Macronutrient Solubility in Response to the pH of Soilless Container Substrates

Abstract This study investigated the complex interactions between pH and nutrient availability in soilless substrates, focusing on how different nutrient solution formulations, substrate components, and liming materials influence macronutrient solubility. The objective was to develop a more sophisticated representation of pH-nutrient solubility relationships compared with charts currently used by horticulture professionals. Macronutrient concentration was measured in response to substrate-pH using Ca(OH) 2 and Mg(OH) 2 in four peat-based substrates (70% peat mixed with 30% perlite, pine bark, coconut coir, or vermiculite by volume). A range of lime rates and fertilizer formulations were applied, and resulting pH and macronutrient concentration in a water extract solution was analyzed. Nitrate-N and ammonium-N concentrations decreased at higher pH levels. Phosphorus concentration decreased, especially when limed with Ca(OH) 2 . Potassium availability was affected by substrate type and cation exchange capacity. Calcium solubility increased with Ca(OH)₂ but decreased with Mg(OH)₂ at elevated pH levels, whereas magnesium solubility showed a reverse pattern. Sulfate-sulfur concentration remained stable across treatments. Chemical equilibrium simulations with Visual MINTEQ highlighted the formation of insoluble Ca-P compounds with Ca(OH) 2 as the lime source, and low solubility Mg-P compounds with Mg(OH) 2 as lime, hindering available P. This study illustrates several key trends in pH effects on macronutrient solubility and plant availability that have practical implications for horticulture fertilizer management and emphasize the real-world complexity beyond widely-used pH solubility charts. The review of published plant studies highlights that tissue nutrient concentrations do not always correlate with nutrient solubility. This discrepancy is influenced by factors such as root-secreted compounds and pH modifications caused by nutrient uptake by plant roots.