Dissolved oxygen is the concentration of O₂ gas held in a liquid. Roots respire, using oxygen to power growth and active nutrient uptake. In soil and soilless media they get it from air in the pores; in water culture they depend entirely on what is dissolved. How much the solution can hold depends mainly on temperature: cold water holds more.
In practice
Hydroponic guidance for leafy crops recommends keeping DO at about 7-8 mg/L or more, with stress and failure as levels drop below about 3-4 mg/L. Saturated fresh water holds roughly 9 mg/L at 20 °C (68 °F) and about 8 mg/L at 25 °C (77 °F). Low DO shows as brown, slimy roots, wilting and sudden root rot. Raise DO with cooler solution, more aeration and circulation, and fewer dead spots; keep root mass healthy, since rotting roots and microbes consume oxygen.
Units and formula
mg/L (equal to ppm for oxygen in water) or % saturation. A handheld optical DO meter is the most reliable test.
Don't confuse with
- Hydrogen peroxide: releases oxygen briefly as it breaks down; not a sustained DO source.
- Aeration: the process; DO is the measured result.
