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 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; 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) |
| 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.being reviewed
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.being reviewed
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 |
| B | <0.3 mg/L | >0.5 mg/L | Count it in the feed.being reviewed |
| Ca, Mg | Note the values | Subtract from recipe | |
| Chloramine | 0 for living soil | Any, if using biology | [3] |
How to treat high alkalinity
- Test alkalinity (lab report or a drop-count alkalinity kit).
- Decide how much to remove. Leave about 0.5–1 meq/L as a buffer in hydro [6]; neutralise the rest.
- 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.
- 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.
- 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).
- 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.
