Effect of Foliar Application of Process Water from Hydrothermal Carbonization of Sugar-Energy Byproducts on Tomato Plant Growth and Chlorophyll Content

Process water (PW) from hydrothermal carbonization (HTC) of agro-industrial residues is rich in carbon and nutrients yet underexplored as a foliar fertilizer. Two PWs from HTC of sugar cane bagasse and vinasse using H3PO4 (PWA) and KOH (PWB) were employed as foliar treatments (at 50, 100, and 300 mg TOC L–1) with/without commercial fertilizer (CF) on tomato growth. PWA exhibited greater hydrophobicity, whereas PWB was more hydrophilic and richer in organic compounds. Plant growth and chlorophyll responses were strongly dose-dependent, with phytotoxicity observed at higher TOC levels (300 mg TOC L–1), whereas 100 mg TOC L–1 yielded optimal biostimulatory effects. PWB at 100 mg TOC L–1 with CF produced the largest growth response, increasing shoot length by up to 29% relative to the commercial fertilizer control. PWA at the same concentration with CF increased shoot length by 11% compared to the commercial fertilizer control. Relative to distilled water and fertilizer controls, chlorophyll content increased by 60% and 49% with PWA (at 100 mg TOC L–1), and by 11% and 9% with PWB, respectively. These results indicate that foliar application of PW at 100 mg TOC L–1 is promising as a foliar fertilizer supplement to stimulate chlorophyll production and tomato plant growth.