Aggregate stability as affected by polyacrylamide molecular weight, soil texture, and water quality

The favorable effects of the environmentally friendly, nontoxic, anionic polyacrylamide (PAM) as a soil conditioner have long been established. Some uncertainties exist, however, regarding the effects of PAM molecular weight (MW) on its performance as a soil amendment and its ability to penetrate into aggregates and stabilize interior surfaces. We studied the effects of two anionic polymers, a high-MW (12 x 10(6) Da) and a medium-MW (2 x 10(5) Da) PAM, using deionized water (electrical conductivity of 0.004 dS m(-1)) or a 15 mmol L-1 gypsum solution, on the stabilty of aggregates from four smectitic soils varying in clay content. Penetration of PAM into the aggregates was estimated from treating 0.5- to 1.0- and 1.0- to 2.0-mm aggregates with PAM and thereafter comparing the stability of the small aggregates to that of the large aggregates after the latter had been crushed and sieved to 0.5- to 1.0-mm size. The stability ratio (SR) ranged from 0.090 to 0.900 and tended to (i) increase with the increase in soil clay content, (ii) maintain, in the absence of PAM, a greater level with electrolyte solution than deionized water, and (iii) be greater for the PAM-treated aggregates than the control. In the finer textured soils, the SR of the initially small aggregates was generally greater than that of the initially large aggregates, indicating that most of the PAM was adsorbed on the exterior surfaces and only a small fraction of the PAM added, if any, entered into pores. A significant interaction among the treatments tested (PAM MW, aggregate size, and solution ionic strength), with respect to their effect on the SR, was identified. Consequently, neither of the two PAM polymers tested could have been singled out as preferable.