The Kratky Method: How Passive Hydroponics Works

The Kratky method is a way of growing plants in water and nutrients with no pump, no timer and no electricity. Every Kratky hydroponics kit is built around the same simple idea, and understanding it makes every other step easier.

Where the Kratky method comes from

The method is named after Dr. Bernard Kratky, a horticulture researcher at the University of Hawaii. In the 1990s and 2000s he published work on what he called non-circulating hydroponics, including "A suspended pot, non-circulating hydroponic method" (Acta Horticulturae 648, 2004). The aim was a system that small growers could run without power or expensive equipment. The same design now fills mason jars on kitchen windowsills and the reservoirs of most beginner hydroponic kits.

How a Kratky system works

A Kratky system has four parts: a container, a lid, a net pot and a nutrient solution. The container holds the solution. The lid has a hole that holds the net pot. The net pot holds the plant in a small amount of grow media such as rockwool or clay pebbles. At the start, the solution touches the bottom of the net pot, so the young roots can reach it.

As the plant grows, three things happen at once:

1. The roots grow down into the solution and take up water and nutrients.
2. The plant uses the solution, so the water level slowly falls.
3. The space between the lid and the falling water fills with humid air, and the upper part of the root system grows into that space.

That last point is the heart of the method. Roots need oxygen as well as water. In other hydroponic systems, an air pump bubbles oxygen into the water. In a Kratky system, the roots that hang in the moist air gap take in oxygen directly, while the lower roots keep drinking. The plant effectively grows its own aeration as the level drops.

Why the water level must drop

The most common mistake with a Kratky kit is topping the container back up to the original line. It feels helpful, but it floods the air roots that the plant grew to breathe. Those roots can suffocate and rot, and the plant may wilt even though it is standing in water. The rule is simple: let the level fall. If a long-season plant needs more solution later, add it to a lower level so the upper roots stay in air. The article on the Kratky air gap covers this in detail.

What the Kratky method does well

• Nothing to plug in. No pumps or timers means nothing to fail, no noise and no running cost.
• Very little maintenance. Many leafy crops can grow from transplant to harvest on a single fill of solution.
• Cheap to start. A jar, a net pot and a bottle of hydroponic nutrient are enough to try it.
• Easy to scale. One jar on a windowsill and a row of totes in a garage follow the same rules.

Where it has limits

• Big, thirsty plants need big containers. A fruiting tomato or pepper can drink far more than a small reservoir holds. See Kratky containers for sizes.
• The solution is not adjusted much. Because the batch stays in place, the nutrient strength and pH you start with matter. The Kratky nutrients guide and Kratky pH guide explain how to get them right at the start.
• Warm water holds less oxygen. A container in hot sun can overheat, which raises the risk of root problems.

Kratky compared with other hydroponic methods

Deep water culture (DWC) also suspends roots in solution, but uses an air pump and air stone to keep the water oxygenated, so the level can stay high. Nutrient film technique (NFT) and ebb and flow systems use pumps to move solution past the roots. These active systems can support bigger plants and faster growth, but they need power and more attention. The Kratky method trades some of that capacity for simplicity. For a side-by-side look, see Kratky vs DWC kits.

Kratky compared with other hydroponic systems

System	Pump or power	How roots get oxygen	Water level
Kratky	None	Air gap above the falling solution	Falls during the crop
Wick	None	Airy grow media	Kept topped up
Deep water culture (DWC)	Air pump	Air stone bubbles	Kept full
Nutrient film technique (NFT)	Water pump	Thin film of moving solution	Constant flow
Ebb and flow	Water pump and timer	Media drains between floods	Floods and drains

Unfamiliar terms are explained in the Kratky hydroponics glossary.

What this means for your kit

A Kratky hydroponics kit is simply this method in a box: a reservoir sized for the plants it is meant to grow, a lid that holds net pots, some grow media, and usually a nutrient and pH supplies. Knowing how the method works lets you judge whether a kit is suited to what you want to grow, and how to run it once it arrives. The next steps are choosing a Kratky kit and setting it up.

Go deeper: the method itself, beyond what a kit owner needs, is covered at Kratky Method Hydroponics, our sister site on hydroponic gardening with the Kratky method.

Frequently asked questions

Key points

• The Kratky method is passive hydroponics: no pump, no timer, no power.
• Roots drink from the solution below and breathe in the air gap above it.
• The water level is meant to fall. Do not refill to the top.
• Container size, nutrient strength and pH are set at the start, so get them right.
• Leafy greens and herbs suit the method best; larger plants need larger containers.