Closed Lettuce Cultivation System Using an Array of Ion-Selective Electrodes

Abstract

<abstract> <bold>Abstract.</bold> A recirculating hydroponic system based on EC-control has limited the ability to manage the composition of individual ions in the solution. This study reports on the automated management of hydroponic nutrients using an array of ion-selective electrodes (ISEs) and a nutrient dosing algorithm used in a closed lettuce growing system. The ability of three PVC membrane-based ISEs for simultaneous determination of NO₃^-, K⁺, and Ca²+ ions in the closed soilless system was evaluated. A computer program was developed with the LabVIEW program, which could calculate the volumes of individual nutrient stock solutions to be supplied based on real-time measurements of NO₃-K-Ca ions, and vary the injection times of the metering pumps working at constant injection rates. Changes in nutrient concentrations and water uptake were continuously monitored and their amounts were variably controlled when needed. The results showed that the NO₃–K-Ca ISEs exhibited acceptable measurement performance, indicating that the ISEs provided concentrations within 12% of those for the standard analyzers, i.e., 90.6 ±7.1%, 88.8 ±8.4%, and 89.6 ±4.0% for NO₃, K and Ca ions, respectively. Using the automated management system, concentrations of the three ions were effectively maintained with a balanced concentration ratio of the three ions for two weeks. However, an improvement in the nutrient dosing algorithm and the identification of more nutrient salts were needed for better independent management of the three ions.