A comparison of sulfur extractants for weathered acid soils

Abstract

The increasing incidence of S deficiency throughout the world has focused attention on ways of assessing the S status of soils. An incubation experiment was conducted using three acid soils varying in S sorption capacity. The soils were a gleyed podzolic (Aquic Haplustalf), a red earth (Haplohumult) and krasnozem (Haplohumult) with a S sorption (at 5 µg S mL(^-1)) of 13, 48 and 135 µg S g(^-1)) soil, respectively. A factorial combination of S (0 and S applied to 5 µg S mL(^-1)) in soil solution), P (0 and P applied to 0.2 µg P mL(^-1)) and lime (0, 1.5 x exch. Al) was applied to the soils which were incubated at 70% of field capacity at 25°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 extracted (µg S g^-1) soil) varied between soils, treatments and extractants. In the krasnozem, the order was MCP > water > CaCl2. In the gleyed podzolic 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 ³⁵S-labelled sulfate allowed a calculation of the recovery of added S and this differed between soils and extractants. The results indicate that on low S-sorbing soils the extractants used were not sensitive enough to account for differences brought about by P and liming and that care is needed in interpreting results from such experiments.