You can grow a head of lettuce in a single jar of nutrient solution, with no pump, air stone, or electricity for the system itself. That is the whole point of kratky hydroponics diy: you fill the container once, set the seedling in the lid, and let the falling water level create the air space the roots need. It matters because it removes the cost and noise that stop most beginners from trying hydroponics at all. After reading, you will be able to size a container for your crop, mix the solution correctly, set the starting water level, and know when to leave the plant alone instead of topping it up.
What Is the Kratky Method, and Why Does the Air Gap Matter?
The Kratky method is a passive, non-circulating form of hydroponics where roots grow down into still nutrient solution while the upper roots stay in moist air. It was developed by Bernard A. Kratky at the University of Hawaii for short-term vegetables grown in a simple kit that, according to the University of Hawaii publication A Simple Hydroponic Growing Kit for Short-Term Vegetables, does not require electricity or a pump.
Hydroponics means growing in nutrient solution instead of soil. Soil normally holds the plant upright, stores water, and stores nutrients. In this system the lid and net pot hold the plant, and the solution does the other two jobs.
The air gap is the oxygen supply. At the start, most roots reach the solution and take up water and nutrients. As the plant drinks, the level drops. The roots left above the new level adapt to taking in oxygen from the humid air in the container, while the lower roots keep drinking. Fill the jar back to the top and you submerge those air roots, which removes the balance the method depends on.
What Can You Grow Well in a Kratky Hydroponics DIY Setup — and What to Skip
Short, fast, leafy crops work best because they finish before the solution runs out or its nutrient balance drifts too far. Long-season and heavy-drinking crops need a much larger reservoir and careful refilling, which changes the method from set-and-leave to actively managed.
Choose your first crop by harvest time and water use, not by what looks impressive.
| Crop | Good first choice? | Practical starting container | Why |
|---|---|---|---|
| Leaf lettuce | Yes | 1 gallon (3.8 L) per plant | Fast, shallow-rooted, and harvested whole in one cycle |
| Basil | Yes | 1/2 to 1 gallon (1.9 to 3.8 L) per plant | Leafy and productive, but drinks more as it gets bushy |
| Pak choi, kale, and other greens | Yes, with space | 1 gallon (3.8 L) or a tote with spacing | Leaves need room and airflow to stay clean and dry on top |
| Spinach | Sometimes | 1 gallon (3.8 L) per plant | Prefers cool conditions and can bolt quickly in a warm room |
| Tomato or pepper | Not for a first classic setup | 5 gallons (19 L) or larger, with refilling | Long season, high water use, and roots that outgrow a jar |
| Carrots and other root vegetables | No | Use soil or another method | The edible part needs to form in a solid medium, not in open air and water |
A tote works better than a row of jars once you want four to six lettuces at once, because one larger reservoir is easier to mix and check than six small ones. A jar works better for learning, because you can see exactly how fast one plant uses solution.
UF/IFAS Extension also lists lettuce as a crop suited to small home hydroponic systems in Growing Lettuce in Small Hydroponic Systems, which covers several small-system options for home growers.
What You Need for a First Jar or Tote
You need a container, a way to hold the plant in the lid, a growing medium, a complete hydroponic nutrient, and a seedling. Nothing on this list needs to be specialised beyond the nutrient and the net pot.
- Opaque, food-grade container with a lid: A 1-gallon (3.8 L) jar, a 5-gallon (19 L) bucket, or a storage tote. Opaque matters because light reaching the solution feeds algae. Do not use a container that previously held fuel, pesticides, or cleaning chemicals.
- Net pot that fits the lid: Usually 2 or 3 inches (5 to 7.6 cm) for lettuce and herbs. The pot holds the medium and lets roots pass through.
- Growing medium: Expanded clay pebbles, a rockwool cube, or a hydroponic sponge. Its job is anchoring and holding a little moisture around the young seedling, not feeding the plant.
- Complete hydroponic nutrient: Use a product made for hydroponics and follow its label rate. Ordinary garden fertiliser often lacks the calcium, magnesium, and trace elements in the soluble forms a soilless system needs, because soil would normally supply or buffer some of them.
- pH test and adjustment supplies: Test strips are enough to start, and a basic pH meter is more accurate. You also need pH up or pH down only if your mixed solution falls outside the target range.
- Seedlings: Start seeds in the medium in a separate tray and move them once roots are visible at the bottom of the cube or pot. Starting this way stops loose seeds from falling into the reservoir.
- Light: A bright window may work for a short trial in summer, but most indoor growers need an LED grow light because weak light slows both leaf growth and water use, and water use is what creates the air gap.
Buy the nutrient before you buy extra hardware. A correct solution in a plain bucket grows lettuce. A weak or incomplete solution in an expensive container does not.
How to Set Up a Kratky System, Step by Step
Set the starting level so the bottom of the net pot just touches the solution, then leave the level to fall on its own. That single level decision decides whether the young roots can drink and the older roots can breathe.
