EC stands for electrical conductivity. Distilled water barely conducts electricity; what conducts are the dissolved ions — potassium, calcium, nitrate, sulphate, magnesium. The meter passes a weak current between two electrodes and calculates from the resistance how many ions are in the water. More salt, more conductivity. That is the entire measurement.
What EC tells you — and what it does not
EC is a sum, not a list. A solution reading 1.4 can be a perfectly balanced nutrient mix — or tap water full of sodium and bicarbonate with not a gram of nitrogen in it. The meter shows the same number. So treat EC as a measure of strength, never as a deficiency diagnosis.
- EC tells you: how strong the solution is, whether water has evaporated, whether you overdosed, whether uptake has slowed down.
- EC does not tell you: whether iron is missing, how much nitrogen is left, what the calcium-to-potassium ratio looks like, or what the pH is.
Units: µS/cm, mS/cm and the ppm confusion
The base unit is siemens per centimetre. Two magnitudes are in circulation: 1 mS/cm = 1000 µS/cm. When someone says „EC 1.4“ they mean 1.4 mS/cm, i.e. 1400 µS/cm. Trouble starts with cheap TDS meters that display ppm — ppm is not a measurement but a conversion, and three different conversion factors are in use.
| EC (mS/cm) | µS/cm | ppm scale 500 | ppm scale 640 | ppm scale 700 |
|---|---|---|---|---|
| 0.8 | 800 | 400 | 512 | 560 |
| 1.2 | 1200 | 600 | 768 | 840 |
| 1.6 | 1600 | 800 | 1024 | 1120 |
| 2.0 | 2000 | 1000 | 1280 | 1400 |
| 2.4 | 2400 | 1200 | 1536 | 1680 |
The second trap is temperature. Conductivity rises by roughly 2 % per degree Celsius. Meters marked ATC correct back to the 25 °C reference; without ATC the same solution read at 15 °C shows about 18 % too low. Measure in the tank, not in a cup that has been sitting in the sun.
Target values by crop and phase
| Crop | Seedling | Vegetative growth | Fruit / harvest |
|---|---|---|---|
| Lettuce, lamb's lettuce | 0.4–0.8 | 0.8–1.2 | 1.2–1.4 |
| Rocket, spinach, chard | 0.6–0.8 | 1.2–1.6 | 1.6–1.8 |
| Basil, parsley, coriander | 0.6–0.8 | 1.0–1.6 | 1.4–1.8 |
| Pak choi, kale | 0.6–0.8 | 1.2–1.8 | 1.8–2.0 |
| Strawberry | 0.8–1.0 | 1.2–1.6 | 1.6–2.0 |
| Tomato | 0.8–1.2 | 1.8–2.2 | 2.2–2.6 |
| Pepper, chilli | 0.8–1.2 | 1.6–2.0 | 2.0–2.5 |
| Cucumber | 0.8–1.2 | 1.5–1.8 | 1.8–2.2 |
In a vertical tower every plant shares one tank, so the rule is: plan for the most sensitive crop. Lettuce at 1.0 and tomato at 2.4 do not belong in the same reservoir — the lettuce will show brown leaf margins long before the tomato gets going. If you do mix, stay at 1.4–1.6 and accept that nobody is in their optimum.
In heat above 28 °C, drop the target EC by 0.2–0.4. The plant drinks a lot of water while the salts stay behind, so the concentration around the roots is genuinely higher than the meter in the tank suggests.
Count your tap water into the number
Measure the bare water first, before adding anything. Across most of Europe tap water sits at 0.3–0.8 mS/cm, higher in limestone regions; rainwater and RO water are below 0.05. If your water reads 0.5 and the lettuce target is 1.2, the fertiliser may only add 0.7 — meaning you dose less than the label says, because the label assumes clean water.
- Measure your tap water EC and write it down — it shifts with the season, so recheck every two or three months.
- Target EC = water EC + fertiliser contribution. Never add the full dose on top of hard water.
- Above 0.8 mS/cm consider a 50:50 mix with rainwater; above 1.0 hydroponics gets stubborn, because half your headroom is taken by calcium and sodium the plant does not need.
- Sodium is never consumed. It stays in the tank and makes up a growing share of the measured EC with every top-up — another reason for a full solution change.
Hardness, chlorine and bicarbonate are covered in Water quality.
Why EC in the tank climbs on its own
This is the most common question and the most common mistake. The plant drinks water faster than it takes up salts, and some water simply evaporates. The salt load stays the same in less water — so the concentration rises. A rising EC means top up water, not add fertiliser.
Concretely: a 40 L tank at EC 1.2 loses 10 L over a hot week. What remains is 30 L holding the same salt, and the EC jumps to about 1.6. The same loss in a 50 L tank gives 1.5, because 10 L is a smaller share of the volume. A bigger tank does not just mean fewer refills — it keeps the solution calmer.
- Top up with plain water to the mark, stir, and measure only after 15–30 minutes.
- If the EC is still too high after topping up, remove 20–30 % of the solution and replace it with water.
- If EC drops fast (cloudy, cool, rapid growth) the plants are eating the salts — feed back in steps of 0.1–0.2, never in one go.
- Change the whole solution every 2–3 weeks regardless of how healthy the EC looks.
You can work out the target EC, the exact dose and the pH for 67 plants in our nutrient calculator: pick the plant, the growth phase, the fertiliser line and the tank size. Always read EC together with pH — a high EC at the wrong pH still means the plant cannot take up what you added.