# pH and nutrient availability: what pH really controls, by medium

> pH controls how soluble each nutrient is and how well roots absorb it. The accepted ranges are soil about 6.0–7.0 and soilless/hydro about 5.5–6.5 (sweet spots: 5.5–6.2 in hydro, 5.8–6.3 in coco and rockwool, 6.2–6.8 in soil), and drift within the range is normal. High pH starves plants of iron, manganese, zinc, boron and phosphorus; low pH makes iron and manganese overly available and reduces calcium, magnesium and molybdenum. The classic availability bar chart was built for mineral soils and overstates the effect in chelated soilless feeds. The ranges are broadly agreed accepted practice, from extension and grower consensus.

- Canonical: https://genetixbay.com/knowledge/ph-ec/ph-nutrient-availability.html
- Category: pH and EC · Level: Beginner
- Evidence: Established · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Any stage · Systems: Any system
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**What pH should I use for cannabis in soil, coco and hydro?** Cannabis grows best at about pH 6.2–6.8 in soil, within the accepted 6.0–7.0 range; about 5.8–6.3 at the feed in coco and rockwool, with runoff at 5.8–6.5; and 5.5–6.2 in DWC, [RDWC](https://genetixbay.com/knowledge/glossary/rdwc.md), NFT and aeroponic reservoirs. These ranges are accepted practice from extension guidance and grower consensus, and the GenetixBay house position.

**What is nutrient lockout from pH?** Nutrient lockout from pH happens when root-zone pH moves far enough out of range that nutrients become insoluble or roots absorb them poorly, so the plant shows [deficiencies](https://genetixbay.com/knowledge/glossary/nutrient-deficiency.md) even though the feed contains them. At high pH, iron, [manganese](https://genetixbay.com/knowledge/glossary/manganese.md), zinc and copper form insoluble compounds; at low pH, calcium, magnesium and molybdenum drop. Measure before blaming every deficiency on lockout.

**Why is the hydro pH range lower than soil?** The hydro pH range is lower than soil because soilless media and hydroponics lack the clays and aluminium chemistry of mineral soils that make pH 6.0–7.0 best, and their [micronutrients](https://genetixbay.com/knowledge/glossary/micronutrient.md) are supplied as chelates. Extension sources give about 5.4–6.2 for typical soilless crops, so cannabis hydro sits at about 5.5–6.2 and coco at 5.8–6.3.

**Is the pH nutrient availability chart accurate?** The classic pH nutrient availability chart is accurate only as a direction, not a law: it was drawn for mineral soils and overstates the effect in chelated soilless feeds. In coco and hydro, [chelated iron](https://genetixbay.com/knowledge/glossary/iron-chelate.md), manganese, zinc and copper stay available further up the scale; for example, iron-EDTA holds to about pH 6.5, iron-DTPA to about 7.5 and iron-EDDHA to about 9.

**What happens if my pH is too high?** If root-zone pH is too high, above about 6.5 in soilless media or about 7.3 in soil, cannabis runs short of iron, manganese, zinc, boron and copper, and phosphorus precipitates with calcium. The first sign is usually interveinal chlorosis on new growth. Bicarbonate-rich alkaline water makes it worse by reducing iron uptake directly and pushing pH up over weeks.

**What happens if my pH is too low?** If root-zone pH is too low, below about 5.3 in soilless media or 5.8 in soil, iron and manganese become overly available and can reach toxic levels, while calcium, magnesium and molybdenum run short and root membranes are damaged. Typical signs are leaf speckling, necrosis along old-leaf margins and distorted new growth.

**Is it OK if my pH drifts?** Yes, pH drift is fine as long as it stays within the accepted range, about 5.5–6.5 for soilless and hydro or 6.0–7.0 for soil; drift within the range is normal and healthy, and some movement helps. Treating pH as one perfect number is a common mistake. Act on a sustained trend outside the band, measured in the [root zone](https://genetixbay.com/knowledge/glossary/root-zone.md) with a calibrated meter.

**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:

1. **Solubility and precipitation.** At high pH, iron, manganese, zinc and copper form insoluble hydroxides and [phosphates](https://genetixbay.com/knowledge/glossary/phosphate.md); calcium phosphate precipitates. At low pH, aluminium and manganese dissolve (in soils) and molybdate binds to iron oxides [1][6][7].
2. **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].
3. **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](https://genetixbay.com/knowledge/glossary/living-soil.md) | 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](https://genetixbay.com/knowledge/glossary/alkalinity.md) [1] |

## What to check first

1. **Calibrate the meter** (see [meter calibration](https://genetixbay.com/knowledge/ph-ec/meter-calibration.md)). A drifting probe causes more "pH problems" than chemistry does.
2. **Measure the root zone,** not only the input: reservoir, runoff, or pour-through.
3. **Know your water alkalinity.** Alkaline water pushes pH up over days and weeks regardless of the input pH [1] (see [water quality](https://genetixbay.com/knowledge/ph-ec/water-quality.md)).
4. **Look at N form.** Nitrate feeds push pH up; ammonium pushes it down (see [pH drift](https://genetixbay.com/knowledge/ph-ec/ph-drift-hydro.md)).

## How to fix it

- **Hydro:** adjust with acid (phosphoric, nitric or sulfuric) or base (potassium hydroxide, [potassium silicate](https://genetixbay.com/knowledge/glossary/potassium-silicate.md)) 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](https://genetixbay.com/knowledge/glossary/sulfur-burner.md); 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](https://genetixbay.com/knowledge/glossary/root-zone-flush.md) 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](https://genetixbay.com/knowledge/harvest-cure/flush-debate.md)).
- **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.

## Sources

1. Lopez RG, Mickelbart MV, Pasian C. Alkalinity management in soilless substrates. Purdue Extension HO-242-W. https://www.extension.purdue.edu/extmedia/HO/HO-242-W.pdf
2. Michigan State University Extension. pHix it! Getting substrate pH just right. https://www.canr.msu.edu/news/phix-it-getting-substrate-ph-just-right
3. Michigan State University Extension. pH and EC issues still a problem in some greenhouses. https://www.canr.msu.edu/news/ph_and_ec_issues_still_a_problem_in_some_greenhouses
4. HortScience. 2025. Iron chelates in high-pH soilless media. HortScience 60(3):404. https://journals.ashs.org/view/journals/hortsci/60/3/article-p404.xml
5. Velechovsky J, Malik M, Senkyrik JB, Tlustos P. 2024. Frontiers in Plant Science 15:1322824. https://doi.org/10.3389/fpls.2024.1322824
6. Kaiser BN, et al. 2005. The role of molybdenum in agricultural plant production. Annals of Botany 96(5):745-754. https://doi.org/10.1093/aob/mci226
7. Marschner P (ed.). 2012. Marschner's Mineral Nutrition of Higher Plants, 3rd ed. https://doi.org/10.1016/C2009-0-63043-9