Macronutrient solubility in response to the pH of soilless container substrates

Authors: HUANG, JINSHENG - University Of Florida; FISHER, PAUL - University Of Florida; ARGO, WILLIAM - Blackmore Company; JEONG, KA YEON - Sun Gro Horticulture; Altland, James

Submitted to: Journal of Soil Science and Plant Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/13/2025
Publication Date: 3/6/2025
Citation: Huang, J., Fisher, P., Argo, W., Jeong, K., Altland, J.E. 2025. Macronutrient solubility in response to the pH of soilless container substrates. Journal of Soil Science and Plant Nutrition. https://doi.org/10.1007/s42729-025-02331-0.
DOI: https://doi.org/10.1007/s42729-025-02331-0

Interpretive Summary: Substrate-pH affects nutrient solubility and resulting availability for plant uptake by roots. For most floricultural crop species grown in a soilless substrate, the optimum pH range is 5.6 to 6.2. Lime from calcium (Ca) and magnesium (Mg) carbonates are used to adjust pH in soilless substrates, however, these materials can also influence solubility of other mineral nutrients. Further investigation is needed to quantify the impact of substrate-pH on nutrient solubility, considering substrate components, and methods for altering substrate-pH (to eliminate confounding effects with Ca when using Ca(OH)2 as the lime source). The objective of this research was to quantify the effects of substrate-pH on available nutrients in soilless substrates using a variety of substrates, lime sources, and nutrient solutions. Macronutrient concentration was measured in response to substrate-pH using reagent Ca(OH)2 and Mg(OH)2 as lime sources in four peat-based substrates (70% peat:30% perlite, 70% peat:30% pine bark, 70% peat:30% coconut coir and 70% peat:30% vermiculite, by volume). Increasing Ca or Mg based lime both increased pH and also increased the concentration of soluble Ca or Mg originating from the lime itself. Increasing an Ca-based lime resulted in higher soluble Ca at high pH because of additional applied Ca, but lower Mg because of reduced Mg solubility at high pH. Increasing either Ca or Mg-based lime decreased soluble P at high pH. Nitrate-N levels decreased when substrate-pH exceeded 7, potentially due to denitrification processes that resulted in N losses. Ammonium-N levels decreased as substrate-pH increased, which was attributed to pH-dependent nitrification and ammonia volatilization at high pH. The results highlight the importance of considering specific lime sources and substrate types to achieve desired pH levels and nutrient solubility for plant growth.

Technical Abstract: In soilless substrates, there are complex interactions between substrate-pH and macronutrient availability to plants. Our objective was to quantify the effects of nutrient solution formulation, substrate components, and liming materials on the solubility of macronutrient ions in soilless substrates in order to provide a more sophisticated representation of pH solubility compared with charts currently used by horticulture professionals. Macronutrient concentration was measured in response to substrate-pH using reagent Ca(OH)2 and Mg(OH)2 as lime sources in four peat-based substrates (70% peat:30% perlite, 70% peat:30% pine bark, 70% peat:30% coconut coir and 70% peat:30% vermiculite, by volume). A range of lime rates and fertilizer formulations were applied, and resulting pH and macronutrient concentration in a deionized water extract solution was analyzed. The pH rose with increasing Ca(OH)2 lime dose to a maximum pH of 7.6, and pH 9.0 with Mg(OH)2, although this pH response varied with substrate type. Nitrate-N levels decreased above pH 7, potentially due to denitrification. Ammonium-N decreased with increasing pH, probably due to pH-dependent nitrification and ammonia volatilization. Phosphorus concentration decreased with increasing pH, particularly when Ca(OH)2 was the lime source, indicating the likely formation of insoluble Ca-P compounds. Potassium levels were influenced by substrate type and lime rate, with peat/coir having the highest K concentration and peat/vermiculite the lowest K. Calcium concentration increased with increasing lime rate and substrate-pH when Ca(OH)2 was used as the lime source, whereas Mg(OH)2 reduced Ca solubility at high pH. Similarly, Mg solubility was affected by lime type and substrate-pH when Ca(OH)2 was the lime source, with decreased Mg solubility at high pH. Sulfate-sulfur concentration remained stable with increasing lime and pH. 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 highlights the complex interactions between lime sources, substrate-pH, and nutrient solubility, providing insights for optimizing horticultural practices. The findings emphasize the importance of considering specific lime sources and substrate types for achieving desired pH levels and nutrient availability for plant growth.