The Magic Of Cal-mag: What Is Cal-mag And How To Use It

Brock Ingham

This post follows our research editorial guidelines.

Brock Ingham

Everyone has a “magic potion” for getting the most from their gardens, and for many that magic comes from Cal-Mag. I personally like to know exactly what I’m putting in my precious garden, and am naturally wary of any product or treatment that’s said to cure all ills.

Cal-mag is no different – it has its role to play in my garden, especially in my hydroponic setups, but it’s not magic. It’s science, and like all science it pays to take the time to learn what exactly is in that potion, and how I can make the most of its magic.

Calcium Magnesium deposits at yellowstone
Calcium Magnesium deposits at Yellowstone

Why use Cal-mag?

Cal-Mag provides vital micro-nutrients for heavy feeding plants, especially those that produce large amounts of blossoms or fruit. Apply Cal-Mag as a foliar spray or as a root drench during the growing season to plants to prevent deficiency diseases, or to treat those impacted by nutrient deficiencies.

What is Cal-Mag made of?

The name says it all – Cal Mag is a liquid plant food containing bioavailable calcium and magnesium. While there’s lots of do-it-yourself recipes for Calmag out there, it’s best to look at commercial formulas. They’re uniformly more predictable, reliable and to be totally frank a whole a lot less mucking about.

Most are a liquid or powder form, to be added to water. They contain soluble magnesium and calcium, and many brands include other nutrients and minerals. Let’s break down what each of those parts means for you as a gardener.


Calcium is a vital nutrient, performing a large number of vital roles in plant biology. It’s a crucial component in plant cell walls, and helps transport other minerals from one side of cell membranes to the other. It’s also involved in some enzyme function.

It’s what’s known as an immobile nutrient – once the plant has put it to use in one part of its structure, it can’t be relocated. That’s why we see deficiency in young leaves first – even if old leaves have more than enough, the calcium is fixed and can’t travel to where it’s needed.

Without enough calcium, those membranes become weak. The cell walls can’t control their permeability, resulting in the leeching of vital nutrients and an eventual waterlogging of affected cells. Mostly we see it as yellowing leaves, especially in newer growth, and fruit that becomes soggy and sodden from too much moisture.


Magnesium powder
Magnesium powder

Magnesium is just as important. It’s a key component in the construction of chlorophyll, arguably the most important of all chemicals inside a plant. That chlorophyll is the powerhouse of the plant. It’s responsible for turning oxygen and water into sugar, fueling all the plants growth. Without it, there’s no chance of vigorous growth at all

Unlike calcium, magnesium is mobile and can be redeployed, so to speak, if the plant becomes deficient. As a result, magnesium deficiencies show in older leaves first, as the plant shifts its dwindling supplies to new growth.

Chlorosis is the defining trait of magnesium deficiencies. Leaves turn yellow, from the oldest to the youngest. It makes sense – after all, no magnesium, no chlorophyll.

Other Nutrients

Many Calmag solutions include iron, usually as a chelate. This is because many of the conditions that lead to soils poor in calcium and magnesium can also lead to low levels of iron, so it pays to cover all bases. Iron deficiencies also cause the same sort of chlorosis as magnesium deficiencies, so it sometimes pays to apply both at once.

Others will include nitrogen, too, presumably because plants need a fairly consistent supply of the stuff, and a deficient plant is likely to spring to life, hungry and ready to grow, once the deficiency is corrected. This is not the case for all brands, so it pays to check – there are plenty of cases where a low or nitrogen fertilizer is preferred.

Keeping Balance

Calcium and magnesium work in concert within the plant, and so for many years it was assumed you had to ensure a good ratio of calcium to magnesium in order to get good growth from your plants.

We now know that it’s both simpler and more complicated than that. The ratio of calcium to magnesium in the soil isn’t terribly important, provided there’s enough of both for whatever is growing.

But too much calcium can cause a drop in available magnesium. The two get along, and readily bind to each other. You may well wind up with a magnesium deficiency if you go too hard with a purely calcium-based amendment. It’s why Cal-mag fertilizers are so useful – they prevent magnesium depletion while addressing both deficiencies at one.

