A nutrient solution is the sum of two things: what you added and what was already in the water. Ignore the second part and every other measurement is off. That makes the water analysis the first actual step, not paperwork — and it is free: every utility publishes hardness, sodium, chloride and bicarbonate once a year.
Base EC and why you subtract it
Conductivity measures the total amount of dissolved salts. Distilled water sits near zero; tap water never does. Calcium, magnesium and bicarbonates from the catchment already give a measurable reading — and the meter cannot tell useful potassium from useless sodium. To the meter they are identical.
| Water source | Base EC | Note |
|---|---|---|
| Rainwater (clean roof, first flush discarded) | 5–50 µS/cm | Almost mineral-free — calcium and magnesium must be added |
| Reverse osmosis (RO) | 5–30 µS/cm | Clean start, but zero pH buffering |
| City water, soft region | 250–450 µS/cm | Ideal; usable with no treatment at all |
| Tap water in karst / limestone regions | 500–800 µS/cm | Usable, but pH keeps drifting upward |
| Well or private spring | 600–1500 µS/cm | Do not use without an analysis — nitrate and sodium |
| Softened water (ion exchanger) | 400–900 µS/cm | DO NOT use: calcium has been swapped for sodium |
The arithmetic is simple. Lettuce wants roughly 1.2 mS/cm. If your water already sits at 0.45 mS/cm, you only add 0.75 mS/cm worth of fertiliser, not the full 1.2. Miss this and you run the plant 30–40 % above target permanently — visible as brown, dry leaf margins. Target values per crop are in EC value.
Chlorine off-gasses, chloramine does not
Chlorine in tap water is typically 0.1–0.5 mg/l. That does not harm roots, but it does knock back the beneficial microbes. Free chlorine leaves on its own: 12–24 hours in an open container, or just 1–2 hours with an air stone.
Chloramine is a different animal. It is chlorine bonded to ammonia, used by utilities that need lasting disinfection across the network, and it does not evaporate — it stays for days. If your utility uses chloramine (the annual report says so), you need an activated carbon filter or a dose of ascorbic acid, roughly 1 g per 100 litres. Note that breaking it down releases ammonia, which then counts toward the nitrogen in your solution.
Hardness: friend and enemy in the same bottle
- Calcium and magnesium in hard water are nutrients — with hard water you need less Cal-Mag, sometimes none at all.
- Bicarbonate (HCO₃⁻) is the problem: above 150 mg/l the pH climbs back within a day of every correction.
- The sweet spot is 30–70 mg/l HCO₃⁻ — enough buffer to stop pH swings, too little to lock it in place.
- Reference: 1 °dH ≈ 17.8 mg/l CaCO₃. Water at 8–14 °dH is perfectly fine for hydroponics.
- Above 20 °dH it pays to blend with rain or RO water — otherwise you burn through pH Down by the litre.
If your pH climbs every single day despite corrections, the cause is almost never the plants — it is the carbonate load of your water. How to tell them apart is covered in pH value.
Sodium — the quiet problem in a closed loop
Sodium is the one parameter that genuinely deserves attention. The plant barely takes it up, while water keeps evaporating from the tank. In a closed system such as a vertical tower that means sodium accumulates over weeks while potassium and nitrogen are consumed.
- Up to 50 mg/l Na — no concern for lettuce, herbs and leafy greens.
- Above 50 mg/l — visible on sensitive crops (strawberry, bean, basil) as leaf-margin necrosis.
- Above 100 mg/l — sodium competes with potassium at the root and growth slows measurably.
- Chloride should stay below 50 mg/l for the same reason.
The most expensive beginner mistake: feeding the system from a water softener. An ion exchanger swaps calcium for sodium — you get exactly what the plant does not need and lose what it does.
When RO actually pays off
An RO unit costs 60–150 €, wastes 2–4 litres per litre of clean water and needs a new membrane every 2–3 years. For a 40 or 50 litre tank topped up every two to three weeks that is reasonable but not mandatory. Switch to RO if at least one of these applies:
- base EC above 0.8 mS/cm — there is barely any headroom left to your target;
- sodium above 60 mg/l or chloride above 70 mg/l;
- bicarbonate above 200 mg/l and you are buying pH Down in bulk;
- you grow salt-sensitive crops (strawberries, rocket, baby leaf).
Water temperature: 18–22 °C
Warm water holds less oxygen, and in hydroponics the root gets its oxygen exclusively from the water. That makes tank temperature every bit as important as EC.
| Water temperature | Dissolved oxygen | What happens |
|---|---|---|
| 14 °C | ≈ 10.3 mg/l | Plenty of oxygen, but growth slows and phosphate uptake drops |
| 18–22 °C | 8.7–9.5 mg/l | Optimal range for roots and microbiology |
| 24 °C | ≈ 8.4 mg/l | The threshold — Pythium (root rot) becomes active |
| 28 °C and above | < 7.8 mg/l | High risk: slimy brown roots and a sour smell |
- Keep the tank shaded — direct summer sun pushes 40 litres past 30 °C.
- More volume means more inertia: 50 litres heat up far more slowly than 40, a real advantage in summer.
- A frozen 0.5 l PET bottle dropped into the tank buys you 2–3 °C for a few hours — cheap emergency fix.
- Never top up with hot water from the boiler: it carries copper, nickel and scale.
- On a winter balcony, growth stalls below 12 °C — that is physiology, not a fault.
Topping up: the most common trap
Only water evaporates — the salts stay behind. Top up with full-strength nutrient solution and your EC creeps up week after week. Rule: top up with plain water and only add fertiliser once the EC drops below target. A full solution change belongs in the routine every 2–3 weeks; details in Tower maintenance.