# Phosphate

> Phosphate is the ionic form in which plants absorb phosphorus, mainly H2PO4- at the normal root-zone pH of 5.5-6.5.

- Canonical: https://genetixbay.com/knowledge/glossary/phosphate.html
- Also written: H2PO4-, HPO4 2-, PO4, orthophosphate, phosphate ion
- 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"

Phosphate is phosphorus combined with oxygen, present in solution as H2PO4- or HPO4 2- depending on [pH](https://genetixbay.com/knowledge/glossary/ph.md). Below about pH 7 the H2PO4- form dominates, and that is the form roots take up most readily. Phosphate is also a buffer: it resists pH change somewhat in nutrient solutions.

## In practice

Phosphate comes from [monopotassium phosphate](https://genetixbay.com/knowledge/glossary/monopotassium-phosphate.md), [monoammonium phosphate](https://genetixbay.com/knowledge/glossary/monoammonium-phosphate.md) or [phosphoric acid](https://genetixbay.com/knowledge/glossary/phosphoric-acid.md). It reacts with calcium to form insoluble calcium phosphate when both are concentrated, or when pH rises well above 6.5. That is why phosphate and calcium sit in separate [A and B](https://genetixbay.com/knowledge/glossary/a-b-nutrient.md) stocks, and why a cloudy reservoir after a pH spike can mean phosphate has dropped out.

## Units and formula

Reports may quote PO4, P or P2O5. P = PO4 × 0.326; P = P2O5 × 0.436. Compare like with like.

## Don't confuse with

- **Phosphite (phosphonate)**: a related ion sold as a fungicide or "[biostimulant](https://genetixbay.com/knowledge/glossary/biostimulant.md)"; plants cannot use it as a phosphorus source.
- **Phosphorus**: the element itself, counted the same whichever ion carries it.

## Related terms

[Phosphorus](https://genetixbay.com/knowledge/glossary/phosphorus.md), [Monopotassium phosphate](https://genetixbay.com/knowledge/glossary/monopotassium-phosphate.md), [Monoammonium phosphate](https://genetixbay.com/knowledge/glossary/monoammonium-phosphate.md), [Phosphoric acid](https://genetixbay.com/knowledge/glossary/phosphoric-acid.md), [Calcium](https://genetixbay.com/knowledge/glossary/calcium.md)

## Read more

- [Phosphorus (P) in cannabis: what it does, deficiency, excess and how much is enough](https://genetixbay.com/knowledge/nutrients/phosphorus.md)
- [pH and nutrient availability: what pH really controls, by medium](https://genetixbay.com/knowledge/ph-ec/ph-nutrient-availability.md)

## Sources

1. Marschner P (ed.). 2012. Marschner's Mineral Nutrition of Higher Plants, 3rd ed. Academic Press. https://doi.org/10.1016/C2009-0-63043-9
2. Sonneveld C, Voogt W. 2009. Plant Nutrition of Greenhouse Crops. Springer. https://doi.org/10.1007/978-90-481-2532-6