# Water quality for cannabis: RO vs tap, alkalinity, bicarbonates and chloramine

> Get a water report before choosing a feed. Alkalinity (bicarbonate), not water pH, is what pushes root-zone pH up over time; above about 100–150 mg/L as CaCO3 (2–3 meq/L) it should be partly neutralised with acid. Tap water may also add useful Ca and Mg, or harmful sodium, chloride and boron. RO water removes nearly everything, which gives precise control but means you must add back Ca and Mg. Chloramine does not evaporate; remove it with long-contact or catalytic carbon or ascorbic acid if you run living soil or beneficial microbes.

- Canonical: https://genetixbay.com/knowledge/ph-ec/water-quality.html
- Category: pH and EC · Level: Intermediate
- Evidence: Supported · 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

**Should I use RO water or tap water for cannabis?** Use RO water for cannabis when your tap water has high [sodium](https://genetixbay.com/knowledge/glossary/sodium.md), chloride or boron, alkalinity you cannot economically acidify, or when you need exact recipes; otherwise good tap water, often 0.2–0.8 [mS/cm](https://genetixbay.com/knowledge/glossary/ms-cm.md), can supply useful calcium and magnesium. RO is under 0.05 mS/cm, so you must add calcium and magnesium back and expect easier [pH](https://genetixbay.com/knowledge/glossary/ph.md) swings. Many growers blend the two.

**What is alkalinity and why does it matter?** Alkalinity is water's capacity to neutralise acid, mostly from bicarbonates and carbonates, and it matters because it, not water pH, pushes root-zone pH up over time. High-alkalinity water acts like adding lime at every irrigation. One meq/L equals 50 mg/L as CaCO₃; extension guidance says to partly neutralise water above about 2–3 meq/L, or 100–150 mg/L.

**How much acid do I need to treat hard water?** To treat high-alkalinity water, 1 meq/L of acid neutralises 1 meq/L of alkalinity, so subtract the buffer you want to keep, about 0.5–1 meq/L for hydro, from the measured alkalinity and dose that difference. Use an extension alkalinity calculator, count the sulfur, nitrogen or phosphorus each acid adds, always add acid to water, and re-test every season.

**Does chloramine hurt cannabis or soil microbes?** Whether chloramine at tap-water levels hurts cannabis is contested: no cannabis study was found. The main concern is soil biology, [compost teas](https://genetixbay.com/knowledge/glossary/compost-tea.md) and beneficial inoculants, which are sensitive to disinfectants, while mineral hydro run with sterile intent is less affected. Living-soil growers should remove chloramine with long-contact or catalytic carbon or ascorbic acid; the US limit is 4 mg/L.

**Does letting tap water sit remove chloramine?** No, letting tap water sit does not remove chloramine. Chlorine gasses off from standing water, but chloramine does not, and boiling or adding salt does not remove it either. Use granular activated carbon with long contact time, about 10 minutes or more, catalytic carbon, or ascorbic acid. Standard RO does not reliably remove chloramine.

**What is a good [EC](https://genetixbay.com/knowledge/glossary/ec.md) for source water?** A good source water EC for cannabis is below about 0.5 mS/cm; above 0.8 mS/cm it leaves little room for nutrients before total feed EC gets too high. Also check the report for sodium, ideally under 30 mg/L, chloride under 30 mg/L and boron under about 0.3 mg/L, since EC alone does not show which ions are present.

**Short answer.** Get a water report before you choose a feed. **Alkalinity** (mostly bicarbonate), not water pH, is what pushes root-zone pH up over time [1]; above roughly 100–150 mg/L as CaCO₃ (2–3 meq/L) neutralise part of it with acid [1][2]. Tap water may add useful Ca and Mg, or harmful Na, Cl and B. **RO** removes nearly everything: precise, but you must add back Ca and Mg. **Chloramine** does not evaporate; remove it with long-contact or catalytic carbon or ascorbic acid if you run [living soil](https://genetixbay.com/knowledge/glossary/living-soil.md) or beneficial microbes [3][4][5].

## What's happening

### pH vs alkalinity

pH is the hydrogen-ion level right now. Alkalinity is the water's capacity to neutralise acid, its buffer, from carbonates and bicarbonates [1]. Water at pH 7.5 with low alkalinity barely affects the [root zone](https://genetixbay.com/knowledge/glossary/root-zone.md); water at pH 7.5 with high alkalinity acts like adding lime every irrigation [1]. Purdue's summary: water pH has minimal effect on root-zone pH; alkalinity has a large one [1].

Alkalinity units confuse everyone [1][2]:
- 1 meq/L alkalinity = 50 mg/L (ppm) as CaCO₃ = 61 mg/L as HCO₃⁻.
- Hardness (Ca + Mg) often tracks alkalinity but is not the same thing.

