# Secondary nutrient

> Secondary nutrients are calcium, magnesium and sulfur: macronutrients needed in substantial amounts but not included in the N-P-K numbers on a label.

- Canonical: https://genetixbay.com/knowledge/glossary/secondary-nutrient.html
- Also written: secondary nutrients, secondary macronutrients, secondary elements
- 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"

Secondary nutrients are calcium (Ca), magnesium (Mg) and sulfur (S). They are true macronutrients, needed in amounts comparable to phosphorus, but historically "secondary" because field soils usually supplied enough and [fertiliser labels](https://genetixbay.com/knowledge/glossary/guaranteed-analysis.md) led with [N-P-K](https://genetixbay.com/knowledge/glossary/npk-ratio.md). The term is about labelling tradition, not importance.

## In practice

In [soilless growing](https://genetixbay.com/knowledge/glossary/soilless.md) they are often the first elements to run short. [RO water](https://genetixbay.com/knowledge/glossary/ro-water.md) strips the calcium and magnesium that tap water brings, coco binds calcium and magnesium on its exchange sites until buffered, and high potassium can crowd them out. A complete hydro base supplies all three: calcium from [calcium nitrate](https://genetixbay.com/knowledge/glossary/calcium-nitrate.md), magnesium and sulfur from [magnesium sulfate](https://genetixbay.com/knowledge/glossary/magnesium-sulfate.md). Fertiliser labels list them under the guaranteed analysis, below N-P-K, when present.

## Don't confuse with

- **Micronutrients**: needed in trace amounts only.
- **Cal-Mag**: a supplement covering two of the three.
- **Primary nutrients**: N, P and K.

## Related terms

[Macronutrient](https://genetixbay.com/knowledge/glossary/macronutrient.md), [Calcium](https://genetixbay.com/knowledge/glossary/calcium.md), [Magnesium](https://genetixbay.com/knowledge/glossary/magnesium.md), [Sulfur](https://genetixbay.com/knowledge/glossary/sulfur.md), [Cal-Mag](https://genetixbay.com/knowledge/glossary/cal-mag.md)

## Read more

- [Cal-Mag: what it is, when you need it and when you don't](https://genetixbay.com/knowledge/nutrients/cal-mag.md)
- [Building a cannabis feed by stage: ppm targets per element and how to hit them](https://genetixbay.com/knowledge/nutrients/feed-by-stage.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