Soil aggregate stability assessment based on ultrasonic agitation: Limitations and recommendations after sixty years (1964–2023)

Soil aggregate stability is a fundamental measure for evaluating soil structure. While numerous tests exist for assessing soil aggregate stability, ultrasonic agitation (UA) is widely recognized for its effectiveness. Nonetheless, a significant limitation of UA is the lack of standardized methodologies and stability assessment criteria, resulting in inconsistency and incomparability across studies. Several critical factors influence the assessment of soil aggregate stability, including sample preparation (e.g., drying, sieving, and settling duration), initial and final aggregate size classes, the definition of final energy form and its calculation, variations in instrumentation and laboratory procedures, and the absence of standardized criteria. Unlike some stability methods, UA produces a broad range of results, with dispersion energy varying significantly (0.5–13 440 J g^-1) across different soil and aggregate types due to divergent procedural settings. These settings encompass factors such as initial power and amplitude, temperature fluctuation, soil/water ratio, probe specification (diameter and insertion depth), and the choice of liquid used during the process. Furthermore, UA faces challenges related to limited reproducibility, raising doubts about its status as a standard stability assessment method. To address these issues, standardization through predefined procedures and stability criteria has the potential to transform UA into a precise and widely accepted method for both qualitative and quantitative assessments of soil stability. In this comprehensive review, we outline the challenges in standardizing UA, elucidate the factors contributing to dispersion energy variation, and offer practical recommendations to establish standardized protocols for UA in soil aggregate stability assessments.