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Title: Automated lettuce nutrient solution management using an array of ion-selective electrodes

item JUNG, DAE HYUN - Seoul National University
item KIM, HAK JIN - Seoul National University
item CHOI, GYEONG LEE - National Institute Of Horticultural & Herbal Science (NIHHS)
item AHN, TAE-IN - Seoul National University
item SON, JUNG-EK - Seoul National University
item Sudduth, Kenneth - Ken

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/18/2015
Publication Date: 10/22/2015
Citation: Jung, D., Kim, H., Choi, G., Ahn, T., Son, J., Sudduth, K.A. 2015. Automated lettuce nutrient solution management using an array of ion-selective electrodes. Transactions of the ASABE. 58(5):1309-1319. doi: 10.13031/trans.58.11228.

Interpretive Summary: Hydroponic production systems grow plants without soil, relying on a circulating solution to provide the necessary nutrients. Maintaining an optimum nutrient balance in this solution is important for maximizing crop growth and yield. Particularly in closed hydroponic systems it is important to monitor solution nutrient concentrations so they can be adjusted to an optimal level before the solution is reused. One technology that can be used for such monitoring is the ion-selective electrode (ISE), which provides a reading based on the concentration of an ion in solution. However, the standard approach of continuous immersion of the ISE in the solution can lead to reduced accuracy over time. We developed an ISE-based, computer-controlled measurement and control system that overcomes this problem through sequential calibration, electrode rinsing, and sample measurement. The system used data from the ISEs to automatically control additions of nitrate-nitrogen, potassium, and calcium to the hydroponic solution used for growing lettuce in a greenhouse. Tests showed that the automated measurement and control system worked well in most cases. Generally, problems were related to the choice of chemical salts used to provide nutrient additions to the system, and ways to overcome these problems were identified. This research is an important step toward a viable automated nutrient monitoring system for hydroponic crop production. With further refinement such a system could improve production efficiency and reduce environmental impacts, benefiting both producers and the general public.

Technical Abstract: Automated sensing and control of macronutrients in hydroponic solutions would allow more efficient management of nutrients for crop growth in closed systems. This paper 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 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 within errors of -7.7+/-28.1, 20.8+/-28.5, and -5.6+/-8.2 mg/L for NO3, K, and Ca ions, respectively.