Ammoxidized kraft lignin has been tested as a slow-release fertilizer for the cultivation of sorghum in pot experiments. In comparison to ammonium sulfate as a conventional inorganic fertilizer, this modified lignin was a satisfactory source of nitrogen when applied in high dosage, producing similar grain and biomass yields as the reference. The soil fertilized with N-functionalized lignin showed essentially lower amounts of NO3- in the percolation water than soil fertilized with ammonium sulfate.