Abstract: Seedlings of lettuce (Lactuca sativa L.) variety Grand Rapids were grown in deep water culture growing units. The units contained growing solution with limited amount of nutrients and different concentrations of humic substances. Six treatments and one control, 12 replications each, were evaluated in a two month period. Only one plant perished from a total of 84, indicating that the growing units were effective. A liquid product containing 13,800 mg L^-1 humic substances of small particulate sizing (1.1 ± 0.64 ?m in mean diameter) was utilized as a source of humic substances. Significant plant (root and leaf) growth was observed at low product rates between 70 and 500 mg L^-1, corresponding to 1 and 7 mg L^-1 humic substances. At increased rates, the growth was reduced. At much higher rates, plant growth was again observed, that was likely caused by the presence of nutrients in the product. These experiments demonstrated the efficacy of humic substances on plant growth, a critical finding in the context of sustainable horticulture, in which maximum yields from minimum input would be desired.