Circularity of growing media use: reuse of nutrients, carbon and bulk material and the role of biochar

Growing media are important for circular horticulture as they allow for optimal use of materials and nutrients, both in terms of blend composition, and in reuse/upcycling spent media at the end of cultivation. The direct reuse or use as feedstock for further processing of spent media makes it possible to extend their use as growing medium and thus provides added value in terms of nutrient use efficiency and sustainability of the soilless cultivation system. Growing media blends based on sustainable materials allow for reduction or complete replacement of non-renewable raw materials such as peat or mineral wool and may also have an added value for plant growth and environment in terms of lower need for fertilizers due to the provision of recycled nutrients, and a lower need for chemical crop protection products due to increased disease resistance of crops. The selection of materials in the blend is key for a successful cultivation as well as for reuse of the growing media. Materials in sustainable growing media blends and the cultivation practice during first use of the growing media affect the characteristics of spent growing media and thus the potential of the spent growing medium to be reused or upcycled. Optimizing the cultivation practices may be a strategy to increase the circularity of soilless horticulture. Biochar has potential to further increase the sustainability of growing media and controlled-environment agriculture. Biochar is a promising material for use in horticultural substrates, and biochar production is a strategy to recycle spent growing media and the included nutrients. The role of biochar as game changer for growing media is discussed in this paper.