Synergistic Effects of Humic Acid, Microbial Biofertilizers, and Seaweed Extract on Growth, Yield, and Essential Oil Composition of Dill (Anethum graveolens L.) under Semi-Arid Conditions

Abstract

Semi-arid agroecosystems impose multiple constraints on aromatic crops, including low soil fertility and limited water availability, ultimately reducing biomass accumulation and essential oil productivity. This study investigated the individual and combined effects of humic acid (HA), microbial biofertilizers, and seaweed extract (SE) on growth performance, yield components, and essential oil quantity and composition of dill (Anethum graveolens L.) under field conditions in Misurata, Libya during the 2024/2025 season. A split-plot design with four replicates was employed, where HA rates (0, 5, 10, and 20 kg ha−1) were assigned to main plots and promoter treatments (microbial biofertilizers, SE, and their combination) to subplots. Results revealed a clear dose-dependent response to HA, with the highest rate (20 kg ha−1) significantly improving plant height (117.38 cm), branch number (17.38 plant−1), fresh biomass (216.81 g plant−1), and fruit yield (2.423 t ha−1). The combined application of microbial biofertilizers and SE further enhanced these parameters, demonstrating strong synergistic effects. The interaction treatment (20 kg HA ha−1 + biofertilizers + SE) produced the highest essential oil content (1.578%) and yield (41.79 L ha−1). GC-MS analysis identified carvone, D-limonene, dillapiole, and (+)-dihydrocarvone as major constituents, with treatment-dependent compositional shifts. These findings highlight the effectiveness of integrated biostimulant-based fertilization strategies in enhancing dill productivity and oil quality under semi-arid conditions, offering a sustainable alternative to conventional inputs.