Kratky Refill: How Often to Top Up a Kratky Kit

The ideal Kratky refill is none at all. The method was designed so that one batch of solution carries the plant to harvest. In practice, longer crops, small containers and strong light can use the solution up early. Here is how to tell when a top-up is needed and how to do it without undoing the air gap.

Why Kratky kits are designed for one fill

In the Kratky method, the falling water level creates the air gap where the upper roots breathe. Each refill to the top would flood those roots. A container sized so the plant drinks it down just as the crop finishes is the ideal. For lettuce and many herbs, that is achievable in a reasonably sized container.

When a refill is needed

• The solution is nearly gone and the plant is still weeks from harvest.
• Only the tips of the lowest roots still reach the solution.
• You are growing a long-season or cut-and-come-again crop, such as basil kept going for months, or a fruiting plant.

Wilting can also mean too little solution, but check for root rot and heat first, as described in Kratky problems and fixes.

How to refill without drowning the roots

1. Mix fresh solution at the label rate, or slightly weaker for a plant that has been growing for a while, and check its pH (see Kratky pH).
2. Pour it in slowly, ideally through a gap at the edge of the lid, without lifting the plant.
3. Fill only part way: enough that a good length of the lower roots is back in solution, but well below the top of the root mass. The upper roots must stay in air.
4. Note the new level and the date.

Some growers top up with plain, pH-adjusted water instead of nutrient solution, especially if leaves show signs of a strong solution. Others use nutrient at reduced strength. Watching the plant tells you which it needs; the Kratky nutrients guide covers the signs.

Changing the solution completely

For a very long crop, the leftover solution can become unbalanced. Some growers replace it entirely: lift the plant and lid out carefully, empty and rinse the container, refill to a level that keeps the air roots in air, and return the plant. This disturbs the roots, so it is a step for long crops rather than routine maintenance.

How to need fewer refills

• Use a bigger container. More solution per plant is the simplest fix. See Kratky containers.
• Plant fewer sites. In a multi-site kit, leaving some holes covered gives each plant more solution.
• Choose quick crops. Lettuce and greens finish before the solution runs out.
• Keep it cool. Heat speeds water loss and stresses roots.

A note on "topping off" advice

You may see Kratky advice that says to "top off" or "replenish" the reservoir to keep roots in contact with the solution. The key detail is the level: adding solution to a lower level for a long crop is fine; refilling to the original line is what causes trouble. The article on the Kratky air gap explains why.

Going away? The post on a Kratky kit on vacation explains how to plan a crop around a trip.

Summary

Size the container so one fill lasts. If a long crop runs low, add solution to a lower level only, keep the upper roots in air, and check pH and strength first. For a refresher on the whole cycle, see the Kratky kit tips checklist.

Frequently asked questions