Aggregation, organic matter, and iron oxide morphology in oxisols from Minas Gerais, Brazil

The characteristic strong aggregation observed in Oxisols is usually attributed to the presence of free aluminium or iron compounds. Previous investigation of Oxisols from Minas Gerais, Brazil, suggested that iron oxide minerals do not necessarily play a role in aggregation. Oxisol profiles developed on different parent materials (rock-saprolites and sediments), and with different degrees of polygenesis, were investigated to assess whether the physical makeup, rather than the iron content, determines aggregation. Oxisols were investigated by means of micromorphology and laser diffraction grain-sizing. Grain-size distribution curves were determined after three pre-treatments: shaking with water; removal of organic matter; and removal of organic matter followed by deferration. Micromorphology indicated that soils developed on rock-saprolites have hematite droplets (discrete, red colored, equidimensional concentrations) in the saprolite, whereas droplets are not found in soils on Tertiary sediments. However, secondary iron accumulations related to periodic water saturation are encountered in the soils on sediments and not in the soils on rock-saprolites. Grain-size distribution curves showed that the Oxisols on rock-saprolites do not have strong aggregation because of iron oxides alone. Conversely, aggregation by iron oxides is evident in the Oxisols on sediments. This indicates that remobilization of iron during soil formation is essential for iron forms to play a role in aggregation. These findings suggest that the mode of formation and iron mineralogy affect aggregation