You add acid — pH drops to 6.0. An hour later it is back at 7.2. You add more acid — it drops again, then climbs back. It feels like the pH meter is lying or there is not enough acid. The real problem is different: the water has alkalinity — a carbonate buffer that physically pulls pH back into the alkaline zone no matter how much acid you add. Until you neutralise the alkalinity, fighting pH with acid is a battle without end.
Quick Glossary
- KH (Karbonathärte, carbonate hardness) — a measure of water alkalinity; the concentration of carbonates and bicarbonates; expressed in degrees of hardness (°dH) or mg/L CaCO₃
- pH — solution acidity; the working range in hydroponics is 5.5–6.5
- Carbonates (HCO₃⁻, CO₃²⁻) — ions that form a buffer system in water and neutralise the acid added for pH correction
- Buffer — a chemical system that resists pH change; the higher the KH, the stronger the buffer and the more acid required to shift pH
How the Carbonate Buffer Holds pH
When you add acid to water with high KH, a reaction occurs: the acid reacts with bicarbonates (HCO₃⁻) and is neutralised. pH temporarily drops — but as soon as the acid is consumed neutralising the bicarbonates, the buffer pulls pH back up.
This is not a faulty instrument or a calculation error. It is chemistry: as long as bicarbonates remain in the water, they will neutralise acid and push pH upward. The only way to stop it is to consume all the bicarbonates. That requires a specific amount of acid determined by the KH of the source water.
In practice this means: at KH 6°dH you first need to "acidify" the water down to pH 4.5–5.0 to fully neutralise the carbonate buffer, and only then will pH stabilise and allow precise correction to 5.8–6.0. If you correct straight to 6.0 without fully neutralising, some bicarbonates remain and will push pH back up within an hour or two.
How to Measure KH and What to Do With It
KH is measured with a drop-count test kit — available at aquarium shops, inexpensive, and takes two minutes. This is a mandatory measurement before mixing the first working solution with any new water source.
Decision guidelines:
KH up to 3°dH — low alkalinity; pH corrects easily and holds stable. The water is suitable for hydroponics with minimal preparation.
KH 3–6°dH — moderate alkalinity. pH will drift if not fully neutralised. Complete acid neutralisation is required before adding the working fertiliser solution.
KH above 6°dH — high alkalinity. The amount of acid needed for neutralisation becomes significant and affects the ionic balance of the solution. Above KH 8–10°dH, consider partial or full dilution with reverse osmosis water.
Which Acid and How Much
Food-grade acids that also serve as pH correctors are used for alkalinity neutralisation: phosphoric acid (adds phosphorus to the solution), nitric acid (adds nitrogen), or citric acid (adds no macronutrients but breaks down quickly).
The exact amount of acid depends on KH and water volume — calculated or determined experimentally. Practical approach: add acid in portions, stir, and measure pH after each addition. When pH stops "bouncing back" after each addition and stabilises below 5.5 — bicarbonates are neutralised. You can now accurately adjust to the working pH.
Three Mistakes That Cost the Most
Correcting pH to 6.0 without fully neutralising alkalinity. The solution looks right, but within an hour or two pH will return. You correct again. pH drifts all day — the plant is constantly moving in and out of the optimal zone. The correct approach: neutralise the buffer first, then correct to working pH.
Not measuring KH when the water source changes. Tap water composition varies seasonally — suppliers may draw from different sources in summer and winter. KH can double between seasons, and your usual amount of acid suddenly becomes insufficient.
Adding a large amount of acid at once to "make sure." Excess acid drives pH below 5.0 — and then you need to raise it with alkali. Every such back-and-forth adds unwanted ions and destabilises the solution. Add in portions and measure after each addition.
How to Know Alkalinity Is Under Control
- After pH correction with full alkalinity neutralisation: pH holds steadily in the 5.8–6.2 range for 12 hours without plants, deviating no more than 0.2 units
- If pH drifts upward within a few hours — alkalinity was not fully neutralised; repeat the procedure or recalculate the acid dose