Grodan - Rockwool Slab (100 x 15 cm)

A stone wool slab of 100 by 15 by 7,5 cm. Stone wool binds no nutrition itself, so the composition of your solution is the composition inside the slab. The most widely used substrate in professional glasshouse growing.

A slab buffers almost nothing, so the soak sets your first week

Stone wool being inert is always presented as an advantage, and so it is: you set the EC to the millilitre. But there is a flip side that rarely gets mentioned. A medium that holds nothing back also corrects nothing. Soil absorbs a mistake, coco partly, stone wool not at all.

So soaking here is not a preparatory step but the first cultivation step. Fresh stone wool is not acidic in itself. Put the slab into use without saturating it properly with water at the right pH and it will pull your solution upward over the first few days, leaving you higher than the meter in your bucket suggests. Let the slab fill completely with water at pH 5,5, give it time to work through, and then measure what comes out.

Before you start, cut drainage slits in the underside, across the full width. A slab that cannot drain fills from the bottom up, and you will not see that coming from above.

Why it is standard in the glasshouse

The fibre structure combines a lot of water with a lot of air, and holds that ratio as long as the slab keeps its shape. On top of that, stone wool is light, packs flat and costs little per litre of substrate. At scale, that is the sum that decides it.

Technical Specifications:

• Type: inert mineral stone wool slab for hydroponics
• Manufacturer: Grodan
• Dimensions: 100 x 15 x 7,5 cm (length x width x height)
• Property: binds and buffers no nutrients itself
• Soaking: saturate fully with water at pH 5,5 before first use
• Drainage: cut slits in the underside across the full width

Frequently asked questions

What pH should I soak at?
Use 5,5. Online you will find both 5,1 to 5,5 and 5,5 to 5,8 quoted; 5,5 falls inside both.

Can I reuse a slab?
Technically yes, as long as it keeps its resilience. In practice it is replaced each crop, because cleaning it is more work than a new slab is worth.

Do I root cuttings in blocks first?
That is the usual method: a stone wool block first, and once it is well rooted you set it on the slab so the roots grow through.

How many plants can I put on it?
That depends on your spacing and your crop. We deliberately give no number, because the figures differ per Grodan line and we cannot establish which line you have in hand.