Rock Fragments as Factor of Soil Structure Formation: an Experimental Study Based on Soil Micromorphology and Image Analysis

Available studies that address the controversial role of rock fragments in soil functioning are often oriented to assess effects of their presence in relation to specific applicative problems like soil erosion or influence in agricultural practices. Experimental research is mostly based on water flow monitoring after rainfall simulation on soil boxes with stones added at different concentrations. However mechanisms explaining the measured changes in soil water flow rate can only be speculated without a direct observation of the modification of the pore network. A lab experiment was carried out here on five natural soils selected for their different physical properties aiming at studying the interaction between different soil matrices and rock fragments by soil micromorphology and image analysis. Two concentrations of three different sizes of rock fragments were mixed with the soil material after sieving and put in flowerpots having diameter of 25cm; then such samples have undergone nine wet/dry cycles. At the end the samples were impregnated with fluorescent resin and soil blocks obtained were examined by image analysis. Results showed that rock fragments induced pore formation contiguous to the stones and fractures trough the soil matrix. Concentration of the stones has shown to be related to the density of the soil fractures, the size of the fragments to the width while the shape of the stones influenced the orientation of the fractures. Shrinkage range and plasticity were the most important physical properties which explained the different reactivity of the five soil matrices in the soil structure formation process.