Ion-Selective Optical Sensors for the Management of Plant Nutrient Solutions
Traditionally, the condition of hydroponic nutrient solutions used in controlled environment or greenhouse plant production is obtained by monitoring solution pH and electrical conductivity. These parameters provide greenhouse operators with an overall sense of the solution quality, but do not provide information on individual components of the fertilizer solution. Considering that plant nutrient uptake requirements vary with crop, growth phase and environment, real-time ion-selective measurements represent a significant technological advance in the management of recirculating nutrient solutions. The capacity for the monitoring and control of individual ions in the nutrient solution is the last remaining environment variable for which reliable feedback control does not exist in controlled environment plant production. This technology development study focussed on optical ion-selective sensors, termed ‘optrodes’, for potassium and calcium sensing within nutrient solution. Bulk optrode absorption-based sensors, tailored explicitly for nutrient ion activities within typical hydroponic solutions were theoretically modelled, fabricated and experimentally tested. Experimental data on sensor response time, measuring range, selectivity, repeatability, lifetime and influence of temperature were compiled. These ion-selective bulk optrode membranes were integrated into a custom instrument combining a light source, mini-spectrometer, optical fibres and other associated components. Monitoring and control software was written that collected and converted optical measurements into estimated nutrient ion activities for use in managing nutrient solution quality. Implemented spectral averaging, normalization and ratiometric computations and techniques resulted in sensors that offered the advantage of auto-calibration. The sensors met selectivity requirements in half strength Hoagland nutrient solution and provided repeatable measurements in solutions of different ion activity levels. Potassium and calcium selective membranes demonstrated lifetimes of 30 to 50 hours when constantly immersed in nutrient solution. Strategies for their operational implementation in plant growth systems based upon experimental testing are proposed as are suggestions for future study. The low mass, low volume, low cost and robust nature of these optrodes offer a promising technology for future space-based plant production systems and give them excellent terrestrial technology transfer potential.