End-of-life stage of renewable growing media with biochar versus spent peat or mineral wool

The composition of horticultural substrates for soilless greenhouse cultivation directly affects the sustainability of the cropping system but has also an indirect effect through the end-of-life stage of the spent media. Biochar amendment in growing media for use as bulk material and source of nutrients may improve the sustainability of controlled-environment agriculture. Horticultural substrates were compared at the end of soilless strawberry and tomato cultivation in six trials at commercial scale. Conventional mineral wool and peat-based blends were compared with peat-reduced and peat-free organic blends with or without 10% v/v biochar amendment. Nutrients, C stability of the growing media and their value as soil improver were measured. The organic growing media had a high potential for reuse and for C storage. Spent mineral wool was significantly richer in total P, K, Mg and Ca and significantly lower in organic C content and C stability than the other blends, with a clearly lower value as soil improver than the organic blends. Biochar amendment in renewable organic blends increases their value as soil improver and their potential for reuse: addition of 10% v/v biochar in the blend significantly increased the C content (90 g C/kg dry matter higher) and the C:N ratio of the spent growing media, but not the C stability. The pH of the biochar in the growing media decreased from 9.1 to 6.2 during cultivation while CEC increased. This research illustrates the feasibility of using biomass and biochar in cascade: first as growing medium and finally as a C-rich soil improver.