Soils and water uptake by vines in pomerol (france). I - Soils distribution and variability

The Pomerol vineyard (France) is located 35 km east from Bordeaux, on the left side of the Isle river. Its area is approximately 800 ha. The geology is made of a system of fluvial terraces, including gravels and stones of various sizes. The underlying substratum is made of tertiary clays and sands. This stratification of terraces of various thickness leads to overlays of very different materials in which various soil development are observed. We studied the distribution of soils and produced a 1/25 000 soil map showing four main soil scapes and 11 soil maping units. The four main soil scapes were : a plateau, mainly characterised by the presence of swelling clays ; a large area (south and west) made of filtering sands on underlying more clayey materials ; the lower terraces with deep sandy soils ; and areas of immature soils from recent colluvial or alluvial origin. The soil survey pointed out the great variability of soils within the Pomerol vineyard. The textural properties exhibited a very broad range from very sandy to heavy clayey ones, with abrupt changes, both laterally and vertically. These various textural changes might induce various water behaviours. All basic soil data were stored into a geographical information system. Besides its interest for map production, this computerization enables a better characterization of mapping units variability. Actually, from a soil map user point of view, the range of variation - and even the statistical distribution - of a given parameter is as important as the mean observed value. This study gives an exarnple of spatial output taking into account this variability. These georeferenced data will also allow the overlay with other sources of geographical information.