Charge balance--a theoretical basis for modulating pH fluctuations in plant nutrient delivery systems

Existing control systems for pH and nutrient supply in hydroponic space plant growth units and bioregenerative life support (BLS) systems require acid and base additions to correct plant-induced charge imbalances in the nutrient solution. Plant growth is dependent on nutrient availability, and the active uptake of ions by roots changes the pH of the nutrient solution. This change in pH is an integrated response to nutrient and organic ion influx/efflux by the plant. Plant nutrient requirements, as a function of crop development and driven by specific environmental conditions, influence the magnitude of the charge balance requirement. Nitrogen is the dominant nutrient taken up by plant species. If the daily crop nutrient (N) requirement at each developmental stage is known, and the daily addition of nutrients (relative addition rate = RAR) to the plant growth system is calculated, controlling the daily addition of ammonium (NH4+) and nitrate (NO3-) offers a mechanism whereby pH fluctuations in the nutrient solution could be largely minimized and controlled, reducing acid or base additions to the system.