Stone wool is chemically inert — it has no natural buffering capacity, adds no ions to the solution, and has zero cation exchange capacity. This is its greatest advantage and its greatest demand: every ion in the root zone is exactly what you put there. EC and pH management in stone wool systems is therefore a direct reflection of your fertiliser programme and water quality.
Understanding EC in Stone Wool
Electrical conductivity (EC) in stone wool systems refers to the total dissolved salts in the root zone solution. Because stone wool itself contributes nothing, the substrate EC approximately equals the supply solution EC, modified by plant uptake and evaporation.
Why Slab EC Rises
Plants take up water faster than they take up nutrients (especially in warm weather). This concentrates remaining salts in the slab, raising EC above the supply level. This is normal and expected — a target drain EC of 0.5–1.5 mS/cm above supply EC is typical in drain-to-waste systems.
EC Targets by Crop
| Crop | Supply EC (mS/cm) | Target Drain EC (mS/cm) |
|---|---|---|
| Tomato — vegetative | 2.5–3.5 | 3.0–4.5 |
| Tomato — fruiting | 3.5–5.0 | 4.5–6.0 |
| Cucumber | 2.0–3.0 | 2.5–4.0 |
| Pepper | 2.5–4.0 | 3.5–5.0 |
| Rose | 1.8–2.5 | 2.5–3.5 |
| Lettuce / microgreens | 1.0–1.8 | 1.5–2.5 |
Understanding pH in Stone Wool
Stone wool has a neutral pH and no buffering capacity. The pH in the slab equals the pH of the irrigated solution, with some modification due to differential ion uptake by roots. Plants preferentially absorb cations (K⁺, Ca²⁺, Mg²⁺, NH₄⁺) relative to anions (NO₃⁻, SO₄²⁻, H₂PO₄⁻), which affects rhizosphere pH:
- High NH₄⁺ uptake → pH drops in slab (roots release H⁺ to balance charge)
- High NO₃⁻ uptake → pH rises in slab (roots release OH⁻/HCO₃⁻)
Target pH Ranges
| Growth Stage | Supply pH | Acceptable Drain pH |
|---|---|---|
| Propagation | 5.5–5.8 | 5.5–6.2 |
| Vegetative | 5.6–6.0 | 5.5–6.5 |
| Generative / Fruiting | 5.8–6.2 | 5.5–6.8 |
pH above 6.8 in the slab causes iron and manganese precipitation, leading to deficiency symptoms even when these elements are present in the solution. pH below 5.0 risks manganese toxicity and calcium/magnesium competition issues.
Measurement Protocol
Where to Measure
- Supply solution: at the irrigation header, before entering the slab
- Drain: collect drain water from multiple slabs (minimum 5 sample points per block) to get a representative average
- Slab squeeze: extract solution from the slab centre by compressing with a clean hand — this represents the actual root zone environment
Measurement Frequency
- Supply EC and pH: daily (or continuous with inline sensors)
- Drain EC and pH: daily in active growing seasons
- Slab squeeze: weekly or when anomalies are detected
Correcting EC Problems
EC Too High in Slab
- Increase drain percentage to 25–35%
- Add a low-EC flush on a cloudy day (supply EC 0.5–1.0 mS/cm for one irrigation)
- Check for blocked emitters creating dry zones with concentrated salts
EC Too Low in Slab
- Reduce drain percentage
- Increase supply EC
- Check if irrigation frequency is too high (over-diluting the slab)
Correcting pH Problems
pH Rising in Slab
Increase the proportion of NH₄⁺ in the fertiliser solution (shift from nitrate-dominated to ammonium-containing formula). Alternatively, acidify the water supply with phosphoric acid or nitric acid.
pH Falling in Slab
Reduce NH₄⁺ proportion; increase NO₃⁻ dominance. Ensure bicarbonate (HCO₃⁻) in source water is not excessively stripped — some buffer can be beneficial.
SPELAND Substrates and EC/pH Stability
Because SPELAND stone wool is 100% inert with zero CEC, it provides a "blank canvas" for nutrient management — what you see in the drain solution exactly reflects what's happening in the root zone. There are no surprises from substrate chemistry. This makes SPELAND substrates the preferred choice for agronomists who want full control over the growing environment.
→ Request SPELAND substrate samples or ask our agronomists a question