Studying hydraulic characteristics of three soils amended with superabsorbent hydrogels

Superabsorbent hydrogels (SAHs) are three-dimensional cross-linked polymeric network, with the ability to absorb large quantities of water (or other fluids), keep their network stable even in the swollen state and  release the water in response to specific environmental stimuli. In last decades, several SAHs have been developed and studied for their potential to improve soil physical properties for agricultural purposes, e. g. by increasing their ability to retain water, increasing the efficiency of water use, increasing the absorption capacity or promoting the uptake of nutrients by the plant. Although various studies have shown that SAHs can contribute to soil microstructural stability and improve soil-water interactions in general, the specific impact of water content on the hydrogel effect in soil remains largely unclear. Specifically, the SAH effects needs to be inquired in dynamic water content conditions.

The aim of this study was to evaluate how the water content affects soil-water-hydrogel interactions in different soils (i.e. sandy, loamy and silty soils). Two different SAHs were studied, a polyacrylamide and a self-made cellulose-based SAH added with humic acids. Untreated and SAH-treated (4 mg g⁻¹) samples were analysed for soil water retention and unsaturated hydraulic conductivity coupling the Wind method (−1 to −80 kPa) with WP4-T dew point (-3000 to 15000 kPa). SAH deswelling dynamic was evaluated measuring the sample volume change from −1 to −7 kPa potential. Volume was measured reconstructing the 3D shape of the soil sample employing a 3D Point Cloud Reconstruction software.

Soil-SAH mixtures changed the water retention characteristics if compared to the controls. SAHs significantly increased the water entrapment and altered the pore-size distribution according to the soil types. It can be hypothesized that in the soil-SAH mixtures there was a texture-dependent rearrangement of soil particles, depending on the various interactions between hydrogel structures and soil constituents as a function of water content. These findings demonstrated the potential of SAHs to advantageously modify the soil hydraulic characteristics, but also showed that further studies are needed to better comprehend the behaviour of hydrogels in dynamic water content conditions in different soils.