Adverse effects of superabsorbent polymers on crop growth and the underlying mechanisms

Application of superabsorbent polymers (SAPs) is an effective measure to improve drought resistance of crops. However, slightly excessive application rate will lead to adverse effects, such as decline of seed germination rate, yield decrease, poor root growth and so on. The indirect causes of the adverse effects could be the competition for soil moisture by crops and reduced soil aeration. The direct cause of the adverse effects is the inhibition of crop growth caused by the residual water-soluble components in SAPs, such as acrylic acid and sodium. Acrylic acid is toxic to animals, but its quick degradation under normal moisture conditions lessens its danger. However, if SAPs absorb water and form gel in soil, the half-life of acrylic acid in the gel will be greatly elongated, and might damage the crops. Most studies on the possible direct damage of monomers in SAPs were conducted under normal soil moisture conditions. Therefore, the safe and effective application of SAPs still needs to be studied under drought conditions. The safe concentration and threshold for crop growth, the degradation kinetics of monomer components under drought conditions, and the toxicity of degraded monomers to soil, plants under drought conditions need to be extensively studied. The degradation, dilution and diffusion characteristics of harmful components in SAPs under different soil conditions and the risk in different crop growing periods are also unclear. The limitations of the residual quantity of monomer should be considered in current industrial standard of SAPs, so as to provide guidance for the safe use of SAPs. In general, the safe and efficient application of polymers relies on the screening of SAPs with low biotoxicity of monomers, the optimized synthesis parameters to reduce the residue of monomers, and the specified application methods.