PEDOGEOMORFOLOGIA E MICROPEDOLOGIA DE UMA SEQUÊNCIA LATOSSOLO-AREIA QUARTZOSA HIDROMÓRFICA SOBRE ROCHAS CRISTALINAS DO ESTADO DO AMAZONAS

Abstract

The pedogenesis of a latosol – plinthic podzolic latosol - hydromorphic quartzous sand sequence wasstudied, with respect to its chemical, physical and mineralogical properties. These soils are developed fromcrystalline substratum of the Guiana Complex, in the Occidental Amazon. The studied area is located inthe Waimiri-Atroari Indigenous reserve, at the Km 160 of the BR 174, which links Manaus to Caracaraí.The toposequence represents a hill formed by clayey soils in the top of the landscape, ending in a broadvalley flat with sandy soils in the bottom, under strong hydromorphism. The temperature and moistureregime of the soils are isohypertermic to hyperthermic, and udic to aquic, from the higher parts to the valleybottom, respectively. Six trenches were open and described in detail; in addition, core samples wereexaminated in six extra sites.Four soil profiles representatives of the segments ( latosolic, transition and hidromorphic) were selectedfor the study of physical , chemical, mineralogical and micromorphological characteristics of theircomponents. The higher part of the sequence is dominated by red-yellow latosol, the transition segment byplinthic podzolic intergrade to red-yellow latosol, and the lower parts by hidromorphic quartzous sand. Theparent material of the soil of all the sequence is heterogeneous, but basically derived from wheatheredgranitic rock, with some localized influence of alteration products of mafic rocks. All soils have a highaluminum saturation. In the higher parts of the sequence the textural gradient is lower and the clay fractionis dominated by kaolinite and crystalline iron oxides. Downslope the textural gradient increases and theiron forms are less crystalline. At the valley bottom rests a deep sandy mantle, where iron oxides are absent,remaining traces of kaolinite. Mica/illite were found in the deeper parts of the soils and hydroxide-interlayeredvermiculite in the upper parts. The sequence of evolution is processed mainly by pedological transformationinvolving seasonal acid and hydromorphic conditions, where ferrolysis occurs. This process leads to alateral degradation of the latosolic material of the top of the landscape, with clay destruction and concentrationof sandy materials dominated by quartz. Micropedological features related to the ferrolysis process arepresented and discussed.