Two soils with contrasting properties were used in this study: (i) a calcareous montmorillonitic sandy clay loam (Lithic Xerorthent) from the Birya forest, N Israel; and (ii) a non-calcareous kaolinitic sandy loam (Typic Xerorthent), from Barbanza, NW Spain. Both soils have very different chemical and structural properties: the presence and amount of montmorillonite clay particles are the main factors responsible for aggregation in the soils from Birya ( Singer, 2010), while the amount and composition of organic matter determine aggregation in the soils from Barbanza, since these soils contain a small amount of clay (mainly kaolinite) that does not contribute significantly to stabilize soil structure ( Benito and Diaz-Fierros, 1992).