Soil and macro-pores under uniaxial compression. I. Mechanical stability of repacked soil and deformation of different types of macro-pores

Soil compaction can seriously damage the physical structure of fertile soil. Soils restored from excavation material and re-used for agriculture are under increased risk of compaction, as they are mechanically destabilized. Impacts on the macro-pores are of particular concern, as these structures play a crucial role for many soil processes such as drainage or aeration. The objective of this study was to test the hypotheses (i) that cylindrical macro-pores may enhance the mechanical stability of soil, and (ii) that macro-pores of different origin, size and orientation also differ substantially in their compressibility.Artificial cylindrical pores of different diameters (2 and 4 mm) and orientations (vertical and oblique) were created in repacked samples of a restored agricultural topsoil by pushing rods of corresponding diameters into the soil. The samples were then conditioned to different initial water potentials (− 1 kPa, − 6 kPa, − 30 kPa) and subjected to uniaxial compression tests. Macro-pore structures were analysed by means of quantitative morphometry of three-dimensional micro-computed tomography images.The mechanical stability of the samples strongly depended on the soil water content, but was not found to be influenced by the cylindrical macro-pores. The vertical cylindrical pores were more stable than the oblique ones. The initial diameter was less important. The cylindrical pores were much more stable than the interaggregate pores originating from the packing of the soil. The mean diameter of the cylindrical pores and the interaggregate pores decreased with increasing compression, whereas the mean diameter of the total pore space (i.e. the volume-based average of both pore types) at first increased under moderate compression due to the loss of the small interaggregate pores, and only decreased under higher loads. Cylindrical pores of moist samples kept their shape throughout compression, but those of samples compressed at low water potential (or water content, respectively) were deformed to ellipsoid shape. The study highlights the important role of cylindrical (bio)pores in soil. Being much less sensitive to compaction than interaggregate pores, they may still enable sufficient soil drainage and aeration, when other pores have already collapsed.