Eco-friendly three-dimensional hydrogels for sustainable agricultural applications: Current and future scenarios

Modern world is searching for water efficient agriculture techniques as irrigation is becoming scarce. Limited water resources and more food demand are the two key challenges in agriculture. Hydrogels which can respond intelligently by pH, temperature, light, ionic strength, osmotic pressure, magnetic or electric field changes are termed as intelligent or smart hydrogels which are analogous to conventional hydrogels in preparatory methods and features. Lag phase, constant release and decay phase are three steps involved in release of nutrients from polymeric hydrogels. In fact, hydrogels act as little reservoirs of water and dissolve nutrients that are released in controlled manner anchored by plant roots via capillary action. Hydrogels also sustain optimum amount of water in water stress conditions and reabsorb water in moist conditions which ultimately increases seedling, seed germination, plant growth and crop yield. Fertilizer and salt release are majorly dependent upon pH and temperature followed by diffusion-controlled mechanism. Cross-linkers, binders and fillers play pivotal role in determining properties, architecture and hydrogel pores. In comparison to potassium (K+) and phosphate (PO4-3) ions, nitrates (NO3-1) and ammonium (NH4+1) ions exhibited faster release rate. This review spotlights application prospects of three dimensional hydrogel in agriculture. Initially, properties of hydrogels, their classification, preparatory methods, effect of natural-synthetic-polymer blending and role of fillers are stated. Afterward, hydrogel functioning, significance, advantages, mechanism of fertilizer release and agriculture specific applications of hydrogels are comprehensively described. In conclusion, extraordinary biocompatible, cheaper, stable, biodegradable, durable, non-toxic and re-wettable characteristics of hydrogel systems motivated their utilization in agronomical applications.