Emerald Harvest Cal-Mag - Calcium and Magnesium Supplement for Coco and Hydro
Best for: growers running coco coir or hydro systems who need extra calcium and magnesium to avoid deficiencies during peak flower.
Our thoughts
Cal-Mag deficiencies are one of the most common issues we see reported by new coco growers, and this is the straightforward fix. It's not glamorous, but a bottle of this sat on the shelf saves a lot of head-scratching over yellowing leaves later in the grow.
Cal-Mag, from California-based Emerald Harvest, is a calcium and magnesium supplement with an NPK contribution of roughly 2-0-0, carrying around 3% calcium and 0.5% magnesium, derived from calcium nitrate, magnesium sulfate, calcium acetate, magnesium acetate and kelp extract. Calcium and magnesium are secondary macronutrients that play an outsized role in plant health: calcium supports cell wall strength and structure, while magnesium is central to chlorophyll production and photosynthesis.
It's particularly important in coco coir, which naturally locks up calcium and magnesium, and in RO or soft-water hydro systems where these elements aren't present in the water to begin with. Cal-Mag also assists with the transport of other nutrients through the plant and helps the rest of your feeding schedule work as intended, especially during the nutrient-hungry peak flowering stage.
How it compares
Cal-mag supplements are near-universal across nutrient brands, and Emerald Harvest's version is a solid mineral-based entry in that category.
- Vs BioBizz Calmag: BioBizz's version is certified organic and contributes no nitrogen at all, while Emerald Harvest's Cal-Mag is a mineral-salt formula with a small NPK contribution alongside the calcium and magnesium.
- Vs General Hydroponics CALiMAGic: a similarly mineral-based product aimed at the same coco and RO-water deficiency, priced comparably.
- Vs Canna's cal-mag options: Canna's coco line often needs less supplementary cal-mag if you're on tap water, but RO growers on any brand will typically need a dedicated product like this one.
It's a straightforward, mineral-based cal-mag supplement priced in line with the rest of the category, and it does the specific job it's designed for.
Usage guidance
Dosage is light, since this is a supplement rather than a base feed.
Irrigation
Use 0.5 to 1.25ml of Cal-Mag per litre of water, mixed in with your regular base feed.
Foliar application
Dilute to 0.75ml per litre for a foliar spray if you need a faster response to visible deficiency symptoms.
When to start
Introduce Cal-Mag from early in the grow if you're on coco or RO water, rather than waiting for deficiency symptoms like interveinal yellowing or weak stems to appear.
Technical specifications
- NPK contribution: approximately 2-0-0
- Calcium: 3%
- Magnesium: 0.5%
- Key ingredients: calcium nitrate, magnesium sulfate, calcium acetate, magnesium acetate, kelp extract
- Dosage (irrigation): 0.5-1.25ml per litre
- Dosage (foliar): 0.75ml per litre
- Best suited to: coco coir, RO water, soft-water hydro systems
- Sizes available: 0.95L, 3.79L, 9.46L, 22.71L
- Vendor: Emerald Harvest (Santa Rosa, California)
Who this is for
- Coco coir growers needing to counter calcium and magnesium lock-up
- Growers on reverse osmosis or very soft tap water
- Anyone running Emerald Harvest's Grow, Micro and Bloom base nutrients who wants the matching cal-mag supplement
- Growers seeing early signs of cal-mag deficiency in peak flower
Best suited to coco and RO-water setups where cal-mag deficiency is a near-certainty without supplementation.
Our take
Essential stuff if you're growing in coco or on RO water, and Emerald Harvest's version is a dependable, straightforward mineral supplement that does exactly the job it's meant to.
One we sell a lot of for new growers moving into coco for the first time.
