Step 4: Check the Kratky water level

Watch the water

The Kratky water level is meant to drop, and understanding why is the key to the whole method.

The water level drops as the plant drinks. Check it every few days, and make sure the lower roots still reach the water.

Top up with care

If the level gets too low before the plant is ready, add a little plain water or nutrient solution, as the label says. Never fill it back to the brim.

The air gap

As the water goes down, it leaves an empty space at the top of the container: the air gap. It is one of the most important parts of the Kratky method. The upper roots hang in the air gap and take in oxygen. The lower roots stay in the solution and feed. A healthy Kratky plant has both.

Two kinds of roots

If you look under the cover, you will often see two kinds of roots. Roots in the air gap tend to be fuzzy and branched. Roots in the water are usually long and smooth. Both are healthy; they are simply doing different jobs.

How to check the level

Lift the flap or the cover and see where the water is. Mark it on a strip of tape on the container, with the date. Each check gets a new mark, which shows how fast your plant is drinking. As the plant grows, it drinks faster, so the marks get further apart.

Topping up without drowning the roots

If you need to add water, do not refill to the top. The air roots near the top cannot take in oxygen under water and can rot. Add only enough to keep the bottom of the roots in the solution, and leave the air gap open. Small top-ups are better than big ones.

Use plain water or nutrient solution, as your nutrient label recommends. A fast-growing leafy plant in a container of the right size often needs little or no topping up before harvest.

Signs to watch for

• Leaves wilt and the lower roots are out of the water: add a little water
• Roots turn brown and slimy and smell bad: the water may be too warm or the air gap too small
• Green growth in the water: light is getting in, so cover the container better
• The water drops very fast: use a bigger container for that plant next time

Keep a water log

Note the date and the water level each time you check. A pattern soon appears. Then see Step 5: Watch it grow for what else is worth recording.

Why the level drops faster later

A young plant has few leaves and uses little water. As it grows more leaves, it uses much more, because leaves release water vapor into the air and pull more up through the roots. Warm, dry, sunny days speed this up too. So the level may barely move in the first week, then drop quickly later on.

A simple water-level record

Keep two columns in your notes: the date, and how far the water has dropped from the first mark. Fill it in every few days. It is a good way to see how fast the plant is growing, often before the change shows in the leaves.

Kratky water level checklist

• Check every few days
• Mark the level and the date
• Lower roots in the water, upper roots in the air
• Top up only a little, never to the brim
• Keep the container covered and out of hot sun

What if the water runs out?

If the container is nearly empty and the plant is not ready, add a little water so the lower roots can feed, and leave the air gap. Next time, use a bigger container for that kind of plant.

Why not keep the container full?

It can seem like a full container is better, but roots need oxygen as much as water. A full container leaves them no room to breathe. A falling level is not a mistake; it is how the Kratky method is meant to work.

How the level should look over time

Early on, the water should sit just at the bottom of the net pot. Midway through, there should be a clear air gap with the upper roots hanging in it. Near harvest, the level may be well down the container, with long roots still reaching the water.