Cascading use of Miscanthus as growing substrate in soilless cultivation of vegetables (tomatoes, cucumbers) and subsequent direct combustion

The demand for sustainable and environment friendly growing media as alternatives for peat or inorganic substrates like Rockwool for vegetable production in the greenhouse is increasing. At the same time growing media have to meet the requirements of the growers with respect to crop performance (yield, product quality) and consistent supply of high quality raw material. In this study we tested Miscanthus shreds, chips and fibers as alternative organic growing substrate for Rockwool in soilless cultivation of mini cucumbers and tomatoes as primary use under practical conditions. Following vegetable production we evaluate the combustion quality of the used growing substrate for a secondary use as solid fuel. Cucumbers and tomatoes grown on the different Miscanthus substrates obtained comparable cumulative yields to Rockwool. Additionally tested plant traits (single fruit length, diameter and weight for cucumbers; plant growth and total soluble solids for tomatoes) were not affected. Gross calorific values of Miscanthus shreds and chips were almost the same after using them as growing substrate compared to the original material. In contrast, the value for fibers decreased significantly. Ash contents were higher in comparison to the original feedstock but were still below 7% for shreds and chips. Ash content of fibers increased to over 10% after primary use as growing medium. In conclusion, Miscanthus is a promising sustainable and renewable alternative to Rockwool in soilless cultivation of vegetables and can be used in a cascade with subsequent direct combustion and reuse of the ash.