Micromorphology of some oxic and argillic horizons in south Brazil in relation to weathering sequences

Abstract

The well-drained tropical soils in south Brazil, derived from the same parent material, can be arranged in development or weathering sequences. In these sequences three main stages can be recognized: the first stage including entisols and inceptisols, the second stage consisting of soils with an argillic horizon (ultisols and alfisols), and the third stage comprising soils with an oxic horizon (oxisols). In this article the attention is focussed on the second and third stages, and especially on the transition from the second to the third stage. The extent to which macropedological and chemical criteria used for the separation of ultisols or alfisols from oxisols are accompanied by differences in micromorphological features is studied. Attention is also given to the question whether argillic horizons from ultisols or a alfisols with strongly weathered soil material and weak horizon differentiation may be regarded as having been less developed than argillic horizons in profiles with stronger horizon differentiation. Six samples of oxisols showed no clay concentration at all, due to illuviation, whereas nine samples of ultisols all had a certain amount of argillans. In addition point-counting showed that oxic horizons are considerably more porous than argillic horizons of the same clay content. Oxic horizons are crumbly and/or granular, argillic horizons subangular blocky or even apedal and compact. Ultisols and alfisols with weak horizon differentiation did not show a lower percentage of clay concentrations in the argillic horizon than strongly differentiated profiles; a phenomenon which militates against the classification of these soils as a separate group on a high level. There is no simple relation between soils with an argillic horizon and oxisols respectively and changes in the plasmic fabric, i.e. changes from braunlehm to rotlehm or roterde. The occurrence of lehm and erde fabrics in the soils dealt with is determined by the degree of desilication (place in the weathering sequences) and by the iron content (the character of the parent material). Therefore, it is possible that some oxic horizons with relatively high iron content possess braunlehm fabric, some argillic horizons rotlehm fabric.