# Dissolved oxygen

> Dissolved oxygen (DO) is the amount of oxygen gas held in water or nutrient solution, measured in mg/L, and roots need it to respire and take up nutrients.

- Canonical: https://genetixbay.com/knowledge/glossary/dissolved-oxygen.html
- Also written: DO, DO levels, oxygen in solution, O2 in water, mg/L oxygen
- Glossary category: Systems and media · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

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](https://genetixbay.com/knowledge/glossary/soilless.md) 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](https://genetixbay.com/knowledge/glossary/root-rot.md). 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.

## Related terms

[Air stone](https://genetixbay.com/knowledge/glossary/air-stone.md), [Chiller](https://genetixbay.com/knowledge/glossary/chiller.md), [DWC](https://genetixbay.com/knowledge/glossary/dwc.md), [Root rot](https://genetixbay.com/knowledge/glossary/root-rot.md), [Reservoir](https://genetixbay.com/knowledge/glossary/reservoir.md)

## Read more

- [Deep water culture (DWC)](https://genetixbay.com/knowledge/systems/dwc.md)
- [Brown roots in hydroponics (DWC, RDWC, NFT): a differential](https://genetixbay.com/knowledge/diagnosis/root-browning-hydro.md)

## Sources

1. Cornell CEA. Hydroponic lettuce handbook. https://cea.cals.cornell.edu/
2. Resh HM, 2022. Hydroponic Food Production, 8th ed. CRC Press. https://doi.org/10.1201/9781003133254