Use of ion‐exchange resin to evaluate the bioavailability of sulfur in the soil

Samples of 22 soils of the State of São Paulo, Brazil were collected from the 0–20 cm layer, dried, sieved, and analyzed for sulfur (S) utilizing the following extracting solutions: (a) calcium phosphate [Ca(H2PO4)2], 500 mg P L‐1 in 2.0 mol acetic acid (HOAc) L‐1, (b) 1 mol ammonium chloide (NH4C1) L‐1, (c) 0.5 mol ammonium acetate (NH4OAC) L‐1 in 0.25 mol HOAc L‐1, (d) ion exchange resin ‐ Resin 1 ‐ (2.5 cm3 of soil + 2.5 cm3 of resin in water and further extraction with 50 mL 0.8 mol NH4Cl L‐1 in 0.2 mol hydrochloric acid (HCl) L‐1), (e) ion exchange resin ‐ Resin 2 ‐ (5.0 cm3 of soil + 5.0 cm3 of resin and 25 mL 0.8 mol NH4Cl L‐1 in 0.2 mol HCl L‐1), (f) ion exchange resin ‐ Resin 3 ‐ (5.0 cm3 of soil + 2.5 cm3 resin and 25 mL 0.8 mol NH4Cl L‐1 in 0.2 mol HCl L‐1). Amounts of 2 kg of each soil, in four replicates were limed in order to reach a base saturation of 70% of the cation exchange capacity (CEC). After a period of incubation of 30 days all nutrients, except S, were added to the soil and corn seeds were planted. Plant tops were removed after 70 days, dried at 50°C, weighed, and analyzed for S determination. The correlation between the bioavailable S in the soil and the yield of dry matter or S uptake was best represented by the linear or the √X models. The coefficient of determination varied from 0.56 (Resin 1) to 0.82 (NH4OAc). Resins 2 and 3 methods showed good efficiency (R2 from 0.63 to 0.79), and can be utilized in the evaluation of S bioavailability in soils.