Automated sensing of hydroponic macronutrients using a computer-controlled system with an array of ion-selective electrodes

Automated sensing of macronutrients in hydroponic solutions would allow more efficient management of nutrients for crop growth in closed hydroponic systems. Ion-selective electrodes (ISEs) are a promising approach because they can directly measure the analyte with a wide range of sensitivity and are small and portable. However, signal drift and reduced accuracy over time are major concerns with an in-line management system based on continuous immersion of ISEs in a solution. An ISE-based, computer-controlled measurement system for automatic sampling, calibration, and electrode rinsing is a viable technique to improve accuracy and precision in the determination of nutrient concentration. This study reports on development and evaluation of such a system for direct measurement of macronutrients in recirculating hydroponic solutions. The sensitivity and selectivity of PVC membrane-based ISEs fabricated with TDDA-NPOE, valinomycin, and calcium ionophore II were satisfactory for measuring NO"3-N, K, and Ca concentrations in single-ion solutions over ranges typical of hydroponic solutions. However, an acceptable Mg membrane was not identified. An ISE array incorporating the selected membranes, in combination with a computer-controlled measurement system, was evaluated for the direct measurement of NO"3-N, K, and Ca concentration in paprika hydroponic nutrient solutions. Using previously developed baseline correction and two-point normalization methods, ISE-measured NO"3-N and K concentrations in spiked or diluted hydroponic nutrient samples were strongly related to those determined using standard laboratory instruments (R^2=0.85). However, the tested Ca electrode did not provide satisfactory results due to reduced sensitivity and poor selectivity in the hydroponic solution. The approach used in this research, where a base solution is used for both referencing the baseline and rinsing electrodes and two calibration solutions are injected to implement the two-point normalization method, could be used in an automated sensing system for hydroponic nutrients in greenhouses. Additional research is required to identify ion-selective membranes for Ca and Mg ions in hydroponic solutions.

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