The Kratky Method: Hydroponics for Beginners with Zero Equipment

What the Kratky method actually is

The Kratky method, developed by Dr. B. A. Kratky at the University of Hawaiʻi in the 1990s, is the simplest hydroponic technique that exists. A plant sits in a net cup at the top of a sealed reservoir. Its roots dangle into a nutrient solution. As the plant drinks the solution, the water level falls — and as it falls, it exposes a growing length of root to humid air inside the reservoir, where the roots take up oxygen directly.

Because the system runs without pumps, the only ongoing cost is nutrients and water. It also runs silently and survives power outages — both genuine advantages over circulating systems for apartment growers.

What you will need to build one

A wide-mouth one-litre mason jar (or any opaque reservoir of similar volume; cover clear jars with foil to block algae).

A 5 cm net cup that sits in the jar's mouth without slipping through.

A growing medium: hydroton clay pebbles, rockwool starter cubes, or coconut coir.

A general-purpose hydroponic nutrient labelled "Part A + Part B" or "all-in-one" — not a regular houseplant fertilizer, which lacks calcium.

A pH test kit and pH Down (most tap water reads 7.5–8.0; lettuce wants 5.8–6.2).

Step-by-step: your first Kratky lettuce

Start a lettuce seed in a moistened rockwool cube. Keep it warm (18–22 °C) and lit until you see two true leaves and roots emerging from the cube — usually 10–14 days.

Mix your nutrient solution per the manufacturer's seedling dilution (often half-strength for the first two weeks). Adjust pH to 5.8–6.2.

Fill the jar so that the bottom 2 cm of the net cup sits in the solution. Place the seedling-in-cube into the net cup, then surround the cube with clay pebbles to support it.

Place under a grow light running 14–16 hours per day, or in a bright south-facing window if you don't have a light yet.

Don't top up the water until the plant is mature. The growing air gap is the system; refilling it drowns the roots.

Common Kratky problems and fixes

Yellowing leaves with green veins usually means an iron lockout from a high pH. Test and adjust pH down to 6.0.

Brown, slimy roots indicate the solution got too warm (above 24 °C) and oxygen levels collapsed. Move the jar somewhere cooler or wrap it in reflective insulation.

Algae blooming on the inside of the jar is a light problem, not a nutrient problem. Use an opaque container or wrap clear ones in foil.