Water Retention and Structure Stability of Semi-Arid Soils As Affected By Polymer Concentration.

In arid and semi-arid regions with unstable soils and limited water resources, studying the effects of polycrylamide (PAM) application on soil structure stability is important for the development of effective soil and water conservation, and management practices for sustaining soil and water quality. Six concentrations of a medium molecular weight anionic PAM (0, 10, 25, 50, 100 and 200 mg l^-1) prepared in tap water (EC~500 µS cm^-1) were tested on semi-arid loam and clay soils to evaluate the effects of PAM and soil texture on soil water retention (at near saturation), and aggregate and structure stability using the high energy moisture characteristic (HEMC) method. The water retention curves obtained by the HEMC method were characterized by a modified van Genuchten (1980) model that provides (i) model parameters α and n, which represent the location of the inflection point and the steepness of the S-shaped water retention curves, respectively, (ii) the volume of drainable pores (VDP), which is an indicator for the quantity of water released by the tested sample over the range of suction studied, (iii) modal suction (MS), which corresponds to the most frequent pore sizes, and (iv) a soil structure index, SI =VDP/MS. The studied treatments had, generally, considerable effects on the shape of the water retention curves (expressed by changes in α and n). Soil type, PAM concentration, rate of aggregate wetting and their interaction had sizeable effects on the stability indices and the model parameters.  The VDP, SI and α increased, and MS and n decreased with the increase in PAM concentration, but the shape of curves were soil and wetting rate dependent, since predominant changes were observed in the macropore and or mesopore or micropore range (pore size > 60 μm) under the used matric potential (ψ), 0-5.0 J kg^-1. Effect of wetting condition was more pronounced at low PAM concentration and in the loam soil. The results indicate that no definite conclusions can be drawn regarding the impact of PAM added to semi-arid soils on water retention and soil structure; therefore prior to its application, soil properties (e.g., texture) and condition (e.g., moisture content) should be  considered. Detailed contribution of treatments on soil structure induces and model parameters are discussed.