- Wash the container, lid, and net pot with hot soapy water and rinse well. Clean gear reduces algae, odours, and root problems that start from residue left by a previous crop.
- Cut or drill a hole in the lid that holds the net pot snugly by its rim. The pot should sit securely and cover most of the hole, so light and mosquitoes cannot reach the solution easily.
- Place the seedling in the net pot and add medium around it to hold it upright. Keep the stem above the solution and the roots or cube pointing down.
- Fill a separate mixing jug or bucket with clean tap water. If your tap water is heavily chlorinated, leaving it uncovered overnight before mixing is a common practical step, but the nutrient label and your local water quality matter more than any single rule.
- Add the hydroponic nutrient at the label rate for leafy greens and stir well. Add any nutrient parts in the order the label states, because mixing concentrates together undiluted can cause some nutrients to precipitate and become unavailable.
- Check pH, and adjust slowly if needed. Retest after a few minutes, because the reading can settle after mixing.
- Pour the solution into your container and set the lid in place. Fill only until the solution touches the bottom 1/2 inch (about 1 cm) or less of the net pot. The medium should get damp, not sit fully submerged.
- Put the container under light and label it with the crop, the date, and the nutrient strength you used. That label prevents the most common record-keeping mistake: forgetting whether a jar was mixed at full or half strength.
- Check every few days that roots are reaching the solution, the level is falling, and no algae or smell is building up. Do not refill a short crop to the original level.
- Harvest leaf lettuce by cutting outer leaves or by removing the whole head. Clean the container before starting the next crop instead of reusing old solution, because the remaining solution no longer has the nutrient balance the plant started with.
Getting pH and Electrical Conductivity Right Without Overcomplicating It
pH is a measure of how acidic or alkaline the solution is, and it controls whether roots can take up nutrients that are already present. Electrical conductivity, called EC, is a measure of the total dissolved salts in the solution, and growers use it as a practical guide to nutrient strength.
A plant can look hungry in a full-strength solution if the pH is wrong, because the nutrients are present but not available. That is why you check pH before you add more fertiliser.
Oklahoma State University Extension gives crop-specific ranges in its Electrical Conductivity and pH Guide for Hydroponics, and notes that soilless growing has no soil buffer, so the grower has to maintain the right conditions directly.
| Crop | EC range (mS/cm) | pH range |
|---|---|---|
| Lettuce | 1.2 to 1.8 | 6.0 to 7.0 |
| Basil | 1.0 to 1.6 | 5.5 to 6.0 |
| Spinach | 1.8 to 2.3 | 6.0 to 7.0 |
| Sage | 1.0 to 1.6 | 5.5 to 6.5 |
| Tomato | 2.0 to 4.0 | 6.0 to 6.5 |
Use those ranges as starting targets for the specific crop, not as a single number for everything. Seedlings are usually started at the low end or below it, following the nutrient label, because young roots are more easily stressed by a strong solution.
You do not need an EC meter for one jar of lettuce if you mix the label rate accurately with a measuring jug. A meter becomes useful when you grow several containers, reuse a water source with high dissolved solids, or need to diagnose why plants stall despite looking fed.
Handle nutrient concentrates and pH adjusters as household chemicals: keep them sealed, labelled, and away from children and pets, wash your hands after mixing, and follow the label directions for eye and skin protection. Never drink the solution, and rinse harvested greens in clean drinking water before eating them.
Light, Temperature, and Placement at Home
A Kratky plant needs enough light to grow leaves and use water at a steady rate. If growth is slow, check light before you change the nutrient.
Place indoor greens under an LED grow light for about 12 to 14 hours a day, at the distance the light manufacturer recommends for seedlings and leafy greens. A timer keeps that schedule consistent, and consistency matters more than moving the container around the house chasing sun.
Keep the container in a spot that stays cool to moderately warm, out of direct heat from a radiator or a hot windowsill. Warm solution holds less dissolved oxygen and encourages root problems, while a cool, bright room suits lettuce and most greens better than a warm, dim corner.
Keep electrical fittings for lights dry and away from the reservoir. The growing method uses no electricity, but a powered light above open water still needs normal care with cables, plugs, and drips.
Outdoors, place containers where they get morning light and some protection from heavy rain. Rain entering the lid dilutes the solution, raises the level over the air roots, and can introduce dirt. A covered porch or balcony solves most of that without extra equipment.
Why You Should Not Top Up a Short Crop — and How to Refill a Long One
For a lettuce that will be harvested in one short cycle, the correct action as the level falls is usually nothing. The falling level is the system working, not a fault to correct.
Refilling becomes necessary only when the crop will outlast its reservoir: a large kale plant, a basil plant you cut from for months, or a fruiting plant in a bucket. In that case, refill with freshly mixed solution, but fill only to about halfway up the root zone, never back to the starting level. That preserves an air space at the top while giving the lower roots more to drink.
