# Phosphoric acid

> Phosphoric acid (H3PO4) is the most common pH-down acid; it lowers pH while adding phosphorus, about 27% P in the usual 85% grade.

- Canonical: https://genetixbay.com/knowledge/glossary/phosphoric-acid.html
- Also written: H3PO4, orthophosphoric acid, pH down (phosphoric)
- 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"

Phosphoric acid is a mineral acid of phosphorus. The 85% technical grade contains roughly 27% P by weight. Diluted retail pH-down products are usually phosphoric acid at a much lower strength. Adding it lowers [pH](https://genetixbay.com/knowledge/glossary/ph.md) and supplies [phosphate](https://genetixbay.com/knowledge/glossary/phosphate.md) at the same time.

## In practice

For most growers with soft or [RO water](https://genetixbay.com/knowledge/glossary/ro-water.md), the phosphorus from pH-down is small. With hard, high-alkalinity water, the acid needed to neutralise bicarbonate can add a meaningful amount of P, sometimes tens of mg/L (ppm). Count it towards the feed's P total, or split acidification with nitric or sulfuric acid. Concentrated acid is corrosive: wear eye and skin protection, add acid to water and never the reverse.

## Don't confuse with

- **Nitric acid**: lowers pH and adds nitrogen instead.
- **Citric acid**: an organic acid that microbes break down, so pH drifts back up.
- **Phosphorous acid (phosphite)**: a fungicidal compound, not a P nutrient.

## Related terms

[Phosphate](https://genetixbay.com/knowledge/glossary/phosphate.md), [Phosphorus](https://genetixbay.com/knowledge/glossary/phosphorus.md), [Nitric acid](https://genetixbay.com/knowledge/glossary/nitric-acid.md), [pH Down](https://genetixbay.com/knowledge/glossary/ph-down.md)

## Read more

- [Water quality for cannabis: RO vs tap, alkalinity, bicarbonates and chloramine](https://genetixbay.com/knowledge/ph-ec/water-quality.md)
- [Phosphorus (P) in cannabis: what it does, deficiency, excess and how much is enough](https://genetixbay.com/knowledge/nutrients/phosphorus.md)

## Sources

1. Sonneveld C, Voogt W. 2009. Plant Nutrition of Greenhouse Crops. Springer. https://doi.org/10.1007/978-90-481-2532-6