You fertilise correctly. The EC value is right. The light cycle is dialled in. Yet the plant is showing classic deficiency symptoms — yellowing leaves, copper-coloured leaf margins, stalled growth. The irrigation water was never measured.
This is not an isolated case. After incorrect watering volume, pH and EC errors in irrigation water are the most common cause of nutrient problems in indoor growing — and the least frequently diagnosed directly, because the error is invisible. The water looks clear. It feels normal. It is wrong nonetheless.
What pH actually measures — and why it is not optional
pH is the negative decadic logarithm of hydrogen ion concentration. In practical terms: a measure of how acidic or basic a solution is. The scale runs from 0 to 14, with 7 being neutral. Each unit corresponds to a tenfold change — pH 5 is ten times more acidic than pH 6, one hundred times more acidic than pH 7.
What matters for the grow: nutrients are only plant-available within a specific pH window. Calcium and magnesium dissolve poorly at low pH. Iron and zinc become insoluble at high pH. Phosphorus has a narrow availability window around 6.5. The substrate plays a decisive role here — because it co-determines the effective pH at the root, regardless of what you pour in.
Soil: 6.0–7.0 · optimal 6.3–6.8
Coco coir: 5.8–6.3 · optimal 5.9–6.2
Perlite / inert: 5.5–6.1 · optimal 5.8–6.0
DWC / water: 5.5–6.5 · optimal 5.8–6.2
These values apply to the incoming irrigation water — not to the substrate pH after weeks of build-up.
Nutrient availability as a function of pH
| Nutrient | Optimal at pH | Problems below 5.5 | Problems above 7.0 |
|---|---|---|---|
| Nitrogen (N) | 6.0–8.0 | Slightly reduced | Strongly reduced |
| Phosphorus (P) | 6.0–7.0 | Strongly reduced | Strongly reduced |
| Potassium (K) | 6.0–8.0 | Slightly reduced | Reduced |
| Calcium (Ca) | 6.5–8.0 | Strongly reduced | Optimal |
| Magnesium (Mg) | 6.0–8.5 | Reduced | Well available |
| Iron (Fe) | 4.0–6.5 | Optimal | Insoluble |
| Manganese (Mn) | 5.0–7.0 | Possibly toxic | Insoluble |
| Zinc (Zn) | 5.0–7.0 | Possibly toxic | Strongly reduced |
| Boron (B) | 5.0–7.0 | Well available | Strongly reduced |
This explains why calcium deficiency symptoms appear so often in coco: coco has a lower natural pH buffer than soil, and watering at pH 6.8 (a soil habit) actively blocks calcium uptake in a substrate that needs more Ca, not less.
EC — what it measures and what it does not
EC (electrical conductivity) measures the ionic concentration of a solution in millisiemens per centimetre (mS/cm) or microsiemens (µS/cm). More dissolved salts = higher conductivity. An EC meter tells you how much is dissolved — not what.
This is the central distinction: an EC of 2.0 can consist of 1.8 EC of nutrient solution plus 0.2 EC of tap water — or 2.0 EC of pure tap-water lime with no nutrients. EC meters measure mass, not quality.
If your tap water has an EC of 0.4 and you target a total EC of 1.8, you fertilise to EC 1.4 — not 1.8. The tap water carries its own ions that the plant must already absorb.
Conductivity targets by phase
| Growth phase | EC target (total) | Note |
|---|---|---|
| Germination / cutting | 0.4–0.8 mS/cm | Tap water only or very weak solution |
| Vegetative (early) | 0.8–1.4 mS/cm | Nitrogen-weighted |
| Vegetative (strong) | 1.2–1.8 mS/cm | Full nutrient solution |
| Early flower | 1.4–2.0 mS/cm | Begin P/K increase |
| Flower | 1.6–2.2 mS/cm | Reduce N |
| Late flower | 0.4–1.0 mS/cm | Flush or strongly reduced |
Tap water vs. RO water — the honest analysis
Tap water in Germany has, depending on the region, an EC between 0.1 and 0.8 mS/cm and a pH between 6.5 and 8.5. Berlin tap water is typically pH 7.2–7.8 and EC 0.3–0.5. This must be measured and corrected before each use.
RO (reverse osmosis) water has EC ≈ 0 and pH ≈ 7.0 — a neutral base with no buffer capacity. That sounds ideal but has a downside: without calcium and magnesium in the water, a pH crash in the substrate can quickly run away because the buffering effect is missing. RO growers therefore usually add a calcium-magnesium supplement (EC ≈ 0.2–0.4) before adding nutrients.
| Water | pH | EC | Effort | Recommendation |
|---|---|---|---|---|
| Tap water (DE average) | 7.2–7.8 | 0.2–0.6 | Adjust pH | Good — when corrected |
| RO water + CaMg | 7.0 → adjust | 0.0 + 0.2–0.4 | Medium | Very good — full control |
| Rainwater | 5.5–6.5 | 0.0–0.1 | Check pH | Seasonally usable |
| Mineral water (bottled) | 7.0–8.0 | 0.3–1.0 | High + costly | Not recommended |
Meters — what actually works
An 8-euro pH meter delivers values with ±0.5 deviation. With a target range of 6.0–6.8, that means: you do not know whether you are at 5.5 or 7.3. For nutrient availability that is an extreme difference.
Minimum requirement: a device with ATC (automatic temperature compensation) and calibration with buffer solution (pH 4.0 and pH 7.0). Regular calibration — at least once a week during an active grow — is not optional.
| Device class | pH accuracy | Calibration | Recommendation |
|---|---|---|---|
| < 15 € pen meter | ±0.3–0.5 | None / 1-point | Not for precision |
| Apera PH20 / similar | ±0.1 | 2-point automatic | Solid entry class |
| Bluelab pH Pen | ±0.1 | 2-point | Recommended — robust |
| Milwaukee MW102 | ±0.02 | 3-point | Very good — pro |
Adjusting pH — the practice
pH-Up (potassium hydroxide / KOH) raises pH. pH-Down (phosphoric acid / H₃PO₄) lowers it. Both are dosed in drops — not millilitres. One drop of pH-Down per litre of water can lower pH by 0.5–1.0 units, depending on the starting buffer.
The correct mixing order:
- Bring water to room temperature (20–22 °C)
- Stir in nutrients — measure EC
- Measure pH
- Add pH corrector drop by drop and stir
- Wait 5 minutes — measure pH again (pH drifts after mixing)
- Measure EC final and document
pH drift in the substrate — the invisible problem
Even if you target pH 6.5 for every litre, the substrate pH changes over time. Nutrient uptake, root metabolic products and microbial activity in the soil shift pH continuously. In soil cultures typically toward acidic (pH falls); with weak nutrient solutions or overwatering it can also rise.
The Growix OS logs pH and EC for every irrigation event. Over several weeks the trend becomes visible — and you can counter-steer before deficiency symptoms appear.