Quick answer: When you first set up a Kratky system, the nutrient solution should touch the bottom of the net cup or growing medium so a new seedling can reach moisture. As the plant grows, let the water level fall naturally. The exposed upper roots become oxygen roots, so never refill a mature system all the way to the net cup.
That sounds simple, but water level is one of the most confusing parts of passive hydroponics. Some instructions say never refill. Others tell you to top off every week. Both can make sense—the right answer depends on whether your reservoir holds enough solution for the plant’s entire crop cycle.
This guide explains the starting level, the mature root air gap, and the safest way to handle a small reservoir that runs low before harvest.
Why Kratky Water Level Matters
The Kratky method is a non-circulating hydroponic system. A plant sits above a reservoir of nutrient solution without an air pump. As the plant drinks, the solution level drops and leaves a humid air space below the lid. Roots in the lower part of the reservoir absorb water and nutrients; roots in the air space absorb oxygen.
That division is the core of the method. According to B. A. Kratky’s description of non-circulating hydroponic methods for lettuce, the system is sized so the nutrient solution is consumed as the crop reaches harvest. In a properly sized reservoir, no pump, circulation, or mid-cycle refill is necessary.
If you flood the established air roots, however, you remove their access to oxygen. That is why “keep the reservoir full” is the wrong mental model for Kratky.
Starting Water Level for Seeds and Young Seedlings
At planting, fill the reservoir until the solution just reaches the bottom of the net cup or moistens the bottom of the starter plug. The young roots are not long enough to reach through an air gap yet.
- Net cup with a starter plug: Let the solution touch roughly the bottom 1/4 inch of the plug or net cup.
- Seedling with roots extending below the cup: The root tips can sit in the solution while the crown and leaves remain dry.
- Established transplant with longer roots: Set the level below the net cup as long as several healthy root tips reach the solution.
The goal is moisture at the roots, not a soaked plant. Keep the stem, crown, and leaves above the lid. A seedling that cannot reach moisture will dry out; a starter plug submerged too deeply can stay waterlogged.
Water Level After the Roots Are Established
Once roots extend into the reservoir, stop trying to maintain the starting level. Let the plant draw the solution down. The growing air gap is intentional.
Do not refill the reservoir to the lid or submerge all of the roots. Mature Kratky roots adapt into two functional zones:
- Lower roots: remain in the nutrient solution.
- Upper roots: remain in the moist air gap and help the plant obtain oxygen.
You do not need to measure a perfect air-gap depth every day. Instead, preserve the portion of the root system that has already been exposed to air.
Should You Refill a Kratky Reservoir?
In the original one-crop approach, no refill is needed. The reservoir is large enough to supply the crop until harvest. Lettuce and other fast leafy greens are especially well suited to this approach.
In a small home container, a partial refill may be necessary. A mason jar, small bucket, warm room, large basil plant, or longer-growing crop can run out before harvest. If the remaining solution becomes dangerously shallow, add prepared nutrient solution only to the lower portion of the reservoir.
A practical rule is to refill to roughly one-third or one-half of the container’s height—not to the original starting level—while leaving the established upper root zone in air. The exact amount depends on the container and root mass, so the visible air roots are a better guide than a universal inch measurement.
Never do this when topping off
- Do not refill all the way to the net cup after the air gap has formed.
- Do not cover roots that have spent weeks adapted to air.
- Do not add concentrated nutrients directly to the occupied reservoir.
- Do not guess at a stronger dose because the plant looks hungry.
Mix any replacement solution in a separate container, following the nutrient manufacturer’s directions, then check its pH before adding it. Our hydroponic nutrient solution guide walks through that process.
How Much Reservoir Does Lettuce Need?
A larger reservoir makes Kratky easier because temperature, pH, and nutrient concentration change more slowly. For a full head of lettuce, many home growers plan around approximately one gallon of solution per plant for the crop cycle, then adjust for variety, temperature, humidity, and expected harvest size.
When using a tote, also leave enough space above the lid for the mature leaves. Crowding lettuce can reduce airflow and contribute to inner-leaf tip burn. The Kratky lettuce storage-tote guide covers the complete crop-specific setup.
Signs Your Kratky Water Level Is Wrong
The level is too high
- Most or all established roots are submerged.
- The plant wilts even though plenty of solution remains.
- Roots become slimy or smell unpleasant.
- The starter plug stays saturated around the stem.
The level is too low
- No root tips reach the solution.
- A young seedling wilts quickly.
- The reservoir is nearly dry well before harvest.
- Nutrient concentration and pH swing rapidly in the tiny remaining volume.
Root color alone is not always conclusive because some nutrients stain roots. Texture and smell matter: healthy roots are generally firm, while rot is often slimy and foul-smelling. Use the hydroponic troubleshooting guide before changing several variables at once.
A Simple Water-Level Routine
- At planting: Let solution touch the bottom of the net cup or starter plug.
- After roots extend: Allow the solution to fall and the air gap to form.
- During growth: Look at the remaining solution weekly without restoring the original level.
- If the reservoir is properly sized: Leave it alone until harvest.
- If a small reservoir may run dry: Add separately mixed solution only to the lower part of the container, preserving the air-root zone.
- Before the next crop: Empty, clean, and reset the reservoir at the starting level.
Kratky Water Level FAQ
Should the net cup touch the water?
At the beginning, yes—the solution should touch the bottom of the net cup or starter plug so the seedling can reach moisture. Once roots extend into the solution, the water level should fall below the cup.
How big should the Kratky air gap be?
There is no single required measurement. The gap grows as the plant consumes solution. The essential rule is to keep established upper roots exposed to moist air rather than flooding them during a refill.
Can Kratky roots dry out?
Yes. The lower root tips still need access to nutrient solution. The air gap is humid, but it is not a substitute for water. Refill a too-small reservoir partially before it runs completely dry.
Do I top off with water or nutrients?
There is no universal answer because water and nutrients are not consumed at identical rates under every condition. For a beginner without EC data, the safest approach is to size the reservoir for one crop and avoid topping off. If a small reservoir must be refilled, use a conservatively mixed, pH-checked solution rather than pouring concentrated fertilizer into the system.
What should I read next?
Start with the complete Kratky beginner guide. When you are ready to build, use the step-by-step Kratky tote instructions.
