Dynamics of sulfur fractions in Brazilian soils submitted to consecutive harvests of sorghum

Different extractants have been used to determine sulfur availability in tr opical soils. Due to variability in their composition, different soil S fra ctions are solubilized and taken up differentially by plants. We studied th e dynamics of S fractions in tropical Oxisols, in the presence and absence of liming, over five consecutive harvests of sorghum [Sorghum bicolor(L.) M oench], Fourteen soil samples (0-20 cm) with differing levels of S adsorpti on capacity (SAC) were characterized for the following S fractions: S-s = i gnition of a soil-sodium bicarbonate mixture; S-a = S available in NH4OAc ( 0.5 M) and HOAc (0.25 M); S-c = S available in CaCl2 (0.01 M); S-p = S avai lable in Ca(H2PO4)(2) (500 mg P L-1 in 2.0 M HOAc); and S-r = S-s - S-a, Co mbined soil-root-shoot results indicate the existence of a shortterm labile organic fraction and a second more recalcitrant organic fraction, In soils with low and medium SAG, the labile organic fraction drives short-term S a vailability. In high-SAG soils, the mineral fractions became the main drive rs of S availability, Soil SAC strongly affected the exractant predictive p otential For long-term S availability. In low-SAG soils, S availability was best correlated with the S-c fraction. In medium-SAG soils, the best predi ctive capacity was also associated with the S-c fraction, followed by the S -a fraction, Conversely in high-SAG soils, S availability was best correlat ed with the S-a fraction, followed by the S-p Fraction. No single S-extract ion procedure appears well suited to predicting long-term S availability ac ross the range of soils studied.