Determination of aggregate stability in kaolinitic subsoils using an energy-based, laser diffraction method

• We present a reproducible aggregate stability method for laser diffraction analyses. • Soil dispersion curve indices are correlated with soil texture and clay mineralogy. • Closed system ultrasonic probe calibrations can be applied to open systems. • Measuring energy used in heating/dispersing fine-textured soils can reduce errors. Traditional aggregate stability methodologies, such as wet sieving, rainfall simulation, and chemical dispersion, measure aggregate size, rather than stability. Sonication methods allow for energy-based measurements of aggregate stability, but most methods involve sieving to obtain gravimetric measurements of particle size fractions, which increases labor and variability compared to volumetric measurements by laser diffraction analyses. One criticism of energy-based methods is that ultrasonic devices are calibrated in a closed vessel containing water whereas the application of energy in routine analyses is commonly done in an open system containing soil and water, without considering the effects of soil mass and specific heat on sonication power or the energy lost from the system by conduction and/or other forces in an open system. Using texturally diverse subsoil samples with low carbon and similar clay mineralogy, we quantified 1) the effects of system type (thermodynamically closed versus open systems) and 2) the effect of assuming energy from calibrations in water versus measuring thermal energy in a soil–water system on soil dispersion curves. We found that these factors do not significantly affect soil dispersion curves of coarse- and medium-textured soils; however, fine samples are affected by system type and the method used to quantify energy. Overall, indices produced from soil dispersion curves are highly reproducible in both open and closed systems and correlate with soil physical properties that impact aggregate stability. These indices may streamline future measurements of aggregate stability and facilitate the inclusion of this important soil property in soil assessments.