First find out whether it's the meter or the solution. Put the probe in fresh pH 7 and pH 4 calibration solution. If it reads both correctly and settles, the probe is fine and your solution really is moving. If it drifts, won't settle or won't hold calibration, clean it, recalibrate with fresh solution and store it properly. Replace a probe that still won't calibrate.
Check the probe
- Use fresh calibration solution, not solution you've already used. Give the reading time to settle.
- If it reads the buffers off, or reads pH 4 low and pH 7 high, clean the probe and its connection, then calibrate again following the meter's instructions.
- If it still won't read the buffers correctly, sort that out (or replace the probe) before you adjust any feed from it.
- Store the probe in storage solution, not calibration solution.
Two meters disagree? Put both in the same well-mixed sample and let them settle. If one still reads differently, resolve that before you touch the tank. Don't just pick the number you usually see.
A probe that sits in a tank with a pH controller needs circulation running while the controller doses. Otherwise it reads a pocket near the dosing point instead of the batch. We've used Bluelab Guardians and Bluelab pH dosers, with American Marine Pinpoint replacement probes.
When the solution is moving
| What you see | What it usually means |
|---|---|
| Recirculating or DWC reservoir with healthy plants, pH slowly rising | Expected as plants take up nitrate. With a controller we hold about 6.0; by hand we start at 5.8 and let it rise to 6.2. |
| Reservoir with plants, pH falling | A warning sign. Check mixing, dosing and roots. |
| Storage or batch tank with no plants, pH rising | Not plant uptake. Look at air stones, splashing and pump heat driving CO₂ out, water alkalinity after a top-off, additives, or residue in reused equipment. |
| Fresh batch still moving | Not fully dissolved or mixed yet. Cake takes longer to dissolve than Veg or Bloom; circulate and measure again. |
| Cake reservoir dropping fast (one grower saw 6.6 to 3.9 in ten hours) | Cake shouldn't drop in pH. Check the probe, then any additions or controller activity. |
| Tank reads right, dripper reads different | Check the nutrient and pH dosing at delivery. |
If the change started when you added a silica product, and the same water and feed were stable without it, remove the silica and remix. We don't use silicates in our reservoirs; see silicates and stability.
The full list of reasons a tank climbs after adjustment, and how to stop it, is in why reservoir pH keeps rising.
Then set the feed
Deal with the cause first, then bring the fully mixed feed to the target for your system. Coco and peat feed about 6.2; the full list is in where, when and how to measure nutrient pH. Use RunClean or regular pH down, a little at a time, mixing each addition through before you measure again. Chasing an unfinished mix with repeated acid can swing it the other way.
Need help applying this?
Tell us what is happening in your garden so we can help with the problem you are trying to solve.
Talk to us