Automated Lettuce Nutrient Solution Management Using an Array of Ion-Selective Electrodes

Abstract

<abstract> Automated sensing and control of macronutrients in hydroponic solutions would allow more efficient management of nutrients for crop growth in closed systems. This article describes the development and evaluation of a computer-controlled nutrient management system with an array of ion-selective electrodes (ISEs) and fertilizer pumps that could effectively manage concentrations of NO₃, K, and Ca ions in closed hydroponic systems. A fertilizer dosing algorithm was developed to calculate the volumes of individual nutrient stock solutions to be supplied based on the measurement of present concentrations in a mixing tank. In a five-step spiking test, the system was able to formulate five different concentrations of NO₃ and Ca ions comparable to the target concentrations, showing almost 1:1 relationships between the actual and target values. However, actual K concentrations were prepared almost 40% higher than target concentrations due to low K estimates. The use of a two-point normalization method in conjunction with ISEs was effective in minimizing signal drifts resulting from in-line measurement of ion concentrations in the closed system. The automated nutrient management system was used to grow lettuce in a greenhouse with the ebb and flow method. After water was automatically added to the mixing tank to maintain the level of water within 10% of the total volume of the tank, the amounts of three different salts, i.e., KNO₃, KH₂PO₄, and Ca(NO₃)₂, were variably supplied to the mixing tank based on real-time measurement of concentrations. The three ions were automatically controlled to reach target concentrations of 280, 140, and 70 mg L^-1 within errors of -7.7 ±28.1, 20.8 ±28.5, and -5.6 ±8.2 mg L^-1 for NO₃, K, and Ca ions, respectively.