A method for cultivating plants under controlled redox intensities in hydroponics

Abstract

An automated, low-cost hydroponic system was developed for growing single plants at controlled redox potentials (Eh) for extended periods. The system features a millivoltmeter with high and low set-point relays and offers the investigator full control of Eh within the entire redox potential range encountered in natural soils (−300 to +700 mV) with an accuracy of ±50 mV around the set-point value. Eh is lowered in the nutrient solution by adding the reducing agent titanium citrate. Conversely, Eh is elevated by bubbling the nutrient solution with air. The system’s ability to control Eh is demonstrated using data from a test experiment involving Phragmites australis plants grown at three redox levels (−150, +150 and +550 mV). Some frequently encountered factors that were potentially responsible for erroneous control are discussed. The buffer capacity and the relationship between pH and Eh were determined for the nutrient solution. These parameters, along with plant-mediated changes in pH, not only determine the time interval between pH adjustments, but also affect the Eh fluctuations in the continuously aerated experimental units. The variability and drift in readings from the platinum (Pt) electrodes, used to measure Eh, was investigated in order to relate electrode reliability to system performance. The variation in Eh readings for clean Pt electrodes was lower (S.E.=4.3 mV, n=23) than that of the calomel reference electrodes (S.E.=6.8 mV, n=23), the difference fixed across solutions of different poise. The response time (drift) of Pt electrodes in hydroponics (low-poised nutrient solution) was compared to pH-buffered quinhydrone (medium-poised) and flooded alluvial silt (well-poised). Drift increased from the low-poised nutrient solution, where Eh readings stabilized within minutes, to high-poised flooded soil, where Eh readings showed a downward drift for ca. 50 h. Overall, it is concluded that both the Pt electrodes and the calomel reference electrodes perform well in hydroponics. The extent to which the high NaCl salinity of titanium citrate (ca. 84‰) may limit the growth of plants is also discussed relative to the proportion of titanium citrate in the nutrient solution.