Abstract
The soil structure transformation from ferralic to nitic horizons was studied in a toposequence on quaternary red clayey sediments and diabase in Piracicaba (SP), Brazil. Morphological and micromorphological studies, image analysis, soil water characteristic curves and monitoring of (total) soil water potential head were used. The presence of polyconcave vughs, clayskins and planar voids shows that the vertical and lateral transition and structural transformation from ferralic to nitic horizons is given by the coalescence of the microaggregates, probably due to tensions created in a drier period in the past. Changes to a more humid climate with a defined dry season and alternate drying and wetting cycles resulted in the fissuration of the previously coalesced material, forming polyhedral aggregates and microaggregates. Simultaneously, clay illuviation filled the voids and together with the compacting action of the biological activity of these soils contributed to the coalescence of microaggregates.
Highlights
Soil surveys and the understanding of processes within soil systems are facilitated by the knowledge on spatial soil distribution in the landscape and the interrelations between horizons of the different soils
At the transition from flat to sloping surfaces these homogeneous soils evolve to soils that are characterized by the presence of a subangular blocky structure in the upper part of B horizons and micro-granular structure in the deeper part of B horizons
Using qualitative and quantitative methods to determine the soil structure organization and soil water dynamics, the purpose of this study was a deeper understanding of the processes of structural transformation from ferralic horizons with a micro-granular structure to nitic horizons with a subangular blocky structure, developed on a toposequence over diabase
Summary
Soil surveys and the understanding of processes within soil systems are facilitated by the knowledge on spatial soil distribution in the landscape and the interrelations between horizons of the different soils. The continuous debate of the last three decades has given rise to a series of hypotheses on the structural transformation from ferralic to argic or nitic horizons in tropical soil genesis (Lepsch & Buol, 1975; Lepsch et al, 1977; Moniz & Buol, 1982; Birkeland, 1984; Bullock & Thompson, 1985; Fedoroff & Eswaran, 1985; Miklós, 1992; Vidal-Torrado & Lepsch, 1993; Vidal-Torrado et al, 1999) These hypotheses vary from the filling of the porosity by illuvial clays deposited in the packing voids between the micro-granular aggregates, to the coalescence of the micro-granular structure by alternate drying and wetting cycles, either under the present humid or under drier climatic conditions in the past. According to Moniz & Buol (1982), Miklós (1992) and VidalTorrado et al (1999) the coalescence of microaggregates that transforms a micro-granular structure into a subangular blocky structure is due to an interaction between soil structure and its hydrodynamic behavior
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