Synthesis and Characterization of Slow-Release Fertilizer Hydrogels for Agricultural Applications

Abstract

The research presented herein focuses on the design and characterization of poly(acrylamide)-g-starch hydrogels and their effectiveness as slow-release fertilizer hydrogels (SRFHGs). SRFHGs have been employed to minimize losses through denitrification and volatilization, which are typically associated with traditional synthetic fertilizers. The application of traditional fertilizers has led to numerous environmental problems such as soil acidification and eutrophication. SRFHGs aim to reduce these issues by exploiting the swelling and controlled-release nature of hydrogels, allowing for an efficient release of fertilizer with time such that losses are minimized at the source. A design of experiments approach was followed, utilizing a 2x3 factorial design to quantifiably investigate the effects of hydrogel synthesis formulations on hydrogel application performance. To maintain reliability between formulations and narrow the scope of the design, the reaction temperature, reaction time and masses of biopolymer were held constant across eight formulations. The three factors of interest were the concentration of N,N’-methylene bis(acrylamide) (MBA) crosslinker, the concentration of the acrylamide (AAm) monomer, and the nutrient loading protocol. The responses of interest were the crosslink density (which was estimated using oscillatory rheometry), the gel content of the materials following Soxhlet extraction (which provided insight about the degree of crosslinking), the degree of swelling, and the cumulative release of urea to evaluate the application performance of the hydrogel materials. Eight formulations were synthesized and characterized following the factorial design of experiments. Of the eight materials evaluated, those loaded following an in-situ loading protocol showed both the highest degree of swelling and the highest cumulative urea release, effectively showing great promise as SRFHGs. Furthermore, among those formulations, the hydrogel synthesized at lower AAm and MBA concentrations showed the most promise, according to the outlined benchmark criteria for slow-release fertilizer materials.