Epi and endosaturation affecting redoximorphic features and pedogenesis in subtropical soils with high textural contrast developed from sedimentary rocks

Abstract

Soils in the Depressão Central and Campanha Gaúcha in Rio Grande do Sul State, Brazil, developed from sedimentary rocks are mainly Alfisols (Luvissolos, Planossolos and Plintossolos) and Ultisols (Argissolos) with high textural contrast between A and/or E horizons and clayey B horizons. Red Ultisols dominate in well-drained areas in the summit. But many soils present redoximorphic features on the backslope and footslope, with mottled and/or nodules similar to plinthite due to the oscillating of the water table above the rock or poorly permeable saprolite. Identifying morphological features, mineralogical and chemical properties, and parent material differences are essential to identify the main processes responsible for their genesis. Knowledge of the relative distribution of different forms of iron in the soil is also of particular interest when interpreting pedogenesis. This study aimed to evaluated the main process responsible for the high textural contrast in soils developed from different sedimentary lithologies, and how the redoximorphic features observed in some soils may be related to genesis of plinthites and ferrolysis processes. The study area is located in the hydrographic basin of Rio Santa Maria, in the city of Rosário do Sul – RS, Brazil (30° 15’ 28” S and 54° 54’ 50” W), average altitude of 132 m and Cfa-type climate. Topolithosequences were defined based on soils developed from lithogies of Piramboia and Sanga-do-Cabral geological formations, by choosing soil profiles according to the source material, variations in relief, altitude and hydrological conditions. Morphological descriptions, particle size determinations, chemical analysis, mineralogy of the clay fraction by XRD analysis, determination of the main forms of iron, tests to identify plinthite were performed. Argiluviation, ferrolysis and plinthization were the most active processes identified in the genesis of most studied soils. The high textural contrast on these soils was not solely due to clay illuviation processes but also ferrolysis and lithological discontinuities in some soils, indicating polygenetic origin. Redoximorphic features in most soil revealed that iron segregation in most soils occurs like mottles and not plinthite, since they did not remain aggregated after the various wetting and drying cycles.