Tarun Kumar Gayen, Mohammad Amdad Ali, Sudhir G. Warkar
Hydrogels have emerged as promising materials for agricultural applications by improving soil water management and nutrient-use efficiency through a controlled nutrient release. However, most commercially available hydrogels are derived from synthetic polymers and raise environmental concerns. Guided by the 12 principles of green chemistry, we developed agro-residue waste-derived biohydrogels for slow-release fertilization. Lignin extracted from mustard straw was blended with chitosan to form a biohydrogel, into which muriate of potash (MOP) was incorporated using an in situ loading strategy. Chitosan was selected because it contains primary amine (−NH 2 ), hydroxyl (−OH), and residual acetamide (−NHCOCH 3 ) groups, which enable interactions with other molecules through hydrogen bonding at neutral pH. The resulting hydrogel exhibited sustained potassium release with ∼91% cumulative release in deionized water over 50 days and a loading efficiency of ∼29%. Release kinetics in both water and soil were best described by the Peppas–Sahlin model ( R 2 = 0.99), indicating diffusion-dominated transport. When applied to soil, lignin–chitosan biohydrogels enhanced water retention for up to ∼65 days, which decreased to ∼45 days upon MOP loading. These results demonstrate the potential of lignin–chitosan biohydrogels as sustainable platforms for simultaneous water retention and controlled potassium delivery in agricultural systems.