Why Fresh Rockwool Raises pH
The fibre surface becomes alkaline during manufacturing (basalt melted at high temperature). When irrigated, drainage pH rises to 7.0–8.0 even when the input solution pH is correct.
Above pH 6.5, iron, manganese, and zinc become unavailable — deficiency symptoms appear despite a correct recipe.
Preparation: Two Steps
Step 1 — Soaking: submerge cubes in a solution at pH 5.0–5.5, EC 1.5–2.0 for 12–24 hours. Plain water is not sufficient — an acidic solution is required.
Step 2 — Drainage check: irrigate several times with the working solution and measure drainage pH. Target: 5.8–6.3. If pH >6.5 — repeat soaking.
Drainage Monitoring
Rockwool has no CEC — drainage directly reflects root zone conditions.
Target parameters:
- Drainage EC: no more than +0.5 above input EC
- Drainage pH: 5.8–6.5
- Drainage volume: 20–30% of irrigation volume
Three Critical Mistakes
- Soaking in plain water without acidification
- Skipping the drainage pH check after preparation
- Ignoring source water alkalinity when preparing the substrate