Fenómenos de convergencia genética en suelos de terrazas fluviales: valle del río Tajo, Madrid-Toledo, España

Zusammenfassung

An analysis of the complex relationships between soils and geomorphology in Pleistocene fluvial terraces (central sector of the Tajo river basin) has been carried out. 47 soil profiles developed on the fluvial terraces have been summarized in eleven prototypes which have been established on the basis of the morphology and horizon sequence, and being representative of the genetic conditions in this area. Taking into account the complexity of soil forming processes, it is interesting to point out that some of them (carbonate leaching, clay illuviation, rubification...) follow general trends. These trends are the responsible that same results be generated, independently of the supposed variation in the soil forming factors, although with a gradation in the development of soil properties with the age. This phenomena that we have called «genetic soil convergence» restricts markedly the final variability of soil profiles, which reflects similar sequences of pedogenetic processes. The main cause of this convergence is the geomorphological stability of the terrace surfaces, and consequently the soil profile evolution is conditioned by the abnormal pedogenetic processes like erosion and superposition. As a result, Xeralfs (rhodic, petrocalcic) predominate in cases of geomorphic stability, and correspond to the best and more evolved profiles where erosion has a minimal intervention. In contrast, Xerochrept (calcixerollic, petrocalcic) are linked to a dominant erosion process representing the simplest convergence phenomena. When superposition processes dominate the pedological evolution degree of the new materials determines once again the convergence to the Xeralfs in most favourable cases, or Xerochrepts when maximal evolution degree is not reached because of the more recent age of the superposed parent material. The application of soil genetic convergence concept in the cartography of soils in fluvial areas can be generalized to other Tertiary basins with similar lithologies, where karstification or neotectonic processes affect the morphology of fluvial sediments.