Rockwool vs Clay Pebbles for Kratky Kits

Rockwool vs clay pebbles is a common question when filling the net pots of a Kratky kit. Both are inert grow media, meaning they hold the plant but supply no nutrients. They do different jobs, which is why many growers use them together.

What grow media does in a Kratky system

In a Kratky system, the net pot hangs above the nutrient solution. The grow media holds the seedling upright, keeps moisture around young roots until they reach the solution, and lets air in. It should not break down, cloud the water or change the pH much.

Rockwool

Rockwool is made by melting rock and spinning it into fibers, then forming it into cubes or slabs.

• Pros: holds water and air well, ideal for germinating seeds, widely available in small starter cubes.
• Cons: usually needs pre-soaking and pH adjustment before use, because it can start out alkaline; fibers can irritate skin, so handle it with care; it is not biodegradable.
• Best for: starting seeds and holding small seedlings.

Clay pebbles

Expanded clay pebbles (often sold as LECA or hydroton) are small balls of baked clay, full of air pockets.

• Pros: drain freely and let plenty of air reach the roots, heavy enough to steady a plant, reusable after cleaning, pH neutral once rinsed.
• Cons: dusty when new and need rinsing; do not hold seeds well, so they are poor for germination on their own; at the very bottom of a net pot they can stay wet and dark, so make sure no light reaches them.
• Best for: filling around a seedling or cube to support it in the net pot.

Using both together

A common approach is to start a seed in a rockwool cube or plug, then place the cube in the net pot and fill around it with clay pebbles. The cube keeps the young roots moist and the pebbles hold everything upright. This combination is used in many Kratky hydroponics kits.

Other options

• Coco coir plugs: made from coconut husk fiber, hold moisture well and are biodegradable. Often sold as ready-made starter plugs.
• Peat plugs: similar use to coco plugs; some come with a mesh wrapper.
• Perlite and vermiculite: light and airy, but small particles can fall through net pot holes into the solution.

Avoid potting soil, which clouds the water and can bring pests and disease.

Preparing grow media

• Rockwool: soak in water adjusted toward the pH you will use, as the packaging directs, then squeeze out excess before sowing.
• Clay pebbles: rinse well to remove dust, then soak before use so they do not wick moisture away from the seedling.
• Plugs: moisten before sowing; many come ready to use.

Which should you choose?

For a first Kratky kit, use whatever came in the box. If you are buying separately, starter plugs or rockwool cubes for germination plus a bag of clay pebbles to support the plants covers almost every crop. The Kratky supplies checklist lists both, and Kratky kit setup shows how to plant with them.

Reusing grow media

Clay pebbles can be cleaned and reused: remove old roots, wash, and let them dry. Rockwool cubes are generally used once. See cleaning and reusing a Kratky kit.

Frequently asked questions