Power needed
None (passive)
Best crops
Lettuce, bok choy, herbs
Fill level
Touch cube, then air gap forms
Target pH
5.5–6.5
Typical EC
0.8–1.4 mS/cm
Ideal room
Cool — under ~75°F solution
The idea behind the Kratky method
The Kratky method is a non-circulating hydroponic technique named after Dr. Bernard Kratky. You plant into a lid over a container of nutrient solution. Roots reach the water; as the plant drinks, the water level falls and an air gap grows. Upper roots become oxygen absorbers in moist air while lower roots continue taking up water and nutrients.
There is no air pump and no water pump. That simplicity makes Kratky ideal for classrooms, closets, and low-cost leafy production — provided temperatures stay moderate and crop cycles stay short enough that the reservoir does not go anaerobic or empty at the wrong time.
Kratky is not magic water. You still mix complete hydroponic nutrients, set pH, and choose opaque containers. The method removes electricity from the equation; it does not remove plant science.
Container setup and starting water level
Use an opaque bottle, bucket, or tote with a tight lid. Light in the solution grows algae. Cut a hole for a net pot, insert a seedling in rockwool or similar media with clay pebbles for support, and fill nutrient solution until it just touches the bottom of the cube or net pot.
Do not fill the container to the brim for the entire grow. The falling water line is intentional. If you constantly refill to the top, you eliminate the air gap and recreate a stagnant, low-oxygen DWC without aeration — a common beginner mistake that invites root rot.
Calculate volume before mixing so your EC matches the label for that many liters or gallons. A quart jar works for a small herb; a several-gallon tote suits multiple lettuce sites. Leave headspace so the lid seals and light stays out.
Nutrients, pH, and what “set and forget” really means
Mix a complete hydroponic formula to roughly 0.8–1.2 mS/cm for lettuce (sometimes up to ~1.4 for hungrier herbs), and adjust pH to about 5.5–6.5 before planting. In true Kratky style, you often do not adjust again until harvest if the reservoir was sized correctly.
“Set and forget” assumes cool conditions, a properly sized tank, and a short-cycle crop. In warm rooms, measure occasionally — pH can drift and pathogens multiply. If the container is undersized, the solution may run out before harvest or concentrate to burning EC as water disappears.
Size the reservoir so the plant finishes roughly when the water is nearly gone, with roots still able to reach what remains. Commercial Kratky lettuce sizing is often based on expected water use for the cultivar and climate; hobby growers learn by logging starting volume and days to harvest.
Crops and climates that suit Kratky
Lettuce, leafy Asian greens, basil, and other quick herbs are the sweet spot. They finish before water quality collapses and do not demand the huge uptake of fruiting vines. Spinach can work in cool conditions but bolts in heat — environment still matters.
Tomatoes, peppers, and large fruiting plants are poor classic Kratky candidates. They drink too much, need higher and changing nutrient profiles, and suffer when oxygen drops in warm stagnant water. Use DWC with aeration or media systems instead.
Keep solution cool — ideally in the mid-60s to low-70s °F (18–22°C). Kratky fails more often in hot garages than from “wrong nutrients.” Shade containers, raise them off hot concrete, and avoid direct sun on dark buckets that absorb heat.
Root health without an air pump
Healthy Kratky roots develop a fuzzy upper zone in the air gap and wetter roots below. They should still look mostly light-colored and smell clean. Brown slime and sulfur smells mean the system went anaerobic — usually from heat, overfilling, or organic contamination.
Because there is no circulation, never drop dead leaves into the tank. Use clean media and washed pebbles. If you must top up in a heat wave, add a smaller amount to preserve some air gap rather than refilling to the net pot every time.
If roots fail mid-cycle, you can sometimes salvage plants by converting to aerated DWC: add an air stone, refresh solution, and lower EC slightly while they recover. That is a rescue, not the standard Kratky design.
Lighting and placement for indoor Kratky
Indoor Kratky still needs grow lights. Target roughly 200–350 PPFD for lettuce for 12–16 hours. A sunny window alone is often uneven and weak in winter. Place jars where temperature is stable — not on a heat vent.
Multiple bottles on a shelf under one LED bar make an excellent micro-farm. Keep lids sealed against light. Label mix date, starting EC, and pH so you can compare batches.
Harvest whole heads or leaf-pick herbs. After harvest, scrub the container, rinse, and remix fresh solution for the next seedling. Reusing old Kratky water for a new plant without measuring EC is a common cause of weak second rounds.
When to choose Kratky vs DWC
Choose Kratky for simplicity, off-grid or silent grows, classrooms, and cool-season leafy greens. Choose DWC when rooms run warm, crops are larger, or you want continuous top-offs and active oxygen control.
Some growers run hybrid approaches — Kratky-style containers with occasional aeration during heat waves. That can work, but understand you are leaving pure Kratky principles for survival.
Master Kratky lettuce once, and you will understand air gaps, reservoir sizing, and why oxygen matters — lessons that transfer to every other hydroponic system.
Frequently asked questions
Do I ever refill a Kratky container?
Ideally the reservoir is sized to last until harvest. In heat or with thirsty plants, small top-offs that preserve an air gap are safer than filling to the top repeatedly. Always use pH-adjusted water and recheck EC if you add a lot.
Why did my Kratky roots turn brown?
Usually warm stagnant water, no air gap (kept too full), light leaks/algae, or dirty organic matter in the tank. Cool the solution, restore an air gap, clean the container, and remix fresh nutrients.
Can Kratky work outdoors?
Yes in mild weather with opaque containers and rain protection so dilution does not wreck EC. Extreme heat outdoor Kratky is risky without shade and careful sizing.
Is Kratky really hydroponics if there is no pump?
Yes. Plants still grow in nutrient solution without soil. Pumps are a delivery choice, not the definition of hydroponics.
Related tools & plant guides
- Reservoir Volume Calculator — Calculate the volume of your reservoir in litres or gallons based on container dimensions.
- Nutrient Mixing Calculator — Calculate exact nutrient amounts for your reservoir based on volume and concentration ratios.
- pH Adjustment Calculator — Estimate how much pH Up or pH Down solution is needed to reach your target pH.
- Hydroponic Lettuce guide — pH 5.5–6.5 (aim 5.8–6.2), EC 0.8–1.2 mS/cm
- Hydroponic Basil guide — pH 5.5–6.5, EC 1.0–1.6 mS/cm
- Hydroponic Spinach guide — pH 5.5–6.6, EC 1.8–2.3 mS/cm
Keep measuring
Pair this guide with free hydroponic calculators, crop targets in plant guides, and meter picks in product reviews.