Effect of rock fragments on soil porosity: A laboratory experiment with two physically degraded soils

There are still few studies on the role that rock fragments (RFs) have in the change in soil structure based on direct observation of the soil-pore system. Physically degraded soil is of particular interest because small RFs might be considered a factor in its remediation. In our laboratory experiment we mixed five different proportions of 4-8-mm sized RFs with a Luvisol and a Regosol that have poor ability to self-structure and were characterized by a massive structure in the field. Nine wetting and drying cycles were applied to repacked samples (15cm in diameter and height) of soil-RF mixtures to facilitate the formation of soil structure. Image analysis was used to quantify development of the pore system at varying RF contents. The physically degraded soils studied in this research initially showed a decrease and then an increase in porosity with increasing amounts of RFs in the soil-RF mixtures. The results indicated that the Regosol responded more than the Luvisol to RF content. Threshold values of RF content at which the mechanism of pore formation prevails over that of pore reduction depended upon the soil type and can be attributed reasonably to small differences in the coefficient of linear extensibility (COLE). We also identified a mechanism for the propagation of porosity downwards from the soil surface with increasing RF content, together with a vertical homogenization effect on porosity. Our results contribute to understanding the mechanisms by which small rock fragments embedded in physically degraded topsoil induce changes in the pore system and confirm the potential of rock fragments to protect soil structure in soil susceptible to compaction. Highlights: Physical interaction between rock fragments and fine earth in degraded soil. Soil porosity examined by combined experimental laboratory approach with image analysis. Soil porosity first decreases then increases with increasing content of rock fragments. Coexistence of two opposing mechanisms: one of porosity reduction and one of formation of new pores