Lactuca sativa L., commonly known as lettuce, represents a vital winter leafy vegetable crop cultivated globally, including Libya, owing to its rich nutritional content and rapid growth characteristics. Enhancing the vegetative growth of lettuce and similar leafy crops often involves employing various horticultural practices, with fertilization being a key component utilizing diverse nutrient sources. Two field experiments conducted at the Department of Horticulture Farm, Omar Al-Mukhtar University, situated in Al-Jabal Al-Akhdar region of Libya in 2021 and 2022 aimed to evaluate the effects of ground-applied urea fertilizer (46% nitrogen) at rates of 0, 60, 120, and 180 kg.ha⁻¹, combined with foliar applications of different concentrations of humic acid (0, 1, 1.5, and 3 ml.L⁻¹), on the fresh and dry weights of leaves and roots, as well as root length of lettuce cv. Nickerson-Zwaan plants under local environmental conditions. Each experiment comprised 16 treatments representing all possible combinations of the two factors and followed a randomized complete block split-plot design with four replications per treatment. Results revealed that increasing levels of mineral nitrogen and humic acid resulted in a significant increase of fresh and dry weights of lettuce leaves and roots across both growing seasons as compared to the control treatment, with consistent effects observed for the interaction between nitrogen fertilization sources and their respective rates. While no significant impact was observed on root length due to urea fertilizer or humic acid spray in either growing season, application of 180 kg.ha⁻¹ urea fertilizer in the second season showed a tendency towards greater root length compared to other treatments. Thus, the findings suggest that the application of mineral nitrogen fertilizer and humic acid spray holds promise for enhancing the vegetative growth of lettuce plants in Al-Jabal Al-Akhdar region of Libya.
