S08 - Session O3 - Real time monitoring of nitrate, ammonium and potassium in closed-loop hydroponics: Customized calibration and compensation strategies of ion-specific electrodes

Authors: Oyvind M. Jakobsen *, Mona Schiefloe, Ann-Iren Kittang Jost

Nutrient monitoring allows improved understanding and optimization of crop production. Closed-loop hydroponics and reuse of nutrients from other processes may result in dynamic nutrient concentrations requiring real-time monitoring to verify and control cultivation conditions. While analysis of most macronutrients can be performed by ion-specific electrodes, their use has been limited by challenges related to drift, selectivity and stability, with biofouling adding complications upon long-term use. The presented work explored different strategies for implementation, calibration and operation of reasonably priced, commercially available ion-specific electrodes for nitrate, ammonium and potassium. Long-term monitoring was successfully demonstrated based on at-line implementation and a customized calibration and measurement strategy, with accuracies superior to those obtained under continuous in-line operation. Built-in features such as compensation of temperature and competing ions were combined with new and customized strategies tuned for hydroponic conditions: Drift was mitigated by idling the electrodes in between measurements in a reference solution with low and known concentrations of ammonium and potassium nitrate, at the same time minimizing long-term biofouling in the absence of other nutrients. Challenges related to selectivity and total ionic strength were mitigated by measurement of a standard solution with known concentrations of macronutrients, and stability issues were mitigated by a 45-minute stabilization and measurement period. These strategies were combined with customized hardware for liquid handling, resulting in a fully automated sampling and calibration system for near real-time analyses of the nutrient solution. During a one-month crop cultivation campaign in closed-loop hydroponics, this system demonstrated accuracies within +- 2 mg nitrogen or potassium per liter, or +- 2 % of the measured value, without human intervention. The paper details the strategies, mathematical data processing and hardware developed, representing a stand-alone analytical system on its own, in addition to principles applicable to other electrode-based systems for similar applications.