Cal-Mag Supplements

When do plants need a cal-mag supplement?

Calcium and magnesium are the two secondary nutrients most often missing from a hydroponic feed, and three situations account for most of the demand: growing in coco, watering with rain or RO water, and running a heavy-feeding crop under strong light. Base nutrients carry some calcium and magnesium, but rarely enough on their own once any of those conditions apply.

Growing in coco coir

Coco holds onto calcium and magnesium as it buffers, so a share of what you feed never reaches the plant. Most coco growers run a cal-mag supplement from the first feed rather than waiting for symptoms, which is why several brands here sell a coco-specific version.

Rain, RO and very soft tap water

Rainwater and reverse-osmosis water contain almost no dissolved calcium or magnesium, and soft tap water in parts of Australia is not far behind. If you are starting from near-zero EC water, a cal-mag supplement replaces what harder water would have supplied.

Reading the symptoms

Magnesium shortage usually shows first on older, lower leaves as yellowing between the veins while the veins stay green. Calcium shortage shows in new growth instead — distorted young leaves, weak tips, and spotting. Because calcium moves poorly inside the plant, damage tends to stay on the growth that formed while it was short.

Our range of cal-mag supplements

Stocked brands include Green Planet (Pro Cal and the Horti Cal powder), Growth Technology, Athena, Canna, Terra Aquatica, FloraMax, Flairform, Med Tek, Plant Mechanics, Cultured Solutions, ISN and Bloom. Sizes run from 250mL trial bottles up to 20L for a commercial room, and Athena's line goes to 18.9L.

Most are liquids that go straight into the reservoir. Green Planet Horti Cal is the exception — a 2.3kg powder for growers mixing larger volumes or wanting a longer-lasting container. Several products combine cal-mag with nitrogen (Growth Technology's Liquid Calcium Magnesium Nitrogen, Plant Mechanics Super N+), which suits vegetative growth but is worth noting if you are already running a high-nitrogen base.

How to choose and use cal-mag

1. Match it to your water. Test your source water first. If it already carries calcium and magnesium, you need less than a grower on rainwater — dosing blind is how growers end up locking out other nutrients.
2. Match it to your medium. Coco almost always needs it. Soil with a decent amendment history often does not. Recirculating systems sit in between and want monitoring rather than a fixed dose.
3. Add it early in the mix. Cal-mag goes into the reservoir before your base nutrients and never into concentrated form alongside them, or the calcium can react and drop out of solution.
4. Watch your EC. A cal-mag supplement adds to total EC. Factor it into the target figure instead of adding it on top of a full-strength feed.
5. Check the nitrogen. Some formulas include nitrogen. In late flower that may be more than you want, so read the analysis before switching brands mid-grow.

If yellowing leaves are what brought you here, Yellow Leaves on Plants? Troubleshoot Common Grow Problems works through the other likely causes before you reach for a bottle, and The Importance of pH in a Hydroponic System covers the lockout that mimics a deficiency. You will find the rest of the range in hydroponic additives and supplements.