### RO vs tap

| | Tap / well | RO |
|---|---|---|
| EC | Often 0.2–0.8 mS/cm | <0.05 mS/cm |
| Ca, Mg | May supply a useful share | ~0 (add back; see [Cal-Mag](https://genetixbay.com/knowledge/nutrients/cal-mag.md)) |
| Alkalinity | Low to very high | ~0; pH swings easily |
| Na, Cl, B | Variable; can be limiting | Removed |
| Chlorine / chloramine | Present in municipal water | Chlorine damages thin-film RO membranes; use a carbon prefilter. RO does not reliably remove chloramine [3] |
| Cost, waste | Low | Equipment, membranes, reject water |

Use RO when your tap water has high Na or Cl, high B, high alkalinity you can't economically acidify, or when you need exact recipes. Many growers blend RO and tap to hit a target.

### Chloramine

Utilities use chlorine or chloramine to disinfect. Chlorine gasses off from standing water; **chloramine does not**, and boiling or adding salt does not remove it either [3]. The US EPA limit is 4 mg/L (as Cl₂) [3]. Removal: granular activated carbon with long contact time (a bed contact time of about 10 minutes or more may be needed with ordinary carbon), catalytic carbon, or ascorbic acid (vitamin C) [3][4][5]. The USDA Forest Service describes ascorbic acid and sodium ascorbate as effective dechlorinators that slightly lower dissolved oxygen in the treated water [5]. Aquarium guides quote roughly 2.5–2.8 parts vitamin C per part chlorine by weight.

Does it harm cannabis? At drinking-water levels, no cannabis study was found. The main concern is for soil biology, compost teas and beneficial inoculants, which are disinfectant-sensitive; mineral hydro with sterile intent is less affected. *Contested.*

## Numbers that matter

| Parameter | Good | Act if | Notes |
|---|---|---|---|
| Source EC | <0.5 mS/cm | >0.8 mS/cm | Leaves little room for nutrients |
| Alkalinity | 0.5–1.5 meq/L (25–75 mg/L CaCO₃) | >2–3 meq/L (100–150 mg/L CaCO₃) | Extension guidance 50–120 mg/L CaCO₃ after treatment for many crops [1][2]; for hydro keep ~0.5–1 meq/L buffer [6] |
| Bicarbonate | <60–120 mg/L HCO₃⁻ | >180 mg/L | Purdue: low <80, moderate 100–200, high >200 mg/L [1] |
| Na | <30 mg/L | >50 mg/L | >5 mM NaCl (115 mg/L Na) may cut cannabis yield [7] |
| Cl | <30 mg/L | >50–70 mg/L | See [chloride](https://genetixbay.com/knowledge/nutrients/chlorine.md) |
| B | <0.3 mg/L | >0.5 mg/L | Count it in the feed. |
| Ca, Mg | Note the values | | Subtract from recipe |
| Chloramine | 0 for living soil | Any, if using biology | [3] |

## How to treat high alkalinity

1. **Test** alkalinity ([lab report](https://genetixbay.com/knowledge/glossary/coa.md) or a drop-count alkalinity kit).
2. **Decide how much to remove.** Leave about 0.5–1 meq/L as a buffer in hydro [6]; neutralise the rest.
3. **Choose the acid** and count the nutrient it adds: sulfuric (adds S), nitric (adds N), phosphoric (adds P; the P limits how much you can use). NC State notes sulfuric is the most common for bicarbonate neutralisation [2]. 1 meq/L of acid neutralises 1 meq/L of alkalinity.
4. **Calculate the dose** with an alkalinity calculator (Purdue, NC State and New Hampshire developed one [1]) or the NC State formula [2]. Add acid to the water, mix, wait, measure pH.
5. **Re-test** every season: well water alkalinity changes with drought and rain [1].

**Safety:** concentrated acids cause severe burns and nitric acid fumes are toxic. Gloves, goggles, ventilation. Always add acid to water, never the reverse.

## Common mistakes

- Adjusting water pH and ignoring alkalinity.
- Running high-sodium softened water (see [sodium](https://genetixbay.com/knowledge/nutrients/sodium.md)).
- Letting chloramine water "stand overnight" for living soil.
- Using a base nutrient designed for hard water with RO, and then blaming the plant.

## Evidence

- Alkalinity, not pH, drives substrate pH: *Established* [1].
- Extension alkalinity targets and acid methods: *Established* for container crops [1][2]; *Supported* (extrapolated) for cannabis.
- Chloramine persistence and removal methods: *Established* [3][4].
- Chloramine harming cannabis at tap levels: *Contested*; no study found.

## 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. NC State Extension. Producing conventional tobacco transplants in greenhouses: water quality. https://content.ces.ncsu.edu/producing-conventional-tobacco-transplants-in-greenhouses-water-quality
3. Massachusetts Department of Environmental Protection. Public notification template when switching from chlorine to chloramines. https://www.mass.gov/doc/public-notification-template-when-switching-from-chlorine-to-chloramines/download
4. Water Quality Association. 2014. Chloramine fact sheet. https://wqa.org/wp-content/uploads/2022/09/2014_Chloramine.pdf
5. USDA Forest Service. Vitamin C dechlorination. https://www.fs.usda.gov/t-d/pubs/html/05231301/05231301.html
6. Sonneveld C, Voogt W. 2009. Plant Nutrition of Greenhouse Crops. Springer. https://doi.org/10.1007/978-90-481-2532-6
7. Yep B, Gale NV, Zheng Y. 2020. Frontiers in Plant Science 11:1169. https://doi.org/10.3389/fpls.2020.01169