Classing the soil skeleton (greater than two millimeters): Proposed approach and procedure

In soil science, analytical procedures apply almost exclusively to the fine earth (<2 mm). Rock fragments or skeleton (>2 mm) are regarded as inert an d discarded during sieving; however, we ha ce found that the clasts display physical and chemical properties that can equal or surpass those of the fi ne earth. These properties depend largely on the degree of alteration of th e clasts. In light of these findings, we developed a method to separate the rock fragments into wreathering classes. This method has been applied to f ive European skeleton-rich soils derived from different parent materials. C olor intensity, roughness and irregularities of the surfaces, cracks, and s urface features of the exposed minerals were considered reliable criteria f or the separation of the clasts. We noticed also that the degree of alterat ion of the clasts corresponds to size: as size decreased, weathering increa sed. Consequently, sieving could be used for separating the weathering clas ses. On the basis of these criteria, clasts were differentiated into highly , moderately, and slightly altered. There are statistically significant dif ferences among the weathering classes in terms of bulk density, porosity, o rganic C, total N, and ration-exchange capacity. There are no statistically significant differences in pH. The results confirm that the procedure sepa rates relatively homogeneous and different classes of rock fragments. We al so compared the characteristics of the soil skeleton to those of the fine e arth and fresh rock. We concluded that not characterizing the skeleton of t he soils may provide distorted information on the capability of these subst rata.