Oxygen at the roots
This plant sits in water. So why is it wilting?
- A · net pot & seedling
- B · roots
- C · air stone
The roots have all the water they want. Something else is missing.
Think of fish in a tank. They live in water, but they still need air mixed into it.
That is why fish farmers often bubble air into their water.
Roots need air too. They take in oxygen, a gas that is part of air.
Like fish, roots in a tub get it from the water around them.
Still water runs low on oxygen. The roots use it up faster than new air mixes in.
So growers give the tub an air pump.
The pump pushes air down a thin tube to a stone. The stone breaks the air into tiny bubbles.
The bubbles help mix air into the water.
- A · air pump
- B · air line
- C · air stone
- D · bubbles
- E · roots
Without enough air, roots start to die. They turn brown and soft.
Growers call this root rotWhen roots turn brown and soft and start to die, often after running short of air.
See it in the glossary.
Warm water holds less air than cool water.
So roots in a warm tub run short sooner. Growers try to keep the water cool.
Your turn. Give the roots air.
Turn the air pump up from off. Watch the roots and the leaves.
Find the setting where the roots stay healthy.
0 · Off
Off. No bubbles. The roots run out of air and turn brown.
Find the setting where the roots stay healthy.
Air pump: what happens in each zone.
- 0 · Off. No bubbles. The roots run out of air and turn brown.
- 1 to 2 · A few bubbles. Some air gets in, but the roots still want more.
- 3 to 4 · Plenty of bubbles. The roots stay white, and the plant stands tall. Right Not quite
Healthy roots in a tub are white.
Some systems give roots air another way. They leave a gap of air above the water.
Check 1 of 3
Why does a deep water tub need an air pump?
Show the answer
The roots need air from the water. Right. The bubbles mix air into the water for the roots.
Check 2 of 3
The air pump broke a while ago. What do the roots look like now?
Show the answer
Brown and soft. Right. Without air, the roots start to rot.
Check 3 of 3
Which water can hold more air?
Show the answer
Cool water. Right. Cool water holds more air than warm water.
You just learned why roots need air, even under water.
Next up: What plants stand in: growing media.
Go deeper: how roots breatheOpenClose
Roots use oxygen to release energy from sugar, much as your own cells do. This is called respiration. The energy helps roots pull in water and nutrients.
The oxygen roots use is dissolved in the water. Bubbles add some oxygen through their surfaces as they rise. They also stir the water, so more of it reaches the surface and picks up oxygen from the air.
Cold water holds more dissolved oxygen than warm water. Water at 10 °C can hold about 11.3 mg of oxygen per litre, but at 30 °C only about 7.6 mg.
Sources differ on the best water temperature. Guides on root disease suggest about 20 to 22 °C for lettuce. One lettuce handbook allows up to about 24 to 25 °C.
Root rot often starts when water molds, such as Pythium, attack roots in warm water with little oxygen.
In a Kratky jar, the water level drops as the plant drinks. The roots in the growing air gap take in oxygen from the moist air.
Sources
- U.S. Geological Survey, Water Science School. Dissolved Oxygen and Water. Link
- Tucker, C. Pond Aeration. SRAC Publication No. 3700. Southern Regional Aquaculture Center, 2005. Link
- University of Arkansas at Pine Bluff, Department of Aquaculture and Fisheries. Aeration. Link
- Lazur, A. Basics of Pond Aeration. University of Maryland Extension. Link
- Wilted plants with lots of water. Southwest Yard & Garden. New Mexico State University, College of Agricultural, Consumer and Environmental Sciences, 2006. Link
- Geist, L. Too Much Moisture Can Hinder Root Development. University of Missouri Extension, 2025. Link
- Brechner, M., and Both, A. J. Hydroponic Lettuce Handbook. Cornell Controlled Environment Agriculture, 2013. Link
- Nelson, M., G. Langellotto, and L. Nackley. Hydro Hints: Deep Water Culture. EM 9455. Oregon State University Extension Service, 2025. Link
- Somerville, C., Cohen, M., Pantanella, E., Stankus, A. and Lovatelli, A. Small-scale aquaponic food production: Integrated fish and plant farming. FAO Fisheries and Aquaculture Technical Paper No. 589. Food and Agriculture Organization of the United Nations, Rome, 2014. Link
- Williams-Woodward, J. Root disease management in hydroponic systems. e-GRO Alert 7(6), 2022. Link
- University of Kentucky Cooperative Extension. Lettuce Diseases in Kentucky. PPFS-VG-53. Link
- Colorado State University Extension. Root Disease Problems and Management in Hydroponic Systems. PlantTalk Colorado 2043. Link
- U.S. Geological Survey. Chapter A6.2, Dissolved Oxygen. Techniques and Methods 9-A6.2, 2020. Table 6.2-2, oxygen solubility in fresh water. Link
- U.S. Environmental Protection Agency. 5.2 Dissolved Oxygen and Biochemical Oxygen Demand. Volunteer Stream Monitoring: A Methods Manual (archived). Link
- Gillespie, D. Root Zone Management of Hydroponic Leafy Greens. Handout, Greenhouse Short Course, The Ohio State University, 2019. Link
- Sutton, J. C., et al. Etiology and epidemiology of Pythium root rot in hydroponic crops. Summa Phytopathologica 32(4): 307-321, 2006. Link
- Kratky, B. A. A suspended pot, non-circulating hydroponic method. Acta Horticulturae 648: 83-89, 2004. Link
- Kratky, B. A. Three non-circulating hydroponic methods for growing lettuce. Acta Horticulturae 843: 65-72, 2009. Copy hosted by the University of Hawaii CTAHR. Link
- Fitriyah, H., Budi, A. S., Maulana, R., and Setiawan, E. Controlling the Nutrition Water Level in the Non-Circulating Hydroponics based on the Top Projected Canopy Area. IJCCS 16(2), 2022. Link