Water Retention As Affected by Aggregate Size of Soils Treated with Polymer.

Studying the effects of amendments application on soil water retention and structure is important for the development of effective soil and water management practices. The contribution of polymers on water retention and structure stability depends, among others, on soil aggregate size, which in turn is linked to soil properties and management. Three aggregate sizes (0.25-0.5; 0.5-1.0; 1.0-2.0 mm) of four semi-arid soils (loam, sandy clay, and  two clays) treated with high-molecular-weight polyacrylamide (PAM) dissolved in tap water (100 mg/l), were used to evaluate an effect of soil texture and aggregate size on water retention and structure stability (at near saturation) using the high energy moisture characteristic (HEMC) method and deionized water. The water retention curves obtained by the HEMC method were characterized using the 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 curve respectively, and (ii) a soil structure index (SI = volume of drainable pores [VDP]/Modal suction [MS]). The SI (i) increased with the increase in aggregate size, clay content, and PAM application, and the decrease in rate of aggregate wetting, and (ii) increased exponentially with the increase in  α and VDP, and with the decrease in MS and n. For all treatments, the relationship between α and VDP, and MS and n was not linear. For non-treated aggregates the parameter α increased with the increase in aggregate size  and  the decrease in wetting rate. However, treating the aggregates with PAM decreased or had no effect on α for small aggregates (0.25-0.5 mm), but increased α for larger aggregates (0.5-2.0 mm). The steepness of the water retention curve (n) decreased with the increase in aggregate size of treated aggregates, while for non treated aggregates, no clear trend  could have been observed. Our results indicate that application of PAM has no trivial effect on soil structure and that its application should be adjusted to field soil properties and condition (e.g. aggregate size distribution, moisture content).