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

Different extractants have been used to determine sulfur availability in tropical soils. Due to variability in their composition, different soil S fractions are solubilized and taken up differentially by plants. We studied the dynamics of S fractions in tropical Oxisols, in the presence and absence of liming, over five consecutive harvests of sorghum [Sorghum bicolor(L.) Moench], Fourteen soil samples (0-20 cm) with differing levels of S adsorption capacity (SAC) were characterized for the following S fractions: S-s = ignition 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 available in Ca(H2PO4)(2) (500 mg P L-1 in 2.0 M HOAc); and S-r = S-s - S-a, Combined 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 availability. In high-SAG soils, the mineral fractions became the main drivers of S availability, Soil SAC strongly affected the exractant predictive potential 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 predictive capacity was also associated with the S-c fraction, followed by the S-a fraction, Conversely in high-SAG soils, S availability was best correlated with the S-a fraction, followed by the S-p Fraction. No single S-extraction procedure appears well suited to predicting long-term S availability across the range of soils studied.