# Calcium

> Calcium (Ca) is an immobile secondary nutrient that builds cell walls and membranes; it travels only in the transpiration stream, so deficiency appears in new growth.

- Canonical: https://genetixbay.com/knowledge/glossary/calcium.html
- Also written: Ca, Ca2+, calcium deficiency, Ca deficiency
- 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"

Calcium is a secondary macronutrient absorbed as Ca2+. It cross-links pectins in cell walls, stabilises membranes and acts as a signalling ion. Calcium moves only upward with water in the xylem and cannot be moved from old leaves to new ones, so it is immobile.

## In practice

Deficiency shows on new and fast-growing tissue: distorted or crinkled young leaves, brown [necrotic spots](https://genetixbay.com/knowledge/glossary/necrosis.md), weak stems and dying [growing tips](https://genetixbay.com/knowledge/glossary/apical-meristem.md). Because Ca rides the [transpiration](https://genetixbay.com/knowledge/glossary/transpiration.md) stream, low transpiration (very high humidity, poor airflow) can cause local shortage even when the feed is fine. Hydro feeds usually carry about 120-200 mg/L (ppm) Ca, most often from [calcium nitrate](https://genetixbay.com/knowledge/glossary/calcium-nitrate.md). [Coco](https://genetixbay.com/knowledge/glossary/coco-coir.md) binds calcium on its exchange sites, which is why coco needs buffering.

## Don't confuse with

- **Cal-Mag**: a product class that supplies Ca and Mg, not the element.
- **pH lockout**: low root-zone [pH](https://genetixbay.com/knowledge/glossary/ph.md) reduces Ca uptake; check pH before adding more.
- **Rust spots from pathogens or light stress**: see the rust-spot article before blaming Ca.

## Myth check

Calcium demand varies mainly by cultivar, water source and medium, not by light type (Evidence: house position, Myth for "LEDs need [Cal-Mag](https://genetixbay.com/knowledge/glossary/cal-mag.md)"). See [Cal-Mag](https://genetixbay.com/knowledge/nutrients/cal-mag.md).

## Related terms

[Secondary nutrient](https://genetixbay.com/knowledge/glossary/secondary-nutrient.md), [Immobile nutrient](https://genetixbay.com/knowledge/glossary/immobile-nutrient.md), [Calcium nitrate](https://genetixbay.com/knowledge/glossary/calcium-nitrate.md), [Cal-Mag](https://genetixbay.com/knowledge/glossary/cal-mag.md), [Nutrient antagonism](https://genetixbay.com/knowledge/glossary/nutrient-antagonism.md)

## Read more

- [Calcium (Ca) in cannabis: deficiency, transport, targets and sources](https://genetixbay.com/knowledge/nutrients/calcium.md)
- [Cal-Mag: what it is, when you need it and when you don't](https://genetixbay.com/knowledge/nutrients/cal-mag.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. Cockson P, Landis H, Smith T, Hicks K, Whipker BE. 2019. Characterization of nutrient disorders of Cannabis sativa. Applied Sciences 9(20):4432. https://doi.org/10.3390/app9204432