Cannabis pH Levels
In soil, cannabis takes up nutrients best when the root zone pH sits between 6.0 and 7.0. In coco coir and water culture, the target is 5.5 to 6.5. Values outside these windows block nutrient uptake even when the feed is correct. A plant can show deficiency symptoms while sitting in a full-strength nutrient solution if pH is off by half a unit.
Cannabis PH Problems: Causes, Symptoms, and Solutions
What pH Measures
pH is the negative log of hydrogen ion activity in solution. Each whole unit change means a tenfold change in hydrogen ion concentration. A drop from 6.0 to 5.0 is a 10x increase in acidity. That is why a reading of 5.4 compared to 5.9 changes what the roots can absorb.
Target Ranges by Medium
- Mineral soil: 6.0 to 7.0. Most garden soils buffer near 6.5.
- Peat-based soilless mix: 5.8 to 6.3.
- Coco coir: 5.8 to 6.2. Coco holds potassium and sodium, so runoff pH shifts.
- Hydroponics, drain to waste: 5.5 to 6.5.
- Hydroponics, recirculating: 5.8 to 6.0. The narrow range limits drift between reservoir changes.
Nutrient Availability
Iron, manganese, zinc, copper, and boron lose solubility as pH rises above 6.5. Iron chlorosis, yellow tissue between green veins on new leaves, shows up first. Phosphorus, calcium, and magnesium lose solubility as pH drops below 5.5. In mineral soil below 5.0, aluminum and manganese become soluble at levels that damage roots.
Testing the Root Zone
- Slurry test for soil: mix one part soil with two parts distilled water by volume.
- Stir the sample and let it stand 15 to 30 minutes.
- Measure the liquid with a calibrated meter.
- Runoff test: pour water through the pot and catch the last 10 to 20 percent that drains.
- Hydroponics: measure the reservoir, then measure again 24 hours later to see the drift.
- Calibrate meters with pH 4.0 and 7.0 buffer solutions before each session.
Adjusting pH
Phosphoric acid and citric acid lower pH. Potassium hydroxide and potassium carbonate raise it. For soil, dolomite lime raises pH over weeks, and elemental sulfur lowers it. Reservoir pH drifts up as plants take up nitrate and other anions faster than cations. A reservoir that climbs 0.3 units per day is normal in flower. Skip adjustment if the meter reads 6.1 in soil or 5.9 in hydro.
Cannabis PH Water: The Ultimate Guide
Key Numbers
Legal cannabis sold in US dispensaries carries a lab report with potency and contaminant data, but no pH value. Root zone pH is a grower measurement. A $40 to $120 meter plus two buffer solutions covers most home setups. Cheap meters drift 0.3 to 0.8 units within weeks, which is enough to mislead a feeding schedule.