extract tended to underestimate S supply at low uptake and overestimate at higher uptak e. The results reported in this experiment showed that the KCl-40 extr act generally performed well for all 3 crops and on both soils. The da ta showed that the KCl-40 extract removed S from pools similar to thos e reached by the crops.