Kratky Hydroponics Glossary: Terms Every Kit Owner Meets

Kit instructions and hydroponic guides use a lot of shorthand. This Kratky hydroponics glossary explains each term in plain language, with a link to the guide that covers it in depth.

The method

Kratky method

A passive, non-circulating hydroponic method developed by Bernard A. Kratky at the University of Hawaii. Plants sit above a container of nutrient solution with no pump; the level falls as they drink, leaving air for the upper roots. See the Kratky method.

Passive hydroponics

Any hydroponic system that runs without pumps, timers or electricity. Kratky and wick systems are the common examples.

Non-circulating

The solution stays still in the container instead of being pumped around. Kratky is a non-circulating method.

Air gap

The space between the lid and the falling solution level. Roots in this space take oxygen from the air. It is the reason a Kratky reservoir should never be refilled to the top. See the Kratky air gap.

Air roots

Roots that grow in the moist air gap. They are adapted to air and can die if flooded.

Water roots

Roots that hang in the nutrient solution and take up water and nutrients.

Fill line

The level of the first fill, usually where the solution just touches the bottom of the net pot. See Kratky kit setup.

Top-up

Adding a small amount of solution late in a crop so the root tips reach it again, kept below the air roots.

The kit

Reservoir

The container that holds the nutrient solution: a jar, tote, bucket or the base of a boxed kit. It should be opaque or covered. See Kratky containers.

Net pot (net cup)

A plastic cup with slotted sides that holds the plant and grow media in a hole in the lid, letting roots grow through into the solution.

Grow media

The material in the net pot that holds the seedling upright and wicks moisture to young roots, such as rockwool, clay pebbles or coco coir.

Rockwool

A spun mineral fiber sold in cubes and plugs for starting seeds. It holds water and air well and is usually soaked before use. Compared in rockwool vs clay pebbles.

Clay pebbles (LECA)

Lightweight expanded clay aggregate: round, baked clay balls that support the plant and stay airy. Rinsed before use and reusable.

Coco coir

Fiber from coconut husks, sold as plugs or loose media, used as an alternative to rockwool.

Seed plug

A small block of media that a seed is started in before it goes into a net pot.

The solution

Nutrient solution

Water with hydroponic nutrient dissolved in it. It supplies every element the plant needs, because there is no soil. See Kratky nutrients.

Hydroponic nutrient

A fertilizer made for growing without soil, containing the major nutrients and micronutrients. Sold as one-part, two-part or three-part products.

Macronutrients

The elements plants need in the largest amounts: nitrogen (N), phosphorus (P) and potassium (K), plus calcium, magnesium and sulfur.

Micronutrients

Elements needed in small amounts, such as iron, manganese, zinc, copper, boron and molybdenum.

pH

A 0 to 14 scale of how acidic or alkaline the solution is. Most Kratky crops prefer about 5.5 to 6.5. See Kratky pH.

EC (electrical conductivity)

A measure of how much dissolved nutrient is in the solution, shown in mS/cm or µS/cm. Higher EC means a stronger solution.

TDS (total dissolved solids)

Another way of showing solution strength, in parts per million (ppm). Meters convert from EC using a scale factor, so ppm readings from different meters are not always comparable.

pH up / pH down

Hydroponic adjusters that raise or lower pH a little at a time.

Nutrient lockout

When a nutrient is in the solution but the plant cannot take it up, often because pH is out of range.

Growing and problems

Photoperiod

The number of hours of light a plant gets each day. See Kratky grow lights.

Leggy

Seedlings that are tall, thin and pale from too little light.

Bolting

When a plant such as lettuce or cilantro starts to flower, stretching upward and turning bitter. Heat and long days trigger it.

Tipburn

Brown, dead edges on leaves, often on inner lettuce leaves, linked to heat and fast growth.

Root rot

Brown, slimy, decaying roots caused by low oxygen and pathogens. See Kratky root rot.

Algae

Green growth that appears when light reaches nutrient solution. See algae in a Kratky kit.

Cut-and-come-again

Harvesting outer leaves or stems so the plant keeps growing. See the Kratky harvest guide.

Hardening off

Gradually moving an indoor-grown plant outdoors so it adjusts to sun and wind.

Other hydroponic systems

DWC (deep water culture)

A hydroponic system where roots hang in solution that an air pump and air stone keep oxygenated. Unlike Kratky, the level is kept full. Compared in Kratky vs DWC.

Air stone

A porous stone connected to an air pump that releases fine bubbles into the solution. Used in DWC, not in Kratky.

NFT (nutrient film technique)

A system that pumps a thin film of solution along channels past the roots.

Ebb and flow

A system that floods a tray with solution on a timer, then drains it.

Wick system

A passive system where a wick draws solution up from a reservoir into the media.

Frequently asked questions