Choose by crop length:
- Choose no refill if the plant will be harvested before the container runs dry.
- Choose a partial refill if the plant is healthy, the solution still smells neutral, and at least the top portion of roots can stay in air.
- Choose a fresh start if the solution smells sour, roots are brown and slimy, or algae is thick throughout. Adding fresh nutrient to a failing reservoir does not fix the cause.
If a container does run dry before harvest, add solution promptly to the halfway point and expect some stress. Undersizing is the usual cause, so use a larger container for that crop next time rather than refilling a small jar more often.
Mistakes That Kill Most First Kratky Plants
Most failures come from the water level, light reaching the solution, or a container that is too small. Work through the symptom you can see, not a guess about the nutrient.
The solution turns green and slimy on the sides
That is algae, and it means light is reaching the water. Move the plant to an opaque container or cover a clear jar fully, including the shoulders, and make sure the lid and net pot leave no open gaps. A thin film can be cleaned off at the next restart, but a heavily green reservoir should be replaced because algae compete for oxygen and nutrients and make the solution smell stale.
Roots are brown, soft, and smell bad
Healthy roots are white or cream and firm. Brown, slimy roots point to low oxygen, warm solution, or contamination. Check that the air gap has actually formed, move the container to a cooler place, and remove badly affected plants so the problem does not spread to clean containers sharing the same tools. Trimming and restarting only helps a lightly affected plant; a plant with a foul smell and collapsed growth is usually better replaced.
The plant wilts even though solution is still there
Check the roots first. If the roots never reached the solution after transplanting, the medium dried out before the plant could drink. Lower the pot slightly at setup next time or keep the medium damp until roots emerge. If the roots are in the solution but brown, the problem is root health, not water quantity.
Leaves turn pale or yellow between the veins
Look at pH before adding feed. A solution outside the crop range in the table above can lock out iron and other nutrients, so leaves yellow even in a correctly mixed reservoir. Correct the pH slowly, retest, and give the plant several days before changing anything else, because changing pH and strength at the same time hides which fix worked.
Leaf edges turn brown and crisp on lettuce
This tip burn is often linked to calcium movement inside the plant, fast growth, and poor airflow around the leaves, not simply to a lack of calcium in the jug. Improve gentle air movement, avoid pushing growth with very strong solution and intense light at the same time, and harvest outer leaves promptly. Affected edges do not recover, so judge the fix by the new leaves.
Tiny flies or mosquito larvae appear around the container
Open solution attracts insects. Keep lids closed, cover any extra holes, and do not leave open mixing jugs standing beside the plants. A tight lid is both a light block and a pest block, which is why loose foil covers fail at both jobs.
Kratky Jar Questions, Answered Straight
Do you ever change the water in a classic Kratky jar?
For a short crop, no. You mix once, harvest, then clean the container and start with fresh solution for the next plant. Changing or constantly topping up the solution during the cycle removes the falling level the method relies on.
Can you grow Kratky plants in a clear mason jar if you like the look?
You can start a seedling in one, but a clear jar is a poor choice for a full cycle. Light reaches the solution, algae grow, and roots get unnecessary light exposure. Wrap the jar completely or choose an opaque container from the start.
Do you need an air stone to make this true hydroponics?
No. An air stone belongs to deep water culture, where roots stay submerged and oxygen is added to the water. In Kratky growing, the exposed upper roots take oxygen from the air gap, so adding a pump changes the system into a different method.
What water should you use for the first mix?
Clean tap water is a practical starting point for most home growers. If your tap water is very hard, a basic EC reading of the plain water helps explain why your mixed reading runs high, because the starting water already contains dissolved minerals. Follow the nutrient label if it gives guidance for hard or soft water.
How long does a Kratky lettuce take?
Timing depends on variety, light, and temperature, so treat any single number as an estimate. Under steady indoor light, leaf lettuce is often ready for first cuts in about four to six weeks from transplanting, and a whole head takes longer in weak light or a cold room. Judge readiness by size and leaf growth, not by the calendar alone.
Can you reuse the net pot and clay pebbles?
Yes, after washing them well and removing all old root material. Reuse is one reason the running cost of this method stays low. Replace sponges and rockwool cubes instead, because they break down and are hard to clean fully.
How this guide was put together
This guide is based on the University of Hawaii description of Kratky's non-circulating kit for short-term vegetables, the Oklahoma State University Extension EC and pH ranges for hydroponic crops, and the UF/IFAS Extension guidance on growing lettuce in small hydroponic systems, all linked above. It does not claim personal testing, expert review, or results for any specific home setup.
Sources
- Kratky, B. A., University of Hawaii, A Simple Hydroponic Growing Kit for Short-Term Vegetables, 2002
- Oklahoma State University Extension, Electrical Conductivity and pH Guide for Hydroponics
- UF/IFAS Extension, Growing Lettuce in Small Hydroponic Systems
