The Kratky method: grow lettuce with zero electricity
B.A. Kratky at the University of Hawaii developed the concept in the 1990s and published the key paper in Acta Horticulturae in 2009. The core idea: fill a container with nutrient solution, put a plant in a net pot sitting in a hole in the lid, and walk away. As the plant drinks, the water level drops, creating an air gap between the lid and the solution surface. Roots in the air gap absorb oxygen. Roots below the water line absorb nutrients. No air pump, no circulation, no electricity.
It works because the air gap grows at roughly the same rate the roots need more oxygen. A lettuce plant that's drinking 200 ml per day is also growing roots that need 200 ml more of air space. The system self-regulates until the reservoir runs out.
The setup
A container: mason jar (1 quart / 1 liter for lettuce, herbs), 5-gallon bucket for peppers or larger plants, storage tote for multiple plants. Opaque containers only; light in the reservoir grows algae.
A net pot that fits in a hole in the lid. 2-inch (5 cm) net pots for lettuce and herbs. 3-inch (7.5 cm) for peppers and tomatoes. The pot sits in the hole with the bottom submerged 1-2 cm in the solution at the start.
Growing media in the net pot: clay pebbles (hydroton), perlite, or rockwool cubes. The media anchors the seedling and wicks solution up to the stem base.
Nutrient solution: any complete hydroponic nutrient (Masterblend 4-18-38 + calcium nitrate + magnesium sulfate is the cheapest; General Hydroponics Flora series works fine). Mix to an EC of 1.0-1.5 mS/cm for lettuce, 1.5-2.0 for herbs, 2.0-2.5 for peppers. pH to 5.8-6.2.
What grows well in Kratky
Lettuce (all types), spinach, arugula, kale, bok choy, Swiss chard, basil, cilantro, parsley, mint, dill, chives. Leafy greens and soft herbs are the sweet spot. They finish before the reservoir runs dry, they don't need heavy root aeration, and they tolerate the declining nutrient concentration as the solution is consumed.
Peppers work in 5-gallon buckets. The reservoir is large enough to last through the fruiting stage (3-4 months). Compact varieties (lunchbox peppers, jalape\u00f1os, small bells) are better than full-size bell peppers, which get top-heavy.
Strawberries work in larger containers. Slow start but continuous production once established.
What doesn't work
Tomatoes. An indeterminate tomato plant drinks 2-4 liters per day at maturity and depletes a 5-gallon reservoir in 3-4 days. The air gap forms too fast, roots dry out, the plant wilts. Determinate (bush) tomatoes in very large reservoirs (15-20 gallons) can work, but at that point a DWC or drip system is simpler.
Cucumbers. Same problem as tomatoes: massive water demand, fast reservoir depletion.
Root crops. No solid medium to shape the roots.
Anything that grows longer than 3-4 months in a single reservoir. The nutrient balance shifts as the plant selectively absorbs some elements faster than others (calcium and nitrogen go first). By the end of a long crop, the remaining solution may be deficient in critical nutrients even though the EC still reads adequate.
Scaling up
Multiple plants in a single reservoir: a long storage tote with holes drilled in the lid, one net pot per hole, spaced 15-20 cm for lettuce. A 27-gallon (100L) tote fits 8-10 lettuce plants. This is the practical limit for Kratky lettuce production without a pump.
Beyond 10 plants, the reservoir volume needed to last the full crop cycle gets unwieldy. That's the natural transition point to DWC (add an air pump to the same tote) or NFT (recirculating channels with a pump). Both handle more plants per square meter with less water.
The nutrient mix calculator computes target concentrations by crop, and the EC/PPM converter handles the unit conversion between the different scales.