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The soil physical parameter S (S-value or S-index) has been proposed as an index of soil physical quality. Soil physical quality is negatively affected by soil compaction (e.g. caused by agricultural field traffic). It has previously been shown that S decreases with increasing soil bulk density. This study investigated whether the relationship between S and soil compactness can be described by a single function that is valid across soil textures when soil compactness is expressed in terms of the degree of compactness (DC), which is relative density expressed as the ratio of bulk density to a reference density. This would provide an alternative measurement for soil physical quality that is more easily obtained than S. We also evaluated different methods for deriving reference density and tested whether reference values for S suggested in previous studies correspond to critical levels of DC reported in the literature. The relationships between S and DC were investigated for the 12 FAO/USDA soil textural classes based on pedo-transfer functions, and compared with data reported in the literature. A strong positive correlation was found between DC and ln (1/S), and a unique function was found between S and DC that is valid across soil textures, with the possible exception of poorly sorted soils with a high sand or silt concentration. Experimental data on S obtained from the literature supported these findings. The reference value of S (0.035) previously proposed as a boundary between good and poor soil physical conditions was found to agree well with the level of DC (87%) reported in the literature as critical with respect to plant growth. Proctor density was found to be the most useful measure of reference density, better than Håkansson reference density, which introduced some texture dependency into the relationship between S and DC. Our findings indicate that 1/S is a good measure of soil compactness and support the usefulness of S as a soil physical quality index. 3 However, our findings suggest that DC can also be used as an index of soil physical quality, and is much easier to obtain than S.

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Core Ideas Acoustic emissions (AEs) reveal information on soil deformation processes. We studied AEs during loading–unloading–reloading cycles under confined compression. The number of AE hits reflected well the soil deformation regimes. Soil compression and swelling indices correlate with AE parameters. The Gutenberg–Richter power law b value varied with wet soil yield stress. Application of mechanical stresses to soil results in deformation, microfracturing, particle motion, and liquid reconfiguration that may release measurable amounts of stored elastic energy in the form of acoustic emissions (AEs). This study aimed to systematically study AE characteristics during confined uniaxial compression of soil samples and link various AE parameters to soil deformation regimes. We hypothesized that variations in AE characteristics could offer new insights into the transition between elastic and plastic soil deformation and noninvasively identify the onset of yield stress. We subjected soil samples at known water contents to a loading–unloading–reloading cycle under uniaxial compression using an oedometer coupled with an AE monitoring system. The soil was a sandy loam equilibrated with two initial gravimetric water contents (12 and 25%) and two initial bulk densities (1.3 and 1.55 Mg m−3) for each water content. Observed AE event numbers (hits) and their energy content (E) varied during stress application to soil samples. Close agreement was found between soil yield stress deduced from normalized cumulative AE parameters and measured stress–void ratio curves (precompression stress, σpc) for the dry soil; however, the yield stress deduced from AE parameters was significantly larger than σpc for the wet soil. Swelling and compression indices were strongly correlated to AE hits and E, respectively. The preliminary results illustrate the potential of monitoring passive AE during load application to characterize soil mechanical parameters (potentially in situ). Nevertheless, additional studies with controlled pore water pressure and a range of soil types would be required to generalize these findings to offer a robust AE‐based method.