Shrinkage analysis of repacked soil samples enables quantifying the soil's potential physical quality

This paper investigates the physical properties of soil samples repacked from aggregate beds and the potential for retrieving representative soil pore properties at the field scale based on shrinkage analysis of a repacked composite sample. This approach was tested on 60 arable fields in Switzerland presenting a large range of soil organic carbon (SOC) and texture. Soil cores constituted from composite samples were repacked at the observed field bulk density. Their pore properties were compared to undisturbed soil samples from the same fields. The soil pore properties were characterized using shrinkage analysis and correlated to soil texture and SOC. The repacking protocol successfully recreated structured samples. The <40-µm equivalent diameter pore and structural pore size distributions were comparable to those of the undisturbed soils. For pores >40 µm, a larger porosity was measured for repacked samples compared to undisturbed ones, limiting the approach to assess the structure and dynamics of larger soil pores. Furthermore, soil clay content as well as SOC were strong predictors for the samples’ physical properties including pore space. The increase in structural porosity in the repacked sample was inversely proportional to the structural porosity already present in the undisturbed samples. Steeper regression slopes of the pore volume to SOC or clay relationship were observed for the repacked samples. These slope changes were comparable to those observed in soil structure restoration experiments and opposite to those observed in soil compaction trials. We conclude that repacking soil from a composite sample allows us to characterize the potential soil structure condition with respect to its SOC and clay content. The method can be used to assess soil structure quality in response to changes in soil management.