Physical Redistribution of Calcium Carbonate in Soil Pore System: An Experiment Using Soil Micromorphology and Image Analysis

Calcium carbonate sedimentation plays an important role also in the soil structure development. Soil structure is a key factor for the soil functions, such as sustaining plant productivity, regulating and partitioning water and solute flow, and maintaining civil engineering works. In order to improve soil structure the calcium carbonate is added to the soil by means liming procedure in order to improve soil chemical and physical properties, neutralizing soil acidity and increasing activity of soil bacteria, and improving the soil structure. A large bulk of scientific literature addresses the relations between the forms of carbonate redistribution in the development of calcic and petrocalcic horizons and the water regime of calcareous hydromorphic soils, but little is still known concerning the underlying physical mechanisms of the effect of carbonate sedimentation on soil pore system. In this work we attempted to investigate physical mechanisms of soil pore development as consequence of the addition of calcium carbonate (CaCO3) and on two soils with different shrinkage-swelling capacity subjected to several wetting and drying cycles. Analysis was conducted using soil 2D image analysis and soil micromorphology. Our results showed changes in the pore size distribution, in some cases very large, and allowed the identification of specific mechanisms of pore modification induced by micrite pedofeatures produced by the mobilization in suspension of CaCO3. These physical mechanisms were triggered: CaCO3 segregations which induced a pore size redistribution fragmenting the pore space, and CaCO3 coatings which seemed to induce a cumulative effect on porosity cementing the walls of newly-formed pores in the soil samples with high shrinkage-swelling capacity. Our results, even if obtained on experimental samples, give a contribution in the understanding of the physical role of CaCO3 pedofeatures in pore formation in soils in field and show the need to reassess physical simulation tests in order to quantitatively investigate combined effects of factors influencing soil structure formation.