Measurement of soil porosity and pores connectivity by different physical approaches and support volumes

The detailed geometrical description of the natural environment at different scales (i.e. from satellite images to micromorphology) is one of the key factors for the understanding of the physical processes affecting soil functioning. This work deals with the analysis of one very important component of the soil system: the porosity, analysed from the microscopy scale to the macroscopy scale which is the scale of a representative sample taken from a soil horizon. The representation of the pore network of soil samples through these scales enable, for instance, the study of very different processes such as biologic processes (different micro habitats inside soil aggregates) and physical processes such as water and solute transport in a soil core. This study has been produced using very different approaches in terms of both the physical methodology employed and the support volume of the measurements. The following methods have been used: high resolution X ray microtomography (SKYSCAN 1072), a mechanical serial sectioning based tomography (developed at CNR ISAFOM) and hydrological analysis such as water retention and hydraulic conductivity measurements. The techniques have enable the analysis of the pore system with scale range of the support volume from cubic millimeter (mechanical serial sectioning based tomography) to cubic decimeter (hydrology). The samples analysed consist of 3 soil horizons (Apg, EBtg, Btg) of a soil from a rice pad classified as Aeric Endoaquent (USDA) o Hydragric Anthrosol (WRB) and a man made sample having an homogeneous particle size (about 70 microns). The results from the particle size distribution and pore connectivity obtained using image analysis approaches based on mathematical morphology algorithms have shown the relevance of the two tomography based techniques which integrate the classical hydrology based approach enabling a better understanding of the transport phenomena at the core scale. Differences between the techniques have also been clearly shown both in terms of the quality of the results obtained (resolution, maximum sample size, etc.) and sustainability (cost, time, etc.). Of course these discrepancies must be related to the differences between the physics behind the methods and technologic restrictions.