Sustainable nutrient substrates produced from spent grains-based hydrochar composite for the growth of bok choy seeds

The development of green, low-cost soilless culture substrates is a critical factor limiting the widespread adoption of soilless cultivation techniques. In this study, spent grains were hydrothermally carbonized with the addition of vermiculite to prepare spent grains-based hydrochar composites followed by preparing hydrochar composite substrates with the involvement of vermiculite and perlite. The results indicate that the spent grains-based hydrochar composites show excellent performance in immobilizing nutrients as well as reducing nutrient loss (NH₄⁺-N, NO₃^--N, and PO₄^3-). Moreover, the hydrochar composite substrates exhibit a lower bulk density, higher porosity, and moderate pH and electrical conductivity. As a result, bok choy shows superior growth performance including germination rate, plant height, and leaf length on the substrates. This study indicates that spent grains have the potential to be developed as a green, cost-effective alternative for soilless cultivation substrates to achieve their resource utilization.