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Cal-Mag
Definition: Cal-Mag is a supplement that supplies calcium and magnesium, two secondary nutrients cannabis needs in larger amounts than micronutrients like iron or zinc. Growers add it mainly when using reverse osmosis or very soft water, coco coir, or strong LED lighting, all of which commonly lead to calcium or magnesium shortfalls.
How Cal-Mag works
Calcium and magnesium play very different roles, and their deficiencies show up in different places on the plant.
Spotting the shortfall
- Calcium builds cell walls and new tissue. It doesn’t move easily inside the plant, so a shortage shows on newer growth as rusty brown spots, crumpled leaf edges, or weak stems.
- Magnesium sits at the center of every chlorophyll molecule. It’s mobile, so the plant pulls it from older leaves first, causing yellowing between green veins on lower fan leaves.
Who usually needs it
Coco coir naturally grabs calcium and magnesium and releases potassium and sodium in exchange, which is why buffered coco is pre-treated and why coco growers add Cal-Mag with most feedings. Reverse osmosis water strips nearly all minerals, so a typical approach is to add Cal-Mag until the water reaches about 0.3 to 0.4 EC (roughly 150 to 200 ppm on the 500 scale) before adding base nutrients. Growers using LEDs also report more frequent calcium and magnesium demand than under older lights.
Who may not
Hard tap water often already carries plenty of calcium. Adding more on top can push the solution out of balance, so test your water before supplementing.
Not every spotty leaf is a Cal-Mag issue
A pH outside the right range can block calcium and magnesium even when they are present. Check pH before reaching for a bottle, or you may be treating nutrient lockout with more of what’s already locked out.