DISPONIBILIDADE DE ENXOFRE EM SOLOS BRASILEIROS: AVALIAÇÃO DO ENXOFRE-SULFATO DISPONÍVEL POR EXTRATORES QUÍMICOS

Abstract

A greenhouse experiment was carried out with surface samples of 12 soils from the States of Sao Paulo, Minas Gerais and Goias (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-SO 4 = was evaluated by four chemical extractants: Ca(H 2 PO 4 ,.H 2 O-500 ppm P in 2.0 N acetic acid: 0.5 N ammonium acetate in 0.25 N acetic acid; CaCl 2 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-SO 4 = thereof. The critical levels were, in ppm of S: a) Ca(H 2 PO 4 )2 H 2 O-500 ppm P in 2.0 N acetic acid - 11; b) 0.5 N NH 4 OAc in 0.25 N acetic acid- 17; c) CaCl 2 0.15% - 19. The critical levels of available S-SO 4 = in soils would be overestimated if evaluated by relative uptake of available sulphur. Turbidimetric determinations of S-SO 4 = 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.