Soil Aggregate Stability as Affected by Clay Mineralogy and Polyacrylamide Addition.

The addition of polyacrylamide (PAM) to soil leads to stabilization of existing aggregates and improved bonding between, and aggregation of adjacent soil particles However, the dependence of PAM efficacy as an aggregate stabilizing agent on soil-clay mineralogy has not been studied. Sixteen soil samples with predominantly smectitic, illitic or kaolinitic clay mineralogy having  loam or clay texture, were studied. Aggregate stability of non-treated or treated with an anionic high-molecular-weight PAM was determined using the high energy moisture characteristic (HEMC) method using deionized water. In this method, the wetting process of the aggregates is accurately controlled, and the energy of hydration and entrapped air are the main forces responsible for aggregate breakdown. An index of aggregate susceptibility to slaking termed stability ratio (SR), is commonly obtained by quantifying differences in the moisture characteristic curves (at 0-500 mm tension) for fast and slow wetting. In the current study the SR of the PAM- treated aggregates was obtained from the differences in the moisture characteristic curves of the fast wetted PAM- treated aggregates and those of the non-treated aggregates.  For  the non- treated aggregates  the SR ranged from 0.240 to 0.800. Generally, SR increased in the order of smectitic<illitic<mixed mineralogy<kaolinitic soils for these samples. In the PAM-treated aggregates the SR ranged from 5.1 to 1.05. The effectiveness of PAM in improving aggregate stability decreased in the order of smectitic>illitic>mixed mineralogy>kaolinitic samples. The results suggest that the less stable the aggregates  the greater the effectiveness of PAM in increasing aggregates stability (i.e., smectitic vs. kaolinitic samples).