"I'll install RO — and all my water problems will disappear." RO does solve problems with hard or alkaline water — but it creates new ones if you do not know what to do with the empty water coming off the membrane. RO water without remineralisation is pH-unstable, has no buffering capacity, and contains no Ca or Mg that plants require. Installing RO without remineralisation is trading one problem for another — less obvious, but no less real.
Quick glossary: RO (Reverse Osmosis) — a water purification method using a semi-permeable membrane that retains 95–99% of dissolved salts, heavy metals, and organics; the output is nearly empty water with EC 0.01–0.05. EC — electrical conductivity, a measure of dissolved salt concentration. Remineralisation — adding Ca, Mg, and other essential minerals to RO water before use; restores mineral balance and stabilises pH.
When RO Is Actually Needed
RO is not a universal answer to "bad water." There are specific situations where it is justified — and situations where simpler solutions are sufficient.
RO is justified when: hardness GH exceeds 12–15°dH — recipe adjustments no longer help, background Ca and Mg are simply too high. Alkalinity KH exceeds 8–10°dH — the amount of acid needed for neutralisation becomes excessive and affects ionic balance. Sodium is present (Na > 50 mg/L) or heavy metals — these cannot be removed with acid or fertilisers, only with a membrane. Well water with an unknown or unstable composition — RO gives a predictable starting point.
RO is unnecessary when: moderate hardness GH 6–10°dH — adjusting the recipe or blending with some RO is sufficient. Alkalinity KH 3–6°dH — neutralised with acid without significantly affecting balance. Water that is already clean and stable by analysis — RO adds complexity without real benefit.
What Happens to RO Water Without Remineralisation
RO water off the membrane is nearly distilled. EC 0.01–0.05, GH ≈ 0, KH ≈ 0. Three problems at once:
Unstable pH. Without a buffer (KH), even a small addition of acid or nutrients changes pH sharply. Instead of gradual correction — spikes. A grower adds a microdose of acid and pH jumps from 7.0 to 4.5 — because there is nothing to hold it in place.
No Ca or Mg. Plants need 100–200 mg/L of Ca and 40–60 mg/L of Mg. In RO water — zero. The nutrient recipe must cover the full requirement — and if the recipe was designed for moderately hard water, it will not provide enough Ca and Mg for empty water.
Aggressive water. Empty water with low EC and zero GH is corrosively active — it leaches minerals from pipes, fittings, and equipment surfaces. Well known in aquaristics. In hydroponics — an additional source of solution contamination.
How to Remineralise Correctly
Remineralisation means restoring mineral balance to a level where the system is stable and the nutrient recipe performs predictably. Two approaches:
Dedicated remineralisation concentrates — ready-made blends of Ca, Mg, and trace elements in correct proportions, formulated specifically for RO water. Simple approach: add the prescribed dose and obtain water with GH 4–6°dH and KH 2–3°dH — a stable base for mixing nutrient solution. Downside: dependence on a specific product and its composition.
Blending RO with source water — 30–50% source water + 50–70% RO gives water with manageable hardness and alkalinity. A simpler and cheaper approach if the source water contains no sodium or heavy metals. With source water at GH 12°dH and 50% blending, you get GH 6°dH — already in the working range.
Targeted remineralisation with Ca and Mg fertilisers — for those calculating the recipe from scratch: CaCl₂ or Ca(NO₃)₂ for calcium, MgSO₄ for magnesium, a small dose of KHCO₃ or Ca(HCO₃)₂ for buffering. Maximum control, but requires understanding of chemistry and precise calculations.
Three Mistakes That Cost the Most
Installing RO and continuing to use the old recipe without adjustments. A recipe that worked well on hard water with GH 10°dH is no longer appropriate for RO water where GH ≈ 0. It will not supply enough Ca and Mg, and pH will be unstable. Changing the water source means reviewing the recipe.
Switching entirely to RO when partial blending would be sufficient. Full RO means more expensive equipment, more waste (RO produces 3–4 litres of drain per 1 litre of clean water), and more complex remineralisation. With GH 8–10°dH and no sodium, 50% blending solves the problem at half the cost.
Not checking the membrane regularly. An RO membrane degrades over time and with dirty water — salt passage gradually increases. The EC of the "clean" output water begins to rise, and the grower cannot understand why the recipe stopped delivering predictable results. Checking the EC after the membrane once a month takes two minutes.
How to Know RO and Remineralisation Are Set Up Correctly
After the membrane: EC 0.01–0.05. After remineralisation: GH 4–6°dH, KH 2–3°dH, EC 0.2–0.4. After mixing the recipe: pH is stable in the 5.8–6.2 range and does not drift more than 0.15 over 12 hours without plants. If pH is still unstable after remineralisation, KH is insufficient and the buffer is too weak — increase the dose of remineralisation concentrate or add a small amount of KHCO₃.
For deeper understanding: Water Preparation: From Analysis to Working Solution — how RO and remineralisation fit into a complete water preparation system for different situations.