Cannabis Science


What is the Cal/Mag Connection in Growing Cannabis?

Understanding nutrients and nutrient deficiencies are critical to a successful cannabis grow. Indeed, nutrient deficiencies can be disastrous in your cannabis garden but are fortunately one of the few major growing problems that can be corrected with relative ease. 

Generally speaking, plant nutrients can be thought of in two ways: macronutrients and micronutrients. Having little to do with size and instead referring to the amount a plant needs, the basic macronutrients are nitrogen, phosphorus, and potassium (NPK). These are the three numbered measurements typically printed on the front of a fertilizer or nutrient product.

Understanding micronutrients, or secondary nutrients, is just as important, however. Though plants will require a smaller concentration of these micronutrients, they nonetheless can substantially impact your grow. The three most significant micronutrients are sulfur, calcium, and magnesium. Of these, Calcium and Magnesium are the most commonly deficient, thus creating a need for cal/mag supplements.

It’s important, of course, not to just throw a bunch of cal/mag into your reservoir and hope for the best. Instead, let’s examine the properties of each of these nutrients individually, as well as how they interact with your plants together.


Calcium’s importance in plant growth cannot be overstated. Known as an immobile nutrient, calcium is uptaken by the plant and put to use in a particular part of the plant. Thus, the plant cannot “borrow” calcium from nearby stores when it is deficient.

Calcium’s role in plant development mainly involves strengthening cell walls. Strong cell walls are more resistant to damage, pests, and disease and are better able to help the plant transport nutrients freely. Through transpiration, plants transport calcium from the roots throughout the plant using water as a vehicle. Because it is transported by water, calcium levels can be affected by any aspect of your grow which impacts moisture, such as humidity or overwatering.

Calcium deficiency manifests in several ways, the most noticeable being the appearance of leaves. Brown spots and “rusty” edges will form along the sides of your plant’s leaves, and stalks and stems will appear weak and unstructured. New leaves may also appear discoloured or misshapen, while growth will slow significantly. In addition, calcium deficiency can affect the root zone as well, leaving roots stunted and unable to properly uptake other nutrients.

Even when calcium is present in your plant’s growing medium and nutrient solution, you can still encounter deficiencies. Often caused by overfeeding magnesium and potassium, nutrient lockout results from nutrient salt buildup in the roots, leaving the plant unable to uptake calcium or other nutrients.


Magnesium is key for proper plant development. It is the central atom of chlorophyll, the molecule responsible for converting light energy into plant growth during photosynthesis. Without adequate levels of magnesium, plants simply cannot convert sunlight into the sugars and carbohydrates necessary for growth.

Often found in quality soils and substrates, magnesium is drawn from the roots of the plant and transported throughout. For this reason, it is important to maintain adequate levels in your plant’s growing medium, an issue that can cause problems for those growing in hydroponic systems.

Unlike calcium, magnesium is a mobile nutrient, meaning a plant has the ability to transport this nutrient wherever it is needed within the plant. Therefore, the main cause for magnesium deficiencies tends to be underfeeding or a lack of availability caused by deficient growing medium or nutrient lockout.

Magnesium deficiency is easier to spot than calcium deficiency, as its effects can be observed on leaves rather quickly. Affecting older, lower leaves first, leaves will begin yellowing around the base, with the colour change spreading throughout the leaves. Eventually, leaf tips will turn brown and brittle, and stalks will begin to turn purple.

A Simple Solution: Cal/Mag

Cal/mag supplements are the simple solution to the problem of calcium and magnesium deficiency, but they must be appropriately used to achieve desired results. The combination of a cal/mag supplement with other fertilizers has the potential to greatly invigorate your plants, spurring on new growth and revitalizing foliage that may have been damaged.

While it will never be possible to bring back leaves that have been affected by a deficiency, new growth will appear healthy and replace older leaves.

