Biochar for Circular Horticulture: Feedstock Related Effects in Soilless Cultivation

Biochar has previously been used in growing media blends as fertilizer or for improving plant growth, disease suppression, and as a sustainable replacement of peat. To achieve optimal circular horticulture, we propose here to reuse the biochar from spent growing media. However, it is unclear to what extent the biochar feedstock determines the mode of action of the biochar and if use of spent growing media biochar may encounter nutrient or salt problems. Differences in chemical characteristics, nutrient release, and interaction in a leaching experiment and effects on plant growth, nutrient uptake, and disease suppression in a strawberry greenhouse trial were studied for 11 biochars either processed from spent growing media or from lignocellulosic biomass. A well-studied biochar produced from oak wood was set as reference. Biochars produced from spent growing media were characterized by higher electrical conductivity, extractable and total nutrient concentrations compared with biochars produced from lignocellulosic biomass. Especially in the first phase of the leaching experiment, all biochars showed nutrient and salt release, with most prominent effects for spent growing media biochars and the reference biochar. The latter biochars were an important source of phosphorus and in particular of potassium. Only for the reference biochar, strawberry plants showed increased uptake of phosphorus, potassium and calcium, and increased chlorophyll concentration. No <i>Bortrytis cinerea</i> disease suppression and no increase in plant growth was observed for the tested biochars. It is concluded that spent growing media can be recycled as biochar in growing media without adverse effects compared to biochars produced from lignocellulosic biomass.