133 Saprolite influence on formation of well-drained and hydromorphic horizons in an acid soil system as determined by structural analysis P. Curmil, Widiatmakaz, J. PellerinJ and A. Ruellan4 1INRA-ENSA Laboratoire de Science du Sol, 65 rue de Saint Brieuc, 35042 Rennes Cedex, France 2Bogor Agricultural University, Dept. of Soil Science, 1 J1. Pajajaran, Bogor 16144, Indonesia 3CNRS, Centre de Gkonwrphologie, rue des Tilleuls, 14000 Caen, France 4CCNEARC, 1101 Av. Agropolis B.P. 5098,34033 Montpellier Cedex 1, France ABSTRACT Curmi, P., Widiatmaka, Pellerin, J. and A. Ruellan, A., 1994. Saprolite influence on formation of well-drained and hydromorphic horizons in an acid soil system as determined by structural analysis. In: A.J. Ringrose- Voase and G.S. Humphreys (Editors), Soil Micromorphology: Studies in Management and Genesis, Proc. IX Int. Working Meeting on Soil Micromorphology, Townsville, Australia, July 1992. Developments in Soil Science 22, Elsevier, Amsterdam, pp. 133-140. A landscape unit was studied on both macroscopic and microscopic scales - emphazising the 3-D relationships between horizons - to understand soil distribution and their genetic and functional relationship. On a granitic saprolite with a silty loam cover in the Armoncan Massif (France), associations of Dystrochrepts, Hapludalfs and Glossaqualfs had developed. Structural analysis of a landscape unit showed these soils form two types of pedological system. In this paper, only systems of one type are studied. They occur upslope and along the slope and associate well drained and hydromorphic soils. In these systems, the following features occur: 1) Thickness of the loamy layer varies independently of topography. 2) Clay migration takes place in this loamy layer. When the layer is thin, clay accumulation forms banded horizons a few centimetres thick within the saprolite, whereas, when the layer is thick enough, clay accumulation develops a textural B horizon inside it. 3) Granite saprolite has sandy and clayey facies which are probably of lithological origin and have an inverted inclination compared to the topographic slope. 4) The distribution of clayey facies of the saprolite induces development of local perched water tables. 5) Groundwater emergence transforms the upper loamy horizons from their base by hydromorphic and degradation processes. INTRODUCTION In the Armorican Massif (France), associations of soils referred to as "Alocrisols", "Luvisols" and "Luvisols-RedoxisolsDegrades", according to the Pedological Reference Base (AFES., 1992) (Dystrochrepts, Hapludalfs and Glossaqualfs, Soil Survey Staff, 1975), are commonly observed in the loamy layer on granite, schist or sandstone (Curmi, 1979a; R o w e l ,