Formulation of slow release NPK fertilizer (cellulose-graft-poly(acrylamide)/nano-hydroxyapatite/soluble fertilizer) composite and evaluating its N mineralization potential

Polymer nano-composite fertilizer formulation has the potential to enhance nutrient use efficiency. Slow release fertilizer (SRF) composite was formulated by incorporating nano-hydroxyapatite (nano-HA) and water soluble fertilizers (urea, (NH4)2HPO4 and K2SO4) into water hyacinth cellulose-graft-poly(acrylamide) polymer hydrogel. Fourier Transform Infra-red spectra revealed existence of chemical interaction between the monomer, cellulose, urea and nano-HA. The release of nutrients was assessed using laboratory incubation experiment. Significantly higher content of mineral nitrogen (MN) was observed in the first 4 weeks in conventional fertilizer (CF) compared to SRF treatments and the control. MN content in SRF treatments increased considerably between the 8th and 12th week, and declined in the 16th week. The values of potentially mineralizable N estimated using first order kinetics model related well to the observed cumulative MN at 16th week. No significant difference was observed between CF and SRF treatments for available P content in the 2nd week. Significantly higher P content was observed in CF compared to SRF treatment in the 4th week, whereas in the 8th week, some SRFs released significantly higher content than CF. Available P peaked in the 8th week in all the treatments and remained constant at 12th and 16th week. Availability of P in SRFs increased with increased content of soluble P and decreased content of nano-HA. Exchangeable K showed less variation during the incubation period, suggesting short release time. The data revealed reduced chances of leaching losses and toxic effect to the plant roots, as well as synchronized nutrient release and requirement by crops. Keywords: Nano-composite, Polymer hydrogel, Chemical interaction, Incubation experiment, Kinetics model