Clay Pellets vs Rockwool

What a growing medium does in Kratky

Clay pellets vs rockwool is the most common growing-medium question for Kratky beginners. In a Kratky net pot, the medium does not feed the plant. Its job is to hold the plant upright, keep the stem steady, block light from the water and let air reach the upper roots. Nutrients come from the solution below. That makes the choice simpler than it is for soil.

Clay pellets

Clay pellets, also called expanded clay, clay pebbles or LECA, are small balls of clay fired so they puff up and become light and porous.

• Reusable many times after rinsing
• Let air move freely around the stem and upper roots
• Generally pH neutral
• Dusty when new, so rinse well before use
• Too coarse to hold a tiny seed on their own

Rockwool

Rockwool is made by melting rock and spinning it into fibers, which are pressed into cubes and plugs. It holds water and air well, which makes it a popular seed-starting medium.

• Very good for starting seeds
• Holds moisture evenly around young roots
• Usually single-use
• Often has a high pH when new, so many growers soak or condition it before use, following the maker's directions
• Fibers and dust can irritate skin and lungs: wear gloves and handle it damp

Side by side

• Starting seeds: rockwool better
• Supporting a grown plant: clay pellets better
• Reuse: clay pellets yes; rockwool usually no
• pH: clay pellets neutral; rockwool may need conditioning
• Handling: clay pellets need rinsing; rockwool needs gloves
• Light blocking: both work if the net pot is well filled

Using both together

Many Kratky growers use both. A seed is started in a small rockwool cube or sponge. Once it has a few leaves and roots showing, the cube goes into the center of the net pot and clay pellets are packed around it to hold it upright and shade the water. This gives the seedling a moist start and the grown plant a stable, airy base. Step 3: Plant a seedling shows the process.

Other options

• Seed sponges or foam plugs: clean and easy for starting seeds, usually single-use
• Coco coir: made from coconut husk, holds moisture well, but can be messy and may fall through net pot slots
• Perlite: light and airy, but small pieces can wash into the water

For a first grow, clay pellets plus a seed cube or sponge is the simplest combination.

Preparing clay pellets

Rinse new clay pellets in a sieve or bucket until the water runs clear, to remove dust that would cloud the solution. Some growers soak them before use. Between plantings, rinse out old roots and debris, wash them and let them dry before storing.

Which should you choose?

If you are starting from seedlings, clay pellets are all you need. If you are starting from seed, add rockwool cubes or seed sponges for germination, then use clay pellets in the net pot. Both are on the Kratky Supplies list, and How to Choose Kratky Supplies covers the rest.

How much medium you need

For a jar, a handful of clay pellets usually fills a small net pot. Fill it enough to hold the plant upright and shade the water, but not so tightly that roots cannot grow through. For seed starting, one rockwool cube or sponge per plant is enough.

Medium and the water level

At setup, the water should just touch the bottom of the net pot, so the medium around the base stays damp and the young roots can reach the solution. Clay pellets wick a little water up; rockwool holds it. As the plant grows and the level drops, the roots take over. More in Step 4: Check the water level.

Storing growing media

Keep clay pellets dry in a bag or box between plantings. Store rockwool in its packaging in a dry place, and handle it damp to keep dust down. Label anything you have conditioned or soaked so you know it is ready to use.

Medium and algae

A well-filled net pot shades the water below it. Gaps let light in, and light causes algae. If you use only a seed cube, add clay pellets around it so no light reaches the solution. See Algae in a Kratky Container.

Reusing clay pellets safely

Before reusing clay pellets, remove old roots, wash them in hot soapy water, rinse well and let them dry completely. Pellets that smell bad after washing, or came from a container with root rot, are worth a second wash or replacing, so problems do not carry over to the next plant. Dry pellets store for a long time in a closed bag or box.