The Kratky method, explained
One container, one net pot, no pump, no air stone, no electricity. It is the cheapest way into hydroponics and it works well for exactly one category of crop.
How it works
Bernard Kratky's method relies on a simple trick. The net pot sits so the roots start in the nutrient solution. As the plant drinks, the level falls, and the upper roots end up in humid air above the liquid — which is where they get their oxygen. You never top the reservoir back up: the falling level is the mechanism, not a problem to correct.
What you need
- An opaque container — light in the solution grows algae.
- A net pot and a substrate that holds moisture: clay pebbles, rockwool or coir.
- Nutrient solution mixed to a lettuce-appropriate strength (EC around 0.8–1.2).
- Nothing else. That is the appeal.
Why it suits lettuce and herbs
Kratky works for short-cycle, lightweight, shallow-rooted crops: lettuce, basil, parsley, rocket, pak choi, microgreens. Those plants finish in four to six weeks using a fraction of the reservoir, and their oxygen demand is modest.
It stops working for anything that wants to fruit. Tomatoes and peppers drink litres a week, drop the level faster than the roots can adapt, and demand far more oxygen than a still reservoir holds. People try it; the plants stall or rot.
The failure modes
- Clear containers. Algae in days. It competes for nutrients and blocks the roots. Wrap the jar in tape or foil if it is glass.
- Warm solution. Above roughly 24 °C, dissolved oxygen falls and root rot takes hold. This is the real limit in summer.
- Topping up. Refilling breaks the air gap that the method depends on. Start a fresh reservoir or a fresh crop.
- Strong nutrients. A small reservoir makes mistakes expensive — errors concentrate as water is taken up. Mix weak.
- No way to measure. Without a pH or EC reading you are guessing; a cheap pen pair pays for itself in saved crops.
Kratky versus a bubbler
A deep water culture bucket with an air pump gives you more oxygen and lets you run larger plants, at the cost of noise and a power draw. Kratky is silent and free to run. For a windowsill of lettuce, Kratky is the sensible default; for anything bigger, aerate.
Crops that suit it, from our catalogue
| Crop | Varieties we carry | Typical days to harvest |
|---|---|---|
| Lettuce | 110 | 10 |
| Herbs | 104 | 16 |
| Microgreens | 57 | 2 |
| Brassicas | 79 | 20 |
| Alliums | 44 | 50 |
The short version
Kratky is a lettuce and herb method. One container, don't top it up, keep it dark and cool, mix weak nutrients. It is not a route to tomatoes.
Questions
Does the Kratky method need a pump?
No. That is the point of it. Roots in the humid air gap above the falling nutrient solution get oxygen without aeration, which is why the method is silent and uses no electricity.
Do you top up the reservoir in Kratky?
No. Refilling destroys the air gap the method depends on. Run the reservoir down and either harvest and restart, or move the plant to a fresh container.
What crops fail in Kratky?
Fruiting crops. Tomatoes, peppers and cucumbers drink too much, drop the level too fast and need more oxygen than a still reservoir provides. Leafy crops and herbs are the appropriate fit.
Why is there algae in my Kratky jar?
Light is reaching the nutrient solution. Use an opaque container or wrap a clear one, and cover the top around the net pot.
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