Cal-Mag Supplements

When do plants need a cal-mag supplement?

Two secondary nutrients drop out of a hydroponic feed more often than the rest: calcium and magnesium. Almost all of the demand comes from three setups — coco as the medium, rain or RO as the water, and a heavy-feeding crop under strong light. Base nutrients already include some of both, but once any of those conditions apply they are rarely enough on their own.

Growing in coco coir

While coco buffers it holds calcium and magnesium, so a share of each feed never reaches the plant. Most coco growers therefore start a cal-mag supplement from the first feed rather than waiting for the leaves to show it. That buffering is why several brands here sell a coco-specific version.

Rain, RO and very soft tap water

Rainwater and reverse-osmosis water hold almost no dissolved calcium or magnesium. Soft tap water in parts of Australia sits close behind. Start from near-zero EC water and a cal-mag supplement is replacing what harder water would have brought in.

Reading the symptoms

A magnesium shortage usually appears first on older, lower leaves: yellow between the veins, veins still green. A calcium shortage appears on new growth instead — distorted young leaves, weak tips, and spotting. Calcium moves poorly inside the plant, so the damage tends to stay on the tissue that formed while it was short.

Our range of cal-mag supplements

The brands on this page are 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 start at 250mL trial bottles and run to 20L for a commercial room; Athena's line goes to 18.9L.

Most of the range is liquid and goes straight into the reservoir. The exception is Green Planet Horti Cal — a 2.3kg powder for anyone mixing larger volumes or wanting a container that lasts longer. A few products combine cal-mag with nitrogen (Growth Technology's Liquid Calcium Magnesium Nitrogen, Plant Mechanics Super N+). Those suit vegetative growth; they are worth noting if the base feed is already high in nitrogen.

How to choose and use cal-mag

1. Match it to your water. Test the source water first. Water that already carries calcium and magnesium needs less than a rainwater setup — dosing blind is how other nutrients get locked out.
2. Match it to your medium. Coco almost always wants it. Soil with a decent amendment history often does not. Recirculating systems sit in the middle and need watching rather than a fixed dose.
3. Add it early in the mix. Pour cal-mag into the reservoir before the base nutrients, and never mix the two as concentrates, or the calcium can react and drop out of solution.
4. Watch your EC. A cal-mag supplement adds to total EC. Build it into the target figure instead of stacking it on a full-strength feed.
5. Check the nitrogen. Some formulas include nitrogen. Late flower may not want that extra N, so read the analysis before you switch brands mid-grow.

Yellowing leaves sent you here? Yellow Leaves on Plants? Troubleshoot Common Grow Problems walks through the other likely causes before you reach for a bottle, and The Importance of pH in a Hydroponic System covers the lockout that can look like a deficiency. The rest of the range lives in hydroponic additives and supplements.