Hydroponic production systems with recirculating nutrient solutions are routinely monitored and adjusted to maintain a target pH value. Supra-optimal or sub-optimal pH values can lead to nutrient deficiencies or toxicities, respectively, reducing crop quality and yields. The objective of our research was to determine appropriate nutrient solution pH ranges for herbs grown in recirculating nutrient solutions. Two week old seedlings of basil (Ocimum basilicum ‘Nufar’), and three week old seedlings of dill (Anethum graveolens ‘Hera’), parsley (Petroselinum crispum ‘Giant of Italy’), and sage (Salvia officinalis), grown in phenolic foam cubes were transplanted into one of six deep-flow technique (DFT) systems in a greenhouse with different pH treatments. Treatments consisted of pH setpoints of 4.5, 5.0, 5.5, 6.0, 6.5, and 7.0. DFT systems contained nutrient solutions made with tempered municipal water supplemented with a complete water-soluble fertilizer (16N-2.2P-14.3K) to maintain a target electrical conductivity of 2.0 dS·m–1. The nutrient solution pH was maintained through a dosing system using 2% sulfuric acid and 2% potassium hydroxide as the acid and alkali, respectively. One-third (by vol.) of the nutrient solution was renewed with freshly mixed 16N-2.2P-14.3K fertilizer each week of production to ensure adequate nutrients in the nutrient solution. Greenhouse target environmental conditions consisted of day and night temperatures of 22 °C and 18 °C respectively, and a daily light integral of 12 mol∙m–2∙d–1. The optimal pH for culinary herb growth varied by species. Basil fresh mass was optimized at pH of 5.5 and decreased by 41.7% when grown at pH 7.0 compared to basil grown at pH of 5.0. Basil grown at pH of 7.0 was 3.38 cm shorter than plants grown at pH of 6.0. Similarly, dill and parsley had the greatest fresh mass when grown at pH of 5.0 and 5.5, respectively, and fresh mass was reduced by 40% and 33 %, respectively, when grown at pH of 7.0 compared to their optimum pH. In contrast, sage growth increased with pH, with a 17% increase in fresh mass between pH of 4.5 and 7.0. The results of this study indicate herbs may be able to grow throughout a broader range of pH values than originally thought, if nutrients do not become limited. Furthermore, when possible, hydroponic culinary herb producers can group species with similar pH requirements to maximize yields.
