The water has dropped below the pot. Should I top it up?

No, that is how the method works. Kratky found the level in a 4-litre bottle typically falls 15-25 cm by harvest. The long roots reach the solution, and the upper roots use the gap to breathe. Raising the level drowns those air roots.

My meter reads ppm. How does that compare to EC?

Check your meter's manual for its conversion factor. One meter maker says basic meters use 0.5 and that fertiliser solutions suit 0.7. So EC 1.5 mS/cm can read as 750 ppm or 1,050 ppm depending on the meter. Working in EC avoids the confusion.

What can I grow this way?

Short-lived crops that use little water: leaf lettuce, other leafy greens and some herbs. Cucumbers and tomatoes have been grown in research, but they need very large tanks; the original paper calls fill-once tomatoes an academic curiosity except in special situations.

Kratky hydroponics: no pump, no power, container, EC, air gap and heat

Kratky is a non-circulating way to grow vegetables in water. You fill a container with nutrient solution once at planting and let the crop grow to harvest with no pump, no electricity and no watering. It was developed by B.A. Kratky, a horticulturist at the University of Hawaii, and works well for leaf lettuce and other short crops. It suits homes with no soil, just a balcony or the space under the eaves.

This lesson in the “Grow your own vegetables” path is for people who have grown in soil and want to try a soil-free corner.

Why it needs no pump

Most hydroponics pumps or bubbles air through the water, because roots sitting in water run short of oxygen. Kratky solves this by letting the water level fall on its own.

A net pot filled with growing medium sits with its bottom 2-3 cm in the solution. Water wicks up and keeps the seedling moist (capillary action).
As the plant grows it drinks more, and the level drops below the pot. By then roots have grown down into the solution.
The space between lid and water becomes a humid air gap. Roots in that gap grow thicker and more branched to take in oxygen; roots below take up water and nutrients.

The one rule you can’t break: the level may stay the same or fall, but it must not rise. Kratky warns that raising it more than about 2 cm after roots have emerged, by rain or by topping up, can injure the plant, because the breathing roots go under.

In a Kratky container the water may fall or hold steady, but never rise.

One 4 L fill; the level falls 15-25 cm by harvest

Choosing a container

Type	Volume	Plants	Note
Plastic bottle	4 L	1 lettuce	The smallest version in the original work
Bucket with lid	20 L	3-4 lettuces	Holes cut in the lid for the pots
Storage box or cool box	Varies	6-8 lettuces	Lid must hold the pots’ weight
Shallow lined tank	About 14 cm deep	20 x 30 cm spacing	The commercial version in Hawaii

Every version needs two things: it must be opaque or painted dark, because light in the solution grows algae, and the lid must close well with no gaps around the pots.

Kratky suggests 4-8 litres of solution per plant and says growers typically aim for 150-250 g lettuces, using about 3-6 litres per plant.

Worked example: one 4-litre bottle

Following Kratky’s 2009 paper:

Fill a 4-litre plastic bottle with water to about 4 cm below the top.
Add 5 g of a complete granular hydroponic fertiliser. The original used one specific product; what you can buy in Thailand will differ, so mix to its label and check that EC is about 1.5 mS/cm.
Wrap the bottle in a black bag or opaque paper.
Sow lettuce in a net pot about 7.5 cm long, set it in the neck, with the bottom 3 cm in the solution.
Leaf lettuce is usually ready about 6-7 weeks after sowing. The level will have dropped 15-25 cm.
Check the numbers: in Hawaii, a 200 g lettuce used about 3-4 litres, so a 4-litre bottle fits one plant. If it runs dry before the lettuce is full size, the bottle was too small or the weather was hot enough to make the plant transpire a lot. Use a bigger container next time.

If you start seedlings in a tray first, the tank method transplants 1-2-week-old seedlings and harvests about 5 weeks later, depending on season and variety.

Reading EC and ppm

EC (electrical conductivity) measures how much dissolved salt is in the water and stands in for fertiliser strength. Kratky used about EC 1.5 mS/cm for lettuce, and 1.5-2.5 mS/cm in commercial tanks. Cornell’s recipe runs at about 1.5 with pH held at 5.5-6.0. Kratky reports that with good starting water, crops usually finish without any pH adjustment.

Many meters show ppm or TDS instead, which is EC multiplied by a factor. One meter maker states the factor is 0.5 for strong ionic solutions and 0.7 for fertiliser solutions.

EC (mS/cm)	Factor 0.5	Factor 0.7
1.5	750 ppm	1,050 ppm
2.0	1,000 ppm	1,400 ppm
2.5	1,250 ppm	1,750 ppm

The same ppm number can mean two different strengths, so when you read a recipe, ask which factor it uses.

Your starting water matters

Nothing is ever drained or changed, so salts already in the water concentrate as the solution is used. Kratky’s example: water with EC 0.5 mS/cm from contaminant salts reaches 2.0 mS/cm when 25 per cent of the solution is left, which can cause tipburn and other problems. Measure the EC of your plain water before adding fertiliser. If it is high, the paper recommends rainwater, or deeper containers emptied after every crop.

Mosquitoes come first in a Thai home

Still water is somewhere mosquitoes can lay eggs. Kratky states plainly that mosquitoes breed in non-circulating solution and are both a health risk and a nuisance.

Close every gap. Pots should fit their lid holes snugly and the lid should seal. The original papers stress that a snug fit helps keep mosquitoes out.
Watch the gap around young seedlings, which may be open while the plant is small. Check whenever you pass.
Look for larvae. If you see any, find the gap and close it.
Research setups have used salt-tolerant fish or Bti, a bacterium that kills mosquito larvae. At home, sealing the container properly is the simplest and most effective step.

Thai heat: the method’s weak spot

Lettuce is a cool-season crop. A University of Florida report cites lettuce thriving at daytime temperatures of about 16-21°C (60-70°F). It found that Florida summer nutrient solution often runs above 32°C (90°F), which lowers dissolved oxygen and raises the risk of Pythium root rot; tipburn becomes more likely, and very hot lettuce tends to bolt and turn bitter. That trial needed a chiller holding the solution near 22°C (72°F), plus shade cloth.

Cornell gives dissolved oxygen of about 7-8 ppm as the target for aerated systems and reports crop failure below 3 ppm. Kratky doesn’t aerate; the air gap does that job, so the gap has to be real.

What a Thai home can do without extra equipment:

Start your first round in the cool season, when the weather is closest to what lettuce likes.
Keep the container under the eaves or in light shade, so the leaves get light but the container isn’t in direct sun heating the water.
Choose lettuce varieties labelled heat-tolerant or slow to bolt.
If roots turn brown and mushy, smell bad, or the plant wilts even with water present, empty the container, clean it and start again.

Common mistakes

Topping up daily like watering a pot. The level rises, the air roots drown, and the plant wilts in a full container.
Clear or pale containers. Light gets in and algae compete for nutrients and oxygen.
Leaving gaps around the pots. You’ve built a mosquito nursery.
Mixing fertiliser by eye without measuring EC, then comparing ppm with a recipe that used a different factor.
Lifting the lid to look at the roots. Kratky warns that once roots have emerged, lifting the lid can damage them.
Big plants in small containers. A 4-litre bottle is enough for one lettuce, not for a fruiting crop.

Sources

General information, not medical advice. If you are pregnant, take regular medication, or have a liver or kidney condition, ask a doctor or pharmacist before taking herbs beyond normal food amounts.