THE INFLUENCE OF ORGANIC SOIL AMENDMENTS ON SULFATE ADSORPTION AND SULFUR AVAILABILITY IN A BRAZILIAN OXISOL

Soil management practices that involve additions of organic materials may influence plant sulfur availability in highly-weathered, acid soil s. This study evaluated the effects of organic additions on sulfate ad sorption and sulfur availability in a limed (3.4 t ha-1) and unlimed T ypic Haplustox soil of the Cerrado Region of Brazil. In unlimed soil, the proportion of applied sulfate (600 kg S ha-1 as gypsum) that was a dsorbed temporarily decreased over two cropping seasons by incorporati on of 10 t dry matter ha-1 crop-1 of guinea grass (Panicum maximum Jac q.) but not when a similar quantity of a tropical legume, feijao de po rco (Canavalia ensiformis L.), was added. Liming reduced sulfate adsor ption and resulted in sulfate leaching to a depth of 30 to 45 cm. Both plant materials temporarily reduced sulfate adsorption in laboratory studies when added to an unlimed soil at a rate equivalent to 40 t ha- 1. Analysis of soil properties affected by organic additions and limin g showed significant correlations between sulfate adsorption and soil pH, extractable aluminum, calcium and magnesium, and surface charge. M aize dry matter yields increased by 1.3 to 3.5 t ha-1 with addition of both, organic materials. However, only the feijao de porco treatment resulted in increases in sulfur uptake for the years in which organic materials were applied. Determining the effects of organic material ad ditions on plant sulfur availability is complicated by the combined ef fects of sulfur mineralization, sulfate adsorption, and the plant's ab ility to utilize adsorbed subsoil sulfate.