Library › Part F · Article 45 in the series · Level 1 · 13 min read

Build: a single-container Kratky system

Standfirst: After reading this article you will be able to build a Kratky container for one lettuce from a parts list, fill it to the right level, mix the solution by weight, and run the crop to harvest without a pump. You will also be able to size the same build as a tote for four to six plants, and to read the common failures from their symptoms.

Before you read: Kratky and other passive systems, Mixing your own solution, Seeds, germination and propagation, Meters and measurement.

Three rules from the mechanism

A Kratky system is a closed, dark container of nutrient solution with a plant held in a hole in the lid. Kratky’s own summary is that the whole crop is grown “with only an initial application of water and nutrients”¹. The build follows from three rules, each explained in Kratky and other passive systems.

1. The pot touches the solution at the start. The bottom of the net pot dips into the solution, and the medium inside is wetted by capillary action, the pull that lifts water up a paper towel, so the seedling is watered automatically¹.
2. The level falls and is never raised. As the plant drinks, a layer of moist air opens under the lid. Roots in that air become “oxygen roots” (Kratky’s term, after Imai) whose main job is aeration, and submerging them again makes the plant “drown”¹.
3. The lid is opaque and tight. Light in the solution grows algae, which compete with the roots for nutrients and for oxygen at night⁵. An open gap lets mosquitoes in to breed⁴.

Every step below serves one of these rules.

Parts list

Use plastic sold for food or drinking water for the container and lid, because the solution and the roots stay in contact with it for weeks.

Step 1: cut the lid hole to the net pot

The net pot has a wide lip at the top and a narrower body. Cut the hole to the outside diameter of the body just below the lip, so the pot drops in and the lip rests on the lid. Kratky’s bulletin cuts the cover holes with a 2-inch hole saw and shows 2-inch net pots sitting in them³. Measure your own pot with a ruler, since sizes vary between makers.

The fit matters for rule 3. In Kratky’s home-garden instructions, the net pot “should fit snugly”, because mosquitoes could enter through a gap and multiply in the solution⁴. With a knife, cut inside a marked circle and enlarge the hole until the pot just drops in.

Step 2: make the container dark

Light that reaches still nutrient solution grows algae, and Resh’s remedy is to build or cover containers with opaque materials⁵. Kratky’s home instructions suggest painting the bottle or placing a bag around it to exclude light⁴. Wrap the container, base included, in foil or black tape, and hold the lid against a bright window: light that passes it will reach the solution.

Leave one narrow vertical strip of the wrap that you can lift, or use a container with a level you can see from outside. You will read the level each week without opening the lid (step 8).

Step 3: fill to a measured level

The solution must touch the bottom of the net pot at transplant, and no more. Kratky immerses the lower 1 to 2 cm of the net pot in his 2009 tanks¹, 2 cm or more in his 2004 paper², and the lower 3 cm in his bottle¹. To find the fill level:

1. Put the empty net pot in the lid and measure how far its bottom hangs below the underside of the lid.
2. Subtract the immersion you want, for example 1.5 cm.
3. The result is the distance from the underside of the lid down to the solution surface.

Mark that line on the inside of the container before you fill it. The worked example below turns the line into litres. Starting too low leaves the medium dry, because capillary wetting needs the pot in the solution¹; starting too high floods the top of the pot and, in this article’s inference, leaves no air space for the first roots.

Step 4: mix the solution

Mix the solution in a separate bucket or jug, not in the container, and pour it in up to the line. There are two routes, both weighed on a 0.1 g scale.

Route A, Kratky’s own formula. Kratky makes two stocks: stock A holds 120 g/L of a commercial hydroponic lettuce fertiliser (8% N, 6.6% P, 29.9% K, plus micronutrients, with no calcium or magnesium in its listed analysis) and 72 g/L of magnesium sulfate; stock B holds 120 g/L of calcium nitrate¹. His bulletin names the fertiliser as Chem-Gro 8-15-36 from Hydro-Gardens³. He adds them in equal volumes to an EC of 1.5 mS/cm¹, and his bulletin gives the dose: 1,500 mL of each stock in 100 US gallons (378.5 L) of water gives about 1.5 mS/cm³. That is 3.96 mL of each stock per litre, so each litre of final solution holds 0.48 g of the fertiliser, 0.29 g of magnesium sulfate and 0.48 g of calcium nitrate, our arithmetic from¹ and³. For one container you can weigh those amounts directly. Dissolve the fertiliser and the magnesium sulfate in one jug of water and the calcium nitrate in another, and pour both into the main volume; never mix the two concentrates, for the reason given in Mixing your own solution.

