Humic-like Substances Extracted from Hydrochar as an Additive for Foliar Fertilization in Tomato Plants

Biomass reuse and the development of fertilization strategies are approaches for enhancing agricultural productivity. Humic-like substances (HLS), extracted from hydrochars of various biomasses, exhibit valuable characteristics such as diverse organic composition and nutrient complexation, which can stimulate plant growth. However, few studies have investigated HLS as a foliar fertilization application. In this work, HLSs were extracted from two hydrochars derived from sugar cane bagasse and vinasse: HLS_A (additive H₃PO₄) and HLS_B (additive KOH), which were then characterized. HLSs were applied by foliar spraying at 50, 100, and 300 mg of TOC L^-1, with and without fertilizer in tomato plants (<i>Solanum lycopersicum</i> L.). Plant development was monitored for 35 days, and growth, chlorophyll (ICF), and micromorphological evaluation (SEM) measurements were recorded. Both HLSs influenced plant growth. HLS_A at 100 mg of TOC L^-1 with fertilizer increased shoot length, dry biomass, and chlorophyll content by 18, 20, and 13.9%, respectively, compared to the control. Similarly, HLS_B at 300 mg of TOC L^-1 showed comparable increases in shoot length (17.4%) and dry biomass (15%) but was more effective in enhancing chlorophyll index (28.2%). Plants treated with HLS_B also showed an increase in root length and root dry weight by 25%, respectively. Treatments using only HLS also produced positive results, suggesting HLS's intrinsic stimulating effects and without signs of anatomical ruptures in the cellular structure. The findings indicate that both HLS are promising for use in sustainable agriculture, endorsing the reuse of sugar-energy industry residues.