SULPHUR AVAILABILITY IN BRAZILIAN SOILS: EVALUATION OF SULPHUR NUTRITIONAL STATUS IN PLANTS

Abstract

A greenhouse experiment was carried out with surface samples of 12 soils from the States of São Paulo, Minas Gerais and Goiás (Brazil). Carbon content varied from 0.50 to 3.03%, total sulphur from 0.0104 to 0.0319% and clay content varied from 8 to 60%. Corn (Zea mays L.) was grown in two successive crops in the presence of tree rates of S applied as gypsum, namely 0, 20 and 40 ppm, previously incubated in absence and presence of lime. For the evaluation of the nutritional status of corn plants total sulphur and N/S and P/S ratios against dry matter yield and sulphur absorption were correlated. In sandy soils the critical level for total S in the tops of the corn plants were 0.14% and 0.20%, respectively, for dry matter and S uptake, in the case of the first cropping; for the second cropping it was 0.14% S. In the case of clayey soils (carbon> 1.2%), correlation coefficients were not significant in the first cropping; in the second one, however, the critical levels were found to be 0.20% for dry matter yield and 0.30% for S uptake. Close relationships were found, both in the first and in the second cropping, when the N/S ratio in the tops was plotted against S uptake: sulphur deficiency is associated with a ratio of 10-12/1. When data from the two croppings were analysed jointly, as well as in the case of the second one, significant correlations between P/S and S absorption were found. It is suggested that the P/S ratio 1/1 should be used as a critical value. The N/S ratio proved to be the most reliable parameter for the evaluation of the nutritional status of the plants with respect to sulphur.

KEY-WORDS: Sulphur in plants; critical levels; N/S ratio; P/S ratio.