The Effect of Oxygenation of Water on Dissolved Oxygen Measurements in Irrigation Water and Container Substrate

Oxygen injection systems are an option to increase dissolved oxygen (DO) in irrigation water.  Two experiments evaluated (1) the persistence of super-saturated DO in irrigation lines, and (2) the DO level in container substrates irrigated with super-saturated water.  An optical oxygen sensor was used to measure DO.  In the first factorial experiment, there were two water types [“oxygenated” (26.0 mg·L^-1) or “ambient” (6.3 mg·L^-1) tap water], and three water delivery points [a holding tank immediately after the oxygen injector or tap (“source”), after being pumped 4.7 m to an outlet (“hose end”), or through a propagation emitter (“mist nozzle”)].  Water type and delivery point interacted in their effect on DO (P<0.0001).  Oxygenated water decreased slightly from the source (26.0 mg·L^-1) to the hose end (24.6 mg·L^-1), but decreased greatly to 8.7 mg·L^-1 at the mist nozzle.  Ambient water did not differ between the holding tank (6.3 mg·L^-1) and hose end (7.0 mg·L^-1), but increased to 8.7 mg·L^-1 at the mist nozzle.  The DO at the mist nozzle was equal for both water types, and close to saturation (8.9 mg·L^-1).  In a second experiment, two different sized containers (20 cm³ “cells” and 400 cm³ “pots”) were filled with a 60:40 (by volume) peat: perlite substrate and irrigated to container capacity with ambient water.  Water type was then poured onto the substrate at 0, 25, 50, 100, or 200% of container volume.  Increasing the applied water volume with ambient water did not affect substrate-DO in either container size.  However, at a 1-cm depth in the cells, substrate-oxygen increased from 8.6 mg·L^-1 to 12.3 mg·L^-1 (a 46% increase in oxygen) as applied oxygenated water increased from 0 to 200% of container volume.  A diminishing returns relationship between substrate-oxygen and applied volume of oxygenated water was observed in both the cells and pots.  In the pots, substrate-oxygen increased by 68% and 79% at the 2-cm and 4-cm depths, respectively, as applied oxygenated water volume increased from 0 to 200%. Substrate-oxygen was 1.3 mg·L^-1 lower at 4-cm compared with 2-cm depth with ambient water, and 2.3 mg·L^-1 lower with oxygenated water.  In conclusion, passing irrigation through a mist nozzle caused DO to reach an equilibrium saturated level in irrigation water, regardless of whether oxygen was injected.  In the substrate, a large volume of oxygenated water was required to increase DO in the substrate.