Hydrogels for agronomical application: from soil characteristics to crop growth: a review

In a growing world population scenario, greater demand for food and in turn agricultural input is expected in the coming decades. The development of innovative sustainable amendments and fertilizers such as hydrogels (HGs) relies on this context. HGs are defined as 3D polymeric networks with the ability to absorb and retain a large amount of water (i.e., swellability). HGs swellability makes them ideal platforms for water- and nutrient-controlled release. The literature reflects a notable diversity in HGs composition, properties, and impacts on agroecosystems, resulting in the dissemination of information across multiple publications. Within this context, the primary aim of this scientific review is to systematically compile, critically evaluate, and synthesize the latest findings and relevant literature pertaining to the application of HGs in agroecosystems. We found that this class of materials was consistently found on soil bulk density, crop available water, soil electrical conductivity, cation exchange capacity and main cereal or legume crops performances particularly under moderate water deficit conditions, suggesting their effectiveness for arid and semi- arid areas. Encouraging results were also found on pesticide, agrochemicals, and microorganisms delivering. Thus, the delivery of agronomic inputs with loaded-HG may play a key role in forthcoming agriculture that would minimize the agronomic impact on the environment. Nevertheless, some critical issues still remain open, concerning: (i) the decreased of HG effectiveness following wetting and drying cycles, (ii) the impact of Na+ contained in HG on crop cells, (iii) the HG effect on soil saturated hydraulic conductivity and pH, and (iv) HG biodegradability and their fate into the environment. In conclusion, HGs seem a promising technology to reach/maintain food security and soil health, but future research should address the development of well-defined protocols for producing biowaste-derived HG that will ensure their biodegradability and non-toxicity and will disclose new insights into a circular economy approach agronomy. One of the biggest challenges would be the synthesis of "smart" HG able to respond to environmental stimuli triggering molecule(s) release in soil.