Of course, it’s also possible to overdo it with cal/mag. Overuse often results in nutrient lockout of one or more vital nutrients, causing the plant to quickly diminish. This is why it is essential to understand the reactions of cal/mag with your basic fertilizer. For example, most flowering nutrients contain large amounts of phosphorous and potassium, used to boost bloom size and enhance terpene production. The combination of high concentrations of potassium and calcium, however, can lead to magnesium lockout.

Understanding the interactions of calcium and magnesium with your base nutrients will be vital to success. Often, the necessity of supplementary cal/mag will depend on the type of grow being conducted and the type of substrate being used.

When is Cal/Mag Necessary?

Many factors influence the necessity (or lack thereof) of cal/mag supplementation. Whether you’re relying simply on the plant’s appearance or basing your determination on calculations regarding your reservoir, it’s essential to consider a few key elements.

The first key consideration is growing medium. Various substrates require varying degrees of supplementation. A quality bagged potting soil, for instance, may contain enough fertilizers and compost to power your plants all the way through their life cycle. Other mediums, particularly hydro mediums like coco coir, require supplementation in order for plants to be able to uptake any nutrients from them. Knowing what your particular medium requires is a great place to start.

Maintaining proper grow medium conditions is necessary as well. Monitoring the pH level of your medium will ensure that it stays within a proper pH range, allowing your plant’s root system to maximize nutes and additives. Some growers opt to use dolomite lime in their soil to supplement calcium and magnesium, as well as to maintain pH levels. Other growers implement the age-old method of applying Epsom salts to their soil for their high magnesium content. Whichever methods are used, it’s important to monitor the pH level of your growing medium.

Another critical factor in determining whether a cal-mag supplement is necessary is the appearance of the plant itself. Carefully examining the plant for discoloured leaves, distorted growth, or weak stalks is a great way to catch a deficiency before it gets out of control. Many popular fertilizers and nutrient lines contain adequate amounts of calcium and magnesium, leaving little for the grower to worry about. Others, though, may choose to mix their own reservoirs from a conglomeration of base elements. For these growers, a cal/mag supplement will be vital.

Still another consideration to take into account is the hardness or softness of the water being utilized. Those who live in areas with very hard water will not need to supplement cal/mag nearly as much as those who live in soft water areas. Filtration systems may affect your water, as well. Reverse osmosis (RO) systems, for example, filter out the vast majority of calcium and magnesium from tap water, requiring the grower to supplement with cal/mag.

A Word on Coco Coir 

Coco coir is a fantastic growing medium, beloved worldwide for its ability to replicate soil while providing hydroponic conditions. It is worth mentioning here, however, because of all substrates, it possesses a particular relationship with cal/mag supplements.

Coco coir is widely renowned for being inert or neutral, meaning that it doesn’t hold onto nutrients salts the way soil will. Like other hydroponic mediums, it provides the grower complete control of what nutrients their plant receives. Unlike other inert mediums, however, coco is unique in that it still possesses cation (positively charged ions) exchange sites like soil.

Unfortunately for coco growers, the cation exchange sites in coir will bond to calcium and magnesium ions, rendering them unavailable for use. For this reason, it is necessary to add cal/mag to coco coir prior to use, in a process known as “buffering.”

Suitably buffered coco coir will allow the plant to access supplemented calcium and magnesium, not leave it simply suspended in the medium. Many soil growers opt to cut off cal/mag application midway through the flowering stage, but coco growers will need to continue supplementing throughout the bloom phase.

Cal/Mag: Asks for Little, Gives a Lot

It’s never a bad idea to keep a cal/mag supplement on hand. It’s useful for curbing minor deficiencies and increasing plant vigour and strength. However, because some nutrient lines already contain supplemented calcium and magnesium, it’s important to do your homework and determine if you’ll need to supplement and how much. You can be assured, though, that if you’re growing in coco coir, you’ll certainly need a calcium-magnesium supplement handy. After all, it’s only taking up a little space on your nutrient shelf.

Post author
Michael Richey
Michael loves to grow cannabis and write about what he learns along the way. His best friends are his dogs and marijuana.
See more from Michael Richey

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