A COMPARISON OF SULFUR EXTRACTANTS FOR WEATHERED ACID SOILS

The increasing incidence of S deficiency throughout the world has focu sed attention on ways of assessing the S status of soils. An incubatio n experiment was conducted using three acid soils varying in S sorptio n capacity. The soils were a gleyed podzolic (Aquic Haplustalf), a red earth (Haplohumult) and krasnozem (Haplohumult) with a S sorption (at 5 mu g S mL(-1)) of 13, 48 and 135 mu g S g(-1) soil, respectively. A factorial combination of S (0 and S applied to 5 mu g S mL(-1) in soi l solution), P (0 and P applied to 0.2 mu g P mL(-1) and lime (0, 1.5 x exch. Al) was applied to the soils which were incubated at 70% of fi eld capacity at 25 degrees C for 3 weeks. At the end of the incubation period the soils were extracted with 0.01 M P as Ca(H2PO4)(2) (MCP), water (water) and 0.01 M CaCl2 (CaCl2). The concentration of S extract ed (mu g S g-l soil) varied between soils, treatments and extractants. In the krasnozem, the order was MCP > water > CaCl2. In the gleyed po dzolic the amounts extracted by the three methods were similar and in the red earth the order was water > CaCl2 > MCP. The addition of P and lime affected the extractans in different ways. The use of S-35-label led sulfate allowed a calculation of the recovery of added S and this differed between soils and extractants. The results indicate that on l ow S-sorbing soils the extractants used were not sensitive enough to a ccount for differences brought about by P and liming and that care is needed in interpreting results from such experiments.