Simpler still is a one-part complete hydroponic fertiliser at the maker’s rate. Kratky’s bottle method uses 5 g of such a fertiliser in a 4-litre bottle¹, which is 1.25 g/L in our arithmetic.

Route B, the recipe from Mixing your own solution at half strength. That article builds Resh’s formulation of 200 mg/L nitrogen, 50 phosphorus, 300 potassium, 200 calcium and 40 magnesium⁵. Kratky’s route A solution contains about 111 mg/L nitrogen, 31 phosphorus, 142 potassium, 104 calcium and 28 magnesium, our calculation from Kratky’s analysis and Resh’s conversion factors and purities^1,3,5. That is close to half of Resh’s recipe, element by element, so half strength is this article’s estimate of the equivalent. The grams for half strength are in the worked example. Masses under 1 g are better made from the A and B stocks of that article than weighed directly.

If you have an EC meter, read the finished solution. Kratky’s rule is that each 1 mL of stock A plus 1 mL of stock B in one US gallon (3.79 L) raises the EC by about 0.1 mS/cm³. Aim for 1.5 mS/cm above your water, and use rain water if your tap water already reads high, because what the plant does not take up concentrates as the level falls¹.

Step 5: check the pH

Kratky’s recommended pH range is 5.5 to 6.5³, and his crops in Hawaii were “typically completed without altering pH” when commercial hydroponic fertilisers and good water were used¹. Measure once after mixing. Paper strips have a precision of 1 pH unit and a liquid test kit of 0.5 unit, which Kratky judges good enough for most growers³; a meter needs periodic calibration³. If the reading is above 6.5³, add pH-down a few drops at a time, stir and measure again. pH in hydroponics explains the range, and acid safety is in Mixing your own solution.

Step 6: transplant

Use a seedling 1 to 2 weeks old¹; Kratky’s commercial bulletin allows 1 to 3 weeks³. Place the empty net pot in the lid first, then set the plug in it. If the plug does not fill the pot, Kratky’s bulletin says “that is okay”³; you can add a little medium around it. The medium should become moist from below by capillary action¹. If it stays dry, the level is too low, or the medium is too coarse to wick; Kratky’s home kit advises adding one or two teaspoons of water to the medium⁴.

Step 7: give it light

Lettuce grows on the total light it receives in a day, the daily light integral (DLI), measured in moles of photons per square metre per day (Light: PPFD, DLI, spectrum and photoperiod). The Cornell lettuce handbook recommends 17 mol m⁻² d⁻¹ for one cultivar of Boston bibb, but only with fans blowing air down onto the plants; without that airflow it could not go above 12 mol m⁻² d⁻¹ before tipburn⁶. A still-air container at home should aim at the lower figure. Under a lamp, divide the DLI by the light intensity your lamp gives at the plant, in µmol m⁻² s⁻¹, times 0.0036 to get the hours; the factor 0.0036 is 3,600 seconds per hour divided by one million micromoles per mole. A lamp giving 250 µmol m⁻² s⁻¹ at leaf height needs 12 / (250 × 0.0036) = 13.3 hours a day for 12 mol, our arithmetic using the Cornell still-air figure⁶. A window gives less light, and it changes with the season; Choosing and sizing grow lights shows how to measure it. Outdoors, Kratky’s home kit asks for a place with plenty of light that is sheltered from wind and rain⁴, because rain raises the level and drowns the oxygen roots³.

Step 8: leave it, and check once a week

From now on the work is to look, not to act. Kratky’s home instructions are to leave the bottle alone, not to pull the net pot, which damages the roots, and not to add water or fertiliser⁴. Do not lift the lid while the crop grows, because roots may tear³.

Once a week, check four things:

• Level. It should be lower each week, and it falls fastest at the end, since consumption “is very high in the later stage of the crop”³. Mark it on the outside.
• Roots. Through the strip in the wrap, roots should reach the solution before the level falls away from the pot^1,3.
• Colour. Pale old leaves suggest the solution is running short of a mobile element such as nitrogen⁵.
• pH, if you have a meter or kit; no correction is usually needed¹.

Slight wilting at midday is normal, and plants recover by late afternoon³.

The one exception: a crop that will run dry. If it looks as if the container will empty before harvest, Kratky’s bulletin advises adding “about 1/2–1 inch” of water or half-strength solution³, which is 1.3 to 2.5 cm. His 2004 paper warns that raising the level by more than 2 cm after the roots have emerged “may cause plant injury”², so keep any addition at or under 2 cm, our reading of the two sources together. If this happens every crop, Kratky’s advice is a deeper tank³.

