CalMag: Why Is Everyone Recommending It?

CalMag is a fertilizer formulation containing calcium and magnesium, sometimes with additional nitrogen or micronutrients. Both elements are essential, but they perform different functions and do not necessarily become limiting at the same time. A recommendation to add the product is useful only when it addresses the plant's actual supply or uptake conditions.

Before supplementing, consider the complete nutrient solution, irrigation water, substrate and root environment. Similar-looking symptoms can arise from inadequate supply, nutrient interactions or impaired transport, and adding calcium and magnesium together does not distinguish between those causes.

Featured photo: a young-mature cannabis leaf from the 2 mg/L Mg treatment, photographed 30 days after treatments began. Cropped from Figure 1F in Morad & Bernstein (2023), licensed under CC BY 4.0. Cropped from the original figure. This is an experimental example; leaf appearance alone does not establish a nutrient-deficiency diagnosis.

Calcium and magnesium have different functions

Calcium contributes to cell-wall structure, membrane stability, signalling and the development of young tissues. Magnesium is central to the chlorophyll molecule and participates in numerous enzymes and metabolic reactions.

Their redistribution within the plant also differs. Magnesium is relatively mobile, allowing older leaves to supply developing tissues when availability is insufficient. Deficiency commonly produces interveinal chlorosis on older leaves. Calcium is much less readily redistributed, so inadequate delivery tends to affect young, actively developing tissues.

These patterns provide diagnostic clues, but a combined label such as “CalMag deficiency” obscures which nutrient or process may be involved. Controlled cannabis nutrient-disorder studies help establish symptom patterns without making a photograph sufficient for diagnosis (Cockson et al., 2019; Llewellyn et al., 2023).

Magnesium supply in cannabis experiments

Controlled experiments have produced magnesium-deficiency symptoms in cannabis, including interveinal chlorosis, reduced physiological performance and smaller plants. During vegetative growth, Morad and Bernstein (2023) found approximately 35–70 mg/L Mg to be the most favourable range under their experimental conditions.

That range is not a universal fertilizer recipe. Genotype, developmental stage, environment, water quality, substrate and the rest of the nutrient solution affect interpretation. A result from vegetative growth should not automatically be applied to every flowering treatment or growing system.

The study supports an optimum-response interpretation rather than an assumption that increasing magnesium indefinitely will improve growth.

Interactions with calcium and potassium

As magnesium supply increased in the vegetative study, calcium and potassium uptake declined, and movement of calcium from roots to shoots was reduced. These responses illustrate cation antagonism: positively charged nutrients can influence one another's availability, uptake and distribution.

Cannabis nutrition research has also shown that higher potassium supply can reduce leaf magnesium concentration. Calcium, magnesium and potassium therefore need to be considered together rather than adjusted as independent inputs.

If calcium delivery is already a problem, adding a formulation that also supplies more magnesium may not address the cause. The relevant issue may be nutrient balance rather than the total amount of fertilizer present.

Why coir changes the discussion

Coconut coir has cation-exchange sites that hold positively charged ions. Fresh or poorly processed material may contain relatively high potassium and sodium while supplying comparatively little available calcium and magnesium. When it first contacts a nutrient solution, calcium and magnesium can associate with exchange sites as potassium and sodium are released, temporarily changing the balance around the roots.

Washing and buffering are therefore important distinctions between raw coir and horticultural products prepared for cultivation. Buffering often uses calcium-containing solutions before planting. Properties also vary between coir materials, as documented by Abad and colleagues (2002).

The practical question is how the particular substrate was processed and what the complete programme already supplies. The guide to root-zone management in coco develops that context. Coir use alone is not evidence that a large additional CalMag dose is required.

Source water contributes to nutrient supply

Tap water can supply substantial calcium and magnesium, whereas very soft or reverse-osmosis water may contribute little. Two growers using the same fertilizer dose can consequently deliver different total amounts of these elements.

Supplementation may be relevant where source water contributes little and the base formulation does not provide enough. Where water already supplies considerable calcium or magnesium, an automatic additional dose can increase EC and alter nutrient ratios unnecessarily. Assess the combined inputs rather than the contents of one bottle.

Calcium delivery depends on water movement

Calcium transport is strongly associated with the transpiration stream. Water absorbed by roots moves upwards through the xylem, carrying calcium with it. Inadequate delivery to rapidly growing tissue can therefore occur even when calcium is present around the roots.

Low transpiration, poor root function, waterlogging or root damage can interfere with that delivery. Water relations and the root environment need to be considered alongside supply; the guide to pH, EC and nutrient availability explains why adding fertilizer does not address every uptake problem.

Silicon and calcium in other crops

Research in several plant species has linked silicon supply with increased calcium uptake or movement towards shoots, but the response depends on species and conditions. It should not be assumed to occur identically in cannabis or to correct every calcium-related problem.

The relationship is also discussed in fruit-quality research. In strawberries, low fruit calcium has been associated with poor firmness, water soaking and albinism. Silicon and calcium both relate to cell-wall properties, but an association in a fruit crop does not establish a cannabis supplementation recommendation. The broader article on silicon in plants considers that subject separately.

Why an improvement does not confirm the diagnosis

Some CalMag formulations also contain nitrogen, iron or other micronutrients. If a pale plant improves after application, several nutrient inputs may have changed at once. That response does not establish that calcium or magnesium was originally limiting.

Record what was added and what changed in subsequent growth. The practical guide to diagnosing cannabis plant problems explains why the symptom pattern, recent management and root conditions should be assessed together.

When supplementation has a clear rationale

Reasons to investigate additional calcium or magnesium include very soft or reverse-osmosis water, an incomplete base formulation, poorly buffered coir, a confirmed deficiency or analysis indicating inadequate supply. None of these should be assessed without the rest of the growing system.

Check root-zone pH and EC, water availability, root function, potassium and magnesium inputs, and the contribution from source water. Where supply is adequate but transport or nutrient balance is limiting, increasing a combined product may be ineffective or introduce another imbalance.

CalMag can be a useful way to supply essential elements when the formulation fits the need. The decision should follow an assessment of calcium and magnesium individually, including how much is already present and whether either is actually limiting plant function.

References

Morad, D. & Bernstein, N. (2023). Response of Medical Cannabis to Magnesium (Mg) Supply at the Vegetative Growth Phase. Plants, 12(14), 2676. https://doi.org/10.3390/plants12142676.

Morad, D. & Bernstein, N. (2025). From deficiency to toxicity: Magnesium increases cannabinoid and terpene production in cannabis plants. Journal of Cannabis Research. https://doi.org/10.1186/s42238-025-00358-9.

Cockson, P., Landis, H., Smith, T., Hicks, K. & Whipker, B.E. (2019). Characterization of Nutrient Disorders of Cannabis sativa. Applied Sciences, 9(20), 4432. https://doi.org/10.3390/app9204432.

Llewellyn, D., Golem, S., Jones, A.M.P. & Zheng, Y. (2023). Foliar Symptomology, Nutrient Content, Yield, and Secondary Metabolite Variability of Cannabis Grown Hydroponically with Different Single-Element Nutrient Deficiencies. Plants, 12(3), 422. https://doi.org/10.3390/plants12030422.

Abad, M., Noguera, P., Puchades, R., Maquieira, A. & Noguera, V. (2002). Physico-chemical and chemical properties of some coconut coir dusts for use as a peat substitute for containerised ornamental plants. Bioresource Technology, 82, 241–245. https://doi.org/10.1016/S0960-8524(01)00189-4.