# Nitric acid

> Nitric acid (HNO3) lowers pH while adding nitrate nitrogen; growers use it to neutralise hard-water alkalinity, often alongside or instead of phosphoric acid.

- Canonical: https://genetixbay.com/knowledge/glossary/nitric-acid.html
- Also written: HNO3, nitric pH down, pH down (nitric)
- Glossary category: Nutrients · 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"

Nitric acid is a strong mineral acid that supplies [nitrate](https://genetixbay.com/knowledge/glossary/nitrate.md) when it neutralises [alkalinity](https://genetixbay.com/knowledge/glossary/alkalinity.md). Pure HNO3 is about 22% N by weight; commercial strengths are lower. It is the main pH-down acid in commercial greenhouse hydroponics.

## In practice

Growers with hard, high-bicarbonate water often use nitric acid so that acidification adds nitrogen rather than too much phosphorus. The nitrogen it adds must be counted in the feed total, which matters during a late-flower [N taper](https://genetixbay.com/knowledge/glossary/nitrogen-taper.md). Langenfeld and Bugbee studied nitric acid with [ammonium nitrate](https://genetixbay.com/knowledge/glossary/ammonium-nitrate.md) for [pH](https://genetixbay.com/knowledge/glossary/ph.md) control in [recirculating](https://genetixbay.com/knowledge/glossary/recirculating-system.md) hydroponics. Concentrated nitric acid is highly corrosive, gives off toxic fumes and is an oxidiser. Most home growers buy it pre-diluted as a pH-down product, and should keep to that. It may be regulated in some places; check local rules.

## Don't confuse with

- **Phosphoric acid**: lowers pH while adding phosphorus.
- **Sulfuric acid**: lowers pH while adding [sulfur](https://genetixbay.com/knowledge/glossary/sulfur.md).
- **Nitrate salts**: supply nitrate without lowering pH.

## Related terms

[Nitrate](https://genetixbay.com/knowledge/glossary/nitrate.md), [Phosphoric acid](https://genetixbay.com/knowledge/glossary/phosphoric-acid.md), [Nitrogen](https://genetixbay.com/knowledge/glossary/nitrogen.md), [Ammonium nitrate](https://genetixbay.com/knowledge/glossary/ammonium-nitrate.md)

## Read more

- [Water quality for cannabis: RO vs tap, alkalinity, bicarbonates and chloramine](https://genetixbay.com/knowledge/ph-ec/water-quality.md)
- [pH drift in hydro: why reservoir pH rises (and sometimes falls)](https://genetixbay.com/knowledge/ph-ec/ph-drift-hydro.md)

## Sources

1. Langenfeld NJ, Bugbee B. 2025. pH control in recirculating hydroponics with nitric acid and ammonium nitrate. HortScience 60(9):1555. https://journals.ashs.org/view/journals/hortsci/60/9/article-p1555.xml
2. Sonneveld C, Voogt W. 2009. Plant Nutrition of Greenhouse Crops. Springer. https://doi.org/10.1007/978-90-481-2532-6