Many hydroponic fresh-cut herb producers in the United States utilize water sources with a high pH. Nutrient solutions with a high pH can prevent essential nutrients from being accessible for plant growth and development, especially micronutrients. The objectives of our research were to better understand how the growth of culinary herbs is impacted by supra-optimal pH and determine if supplemental micronutrients are an effective mitigation technique. 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. Herbs were grown in nutrient solutions with a pH of 6.0. 7.0, and 8.0, and either with a 1X or 2x micronutrient concentration. The DFT systems contained nutrient solutions made with tempered municipal water supplemented with a complete water-soluble fertilizer (16N-2.2P-14.3K) to create a target electrical conductivity of 2.0 dS·m–1, plus a supplemental micronutrient blend provided from the manufacturer to increase micronutrient concentrations for 2x micronutrient treatments. The nutrient solution pH was maintained through a dosing system using 2% H2SO4 v/v or 2% KOH w/v as the acid and alkali, respectively. One-third (by vol.) of the nutrient solution was renewed with freshly mixed fertilizer each week of production. Target greenhouse environmental conditions consisted of day and night air temperatures of 22 °C and 18 °C respectively, and a daily light integral of 12 mol∙m–2∙d–1. The experiment was replicated four times over time, and data was collected four weeks after herbs were transplanted into DFT systems. There were no significant interactions between micronutrients and pH. While high pH decreased dill, parsley, and sage shoot height, width, and fresh mass, providing supplemental micronutrients in the nutrient solution did not mitigate growth suppression induced by high pH. Additionally, all species had significantly shorter roots at a high pH of 8.0 compared to a moderate pH of 6.0, without a subsequent reduction in root mass. This study indicates nutrient solutions with a high pH can severely limit the growth of culinary herbs. While providing supplemental micronutrients does not alleviate the impact of high pH, producers can rely on other methods, such as acid injection, to lower the nutrient solution pH to maximize nutrient uptake.
