An Embedded System for Automated Hydroponic Nutrient Solution Management

Abstract

Abstract. An ideal closed hydroponic system would include automated preparation of nutrient solutions, including reuse of the drain solution with replenishment of fertilizers and water in proper ratios based on real-time measurement of various nutrient concentrations in the drain solution. The electrical conductivity (EC) based systems commonly used for managing hydroponic nutrients in closed systems do not allow individual corrections to each nutrient deficient for crop growth. To improve the performance of a PC-based nutrient management system originally developed in our previous study, this article describes the development and evaluation of a WinCE-based embedded system with robustness and an easy-to-use user interface, consisting mainly of a micro-control unit (MCU) based embedded nutrient controller and an array of ion-selective electrodes (ISEs) that can automatically control the nutrient replenishment events based on real-time measurements of NO3, K, and Ca concentrations. To overcome the problem of decoupled replenishment among the three nutrients found in the previous study, a robust fertilizer dosing algorithm was designed that can separately calculate the volumes of six nutrient stock solutions to be supplied in a sequential order according to specific ions. In a five-step spiking test, the use of a two-point normalization method prior to each measurement sequence coupled with the robust nutrient dosing algorithm enabled the system to formulate five different concentrations of NO3 and K ions comparable to the target concentrations. However, the five different Ca concentrations prepared were almost 33% higher than the target concentrations because of low Ca estimates due to a change in the sensitivity of the Ca ISE between the different background solutions used for calibration and measurement. This could be overcome with the use of the same background solution or a pre-sensitivity compensation when an initial measurement is made. In a lettuce cultivation test conducted with the ebb-and-flow method over 27 days, the nutrient control performance of the embedded system was improved as compared to the previously developed PC-based system while maintaining the target concentrations of 436, 117, and 80 mg L-1 within errors of -15.0 ±18.1, -5.0 ±10.0, and -9.2 ±4.1 mg L-1 for NO3, K, and Ca ions, respectively. The use of the developed system in closed crop cultivation proved to be feasible for managing individual nutrients in reused solution at desired concentration levels, resulting in the potential of maximizing the efficiency of nutrient use. Keywords: Automatic control, Closed system, Dosing algorithm, Embedded system, Greenhouse, Ion sensor, Nutrients.