Selenium (Se) is an essential trace element for humans and animals with a narrow window between deficiency and toxicity levels (Boyd 2011, Fordyce 2013, Rayman 2000). Application of conventional chemical Se fertilizers to increase the Se content in crops can result in secondary soil and water contamination due to the low utilization rate of Se. Therefore, it may be beneficial to produce slow-release Se-enriched organic fertilizers locally from Se-containing (waste)water. This process may contribute to the worldwide drive for resource recovery. In this study, we aim to evaluate the bioavailability of Se released from two Se-enriched biomaterials (duckweed and anaerobic sludge) added to soil, and to assess the potential of these two biomaterials as Se fertilizers.