The air gap is the space between the lid and the surface of the nutrient solution. It is the one feature that lets a Kratky container work without a pump, and losing it is the fastest way to kill a strawberry plant.
What the Air Gap Does
Roots need oxygen as well as water. In most hydroponic systems a pump or air stone keeps the solution oxygenated. A Kratky container has neither. As the plant drinks, the level falls and the upper roots are left hanging in humid air, where they take in oxygen directly. The lower roots stay in the solution and handle water and nutrients.
How the Gap Forms
You start with the solution touching the bottom of the net cup so the young roots can reach it. From then on the plant lowers the level itself. Within a few weeks there is a band of roots in air and a band in liquid. Roots that grow in air are finer and more branched than the ones below, and they do not cope well with being submerged again.
How Big It Should Be
For an established strawberry plant, aim to keep at least a couple of inches of air under the net cup at all times, and more in a deep container. There is no need to measure precisely. The rule is simply that part of the root mass should always be out of the liquid.
What Goes Wrong Without It
- Refilling to the brim drowns the air roots. The plant wilts even though it is sitting in water.
- Roots turn brown and slimy as they run short of oxygen.
- The solution starts to smell, a sign that rot has set in.
When you top up, stop well short of the lid. That one habit prevents most root problems in passive hydroponics.
Related guide: How the Kratky method works.
