Reservoir pH falling fast without explanation

Likely causes, most likely first

Ranked for this sign on its own. Your stage, medium and readings change the order, so run the checker for your plant.

1. PossibleLow root-zone pH starving calcium and magnesium

   pHDeal with it this weekRead: pH and nutrient availability: what pH really controls, by medium

   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.

   How to confirm it

   • 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.
   • 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.
   • 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.

2. Less likelyAmmonium toxicity (too much NH4 in the feed)

   Too much of somethingDeal with it this weekRead: Nitrogen (N) in cannabis: nitrate vs ammonium, deficiency, toxicity and targets

   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.

   How to confirm it

   • 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.
   • 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.

3. Less likelyRoot rot (Pythium)

   DiseaseAct nowRead: Root rot and Pythium in cannabis

   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.

   How to confirm it

   • 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.
   • 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.
   • 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.
   • 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.
   • 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.

4. Less likelyUncalibrated or failing pH or EC meter

   Physical damageDeal with it this weekRead: pH and EC meter calibration: buffers, slope, storage and when to replace a probe

   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.

   How to confirm it

   • 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.

Look for

pH log over a week and a root photo..