Abstract : Because of arid climate (70 mm precipitations/yr), the Coquimbo region, Chile, (30ªS), is the scene of environmental extreme contrasts. In fact, from a social point of view, this region shows the coexistence of three different kinds of arid lands : a) an irrigated agricultural sector, modern and open to international markets; b) an irrigated sector of traditional use; c) a non-irrigated sector consisting of sparse and eroded areas. The Coquimbo region is especially sensitive to any activities and has not been yet the object of studies to enhance the knowledge of the soil use impact or of the potential climatic variation. In this context, we sample several types of soil from differents sectors of use and preservation of the Coquimbo region. We carried out quantitative and qualitative analyses of the Organic Matter (OM) and a study of its dispersion along the profiles. 82 soil profiles (1 m depth – sampling every each 10 cm) have been sampled along an altitudinal transect ocean-mountain. Granulometry, bulk density, Cationic Exchange Capacity (CEC), Total Organic Carbon content (TOC) and hydrogen (HI) – oxygen (OI) indices of the OM (by Rock-Eval pyrolysis) have been analyzed on samples. Results illustrate a rapid decrease with depth of the OM content and quality. However, distinctions are observed between non-cultivated and cultivated soils, the second one is OM richer than the first one. However, no major differences in OM are observed. It implies a rapid transformation of a noteworthy part of OM, acquired from cultivated vegetable.