Biomass production of fodder sorghum (Sorghum sp.) has been tested in a field trial over two harvesting periods under natural meteorological conditions using ammoxidized kraft lignin (AKL) as a slow-release fertilizer and urea as conventional reference. In the course of the first growth cycle, plants treated with urea gave higher biomass yields because of the better solubility of urea in the initial phase. However, during the second cycle AKL treated plants performed better than urea treated sorghum, indicating that nitrogen from AKL became readily available.