Soil properties and their variations on two contiguous hillslopes in Northeast Brazil

Abstract

Information on spatial distribution of soil properties in a landscape is not only required for soil mapping and classification but also for soil management. Spatial patterns of soil properties in a landscape are the results of systematic variations in pedogenic processes controlled by environmental factors of which landscape position is important. In northeast Brazil, Lithosols (Ustorthents) whose surface horizons are directly underlain by lithic or paralithic contacts occupy upper slope positions with steep gradient; the Cambisols (Ustropepts) occupy the mid- and lower slope positions. In some cases Non-Calcic Brown soils/Planosol (Haplustalfs) occupy lower slopes. We examined the properties of two contiguous hillslopes in granitic and gneissic saprolite to determine whether there were any systematic variations among landscape positions and to interpret pedogenesis in the hillslopes. Particle size distribution, and the Ti content of sand plus silt and saprolite showed that the two hillslopes were formed on different parent materials. At Site 1, soil property variations along the slope, measured by standard deviation from the means, were attributed to variability in parent material rather than to landscape position. Except for solum thickness, which increased systematically downslope, variations in sand, clay, ECEC and pH were not related to landscape position. Increasing solum thickness downslope was explained by the effect of erosion on the upper, and deposition at the lower slopes. Silt and organic carbon contents at Site 1, and Fe o and Fe d contents at Site 2 significantly decreased downslope. One possible explanation for this trend, in addition to erosion and deposition, is the effect of slash and burn agriculture on the lower slopes, and the changes in redox conditions caused by periodic waterlogging.