OptiDose: An optimal control for macronutrient dosing in hydroponics

Achieving closed-loop hydroponics necessitates precise adjustment of individual macro- and micronutrients within the nutrient solution. However, nutrient management in hydroponics remains constrained to electrical conductivity (EC) and pH-based approaches, due to the complexity of steering individual ions and the coupling inherent in multi-element fertilizer formulations. In this study, an optimal control framework, termed OptiDose, is implemented to optimize daily fertigation strategies for hydroponically grown lettuce. The system integrates six fertilizer sources—calcium nitrate, magnesium sulfate, monopotassium phosphate, potassium nitrate, magnesium nitrate, and potassium sulfate—to maintain the concentrations of the macronutrients nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S) within crop-specific adequacy ranges. Five scenarios are tested in the simulator to evaluate system performance under varying operational constraints. Results indicate that OptiDose maintained suitable nutrient concentrations for plants throughout the growth cycle—without nutrient deficiencies or toxicities—while markedly improving resource-use efficiency. Relative to a single-shot nutrient preparation (baseline), the strategy using properly sized solution tanks with daily recipe adjustment (Scenario 1) increased water-use efficiency sixfold and doubled fertilizer-use efficiency, achieving 32.3 ± 1.4 g/L and 12.3 ± 0.3 g/g, respectively. Additionally, water and fertilizer costs decreased significantly (p < 0.05), by approximately 76% and 51%, respectively. The results underscore the promise of element-specific fertigation and optimization for precision nutrient management in controlled environment agriculture.