ASHS 2025 Conference

The pH of the root zone plays a crucial role in influencing the solubility of nutrients within a soilless substrate or hydroponic solution, affecting their uptake into plant tissue. Our objective was to re-evaluate and update the understanding of how root zone pH affects nutrient solubility in soilless substrates and hydroponic systems using chemical equilibria software, laboratory testing, and a literature review. The aim was to provide horticulture practitioners with an understanding of the trends that should be considered when managing substrates and fertilization. Calcium and magnesium have decreasing solubility with increasing pH. However, dolomitic limestone added for neutralization of acidic substrates such as bark and peat increase Ca and Mg under alkaline conditions. Because potassium salts are usually the liming sources in hydroponics, Ca and Mg levels decrease at high pH. Phosphorus availability is limited primarily because of reactions with Ca at high pH. Results demonstrated substantial differences in micronutrient solubility between sulfate-based and chelated micronutrients. In hydroponic simulations (GEOCHEM-EZ), sulfate-based micronutrients rapidly became insoluble at higher pH (>6.0), whereas chelates, particularly Fe-EDDHA (ethylenediamine-N,N-bis(2-hydroxyphenylacetic acid), maintained solubility even under alkaline conditions (up to pH 10.0). Visual MINTEQ simulations highlighted the role of DOM in stabilizing micronutrient availability in soilless substrates through the formation of metal-organic complexes, especially for metal micronutrients Fe, Cu, and Zn. Laboratory measurements were consistent with chemical equilibrium simulations, demonstrating higher water-extractable micronutrient solubility with EDTA (ethylenediaminetetraacetic acid) chelates compared with sulfate forms, particularly at elevated pH. Fe and Mn solubility sharply declined with increasing substrate pH, whereas Zn and Cu solubility peaked at intermediate pH levels (5.5 to 6.5). Boron availability decreased as substrate pH increased, whereas Mo solubility increased under alkaline conditions. A review of trends in plant tissue micronutrient concentration at different pH levels in hydroponics and soilless substrates emphasized that translating micronutrient solubility to plant availability is complex. For example, absorption of cations by roots at very low pH may be impeded in soilless substrates hydroponics by a combination of growth inhibition, nutrient disorders, and direct pH effects of H concentration. Plant uptake involves numerous biological processes, including root exudation, microbial interactions, nutrient competition, redox conditions, and plant-specific nutrient strategies. Updated pH solubility charts were developed to help visualize micronutrient solubility and plant uptake under different scenarios that have practical implications for horticulture fertilizer management.