Supplement calcium and magnesium when your plant’s water, substrate and normal fertiliser do not provide enough.

CalMag is a powdered mineral supplement derived from calcined algae. Calcium contributes to developing plant tissues and cell structure, while magnesium is a central component of chlorophyll.

This is a targeted supplement rather than a product every plant needs routinely. Check the existing nutrient supply and growing conditions before adding it.

The declared composition is:

• 80% calcium carbonate
• 12% magnesium carbonate
• Natural calcined algae

These percentages describe the carbonate compounds, not the amount of pure elemental calcium and magnesium.

Because the minerals are supplied as carbonates, the product can neutralise acidity and raise substrate pH. This may be useful in an acidic mix, but repeated use may be unsuitable when the substrate already has an appropriate or high pH.

CalMag does not replace a complete fertiliser. Plants still require nitrogen, phosphorus, potassium and other essential nutrients.

Consider additional calcium and magnesium when:

• Using rainwater, reverse-osmosis water or another water source with low mineral content
• Growing in a substrate that supplies little calcium or magnesium
• Using coco or another medium whose nutrient programme requires additional CalMag
• Your regular fertiliser does not already supply sufficient amounts

Possible deficiency symptoms worth investigating include distorted new growth or damaged young leaf margins where calcium supply or uptake may be involved, and yellowing between the veins of older leaves where magnesium may be involved.

These symptoms can also result from root damage, unsuitable pH, watering problems, pests or other nutrient imbalances. Existing damaged leaves will not repair themselves, so assess whether subsequent growth develops normally.

Do not add the powder directly to LECA, pon or hydroponic reservoir water. The carbonate material can settle and alter pH, so it should not be treated as a completely soluble reservoir nutrient.

Choose one application method rather than automatically combining full doses from several methods.

Mixing into substrate

Mix 30–50 g per 10 L of substrate, equivalent to 3–5 g per litre.

For indoor pots, begin at the lower end of the range. Check whether the substrate already contains lime, calcium or magnesium before adding more.

Applying through watering

Mix 5 g per litre of water and apply through the substrate from the top.

When supplementation is required, the stated maximum frequency is once per week. Apply it as part of normal watering rather than adding an unnecessary extra watering.

The carbonate powder may settle rather than dissolve completely. Stir thoroughly, prepare only what you need and stir again during application if required.

For self-watering pots, apply the mixture from the top through the substrate. Allow it to drain, then empty and rinse the reservoir before returning to the normal reservoir routine.

Foliar application

Mix 5 g per litre of water and apply no more frequently than every 7–10 days when foliar supplementation is appropriate.

Test a small area first, especially on delicate Anthurium and Alocasia foliage. Avoid newly unfurling leaves, direct sunlight and strong grow lights during application. Provide enough airflow for the leaves to dry.

Surface application

Apply 70–120 g per m² once per month and water it into the substrate. This method is more practical for larger growing areas than individual houseplant pots.

For plants on moss poles, apply CalMag to the substrate in the pot. Feed the pole separately with an appropriate diluted liquid fertiliser.

Storage and handling

• Store closed and protected from direct sunlight at 0–30°C.
• Avoid inhaling the dust or getting it in your eyes.
• Keep out of reach of children and animals.

One 1 kg pouch of CalMag calcium and magnesium supplement.

A 1 kg pouch provides approximately:

• 200–333 L of substrate at the stated mixing rate
• 200 L of watering mixture at 5 g per litre