Step 9: harvest and clean

Leafy or semi-head lettuce is ready about 5 weeks after transplanting¹, or 4 to 6 weeks depending on the age of the transplant, the season and the cultivar³. Cut it with a clean knife or scissors^1,3. Kratky ends a crop when less than 10% of the original solution remains².

For cleaning, Kratky leaves the roots in the net pots for 1 to 2 weeks to decay, then soaks the pots in a 10% bleach solution, rinses and dries them³. Before each planting, his home kit asks only for the bottle to be rinsed twice with water and not bleached, with several rinses if soap is used⁴. Hygiene, cleaning and crop turnover covers cleaning between crops.

Worked example: one lettuce in 10 L, and a six-plant tote

The inputs come from the sources cited; the container sizes and the 5 cm depth of the pot below the lid are our choices, and every result is our arithmetic.

One lettuce. The container is 20 × 20 cm inside and 25 cm deep to the underside of the lid, so its cross-section is 400 cm² and it holds 10.0 L to the brim.

1. Fill level: the net pot hangs 5 cm below the lid; with 1.5 cm immersed, within Kratky’s 1 to 2 cm¹, the solution surface sits 3.5 cm below the lid, so the solution is 25 − 3.5 = 21.5 cm deep.
2. Fill volume: 400 cm² × 21.5 cm = 8,600 cm³ = 8.6 L, just above Kratky’s 4 to 8 L per plant¹.
3. Salts, route A: 8.6 L × 0.48 g/L = 4.1 g of the lettuce fertiliser, 8.6 × 0.29 = 2.5 g of magnesium sulfate and 4.1 g of calcium nitrate^1,3.
4. Consumption: Kratky reports that a 200 g head uses about 3 to 4 L in Hilo, Hawaii¹. That lowers the level by 3,000 / 400 = 7.5 cm to 4,000 / 400 = 10 cm.
5. At harvest: 4.6 to 5.6 L remain, 53% to 65% of the start, and the oxygen roots hang in an air gap of 11 to 13.5 cm below the lid.

The same method as a tote. A box 55 × 36 cm inside and 19 cm deep to the lid has a cross-section of 1,980 cm².

1. The same net pot and 1.5 cm immersion¹ give a solution depth of 19 − 3.5 = 15.5 cm, and 1,980 × 15.5 = 30.7 L.
2. Salts, route A: 30.7 × 0.48 = 14.7 g of fertiliser, 30.7 × 0.29 = 8.9 g of magnesium sulfate and 14.7 g of calcium nitrate^1,3.
3. Spacing: 4 plants get 495 cm² each and 6 plants 330 cm² each. Kratky’s bulletin gives two common densities, 1.5 and 1.9 lettuce plants per square foot, which is about 490 to 620 cm² per plant in our conversion, and keeps higher densities for smaller cultivars or younger harvests³. Four plants match his spacing; six suit a small cultivar.
4. Water: 6 plants at 3 to 4 L each¹ use 18 to 24 L and leave 6.7 to 12.7 L, 22% to 41% of the start; the level falls 9.1 to 12.1 cm. Four plants use 12 to 16 L and leave 14.7 to 18.7 L. Both stay above Kratky’s 10% stopping point² at the Hilo figure. At the top of Kratky’s general range of 3 to 6 L per plant¹, six plants would use 36 L, more than the tote holds, while four would use 24 L and leave 6.7 L, 22% of the start.

Route B in the 10 L container. Resh’s recipe at half strength is 100 mg/L nitrogen, 25 phosphorus, 150 potassium, 100 calcium and 20 magnesium, and his conversion factors and purities give these masses, our arithmetic⁵:

The other micronutrients are too small to weigh for one container; take them from the stocks in Mixing your own solution.

Troubleshooting

Most problems trace back to one of the three rules.

Safety

The method needs no electricity¹, so there is no mains power near the water. Cut away from your hand, with the lid on a firm surface. Wear gloves when you weigh and dissolve fertiliser salts, keep the containers labelled and closed, and keep the solution and the salts out of reach of children and pets. At the end of a crop, Kratky’s advice is to use the remaining solution to water plants growing in soil, spread over an area rather than poured in one spot, because it still contains nutrients³. If you have no garden, ask your local water authority how to dispose of it rather than pouring a container of fertiliser down a drain.

What we have seen in passive containers

PonicLabs has run passive, Kratky-style containers with organic nutrient sources, which are outside this article. In a 2020 radish trial in two 10 L containers fed with vermicompost powder, one with an air pump and one without, the plants without air seemed to grow the most, though, as the video says, the trial was “not really representative to make a conclusion”¹⁰. In 2023, with a liquid fertiliser made from vegetable waste, basil in passive pots of sand and a micro-dwarf tomato in passive pots of coir and perlite grew healthily, while basil in passive water culture with the same fertiliser gave slow growth, necrosis and early flowering¹¹. Both are single trials, reported as observations. Our passive hydroponics page has more.

