"pH 6.0 — corrected, all good." But an hour later it is 6.8 again, the plant is showing chlorosis, and you cannot understand what is happening with a "correct" recipe. pH is not a number you maintain for the sake of order. It is the parameter that determines what chemical form nutrients exist in within the solution — and whether the root can absorb them at all.
Quick glossary: pH — the acidity scale from 0 to 14; the working range in hydroponics is 5.5–6.5. Nutrient availability — the root's ability to absorb a specific nutrient element; depends on solution pH, because at different pH levels the same element exists in different chemical forms — some the root absorbs easily, others it can barely take up.
Why the Same Recipe Gives Different Results at Different pH
Iron in solution at pH 6.5 and at pH 7.0 are two different states. At 6.5, iron is dissolved and available to the root. At 7.0, it converts to insoluble forms — precipitating or forming compounds the root cannot absorb. You see chlorosis on young leaves and think "iron deficiency" — even though there is plenty of iron in the solution. The problem is not the recipe; it is pH.
The same applies to manganese, zinc, and boron: above pH 6.5 their availability drops sharply. Calcium and magnesium work the other way — at very low pH (below 5.5) they become excessive and block the uptake of other elements. Phosphorus has a narrow availability window — 6.0–7.0 — and when pH drifts in either direction the plant is deficient in it even with correct EC.
The Working Range and Why It Is What It Is
The 5.5–6.5 range is a compromise at which most macro- and micronutrients are simultaneously available to the root. There is no perfect pH at which everything is ideal — there is a range at which nothing is critically blocked.
Some growers deliberately drift pH within the range — for example between 5.8 and 6.3 over the course of a week. The logic: different elements have availability peaks at different points within the range, and a small drift gives the plant access to a broader spectrum. This makes sense when the system is stable and the drift is controlled, not chaotic.
Why pH "Won't Hold" and What to Do About It
pH drifts upward for two reasons — and they are different problems with different solutions.
Reason one — source water alkalinity (KH). Carbonates in the water physically buffer the solution toward alkalinity. No matter how much acid you add, the carbonate buffer pulls pH back up. The solution is to neutralise the alkalinity — not to keep correcting pH with acid indefinitely.
Reason two — the plant changes pH itself. Roots absorb ions unevenly and in the process release H⁺ or OH⁻, shifting the solution pH. During active nitrate (NO₃⁻) uptake, pH rises. During ammonium (NH₄⁺) uptake, it falls. This is a normal process, and a small pH drift over the course of a day is not a problem — it is a sign the plant is actively feeding.
If pH steadily climbs several points per day — the problem is water alkalinity. If it fluctuates within 0.3–0.5 — the plant is working normally.
Three Mistakes That Cost the Most
Correcting pH with acid without checking water alkalinity. If KH is high, acid neutralises only the current pH — not the buffer. Within a few hours pH returns. The grower adds more acid, ionic balance shifts, the system becomes unstable — and pH still "won't hold."
Seeing chlorosis and immediately adding micronutrients. Chlorosis on young leaves with a correct recipe almost always means pH above 6.5 — not an actual iron deficiency. Check pH first, then decide whether additional elements are needed.
Calibrating the pH meter infrequently or ignoring electrode drift. A pH meter reading "6.0" may actually be reading 6.8 if it has not been calibrated for a week. All decisions are based on measurements — if measurements are inaccurate, decisions will be wrong.
How to Know pH Is Under Control
After mixing the solution, pH settles in the 5.8–6.3 range and does not shift more than 0.3 over 12 hours without plants. With plants, fluctuations of up to 0.5 per day are considered normal as long as they stay within the working range. No chlorosis on young leaves, micronutrients are in the recipe — meaning they are available.
For deeper understanding: Water Alkalinity: Why pH "Bounces Back" and What to Do About It — explains the main cause of unstable pH and how to eliminate it permanently, rather than correcting it every day.