Substrate Amended with Solid Humic Substances Improved ‘Micro-Tom’ Tomato (solanum Lycopersicum L.) Growth

Humic substances (HS) have been widely used and investigated in several horticultural crops. Most HS used are in liquid form with fast and early releasing effects, while studies on HS products in solid forms and their effects on plant growth components across the complete growth cycle have been less reported. In this study, we selected five commercial solid HS sources derived from different raw materials (lignite, leonardite, and sub-bituminous coal) and tested their effects as substrate amendments on cherry tomato (Solanum lycopersicum cv. 'Micro-Tom') growth. Results showed that solid HS products improved plant growth, compared to controls, in plant height, canopy area, and dry biomass accumulation. HS treatments had profound effects on root growth, as seen in the increased root relative growth rate during the late growth period (12 weeks after seeding). HS treatments did not significantly improve total fruit fresh or dry weights compared to controls or within different HS products. However, leonardite-derived HS treated plants showed a higher fruit number than other HS product treatments. These results demonstrate that as substrate amendments, different solid HS sources can be considered for specific applications to improve tomato growth performance. In addition to exploring the substrate environmental changes due to HS application, future mechanistic studies are needed to better understand how different HS sources induce either positive, neutral, or negative effects on plant growth.