Key numbers

Common mistakes

1. Topping the container up to the line each week. The roots above the old level are built for air, and flooding them makes the plant “drown”¹.
2. Filling to a guess instead of a measured line. If the pot does not touch the solution, capillary wetting never starts and the seedling dries¹.
3. Cutting the lid hole larger than the pot body. The gap lets light and mosquitoes into still solution^4,5.
4. Lifting the lid or the net pot to look at the roots. Roots tear, and the plant loses the ones it needs^3,4.
5. Using salty tap water. Salts the plant does not take concentrate as the volume shrinks¹.
6. Putting six plants in a tote sized for four. The plants share the same water and space, so heads stay small or the container runs dry^1,3.

Summary

A Kratky container works because the plant lowers the level by drinking, and the moist air it leaves behind feeds oxygen to the upper roots. The build has to serve three rules: the pot touches the solution at the start, the level is never raised, and the container is dark and closed. Cut the lid hole to the net-pot body, wrap the container, and fill to a line measured from the bottom of the pot. Mix the solution to about 1.5 mS/cm, check the pH once, transplant a seedling one to two weeks old, and give it light^1,3. Then leave it, check it weekly from outside, and harvest lettuce about five weeks later. In our worked example, a 10 L container holds water to spare for one plant, and a tote of about 30 L suits four plants at Kratky’s spacing^1,3.

Next in the series: Build: a deep water culture tub.

References

1. Kratky, B. A. (2009). Three non-circulating hydroponic methods for growing lettuce. Acta Horticulturae, 843, 65–72. https://doi.org/10.17660/ActaHortic.2009.843.6
2. Kratky, B. A. (2004). A suspended pot, non-circulating hydroponic method. Acta Horticulturae, 648, 83–89. https://doi.org/10.17660/ActaHortic.2004.648.10 Author's PDF: https://www.ctahr.hawaii.edu/hawaii/downloads/A_Suspended_Pot_Non-circulating_Hydroponic_Method.pdf
3. Kratky, B. A. (2010). A Suspended Net-Pot, Non-Circulating Hydroponic Method for Commercial Production of Leafy, Romaine, and Semi-Head Lettuce. University of Hawaii CTAHR, Vegetable Crops VC-1. https://www.ctahr.hawaii.edu/oc/freepubs/pdf/vc-1.pdf
4. Kratky, B. A. (2002). A Simple Hydroponic Growing Kit for Short-Term Vegetables. University of Hawaii CTAHR, Home Garden HG-42. https://www.ctahr.hawaii.edu/oc/freepubs/pdf/HG-42.pdf
5. Resh, H. M. (2013). Hydroponic Food Production: A Definitive Guidebook for the Advanced Home Gardener and the Commercial Hydroponic Grower, 7th ed. CRC Press, Boca Raton. ISBN 978-1-4398-7869-9.
6. Brechner, M., Both, A. J. and CEA Staff (2013). Hydroponic Lettuce Handbook. Cornell University Controlled Environment Agriculture Program, Ithaca, NY. https://cpb-us-e1.wpmucdn.com/blogs.cornell.edu/dist/8/8824/files/2019/06/Cornell-CEA-Lettuce-Handbook-.pdf
7. Jones, J. B. Jr. (2005). Hydroponics: A Practical Guide for the Soilless Grower, 2nd ed. CRC Press, Boca Raton.
8. Lucas, J. S. and Southgate, P. C. (eds) (2012). Aquaculture: Farming Aquatic Animals and Plants, 2nd ed. Wiley-Blackwell, chapter 3, Table 3.2 (reproduced from Colt 1984).
9. Taiz, L. and Zeiger, E. (2002). Plant Physiology, 3rd ed. Sinauer Associates, Sunderland, MA. Chapter 20, Gibberellins.
10. Sánchez, H. (2020). Turning good waste into radish in organic hydroponics. PonicLabs video (transcript).
11. Sánchez, H. (2024). Method of Up-cycled Non-Circulating Hydroponics (MUNCH): a proof-of-concept experiment with passive organic hydroponics using JADAM Liquid Fertilizer. PonicLabs technical report (preprint). https://doi.org/10.13140/RG.2.2.26735.65444

This article was drafted with the help of AI tools and reviewed and edited by PonicLabs. Sources are listed above; tell us if you find an error.