Emerald Harvest Cal-Mag is a targeted calcium and magnesium supplement designed to help prevent and correct common nutrient deficiencies while supporting stronger growth, greener leaves and better overall plant performance.
It’s especially useful in coco coir, where calcium can get tied up in the medium before plants can access it.
Emerald Harvest also positions Cal-Mag as a flexible supplement for other growing media when plants are growing fast, transitioning hard, or showing signs they need extra calcium and magnesium.
Please note: The 9.46 and 22.71 litre bottles are purchase to order so extended delivery times will apply.
Calcium and magnesium are secondary nutrients, but they’re absolutely essential. Calcium supports cell wall strength and overall plant structure, while magnesium sits at the centre of the chlorophyll molecule and is directly tied to photosynthesis. Emerald Harvest specifically highlights both of these roles in its official product information.
This matters most when plants are growing aggressively, moving into flower, or growing in coco. Emerald Harvest’s product guide specifically recommends using Cal-Mag for hungry plants during rapid vegetative growth, transition and peak flowering, and also recommends soaking coco coir or other soilless media with Cal-Mag before planting.
Use as needed, rather than automatically adding it to every feed. Emerald Harvest’s official guidance says to use it from early vegetative growth through late flowering, with a dose range of 0.5 - 1.25mL per litre, and recommends using it on water days only. Emerald Harvest also says not to use Cal-Mag and Sturdy Stalk in the same watering.
A practical way to use it is:
Caution: Do not aerate or use in bubbler systems.
| Stage | Suggested Use | Typical Rate |
|---|---|---|
| Early Veg | Use when needed | 0.5 - 1.25mL/L |
| Late Veg | Use when needed | 0.5 - 1.25mL/L |
| Transition | Use when needed | 0.5 - 1.25mL/L |
| Flowering | Use when needed | 0.5 - 1.25mL/L |
| Late Flower | Use when needed | 0.5 - 1.25mL/L |
Best used on water days only. Do not combine with Sturdy Stalk in the same watering.
Calcium helps plants build stronger cell walls and maintain structural integrity. When calcium is low, plants often struggle more with environmental stress and can become more vulnerable to wider feeding issues.
Magnesium is critical for chlorophyll production, which means it plays a direct role in how efficiently a plant can convert light into energy. That’s one reason magnesium shortages can show up fast under intense lighting or during rapid growth. These functions are consistent with Emerald Harvest’s own product and guide materials.
If you’re growing in coco, Cal-Mag is one of those products that can save a lot of headaches. Pre-treating coco or supplementing when needed can help stop minor issues turning into bigger deficiencies later on. Emerald Harvest specifically recommends soaking coco coir and other soilless media with Cal-Mag before planting.
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