Cal-Mag Supplements: A One-Two Punch for Plants

By Cory Hughes
Published: November 1, 2016 | Last updated: May 4, 2021 06:10:20
Key Takeaways

If the leaves in your garden are starting to grow in misshapen patterns or are showing spotty, discolored patches, odds are you have a nutrient deficiency. Identifying deficiencies can be a challenge, but once you have eliminated your primary macronutrients from the list, your problem probably lies with your secondary nutrients—calcium and magnesium. Fertilizing your garden with a cal-mag supplement will strengthen your plants and ensure a lush, green hue returns to your beauties.

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Calcium and magnesium are two of the three secondary nutrients required by plants, the third being sulfur. They are secondary in as far as quantities required for growth, not importance of function. Simply put, calcium and magnesium are necessary for plant growth and development and are easy to fix in the case of a deficiency. While nitrogen stands as requiring the highest levels in a nutrient feed, secondary nutrients calcium and magnesium only require about eight to 10 per cent of that amount. While their required levels are much lower, their importance can’t be understated.



Calcium is important for strengthening a plant’s cell walls and maintaining its structure. The importance of calcium becomes evident when you see degradation of the leaves, which eventually shows up as discoloration and canker-like sores. When calcium is absent, the integrity of the cell wall falls apart. Calcium is not mobile inside the plant, so the plant cannot borrow from nearby areas to make up for local deficiencies. Instead, calcium depends on transpiration to reach its destination.

Once absorbed by the root system, calcium is then pulled through the plant and deposited where needed as excess water, which played a role in the transpiration process, escapes the plant through tiny pores called stomata. As the process depends on the flow of water through the plant, any factor that affects hydration levels, such as humidity and temperature, can have a significant impact on calcium levels, even if your feed blend is properly mixed.


Calcium deficiencies also result in stunted root growth. Many times, even if the soil is rich in calcium, it may not be in a form readily available for absorption by the plant. This is where supplementation comes in. By supplementing calcium in a form that is geared toward maximum uptake, you increase the amount of bio-available calcium in the soil, which results in a much-needed boost for your plants. A plant with increased calcium levels will result in a higher than usual pH in the soil. However, finding a higher than usual pH level is not necessarily an indicator of well-balanced calcium levels.


Magnesium deficiencies are easily identified by a yellowing of the leaves with distinct green veins. As magnesium can float freely throughout the plant, the veins remain green as newer leaves are actively sucking up what magnesium is available. This results primarily in deficiencies in older leaves.

Magnesium is the key element in chlorophyll, which gives plants their vibrant green color. This secondary plant nutrient also plays numerous roles in the photosynthesis process, including the activation of enzymes, and the creation and transportation of sugars throughout the plant. The most common cause of magnesium deficiencies is simply a lack of adequate application.


Enter the One-Two Punch

Balancing your magnesium and other secondary nutrients is vital since disproportional amounts can limit uptake. Fortunately, treating calcium and magnesium deficiencies can be handled in one shot. Adding a cal-mag supplement to your plants can help alleviate many of the symptoms showing up in your garden. There are a handful of approaches to cal-mag supplementation. You can head to your local grow shop and pick up one of a handful of excellent quality fertilizers, or you can mix up a batch at home. While supplementing with calcium by itself, it is possible to use a foliar application, while the vast majority of cal-mag supplements require introduction to the root system.

Mixing your own cal-mag supplement at home is as easy as measuring out raw nutrient salts. Each of the commercial manufacturers has their own formulation, but they all follow similar methods, tweaking things here and there in order to achieve specific results. If going the route of mixing up your own cal-mag, start with magnesium sulfate, otherwise known as Epsom salts. Epsom salts have been a staple of the gardening community for magnesium supplementation and make the perfect base for your cal-mag mix.


The next ingredient is calcium nitrate. Calcium nitrate is a common plant fertilizer used in standard feed mixes. As both calcium and magnesium come in a chunky, powdery form, they need to be dissolved in water at specific ratios. Taking into consideration personal preference, your calcium should be at a parts per million of at least double that of magnesium.

One example of a homemade cal-mag has a ppm ratio as follows: magnesium 120 ppm and calcium 260 ppm. This breaks down per gallon to 5.8 grams of calcium nitrate and 4.6 grams of magnesium sulfate (Epsom salts). You can always adjust your mixture to compensate for specific deficiencies, depending on the symptoms you are seeing in your garden.

If all of this sounds like too much work, there are a ton of cal-mag formulations out there that are ready to go. Many of them include additional nutrients like iron or nitrogen, but they all stay within certain parameters regarding their levels of calcium and magnesium.

Nutrient deficiencies in the garden can arise for a multitude of reasons. Everything from high temperature and humidity, to an imbalance in your fertilizer can result in calcium and magnesium deficiencies. The decaying of leaf structure, fading of leaves, and yellowing with green veins are indications your calcium and magnesium levels are low.

As these two nutrients play such an important role in plant growth and development, it is important to be able to recognize the symptoms of deficiency quickly and respond appropriately. Really coming to understand the role that calcium and magnesium play in your garden is vital in knowing how to correct problems that arise from deficiency.


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Written by Cory Hughes | Commercial Grower

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Cory Hughes is a former police officer turned full-time commercial grower in Denver, Colorado.

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