Evaluation of polymers for controlled‐release properties when incorporated with nitrogen fertilizer solutions

Three polymers (polyacrylate, vinyl‐alcohol, starch‐based) were evaluated for controlled‐release properties when expanded in urea, ammonium sulfate ((NH4)2SO4), and potassium nitrate (KNO3) solutions, at five nitrogen (N) concentrations (0,10,15, 20 g N/L, and saturation). The expansion capacity (mL solution absorbed/g dry polymer) of each polymer varied and was dependent on the type and concentration of fertilizer solution. On average, polymers incorporated with urea, (NH4)2SO4, and KNO3 fertilizer solutions had expansion capacities of 275, 24, and 30 mL/g, respectively. All three polymers reacted with ammonium ions in solution and resisted normal extraction procedures of ammonium with 2N KCl. To determine gel characteristics when applied to a soil medium, selected gel treatments were incubated in containers of loamy sand soil up to 28 days and then assessed for the quantity of gel recovered, N content, and N concentration. Although most gels released a large portion of N after only 7 days, some gels slowed diffusion better than the dry fertilizer controls up to 28 days.