The method was developed by Dr. Bernard A. Kratky at the University of Hawaii at Mānoa. From the early 1990s he published a series of papers on “non-circulating hydroponics” — his goal was lettuce production for smallholders with no grid power and no budget for pumps. The result is the cheapest possible entry into hydroponics: a jar, a lid with a hole, a net pot and a litre of nutrient solution.
How it works without a single pump
At the start, the bottom 1–2 cm of the net pot sits in the nutrient solution. Roots grow downward, the plant drinks and transpires, and the level falls — typically 1–3 cm per day for lettuce, depending on temperature. A humid layer of air forms between the water surface and the lid.
That layer is the engine of the whole system. The root system splits in two: the lower part stays in the water and takes up water and nutrients, while the upper part — white, fuzzy air roots — hangs in the humid air and absorbs oxygen directly. That is why Kratky needs no air pump, whereas a classic DWC system collapses within days without one.
Raise the water level later and you drown the air roots. They die back in two or three days and the plant stalls. The air gap is a feature, not a mistake.
Step by step, with real measurements
- Container: opaque — a 1 L jar for herbs, a 5 L bucket for a head of lettuce. Wrap clear containers in black tape or foil, or you will have algae within 7–10 days.
- Lid: a 50 mm hole for a 5 cm net pot, 70 mm for a 7.5 cm net pot. A hole saw does it in 20 seconds.
- Sowing: a 25 × 25 mm rockwool cube soaked in pH 5.5 water, two seeds per cube. Lettuce germinates in 3–5 days at 18–20 °C, basil in 5–8 days.
- Transplant: once roots poke out of the cube, usually 10–14 days after sowing. Add 1–2 cm of clay pebbles around the cube for moisture and shade.
- Solution: EC as per the table below, starting pH 5.5–5.8. Fill so the bottom 1–2 cm of the net pot is submerged — not a millimetre more.
- Seal every other opening — standing water plus an open container means mosquito larvae within a week.
- Light: partial shade outdoors in summer; indoors LED for 12–16 hours a day, around 150–200 µmol/m²/s for lettuce.
- Then do nothing. No topping up, no measuring, no stirring. Harvest when the level is near the bottom.
How much solution one plant needs
This is the number beginners get wrong most often. The container has to hold the entire water supply for the entire cycle, because you do not refill. Too small a jar means the plant runs dry in the last week before harvest.
| Crop | Solution per plant | Net pot | Transplant → harvest | EC (mS/cm) |
|---|---|---|---|---|
| Baby leaf, rocket, lamb's lettuce | 1–1.5 L | 5 cm | 3–4 weeks | 0.8–1.0 |
| Head lettuce | 4–5 L | 7.5 cm | 4–6 weeks | 1.0–1.2 |
| Basil, parsley | 3–4 L | 7.5 cm | 5–7 weeks | 1.2–1.6 |
| Pak choi, chard | 4–6 L | 7.5 cm | 5–7 weeks | 1.2–1.4 |
| Tomato, cucumber | 25–40 L (not recommended) | 10 cm | 3–4 months | 2.0–2.6 |
Where the method genuinely runs out
- ⚠️ One cycle and done. After harvest you rinse and refill. Kratky is not a system that runs for months.
- ⚠️ No corrections possible. pH typically climbs from 5.6 to 6.5–7.0 over the run. On longer crops that blocks iron uptake and the young leaves turn yellow.
- ⚠️ No mixing. Salts stratify; the lower zone ends up with a noticeably higher EC than the top.
- ⚠️ Heat is the enemy. Above 24–25 °C solution temperature the dissolved oxygen drops and Pythium gets its chance — roots turn brown and slimy. A dark container in summer sun easily hits 30 °C.
- ⚠️ Large plants do not work. A tomato empties 5 litres in two or three days; to last a season it would need 30+ litres and constant aeration.
So Kratky is excellent as a first experiment and as a permanent herb spot on a windowsill. For continuous harvest across a whole season you need a system that circulates, aerates and lets you correct the water — a vertical tower with 32 sites and a 40 or 50 litre tank, for example. Same principle; the difference is that there you can fix what you got wrong.