SULPHUR AVAILABILITY IN BRAZILIAN SOILS: AVAILABLE SULPHATE-SULPHUR EVALUATION BY CHEMICAL EXTRACTANTS

Abstract

A greenhouse experiment was carried out with surface samples of 12 soils from the States of São Paulo, Minas Gerais and Goiás (Brazil), with carbon contents ranging from 0.5 to 3.03%, sulphur from 0.0104 to 0.0319% and clay from 8 to 60%. The available S-SO₄= was evaluated by four chemical extractants: Ca(H₂PO₄,.H₂O-500 ppm P in 2.0 N acetic acid: 0.5 N ammonium acetate in 0.25 N acetic acid; CaCl₂ 0.15% and 0.05 M HCI. Corn (Zea mays L.) was grown in two successive crops in the presence of three rates of S (0, 20 and 40 ppm S), in absence and presence of liming. Among the chemical extractants, calcium phosphate in 2.0 N acetic acid provided the best performance both in operational terms and with respect to correlation with dry matter yield and S uptake. Dilute 0.05 M HCl proved to be thoroughly inadequate due to the impossibility of giving clear uncolored extracts suitable for the turbidimetric determination of S-SO₄= thereof. The critical levels were, in ppm of S: a) Ca(H₂PO₄)2 H₂O-500 ppm P in 2.0 N acetic acid - 11; b) 0.5 N NH₄OAc in 0.25 N acetic acid- 17; c) CaCl₂ 0.15% - 19. The critical levels of available S-SO₄= in soils would be overestimated if evaluated by relative uptake of available sulphur. Turbidimetric determinations of S-SO₄= have small sensibility in low concentrations of sulphates in soil extracts. One suggest the refining of analytical techniques for sulphates, including several soil/solution ratios.