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

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.