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Adding anionic polyacrylamide (PAM) to soils stabilizes existing aggregates and improves bonding between and aggregation of soil particles. However, the dependence of PAM efficacy as an aggregate stabilizing agent with soils having different clay mineralogy has not been studied. Sixteen soil samples (loam or clay) with predominantly smectitic, illitic, or kaolinitic clay mineralogy were studied. We measured aggregate sensitivity to slaking in soils that were untreated or treated with an anionic high-molecular-weight PAM using the high energy moisture characteristic (HEMC) method and deionized water. The index for aggregate susceptibility to slaking, termed stability ratio (SR), was obtained from quantifying differences in the water retention curves at a matric potential range of 0 to -5.0 J kg(-1) for the treatments studied. For the untreated soils, the SR ranged widely from 0.24 to 0.80 and generally SR of kaolinitic > illitic > smectitic soils. The SR of PAM-treated aggregates exhibited a narrow range from 0.70 to 0.94. The efficiency of PAM in improving aggregate and structural stability relative to untreated soils ranged from 1.01 to 3.90 and the relative SR of kaolinitic < illitic < smectitic samples. These results suggest that the less stable the aggregates the greater the effectiveness of PAM in increasing aggregates stability (i.e., smectitic vs. kaolinitic samples). The effectiveness of PAM in improving structure and aggregate stability was directly related to clay activity and to soil conditions affecting PAM adsorption (e.g., electrolyte resources, pH, and exchangeable cations) to the soil particles and inversely to the inherent aggregate stability.