Super-absorbent polymers (SAPs), a group of new water-saving materials and soil conditioners, have been widely adopted in agriculture, but little is known about their effects on soil physical and chemical properties under alternating dry and wet conditions. This study assesses soil moisture, bulk density, pH, electrical conductivity (EC) and available P and K after different wetting and drying cycles. Four types of SAPs, labelled BF, JP, BJ and WT, with organic macromolecules were mixed with sandy soils to give concentrations of 0%, 0.05%, 0.1%, 0.2% and 0.3%, with the aim of determining water retention and soil properties after amendment with the SAPs. Soil moisture increased by 6.2-32.8% with SAP application, while soil bulk density was reduced by 5.5-9.4% relative to the control, especially with a moderate water deficit when the relative soil moisture contents were about 40-50%. The largest increase in soil moisture and the greatest reduction in bulk density resulted from the WT treatment. The effects of SAPs on soil pH and EC were contrary. Soil available P increased slightly while available K significantly decreased, except following the first wetting and drying cycle. Available K increased with drying, but the opposite effect was observed for available P. Particular SAPs (JP and WT) seem more suitable under alternating dry and wet conditions. The effects on soil-water retention and other soil properties varied according to the structure of the SAP and soil moisture.

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This paper studied the strengthening effects of silica nanoparticles on the polyacrylamide (PAM)/hydroquinone (HQ)–hexamethylenetetramine (HMTA) composite gel. Pure PAM/HQ–HMTA gel and PAM/HQ–HMTA gels containing silica nanoparticles up to 0.3 wt % were prepared at 110 °C. Influences of silica nanoparticles on gelation performances were systematically evaluated. By the addition of silica nanoparticles, the gelation time became shorter and the gel strength was improved observably. Rheological measurements showed that silica nanoparticles enhanced both elasticity and viscosity of the gel significantly. Thermal stability of the gel was studied by differential scanning calorimetry (DSC) measurements. The maximum tolerated temperature of the gel was improved from 137.8 to 155.5 °C by the addition of silica nanoparticles with a concentration of 0.3 wt %. Furthermore, to study the strengthening mechanisms of silica nanoparticles to the gel, the microstructure and existing state of water within the gel were investigated by environmental scanning electron microscopy (ESEM) and DSC measurements. Micrographs of the gel showed that massive aggregations and arrangements of silica nanoparticles existed in uniformly distributed three-dimensional network structures of the gel, which greatly improved the structural strength of the gel. Moreover, the mass fraction of bound water within the gel increased from 22.5 to 39.9% by the addition of silica nanoparticles with a concentration of 0.3 wt %. The hydrogen bonds and electrostatic attractions between silica nanoparticles and water molecules/hydronium ions make a higher bound water ratio, which contributes to better water holding capacity and thermal stability of the gel.