Kratky Method — Passive, Pump-less Hydroponics

Quick answer

The Kratky method is passive hydroponics: plant a seedling in a net cup through a lid, fill the jar with nutrient solution so the bottom of the cube touches it, and walk away. As the plant drinks, the water level drops, exposing aerial roots to oxygen-rich air while submerged roots continue absorbing nutrients. A single mason jar of lettuce reaches harvest in 25–35 days with no pump, no electricity, and one initial fill.

Why Kratky works — the science

Plants need three things at the root: water, nutrients, and oxygen. Active hydroponic systems (DWC, NFT, drip) use pumps to deliver all three. Kratky lets the receding water level do the oxygenation work for free.

The mechanism, step by step:

1. Day 0: Reservoir is full. Roots are submerged. Plant takes up water and nutrients through the submerged root surfaces.
2. Week 1–2: Plant drinks. Water level drops 2–3 cm. New roots grow into the air gap that forms between the water surface and the underside of the net cup.
3. Week 2–4: The plant now has two root systems — submerged feeder roots that absorb nutrients, and aerial roots in the air gap that absorb atmospheric oxygen.
4. Week 4–5: Water level near the bottom of the reservoir. Plant is ready to harvest.

The system is self-balancing. The harder the plant drinks (taller, larger, more leaves), the more air gap it creates, the more oxygen it gets. [KRATKY-ORIG]

What you need (the parts list)

• A 1-gallon (or larger) opaque container with a removable lid. Black is best — blocks light, prevents algae.
• A 2-inch (or larger) net cup fitted through a hole in the lid.
• A rockwool starter cube (pre-soaked to pH 5.5) or a seedling already established.
• Nutrient solution mixed to target EC and pH.
• A lamp — Kratky still needs light, just no electrical pumps.

That's it. Total cost: $5–15 per Kratky station for hobby use.

Step-by-step setup

1. Drill or buy a lid with a hole sized to fit the net cup snugly (typically 2-inch hole for 2-inch cup).
2. Soak the rockwool cube in water at pH 5.5 for 30 minutes (rockwool starts at pH 8 — see coco vs perlite vs rockwool).
3. Plant a seedling into the rockwool cube (or sprout a seed directly in the cube).
4. Mix nutrient solution at the target EC for the crop:
   • Lettuce: EC 1.0, pH 6.0.
   • Basil, herbs: EC 1.4, pH 6.0.
   • Kale, chard: EC 1.2, pH 6.0.
5. Place the rockwool cube into the net cup, surrounded by clay pebbles to anchor it.
6. Fill the reservoir with nutrient solution until the bottom 1 cm of the cube is submerged.
7. Cover the cube lightly with more clay pebbles (blocks light from reaching the cube).
8. Place under light — DLI target depends on crop (see PPFD and DLI).
9. Walk away. Check weekly but don't refill.

Best crops for Kratky

Leafy crops with short cycles thrive in Kratky:

• Lettuce — the classic Kratky crop. 25–35 day cycle, fits perfectly in a 1-gallon jar.
• Basil — works with one mid-cycle top-up.
• Spinach, arugula, bok choy, mizuna — all excellent.
• Kale, swiss chard — fine for shorter cycles; harvest before water runs out.
• Mint, cilantro, parsley — fine for one short cycle.

Crops that don't work well in Kratky:

• Fruiting crops (tomato, pepper, eggplant) — too long a cycle.
• Cucumbers, squash — water demand exceeds reservoir capacity.
• Root crops (radish, carrot) — no root expansion room.
• Watercress — actually wants submerged roots, defeats Kratky's design.

Variants and scaling

Mason jar (the original)

A wide-mouth half-gallon to 1-gallon mason jar. Black-painted exterior or wrapped in foil. The simplest version. One lettuce head per jar.

5-gallon bucket Kratky

Scale up to a single 5-gallon food-safe bucket. Same principle, larger volume. Grows 1–2 lettuce plants over a single cycle. Used by Bernard Kratky in his original research.

Bin or storage tote

A 27-gallon HDPE storage tote with a lid cut for 4–6 net cup positions. Grows 4–6 lettuce plants simultaneously over a single cycle. Common classroom and small-grower setup.

Vertical Kratky towers

Larger containers stacked with multiple net cup positions. Larger volume per plant lets the system support fruiting crops marginally. Niche application.

Operation — what to do (and not do)

Do

• Mix nutrient at the right EC and pH at fill time.
• Block all light from reaching the solution (algae is the silent killer).
• Check air gap weekly — make sure roots are reaching into it.
• Plan for one cycle then drain, sanitize, and refill for the next.

Don't

• Don't top up mid-cycle. Topping off submerges the air-gap roots and kills them. Plants look fine for 3–4 days then collapse.
• Don't open the lid unnecessarily. Each opening introduces light and contamination.
• Don't use clear or translucent containers without light-blocking.
• Don't use Kratky in warm rooms without temperature management — water temp above 24 °C breeds Pythium and root rot fast.

Failure modes

Closed air gap (most common)

Symptoms: plant fine for 2 weeks, then yellows and wilts.

Cause: water level didn't drop because the plant isn't drinking enough, OR you topped off mid-cycle.

Fix: don't refill. If the plant isn't drinking, check temperature, light, and root health.

Algae bloom

Symptoms: green tint to water; green film on container walls.

Cause: light reaching the nutrient solution.

Fix: opaque container, block all light leaks.

Root rot at high temperature

Symptoms: roots brown, mushy, sulfurous smell, plant wilting.

Cause: water temperature above 24 °C breeds anaerobic bacteria and Pythium.

Fix: cool the room or move Kratky to a cooler location. See root rot.

Mosquito larvae in outdoor Kratky

Symptoms: visible "wrigglers" in the water.

Cause: open lid in outdoor environment.

Fix: secure the net cup tightly in the lid hole; cover any exposed water with foam, mesh, or oil.

Kratky vs DWC

Two ways to grow lettuce: passive (Kratky) and active (DWC). Different strengths.

• Kratky: no pump, no electricity, simpler. One cycle, then reset.
• DWC: active aeration, continuous production, more forgiving of mistakes.

See DWC vs Kratky for the deep comparison.

See also

• DWC system
• DWC vs Kratky
• Lettuce — the classic Kratky crop
• Basil
• Choosing a reservoir
• pH management