Short answer. pH sets how soluble each nutrient is and how easily roots absorb it. The accepted ranges are soil 6.0–7.0 and soilless/hydro 5.5–6.5; within them, aim for about 5.5–6.2 in hydro, 5.8–6.3 in coco and rockwool, and 6.2–6.8 in soil. Drift within the range is normal and healthy. High pH starves plants of Fe, Mn, Zn, B and P; low pH makes Fe and Mn overly available and cuts Ca, Mg and Mo [1][2][6]. The classic availability bar chart was drawn for mineral soils and overstates the effect in chelated soilless feeds. These ranges are accepted practice, from extension and grower consensus.
What's happening
pH is the hydrogen-ion activity of the solution around the roots. It changes nutrient chemistry in three ways:
- Solubility and precipitation. At high pH, iron, manganese, zinc and copper form insoluble hydroxides and phosphates; calcium phosphate precipitates. At low pH, aluminium and manganese dissolve (in soils) and molybdate binds to iron oxides [1][6][7].
- Chemical form. Phosphate shifts from H₂PO₄⁻ (preferred) to HPO₄²⁻ above about pH 7.2. Chelates hold metals only up to a ceiling: Fe-EDTA to about 6.5, Fe-DTPA to about 7.5, Fe-EDDHA to about 9 [4].
- Root function. Very low pH damages root membranes and reduces Ca and Mg uptake; very high pH reduces Fe uptake directly, especially with bicarbonate present [1].
Why soil and soilless differ. Mineral soils have clays and aluminium chemistry that make pH 6.0–7.0 best. Soilless media (peat, coco, rockwool, hydro) lack them, and micronutrients are supplied chelated, so the best range sits lower: extension sources give 5.4–6.2 for typical soilless crops [3], with iron-efficient species tolerating 6.0–6.6 [1].
The availability chart caveat. The familiar chart with tapering bars was built for mineral soils. In soilless systems with chelated micros, Fe, Mn, Zn and Cu stay available further up the scale than the bars suggest [4]. Use it as a direction, not a law.
High pH vs low pH
| Root-zone pH | Likely problems | Who's affected first |
|---|---|---|
| Above ~6.5 (soilless) / ~7.3 (soil) | Fe, Mn, Zn, B, Cu shortage; P precipitation with Ca | New growth: interveinal chlorosis [1][2] |
| 5.5–6.3 (soilless) | Broad availability | Target |
| Below ~5.3 (soilless) / ~5.8 (soil) | Fe and Mn toxicity; Ca, Mg, Mo shortage; root damage | Speckling, old-leaf margin necrosis; distorted new growth [1][2][6] |
Numbers that matter
| System | Target pH | Measure where | Notes |
|---|---|---|---|
| DWC, RDWC, NFT, aero | 5.5–6.2 | Reservoir | Recommendation cited for cannabis hydro 5.5–6.0 [5]; let it drift within the band |
| Coco, rockwool, drip | 5.8–6.3 (feed), runoff 5.8–6.5 | Feed and runoff | Soilless recommendation 5.5–6.5 [5] |
| Peat-based mixes | 5.8–6.4 | Pour-through | Extension general group 5.8–6.4 [1] |
| Soil and living soil | 6.0–7.0 (sweet spot 6.2–6.8) | Soil slurry or pour-through | Accepted practice; don't pH-adjust living soil inputs aggressively |
| Irrigation water | 5.0–7.0 before nutrients | Source | Water pH matters less than alkalinity [1] |
What to check first
- Calibrate the meter (see meter calibration). A drifting probe causes more "pH problems" than chemistry does.
- Measure the root zone, not only the input: reservoir, runoff, or pour-through.
- Know your water alkalinity. Alkaline water pushes pH up over days and weeks regardless of the input pH [1] (see water quality).
- Look at N form. Nitrate feeds push pH up; ammonium pushes it down (see pH drift).
How to fix it
- Hydro: adjust with acid (phosphoric, nitric or sulfuric) or base (potassium hydroxide, potassium silicate) in small steps; recheck after 15–30 minutes of circulation.
- Coco/rockwool: adjust each batch of feed; if runoff keeps climbing, acidify the water to reduce alkalinity, or increase the ammonium share slightly (stay ≤20% of N).
- Soil: raise pH with agricultural or dolomitic lime, lower with elemental sulfur; both take weeks. Do not chase soil pH with acid in every watering.
- Lockout with high runoff EC: if salts have built up as well, a root-zone flush resets the medium: 2–3× the container volume of a light, correctly pH-adjusted solution (coco, rockwool) or pH-adjusted water (soil), then check runoff pH and EC and resume normal feeding (see the root-zone flush).
- Safety: concentrated acids and hydroxides burn skin and eyes. Gloves and goggles; add acid to water.
Common mistakes
- Treating pH as one perfect number. A band is fine; drift within the range is normal and some movement helps.
- Adjusting pH before adding nutrients (the salts change it).
- Measuring pH of plain water with a meter and assuming that's the root zone.
- Blaming every deficiency on "pH lockout" without measuring.
Evidence
- pH effects on micronutrient and P solubility and chelate ceilings: Established [1][2][4][6][7].
- Soilless crops do best at lower pH than soils: Established in container horticulture [1][3].
- Cannabis pH ranges (soil 6.0–7.0, soilless/hydro 5.5–6.5): Established as accepted practice from extension and grower consensus [1][3][5]; GenetixBay house position.
