Soil structure has an important role in a soil's agricultural performance because of its influence on many soil properties. The objective of this work was to evaluate aggregate stability and relate it to different organic fractions of a red-yellow Ultisol under forest and cultivated with rubber trees, orange trees and under degraded pasture. Samplings were carried out in soil pits at the depths of 0-10, 10-20 and 20-30 cm. The following characteristics were analyzed: texture, water-dispersible clay, flocculation degree, soil density, total organic carbon (TOC), total nitrogen (TN), fulvic acids (FA), humic acids (HA), humin (HN), carbon in microbial biomass (C-Micr) and light organic matter (LMO). Water-stable aggregates and stability indexes were also calculated. Results showed differences in soil density but no differences in aggregate stability, at 0-10 and 10-20 cm both in the decreasing order: forest < rubber plant < orange < pasture. At 20-30 cm, the aggregate stability was different, following the same order. In the soil under forest, rubber trees and orange trees, aggregate stability was related to higher TOC, TN, FA, HA, HN, C-Micr and LMO with lower C/ N ratio, whereas under pasture it was chiefly related to wetting and drying processes.
Organic Matter; soil aggregate; wetting-and-drying cycles
