Nutrients, either directly lost after application or taken up by plants and consumed by humans and animals, end up in the environment as solid waste or in water or air. Recycling manure and offal and recapturing nutrients from water and air prevent environmental damage and make the nutrients available for plants. These nutrients may appear in organic forms of fertilizers that comprise multiple nutrients, including macro- and micro-nutrients. These may be of added value compared to standard mineral fertilizers comprising single or few nutrients, but they may also inherit harmful components such as bacteria and unbalanced or toxic levels of (micro) nutrients. Proper processing and sound experimentation, disentangling the many factors affecting plant growth, are essential to prove the value of recycled fertilizers.