Tap Water EC: When You Need a Reverse Osmosis System
Many growers measure their tap water and see 0.6, 0.8, sometimes over 1.0 mS/cm – and then wonder about pH drift, burnt tips and numbers that don't add up. Her…
One question keeps coming up under my videos: "I do everything the way you do, and my plant still has problems." When I then ask for the EC of their tap water, the answer is often 0.7. 0.9. Sometimes over 1.0. And that alone explains a large share of the problems before we even talk about nutrients, light or climate. This one number sits at the start of every nutrient solution – and if it's wrong, nothing after it is quite right either. 🌱
What the number actually means
EC stands for electrical conductivity. Pure water conducts practically no current. Everything dissolved in it – salts, minerals, fertiliser – makes it conduct. The more is dissolved, the higher the number. It's measured in millisiemens per centimetre, and because every manufacturer prefers a different scale, here's the conversion once and for all: 0.5 mS/cm = 500 µS/cm = 250 ppm (500 scale) = 350 ppm (700 scale). If someone gives you a ppm value, always ask which scale. Otherwise you're talking past each other.
For reference: rainwater sits at around 0.02 to 0.05. RO water the same. Tap water in Germany ranges from 0.2 to 1.2 depending on the region – coastal and upland areas are often soft, limestone regions like large parts of Bavaria, Lower Saxony or the Münsterland are properly hard. Much of the UK, the Netherlands and the US Midwest is in the same hard-water boat.
To know your number, you need an EC pen. Look for automatic temperature compensation – EC rises with temperature, and without compensation you'll read too low in cold winter water. My tip, good and affordable: the Apera EC20 (affiliate link, German Amazon store), which comes as a kit in a case. It measures your tap water, later your RO water – and whether the membrane is still good.
And now the point that decides everything: EC tells you how much is dissolved – but not what. Your fertiliser is dissolved salt, and your tap water is also dissolved salt. The meter can't tell them apart. Tap water mostly contains calcium and magnesium (that's the limescale), hydrogen carbonate (the carbonate hardness), sodium, chloride, sulphate and a bit of nitrate. Of those, the plant can use calcium and magnesium – in the wrong ratio and in an unknown amount. The rest is ballast.
Why 0.8 from the tap isn't simply "0.8 of fertiliser saved"
This is the most common mistake in thinking: "My water is at 0.8, so I just need less fertiliser." No. There are three separate problems, and each one on its own is enough.
1. You're feeding blind
Say you want to feed at 1.6. Your water is at 0.8. Now you have two options, and both are wrong. Either you add nutrients until the meter shows 1.6 – then you have 0.8 of fertiliser in there and 0.8 of unknown stuff. The plant gets half of what you think it's getting. It starves at a perfect reading. Or you add the full dose as printed on the bottle – then you're at 2.4, the tips burn, the plant drinks less, and everyone tells you "overfeeding" even though you followed the chart to the letter.
The number you're steering by lies in both cases. What you can at least do: EC after feeding minus EC before = what you actually added. It's not perfectly exact, but it's the difference between blind and half-blind.
2. Carbonate pulls the pH up – every single day
Hard water carries a lot of hydrogen carbonate, and that's a buffer. A buffer resists any change. You set your tank to pH 5.8, next morning it reads 6.5. So you add pH down. The acid neutralises the hydrogen carbonate – and in doing so the acid turns back into salt, which stays in the tank and pushes the EC up. You're fighting your own water and paying for it in EC.
In the substrate it carries on. Every shot brings in fresh carbonate, the pH in the root zone drifts upwards, and above pH 6.5 iron, manganese and phosphorus become poorly available. The young leaves turn yellow, everyone shouts "iron deficiency", you add more fertiliser – and the EC climbs further. The deficiency was never a deficiency, it was the pH. And because calcium precipitates with phosphate at high pH, your drippers clog up on top of it. Anyone running drip irrigation on hard water knows the white crusts.
As a rule of thumb: from about 100 mg/l hydrogen carbonate in your water report, pH drift is no longer coincidence but routine.
3. Ballast builds up in the substrate
The plant takes what it needs. Sodium, chloride, excess sulphate and calcium are left behind. In soil, the medium buffers that away for a while. In coco and rockwool it doesn't – everything sits openly in the pore solution. And that's exactly where crop steering gets uncomfortable: the dryback concentrates the pore solution, every shot brings in new ballast, the runoff flushes part of it out, but with 0.8 from the tap the EC in the block climbs faster than you can steer it.
My controller measures pore EC directly in the block. It sees salt – it doesn't see whether that salt is fertiliser or tap water. Anyone steering on hard water is steering by a readi …
Tap Water EC: When You Need a Reverse Osmosis System