A rapid and precise method for determination of SO(,4)-S in soils was developed. The method involves the reduction of SO(,4)-S to H(,2)S in 30 min of digestion of soil extract with Sn and H(,3)PO(,4) mixture and subsequent determination as methylene blue. Use of this method to reduce S model compounds showed that, in addition to reduction of SO(,4)-S, it is capable of reducing partially or completely different inorganic or organic S compounds;Application of the Sn and H(,3)PO(,4) mixture to reduction of organic S in 13 Iowa and 7 Chilean surface soils and 5 Iowa soil profiles showed that most of the reduction occurred after distillation for 1 to 3 h, but some samples continued to release H(,2)S up to 10 h of distillation. Calculation of the amount of organic S in soils potentially reducible (S(,r)) with Sn and H(,3)PO(,4) showed that the S(,r) values ranged from 36 to 81% and from 27 to 95% for the Iowa and Chilean surface soils, respectively;In three greenhouse experiments, the soils were used to study the uptake of S by three successive croppings of corn (Zea mays L.) or soybean (Glycine max (L.) Merr.) at 40- or 60-day intervals, respectively, or three cuttings of ryegrass (Lolium multiflorum L.) at 30-day intervals;Calculation of the potentially mineralizable S (S(,o)) from the results obtained in incubating soil-glass beads mixture in leaching tubes under aerobic conditions at 20 or 30(DEGREES)C showed that the S(,o) values varied with incubation temperature. The S(,o) values calculated from the SO(,4)('2-) released on incubation of soil-glass beads mixtures at 20 or 30(DEGREES)C were not significantly correlated with dry matter or S yields of plants;Nine chemical and three biological methods were evaluated as indexes of plant response to available S in soils. Linear regression analysis showed that the amount of S mineralized in 14 weeks was significantly correlated with S yield (tops + roots) of corn (r = 0.67**, 20(DEGREES)C; r = 0.78**, 30(DEGREES)C) and of soybean (r = 0.76**, 20(DEGREES)C; r = 0.81***, 30(DEGREES)C) and ryegrass tops (r = 0.87***, 20(DEGREES)C; r = 0.84***, 30(DEGREES)C). In general, the corresponding values for the Chilean surface soils were also significantly correlated but lower in r values (ranged from 0.59 to 0.91**). Results obtained by using the chemical indexes showed that the amount of SO(,4)-S extracted by the following reagents were significantly correlated (P < 0.001) with S uptake by corn, soybean, and ryegrass; 0.1 M LiCl, 500 mg L('-1) P as Ca(H(,2)PO(,4))(,2), and 0.5 M NaHCO(,3).