Conventional and Rock-Based Potassium Fertilization Improves Fennel (Foeniculum vulgare Mill.) Performance Under Extended Irrigation Intervals

Water scarcity and rising fertilizer costs challenge the sustainable cultivation of medicinal and aromatic plants in arid regions. This study evaluated the interactive effects of irrigation intervals (21, 28, and 35 days) and potassium sources (potassium sulfate and feldspar) and rates on growth, yield, essential oil productivity, and nutrient status of fennel (Foeniculum vulgare Mill.) over two consecutive seasons in Middle Egypt. Extending irrigation intervals significantly increased soil electrical conductivity while reducing soil-available potassium, whereas soil pH, organic matter, and bulk density remained unaffected. Frequent irrigation (21 days) markedly enhanced vegetative growth, yield components, seed yield, and essential oil yield, producing up to 69.7 L ha−1 oil compared with 50.5–52.0 L ha−1 under 35-day intervals. Potassium fertilization significantly improved plant performance across all irrigation regimes, with potassium sulfate at 120 kg K2O ha−1 consistently producing the highest plant height (≈173 cm), number of umbels (≈45 plant−1), 1000-seed weight (≈13 g), seed yield, and oil yield. Potassium sulfate at 120 kg K2O ha−1 consistently outperformed feldspar, though high-rate feldspar (572 kg K2O ha−1) significantly improved performance over the control, indicating potential as a supplementary source. Extending irrigation to 28 days reduced water application by approximately 23% compared to 21-day intervals, with acceptable yield levels when combined with adequate potassium supply. Potassium application enhanced seed and herb N, P, and K concentrations and mitigated the adverse effects of prolonged irrigation intervals, particularly under moderate water stress (28 days). Significant irrigation × potassium interactions confirm that optimal potassium nutrition improved water-use efficiency and reproductive performance. Overall, integrating frequent or moderately extended irrigation with an adequate potassium supply—especially soluble potassium sulfate—offers an effective strategy to sustain fennel productivity and essential oil yield under water-limited conditions.