Aim <p>This study investigated how the chemical structure of dissolved organic matter (DOM) derived from various organic amendments (OAs) influences growth, nutrient accumulation, and ascorbic acid content in lettuce.</p> Methods <p>Water-extractable organic matter (WEOM) was obtained from bark compost (BC), coffee residue compost (CRC), cattle manure compost (CMC), sewage sludge compost (SSC), fish cake (FC), and rapeseed oil cake (ROC), and their chemical structure was characterized using high-performance size exclusion chromatography and solid-state ¹³C nuclear magnetic resonance spectroscopy. Lettuce seedlings (7-days old) were grown hydroponically in nutrient solutions supplemented with 20 mg C L^–1 concentration of WEOM from each OA as DOM or in nutrient solution (Control). After 28 days, lettuce was harvested and shoot and root growth, nutrient content, and ascorbic acid content were measured.</p> Results <p>SPAD at harvest (42 − 50), shoot fresh weight (SFW; 17 − 24 g), crop growth rate (CGR; 0.043–0.070 g cm^–2 day^–1), and relative growth rate (RGR; 0.248–0.266 g g^–1 day^–1) were highest in the BC-DOM treatment. The SPAD positively correlated with % aromatic C-O (<i>P</i> &lt; 0.05). Conversely, SFW, shoot dry weight, leaf area, maximum leaf width, CGR, and RGR, as well as accumulation of nitrogen, phosphorus, calcium, magnesium, molybdenum, and boron were negatively correlated with % alkyl C and/or % methoxy C (including amino acid Cα) in WEOM (<i>P</i> &lt; 0.01 or &lt; 0.05). Ascorbic acid content in lettuce, 30–40 mg g^–1 FW, was highest in ROC-DOM treatment.</p> Conclusions <p>These findings highlight that the abundance of C functional groups could be a key to predict the positive or negative effects of WEOM in OAs on the growth and nutrient accumulation in lettuce cultivation.</p>