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Emerald Harvest Cal-Mag

Original price was: £20.00 – £55.00.Current price is: £14.00 – £38.50.

Emerald Harvest Cal-Mag calcium-magnesium supplement keeps your plants productive and green, particularly when growing in certain media such as coco coir.

Original price was: £20.00 – £55.00.Current price is: £14.00 – £38.50.

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Emerald Harvest Cal-Mag - 950ml:
Emerald Harvest Cal-Mag - 3.79L:



Keep your plants productive and green in any growing medium with our top-quality calcium-magnesium supplement.

Cal-Mag is the ultimate addition to growing in coco. Use in harmony with your favourite PK fertiliser to replenish essential calcium and magnesium and prevent nutrient deficiencies.

Replenishes calcium lost in coco

Coco coir is a rich source of potassium, but it’s important to supplement the macronutrients that it’s less kind to. Bright grow lights or rapidly growing plants are prone to magnesium deficiencies. It’s one of the key components of chlorophyll, which keeps leaves green and strong and is vital for photosynthesis. Coco substrates have a reputation for holding onto calcium and making it unavailable for plants to use, so a calcium supplement is always recommended.

A little help from kelp

As well as these important secondary nutrients, Cal-Mag contains kelp extract for a boost of natural goodness. Kelp is known to fortify the structure of plant cells and stimulate shoot growth, making it especially useful in the vegetative stages. It’s a bio stimulant that increases plants’ ability to draw available nutrients from the substrate.

Young plants will show increased tolerance to stress and develop strong, robust roots and shoots in early growth.


Cal-Mag is especially important for pre-empting deficiencies when growing in certain media, such as coco coir, but any garden can benefit.

  • Lush, vibrant, green growth and plant development
  • Optimal chlorophyll production to support photosynthesis
  • Less risk of calcium or magnesium deficiencies due to coco coir

How to use Emerald Harvest Cal-Mag?

Cal-Mag has no specific feeding routine: apply as needed from the early vegetative stage through to late flowering. It is recommended for use on watering days only.

Depending on your growing medium and plant necessity (showing signs of calcium and magnesium deficiency, for example), growers should dilute between 0.5ml and 1.25ml of Cal-Mag per litre of nutrient solution.

Do not use Emerald Harvest Cal-Mag in conjunction with Emerald Harvest Sturdy Stalk.

Where to use Emerald Harvest Cal-Mag?

Emerald Harvest Cal-Mag is particularly beneficial in coco-grown plants, since coco coir is known to retain calcium and make it inaccessible to plants.

However, it is a soluble liquid nutrient and will provide safe, effective results in any growing medium, including in soil and in hydroponic systems, tanks, and reservoirs.

Cal-Mag may also be diluted and used as a foliar spray. However, it is recommended that you do not apply other products directly before or after as this may lead to issues with mould and/or mildew.


Emerald Harvest was founded in 2015 in Santa Rosa, California, by industry insiders with decades of experience. Over many years, our founders had listened to the growing community talk about what they wanted and needed from a nutrient line. So, we set out to bring growers superlative yields for significant profits at a reasonable price point while reducing the mess and fuss and waste of other nutrient lines. The result? Our science-backed, streamlined premium base nutrients and supplements for hydroponic plants.

  • Proudly based in Santa Rosa, California
  • Founded after years in the industry
  • Nutrients designed with growers in mind

Emerald Harvest logo


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