Micronutrient Solubility in Response To Root Zone pH for Soilless Plant Culture: Simulation of Chemical Equilibria
摘要
Root zone pH is a key factor governing micronutrient solubility in soilless plant culture, directly influencing nutrient uptake and crop health. This study examined how pH, fertilizer form, and dissolved organic matter interact to influence micronutrient solubility in hydroponic and organic soilless substrate systems. The objective was to quantify these effects using equilibrium modeling to provide a mechanistic basis for pH and micronutrient management. This study investigated the complex interactions between pH and nutrient availability in nutrient solutions with and without organic soilless substrates, focusing on how metal micronutrient forms (sulfate vs. chelates), dissolved organic matter (DOM) and cation exchange in soilless substrates, and pH adjustment methods (acid and base adjustment in hydroponics versus liming of soilless substrates) influence micronutrient solubility. Micronutrient solubility was simulated and analyzed using a chemical equilibrium model (GEOCHEM-EZ) under conditions representative of a typical hydroponic system, excluding organic matter and with constant calcium (Ca) levels. Subsequent simulations were conducted under conditions resembling soilless substrate systems using Visual MINTEQ, incorporating the presence of organic matter and varying Ca contribution from Ca(OH)2. 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 organic soilless substrates through the formation of metal-organic complexes, especially for metal micronutrients Fe, Cu, and Zn. Boron availability decreased slightly as substrate pH increased, whereas Mo solubility slightly increased under alkaline conditions. These results collectively illustrate a clear relationship that chelated micronutrients and organic matter interactions significantly improve nutrient availability in both hydroponics and organic soilless substrates, particularly at higher pH values where sulfate-based micronutrients quickly become insoluble. Practically, this emphasizes the importance of carefully selecting appropriate micronutrient fertilizers (especially stable chelates such as Fe-EDDHA at higher pH) and managing root zone pH precisely to optimize plant nutrient availability.