# Alkalinity

> Alkalinity is water's capacity to neutralise acid, mainly from dissolved bicarbonate and carbonate, reported in mg/L as CaCO3; it decides how much pH Down you need.

- Canonical: https://genetixbay.com/knowledge/glossary/alkalinity.html
- Also written: total alkalinity, water alkalinity, bicarbonate alkalinity, mg/L CaCO3, ppm as CaCO3
- Glossary category: pH, EC and water · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Alkalinity is the [acid-neutralising capacity](https://genetixbay.com/knowledge/glossary/buffering-capacity.md) of water. In irrigation water it comes almost entirely from bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻), with small contributions from hydroxide and other weak bases. It is reported as mg/L (ppm) of calcium carbonate equivalent. Alkalinity is not the same as high [pH](https://genetixbay.com/knowledge/glossary/ph.md): water can be at pH 7.5 with low alkalinity and move easily, or at pH 7.5 with high alkalinity and resist acid stubbornly.

## In practice

Rough guide for container growing:

| Alkalinity (mg/L as CaCO3) | Effect |
|---|---|
| Below 40 | Little buffering; pH moves easily ([RO](https://genetixbay.com/knowledge/glossary/ro-water.md), rain) |
| 40–100 | Comfortable for most container growing |
| 100–200 | Needs acid; media pH tends to creep up |
| Above 200 | Needs significant acidification or RO blending |

Water utilities publish alkalinity in annual water-quality reports, and inexpensive aquarium [KH](https://genetixbay.com/knowledge/glossary/carbonate-hardness.md) test kits give a usable estimate. Acid dosing to neutralise most of the bicarbonate, rather than simply chasing pH, gives a stable result.

## Units and formula

1 dKH = 17.86 mg/L as CaCO3. Bicarbonate (mg/L) ≈ alkalinity as CaCO3 × 1.22.

## Don't confuse with

- **pH**: the acidity level, not the reserve.
- **Hardness**: calcium and magnesium content; often related, but not the same thing.

## Related terms

[Buffering capacity](https://genetixbay.com/knowledge/glossary/buffering-capacity.md), [Carbonate hardness](https://genetixbay.com/knowledge/glossary/carbonate-hardness.md), [pH Down](https://genetixbay.com/knowledge/glossary/ph-down.md), [Tap water](https://genetixbay.com/knowledge/glossary/tap-water.md), [General hardness](https://genetixbay.com/knowledge/glossary/general-hardness.md)

## Read more

- [Water quality for cannabis: RO vs tap, alkalinity, bicarbonates and chloramine](https://genetixbay.com/knowledge/ph-ec/water-quality.md)

## Sources

1. Bugbee B. 2004. Nutrient management in recirculating hydroponic culture. Acta Horticulturae 648:99-112. https://www.ishs.org/ishs-article/648_12
2. Sonneveld C, Voogt W. 2009. Plant Nutrition of Greenhouse Crops. Springer. https://doi.org/10.1007/978-90-481-2532-6