Biomass ash and biochar are gaining increasing interest as alternative or supplementary fertilizers in agriculture and forestry. Both products are made from plant materials. Biomass ash through full combustion of biomass, while biochar through incomplete combustion (pyrolysis) under oxygen-limited conditions. The use of these products fits in with the idea of a closed-loop economy. It allows nutrients to be recycled from plant waste back into the soil, reducing the use of conventional mineral fertilizers and landfill. The following chapter discusses methods for the application of biomass ash and biochar to soil, reviews research results on their effectiveness and provides examples of practical use in different types of crops.