Organic Fertilization and Biostimulants Improve Growth, Yield, and Essential Oil Production in Fennel (Foeniculum vulgare): A Field Study.

The present study was conducted over two successive seasons (2023/24 and 2024/25) at the Experimental Farm of El-Qassassin Horticultural Research Station, Ismailia Governorate, Egypt, to evaluate the response of fennel (Foeniculum vulgare Mill.) plants to organic fertilization and biostimulant applications. The experiment followed a split-plot design in which three compost tea levels (360, 480, and 600 L ha⁻1) were assigned to the main plots, while five biostimulant treatments (control, Azolla, Spirulina, Nostoc, and Chlorella extracts) were allocated to the subplots, resulting in 15 treatment combinations. Vegetative growth parameters, yield components, essential oil (EO) content, and oil yield per plant and per hectare were assessed. Higher compost tea levels, particularly 600 L ha⁻1 (Ct3), combined with the biostimulants Azolla (F2) or Spirulina (F3), consistently resulted in increased oil yield per plant and per hectare, with Ct3 × F3 achieving the maximum yields (55.58 kg ha⁻1 in the first season and 58.85 kg ha⁻1 in the second season). Principal component analysis supported these findings, indicating that yield improvements were associated with coordinated increases in oil-related traits rather than a single contributing factor. These results indicate that Ct3 combined with F2 or F3 was the most effective treatment for improving oil yield and quality under field conditions.