pH measures how acidic a solution is on a scale from 0 to 14. The scale is logarithmic: pH 5.0 is ten times more acidic than pH 6.0 and a hundred times more acidic than pH 7.0. So the gap between 6.0 and 6.5 is far bigger than it looks on paper — which is why you measure pH rather than guess it.
Lockout: why pH controls nutrient availability
In a nutrient solution every element exists as an ion. Whether it stays in a form the roots can absorb depends directly on acidity. Once pH climbs above 6.8, iron, manganese, zinc, boron and part of the phosphorus precipitate as insoluble hydroxides and phosphates. Physically they are still in the tank; chemically they are gone. That is nutrient lockout.
The first visible sign is almost always the same: the youngest leaves turn yellow between the veins while the veins stay green. That is iron deficiency — and in nine cases out of ten there is plenty of iron in the tank, the pH is simply too high. Adding more fertiliser does not fix it, it only pushes EC up.
A detail most guides skip: iron in fertiliser is bound to a chelator, and chelators have limits. EDTA keeps iron in solution only up to about pH 6.5, DTPA up to 7.5, EDDHA beyond 8. Cheap fertilisers using EDTA iron are therefore the first to show trouble when the water drifts alkaline.
The other direction is just as bad. Below pH 5.0 calcium and magnesium uptake collapses, and below 4.5 the acidic solution starts damaging the root membrane — roots go brown, slimy and die off. A brief dip to 4.8 is forgiven; three days at that level is not.
Target range by crop
| Crop | pH range | Note |
|---|---|---|
| Lettuce, lamb's lettuce | 5.6–6.2 | Most forgiving; the tower classic |
| Basil | 5.5–6.0 | Below 5.5 it shows calcium deficiency fast |
| Rocket, pak choi | 6.0–6.5 | Prefers slightly higher than lettuce |
| Spinach | 6.0–7.0 | The exception: tolerates alkaline water |
| Parsley, chives | 5.5–6.0 | Slow to germinate, keep pH steady |
| Strawberry | 5.5–6.0 | Above 6.5 chlorosis appears on new leaves |
| Tomato | 5.8–6.3 | Too low raises the risk of blossom end rot |
| Pepper, chilli | 5.5–6.2 | Reacts more to swings than to the absolute value |
| Cucumber | 5.5–6.0 | Heavy water use → fast drift |
| Mixed tower | 5.8–6.2 | The compromise everything lives with |
If several species share one tower — which in practice they always do — do not chase the ideal value of each individual plant. Hold 5.8–6.2 and every crop in the table performs well. Some growers deliberately let pH wander within 5.5–6.5 over the week, because different elements dissolve best at different points of the range. That only works as long as you never leave the range.
How to measure
| Method | Accuracy | Price | Catch |
|---|---|---|---|
| Paper test strips | ± 0.5 pH or worse | €2–5 | Too coarse for hydroponics — skip it |
| Drop test (bromothymol blue) | ± 0.2–0.3 pH | €5–10 | Colour reading, misleading under LED light |
| Digital pH pen | ± 0.1 pH | €15–40 | Needs calibration and storage solution |
| Inline probe | ± 0.05 pH | €80 and up | Overkill for a single tower |
For lettuce and herbs a drop test is honestly enough. A digital pen earns its place once you grow fruiting crops or your water is very soft. But a pen is not buy-and-forget: the electrode is a glass membrane that must stay wet. Store it in KCl solution — never in distilled water, which strips ions out of the membrane and has the pen lying by half a unit within days. Calibrate with 4.01 and 7.01 buffers every 3–4 weeks; details in calibrating a pH meter.
How to correct it
To lower pH you use pH-Down based on phosphoric acid (usually 30–85 %), to raise it pH-Up with potassium hydroxide or potassium carbonate. Both carry nutrients of their own: the acid adds phosphorus, the base adds potassium. With large doses your EC rises too — more on that in EC value.
- Dose in steps. Roughly 0.5 ml of 30 % phosphoric acid per 40 l of water lowers pH by about 0.2–0.4, depending on water hardness.
- Pre-dilute. Acid into a glass of water, then the glass into the tank. Never pour concentrate next to the pump or the roots.
- Run the pump 15–30 minutes before re-measuring. The solution does not mix in seconds, and measuring too early leads to overdosing.
- Fertilise first, adjust pH second. The fertiliser itself moves pH, often 0.3–0.8 downwards.
- Do not move pH more than 0.5 per day if you can avoid it. A sudden jump stresses roots more than a slightly wrong value.
Why pH keeps drifting
Drift is not a fault, it is the consequence of a living solution. Four main causes:
- Ion balance. When the plant takes up nitrate (NO₃⁻) it releases OH⁻ and pH rises. When it takes up ammonium (NH₄⁺) it releases H⁺ and pH falls. The nitrate-to-ammonium ratio in your fertiliser sets the direction.
- Daily cycle. Plants and algae consume CO₂ from the water during the day, so pH rises and falls again at night. An algae-filled tank can shift pH by 0.5 in a single day — see algae in the system.
- Buffer capacity. Soft water (rainwater, RO, carbonate hardness under 3 °dH) has no buffer and jumps on the smallest dose. Above 12 °dH it barely moves but scales up your pump. Details in water quality.
- Evaporation. On hot days a fully planted 40 l tower loses 2–4 litres daily. Topping up with plain water changes EC and pH at the same time.
How often to measure
| Situation | Measuring rhythm |
|---|---|
| First 3 days on fresh solution | Every day |
| Settled tower, mild temperatures | Every 2–3 days |
| Heat above 28 °C or large plants | Every day |
| After topping up water or fertiliser | Always, 20–30 minutes later |
| Full solution change | Every 2–3 weeks, then measure immediately |
If you are unsure which pH and EC target applies to your plant and its growth stage, enter it in the nutrient calculator — you get the dose for your tank size, the target EC and the pH range.