Polymer hydrogels in agriculture (review)

Polymer hydrogels (PHGs) are formed by swelling three-dimensionally cross-linked hydrophilic polymers and are usually characterized by high water-holding capacity (K. Rop et al., 2019; N. Singh et al., 2021; A. Sikder et al., 2021). Moisture capacity and a prolonged release of fertilizers, pesticides and bio-preparations make them promising for use in agriculture (P. Rychter et al., 2016; A. Sikder et al., 2021). PHGs reduces the need for frequent irrigation, increases seed germination, plant growth, seedling survival, enhances root growth, prevents soil erosion, pesticide and fertilizer overdose (N. Singh et al., 2021). According to their origin, PHGs are divided into synthetic and natural ones: synthetic hydrogels, mainly polymers and copolymers of acrylamide and acrylic acid, have a high water-holding capacity and strength, however, they are weakly degraded in soils (A.V. Smagin et al., 2014; B. Wilske et al., 2014). It is known that microorganisms are able to use PHGs based on acrylic polymers as a source of nitrogen and/or carbon for growth (H. Matsuoka et al., 2002; M. Bao et al., 2010; F. Yu et al., 2015) due to the presence of amidase activity (F. Yu et al., 2015; A. Nyyssölä et al., 2019), ensuring gradual decomposition of PHGs in the soil. Natural hydrogels, among which cellulose-based PHGs predominate, have less strength, but are biodegradable and are environmentally friendly (R. Kundu et al., 2022). In addition to cellulose, collagen (Z.-Y. Hu et al., 2021), alginates (B. Tomadoni et al., 2020), chitosan (A. Zinchenko et al., 2022), and other polysaccharides are used as water-retaining strongly swelling agents of natural origin. Hydrogels are promising as carriers for the prolonged release of fertilizers, mainly urea (P. Rychter et al., 2016; W. Tanan et al., 2021), pesticides (C. Xu et al., 2021; C. Bai et al., 2015; F.E. Baloch et al., 2021; D. Zheng et al., 2022), for the introduction of microbial preparations into the soil, including phosphate-mobilizing and nitrogen-fixing bacteria (C.S. Wu, 2008; A.V. Kovrizhnikov et al., 2021). For a more active introduction of PHGs into practice, it is necessary to reduce their cost, mainly by the creation of composite materials based on agricultural and biotechnology industries waste. It is necessary to combine the positive qualities of synthetic and natural PHGs, synthesizing semi-synthetic hydrogels that are biodegradable and do not pollute the environment, have optimal mechanical strength and water-absorbing capacity. As water-retaining and anti-erosion agents, hydrogels based on polymers and copolymers of acrylamide and acrylic acid are more promising (I.G. Panova et al., 2021; N.B. Sadovnikova et al., 2014; A.V. Smagin et al., 2014), and natural and semi-synthetic PHGs are more promising as carriers of fertilizers and pesticides (P. Jungsinyatam et al., 2022; A. Di Martino et al., 2021). This review summarizes current information on the use of PHGs of various compositions in agriculture, provides data on the positive effect of PHGs on soil water balance, productivity, growth, survival of various crops, seed germination and commercial quality of root crops, as well as the prospects for the PHGs development.