# Reservoir pH falling fast without explanation: likely causes and fixes

> Reservoir pH falling fast without explanation on cannabis is most often Low root-zone pH starving calcium and magnesium, Ammonium toxicity (too much NH4 in the feed) or Root rot (Pythium). How to tell them apart, the fix for each and what recovery looks like.

Also called: pH dropping, pH crashing, pH keeps going down, runoff pH low.

Run the checker with this sign: https://genetixbay.com/knowledge/diagnose/?at=roots&s=ph-falling

## Likely causes, most likely first

### Low root-zone pH starving calcium and magnesium (Possible)

- Kind: pH; urgency: Deal with it this week
- Fix: Raise root-zone pH to range: coco 5.8-6.2, hydro 5.6-6.0, soil 6.2-6.8. Use pH up (potassium hydroxide or carbonate) in small steps; in coco, feed at 6.0-6.2 for a few waterings with Ca and Mg on board. A fast fall in hydro points to dying roots or microbes; check them.
- Recovery: pH holds in range for a week; new leaves spot-free within 7-10 days.
- Confirm: Measure input pH and root-zone (runoff or reservoir) pH: Calibrate the meter with fresh pH 7 and pH 4 buffers, then measure the mixed feed and the runoff (containers) or reservoir (hydro). Soil and living soil should sit at 6.0-7.0, coco, rockwool and peat mixes at 5.5-6.5, hydro at 5.5-6.2. Track 3+ readings; one reading is not a trend.
- Confirm: Check the meters against fresh standards: Put the pH probe in fresh pH 7.0 and 4.0 buffer and the EC probe in a 1.413 mS/cm standard. If pH reads more than 0.1-0.2 off, recalibrate; if it can't be calibrated or the slope is below ~90-95%, replace the probe. Many 'lockouts' are a drifting meter.
- Confirm: Pour-through extraction for container pH and EC: Water normally, wait 30 minutes to 2 hours, pour enough pH-adjusted water on top to collect about 50 mL of leachate (roughly 75 mL for a 15 cm pot, 150 mL for a 4 L pot, 350 mL for 11 L), and measure pH and EC of that 50 mL. Less disruptive than full runoff and more repeatable.
- Article: [pH and nutrient availability: what pH really controls, by medium](https://genetixbay.com/knowledge/ph-ec/ph-nutrient-availability.md)

### Ammonium toxicity (too much NH4 in the feed) (Less likely)

- Kind: Too much of something; urgency: Deal with it this week
- Fix: Keep ammonium at or below about 10-30% of total N (cannabis trials showed toxicity and lost yield above ~30%); switch to a mostly nitrate feed and correct root-zone pH.
- Recovery: pH stops falling and new leaves lose the dark claw within a week.
- Confirm: Measure input pH and root-zone (runoff or reservoir) pH: Calibrate the meter with fresh pH 7 and pH 4 buffers, then measure the mixed feed and the runoff (containers) or reservoir (hydro). Soil and living soil should sit at 6.0-7.0, coco, rockwool and peat mixes at 5.5-6.5, hydro at 5.5-6.2. Track 3+ readings; one reading is not a trend.
- Confirm: Look at the roots: Hydro: lift the net pot and look at the root mass in good white light. Containers: tip the plant out with a hand over the medium and look at the edges and bottom of the root ball. Healthy roots are white to cream with fine fuzzy tips. Brown roots whose outer layer slides off between the fingers leave a bare thread: rot. Firm, evenly tan roots with white tips in coloured solution are staining.
- Article: [Nitrogen (N) in cannabis: nitrate vs ammonium, deficiency, toxicity and targets](https://genetixbay.com/knowledge/nutrients/nitrogen.md)

### Root rot (Pythium) (Less likely)

- Kind: Disease; urgency: Act now
- Fix: Act now. Fix conditions first: solution at 18-21 °C (65-70 °F), strong aeration (6-8+ mg/L DO), no overwatering. Change the reservoir, trim dead roots with clean scissors, clean and disinfect the system. Then either re-establish beneficial microbes (Bacillus, Trichoderma; house-preferred) or commit fully to a sterile regime (e.g. dosed hypochlorous acid or peroxide). Isolate affected plants on recirculating systems.
- Recovery: New white root tips within 3-7 days and wilting stops; brown roots don't recover.
- Confirm: Look at the roots: Hydro: lift the net pot and look at the root mass in good white light. Containers: tip the plant out with a hand over the medium and look at the edges and bottom of the root ball. Healthy roots are white to cream with fine fuzzy tips. Brown roots whose outer layer slides off between the fingers leave a bare thread: rot. Firm, evenly tan roots with white tips in coloured solution are staining.
- Confirm: Smell test of roots, medium and reservoir: Smell the roots, the bottom of the root ball, the drain water or the reservoir up close. Healthy roots smell earthy or of nothing. A sour, rotten, swampy or sewage smell means anaerobic conditions and rot. In hydro, a slimy feel on the reservoir walls or tubing adds to the case.
- Confirm: Measure reservoir and root-zone temperature at the end of the light period: Measure the solution in the reservoir and in the root chamber or bucket, at the warmest time of day. Target 18-22 °C (64-72 °F). Above about 23-24 °C (73-75 °F), dissolved oxygen falls and Pythium risk rises; below 16 °C (61 °F) uptake slows.
- Confirm: Dissolved oxygen check: Measure DO with a meter in the root zone, or check the system: air pump running, stones bubbling evenly, return water splashing, no dead spots. Target 6-8+ mg/L. Without a meter: if stones are clogged or the pump is undersized for the volume, assume low DO.
- Confirm: Send a sample to a plant diagnostic lab: For rots, wilts, leaf spots and viruses you can't separate by eye, send a fresh sample (whole plant with roots or a wilting branch with healthy and diseased tissue) in a sealed bag to a university or commercial plant diagnostic clinic. Results in days to 2 weeks.
- Article: [Root rot and Pythium in cannabis](https://genetixbay.com/knowledge/diseases/root-rot-pythium.md)

### Uncalibrated or failing pH or EC meter (Less likely)

- Kind: Physical damage; urgency: Deal with it this week
- Fix: Calibrate the pH meter with fresh pH 7 then pH 4 buffer before decisions and at least weekly; store the probe wet in storage solution, never in RO water. Check EC against 1.413 mS/cm. Replace probes that won't calibrate (typical life 1-2 years). Drops and strips are too coarse to diagnose with.
- Recovery: Readings match the buffers within 0.1 pH; the 'problem' pH disappears or is confirmed.
- Confirm: Check the meters against fresh standards: Put the pH probe in fresh pH 7.0 and 4.0 buffer and the EC probe in a 1.413 mS/cm standard. If pH reads more than 0.1-0.2 off, recalibrate; if it can't be calibrated or the slope is below ~90-95%, replace the probe. Many 'lockouts' are a drifting meter.
- Article: [pH and EC meter calibration: buffers, slope, storage and when to replace a probe](https://genetixbay.com/knowledge/ph-ec/meter-calibration.md)

## Note

Guidance, not a guarantee. The checker weighs common causes against what you tell it; confirm with the tests before you treat, change one thing at a time and watch the new growth.