Validation testing of an ion-specific sensing and control system for precision hydroponic macronutrient management

Abstract

Ion-specific nutrient management systems would replace conventional electrical conductivity-based management systems for more efficient maintenance of the concentration of each ion in nutrient solutions for closed hydroponic systems. This study reports on the validation testing of a previously developed ion-specific sensing and control system. Specific objectives were to (1) evaluate the ability of the system to maintain the target concentrations of five macronutrients, i.e., NO3-, PO43-, K+, Ca2+, and Mg-2 ions, required for crop growth in closed hydroponic systems based on both real-time measurement of NO3-, K+, and Ca-2 and the use of a concentration ratio method to replenish Mg2+ and PO43-, and (2) investigate the applicability of a cobalt rod based electrode for hydroponic phosphate sensing. In a lettuce cultivation test conducted with the ebb-and-flow method over 21 days, most of the macronutrient concentrations were successfully maintained at the target levels with RMSE values of 25.2 +/- 9.4, 19.1 +/- 8.1, and 11.5 +/- 6.1 mg-L-1 for NO3-, K +, and Ca2+, respectively. In the case of Mg2+ and PO43- ions replenished in a proportional ratio to the supply of Ca2+ and NO3- ions, respectively, the Mg2+ concentrations were maintained between 18 and 25 mg-L-1 at an almost constant level, whereas the PO43- concentrations increased steadily over time with no clear evidence of plant uptake, implying that the use of a concentration ratio method would not be effective in controlling the concentrations of PO43- in hydroponic solutions. As a phosphate sensor, the cobalt electrodes yielded an RMSE result of 10.9 +/- 7.1% from a comparison of the electrode method and standard analysis when tested in hydroponic lettuce samples taken once a day during the period of lettuce growing, thereby offering the potential for use in hydroponic phosphate sensing.