Title: Automated sensing of hydroponic macronutrients using a computer-controlled system with an array of ion-selective electrodes Authors
|Kim, Hak-Jin -|
|Kim, Won-Kyung -|
|Roh, Mi-Young -|
|Kang, Chang-Ik -|
|Park, Jong-Min -|
Submitted to: Computers and Electronics in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 21, 2013
Publication Date: February 28, 2013
Citation: Kim, H., Kim, W., Roh, M., Kang, C., Park, J., Sudduth, K.A. 2013. Automated sensing of hydroponic macronutrients using a computer-controlled system with an array of ion-selective electrodes. Computers and Electronics in Agriculture. 93:46-54. Interpretive Summary: 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 when considering an in-line management system that includes 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 an ISE-based sensing 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 NO3-N, K, and Ca concentrations in single-ion solutions over ranges typical of hydroponic solutions. However, an acceptable Mg ion-selective membrane was not identified. A sensor array incorporating the selected membranes, in combination with a computer-controlled measurement system, was evaluated for the direct measurement of NO3-N, K, and Ca concentration in paprika hydroponic nutrient solutions. Using previously developed baseline correction and two-point normalization methods, ISE-measured NO3-N and K concentrations in spiked or diluted hydroponic nutrient samples were strongly related to those determined using standard laboratory instruments (R2 = 0.85). However, the tested Ca electrode did not provide satisfactory results due to reduced sensitivity and poor selectivity in the mixed 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 can be automatically 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.
Technical Abstract: 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. In this study, we developed an ISE-based, computer-controlled measurement system that overcomes this problem through sequential calibration, electrode rinsing, and sample measurement. We also investigated the suitability of several previously developed ISE membranes for sensing important hydroponic nutrients, including nitrate-nitrogen, potassium, calcium, and magnesium. The membranes for nitrate-nitrogen and potassium gave good results, but results with membranes for calcium and magnesium were not acceptable. The computer-controlled system was used to measure nitrate-nitrogen and potassium concentrations in a hydroponic nutrient solution used for growing paprika, and ion concentrations were strongly related to those obtained with standard lab methods. This research is an important step toward an automated nutrient monitoring system for hydroponic crop production. If ISE membranes for other important nutrients, such as calcium and magnesium, can be identified, such a system improve production efficiency and reduce environmental impacts, benefiting both producers and the general public.