Addressing the demand for agricultural waste valorization, this study comparatively analyzed the regulatory effects of four types of artificial humic acid (HACS, HACL, HAPS, HAMC) derived from corn straw, camphor tree leaves, peanut shells, and microalgae on the soil-plant system. This investigation elucidates the feedstock-specific mechanisms underlying differential improvements in soil amendment and crop growth. A soil incubation-pot experiment system was employed using lettuce (Lactuca sativa) as the model crop cultivated for 61 days to evaluate the effects of artificial humic acids derived from different raw materials on soil physicochemical properties and crop physiological indicators. The results demonstrated that: (1) Artificial humic acidsdemonstrated a more moderate effect on soil pH adjustment while effectively improving soil physicochemical properties and enhancing fertility. Specifically, the HACS treatment applied at 64 mL/kg significantly increased soil dissolved organic carbon by 74.37% compared to CK, and by 23.97% relative to the direct corn straw return (CS) treatment. (2) Artificial humic acids derived from different raw materials all significantly increased lettuce biomass accumulation and promoted shoot/root elongation. Among them, the HACL treatment showed superior growth promotion efficacy, while the HAMC treatment notably improved photosynthesis, transpiration, and vitamin C content in lettuce, resulting in a net photosynthetic rate of 1.75 μmol/(m2·s), a transpiration rate of 0.66 mmol/(m2·s), and a vitamin C concentration of 353.12 mg/kg in lettuce leaves per unit fresh weight. In conclusion, the soil enhancement and plant growth-promoting effects of artificial humic acids depend on the feedstock types. Corn-straw-derived humic acid serves as an effective soil amendment, while microalgae-derived humic acid represents an excellent growth-promoting option for crops due to its photosynthetic regulation function. This study provides a targeted selection strategy for the agricultural functional development of artificial humic acids.