Optimal hydroponic growth of Brassica oleracea at low nitrogen concentrations using a novel pH-based control strategy

Highlights: • Nitrogen spillage from soilless agriculture harms the environment. • Nitrogen concentration control at lower levels is key. • Brassica oleracea exhibits a near constant proton-to-nitrate uptake ratio. • Nitrate concentration is controlled using pH as the sole measurement. • Optimal hydroponic growth is achieved at nitrate concentrations 100 times lower. Aquatic nitrogen pollution from conventional agriculture contributes severely to the degradation of numerous ecosystems and is considered one of the main contributors to earth's alarming rate of biodiversity loss. Soilless agriculture, in contrast to conventional agriculture, has the advantage of discharge control since the nutrient solution is contained. However, periodic replacement of the nutrient solution is dictated by inert build-up over time resulting from transpiration. As the spent solution is usually discharged, the nutrient concentrations are proportional to the load of nutrient spillage to the environment. This study investigates a novel pH-based control strategy to minimise the nitrate concentration while maintaining optimal plant growth and nutrition. Experiments were performed where the nitrate concentration was controlled at 11 mM (representing standard protocol), 1 mM, 0.5 mM, and 0.1 mM. This was accomplished by controlling the pH with a mixture of HNO₃ and NaNO₃. A molar ratio of 3:2 (HNO₃:NaNO₃) resulted in relatively stable nitrate profiles with slow depletion of nitrate in solution, owing to a near-constant ratio between proton dosing required for pH homoeostasis and nitrate absorption. Small manual corrections were made for the 1 mM and 0.5 mM runs, accounting for 8% of the total nitrate absorbed. For the 0.1 mM run, instead of manual correction, an automatic nitrate addition strategy was incorporated, in which nitrate extinction was inferred from a reduction in the rate of change of pH. Zero reduction in plant growth rate and leaf chlorophyll content was detected when comparing the 11 mM run with the other runs, indicating optimal hydroponic performance. A novel nitrate control algorithm is presented that uses pH measurement as the sole input. The experimental results and the control algorithm provide encouraging alternatives for reducing nitrogen spillage from soilless agriculture.