Saving CO2 Emissions by Reusing Organic Growing Media from Hydroponic Tomato Production as a Source of Nutrients to Produce Ethiopian Kale (Brassica carinata)

Large quantities of growing media residues that are rich in nutrients are disposed of after their use in hydroponics. The objective of this study was to investigate the benefits of different organic growing media (wood fibers, hemp fibers, sphagnum moss) residues from hydroponic tomato production as a nutrient source to produce Ethiopian kale. The amount of nutrients that can be reused as fertilizer and the associated CO2 savings have been calculated. Kale was cultivated in sand-residue mixtures, either with 25 or 50 vol% of the mentioned growing media residues. Control treatments with sand with or without nutrient addition were cultivated too. The incorporation of all growing media residues to sand increased the field capacity and growth. Plants that were supplemented with hemp fiber residues showed the strongest growth and highest yields. However, the hemp fiber residues that are used are not suitable for use in the open field due to its excessive content of certain nutrients, which restrict the output quantity. Regarding the fertilization effect of growing media residues, it was calculated that 11–300 kg nutrients ha−1 (N, P, K, Mg, Ca, S), with an average primary energy demand of 90–3435 MJ and 6–317 kg CO2 eq, could potentially be saved when different crops were considered.