When to use Cal-Mag

Cal-mag is best used regularly. As calcium is non-mobile, it needs to be present in the soil for use all through the growing season. As flowers and fruit develop it’s especially crucial to keep everything well fed and clicking along.

This is especially true if the weather has been erratic – plants draw calcium from the soil in water, so if the weather has alternated very wet to very dry, it interrupts that uptake.

I’d suggest you apply Calmag as a supplement for heavy feeders through the growing season, especially if the weather has been sketchy. Depending on your location, this could be anywhere from early spring through to late fall.

Be mindful that plants with low fertilizer requirements won’t benefit from Cal-mag at all, and in fact can be harmed by too much of it in the soil.

How To Identify When to Use Cal-Mag

“The best way to check for a lack of nutrients in the soil is to test a sample of the soil in the laboratory. While there are do-it-yourself kits available, your county’s WVU Extension agent can provide the necessary supplies and technical assistance for proper sample collection and submission. The resulting soil analysis report will provide information on soil deficiencies and will make recommendations on how to correct them.” Quote: Mahfuz Rahman, West Virginia University Extension Specialist.

Calmag is best used before problems arise, and for that it’s a good idea to test your soil.

Most universities have an extension service that will happily perform soil tests for you, giving you a precise understanding of what’s going on beneath the surface. The reports generally give you a broad idea of the soil’s nutritional profile, allowing you to tailor your approach, as well as its acidity. A test can save you a lot of trouble to just work out what’s wrong from the start, and make it much easier to correct.

That said, fast growers and plants that produce lots of harvest – like the aforementioned tomatoes and peppers, and lettuces, some herbs and fruit trees – do well with a seasonal dose. This is especially true for plants grown in containers or raised beds.

You can also use Calmag to treat either magnesium deficiencies, or calcium deficiencies as they appear. Both show up as chlorosis, with magnesium depleting the green from old leaves and calcium from the young.

How Often to Use Cal-mag in Hydroponics?

It can be challenging to get the nutrient mix in your hydroponics systems just right, especially if you’re seeing weird results. Yellowing leaves and poor growth may be the result of a calcium deficiency, especially if you’re growing with coco coir.

Coco substrates have a few unique chemical properties that can cause problems if not treated. Chief among these is the high amount of potassium naturally found in coco. This potassium tends to swap places with calcium in nutrient solutions, resulting in too much potassium and not enough calcium in your system.

Fortunately, treating with Calmag is a good way to remediate this. The magnesium has its own part to play in the complex chemistry happening at the root level, but together they can work to creating a supportive growing environment for your plants.

How often to use Cal-mag does come down to what sort of crops you are growing. Fast fruiters like tomatoes, strawberries and lettuces will need more frequent supplementation than slower growers like rosemary or carrots.

In general, you can add a dose to your solution whenever you change it, and use it as a predatory treatment when setting up new coco coir growing beds or baskets. I also like to add it as a supplement for flowering and fruiting plants.

How Long Does it Take for Cal-mag to Work?

How you apply the Calmag will determine how effective it is, as well as what you’re hoping to achieve.

As a preventative measure, you may never see the Calmag do its work. That’s the point – you are preventing the deficiencies from developing. If applied judiciously, it’s an invisible barrier, protecting you from crop failure and poor growth.

But if used to treat a diagnosed deficiency, the impact will be felt fastest with foliar application. Magnesium deficiencies will correct quite quickly. While already damaged leaves won’t revive, the grim march of yellow will stop almost immediately. Calcium deficiency is slower to spot, as it’s tied to the development of new tissue, but once you’ve corrected the problem the next wave of leaves or blossoms should be in good health.

Soil application takes longer for the plant to process, but it tends to be more enduring. It can take a few days for the minerals to work their way through a large plant, but once they do it’s a long-lasting result.

Can Cal-Mag be used in potted plants?

It’s safe to use Cal-mag on your potted plants. If they’re showing signs of calcium or magnesium deficiencies, it’s an excellent solution. It’s also a good tonic for plants that flower or fruit abundantly.

It’s a good idea to use a weaker solution, however. Unlike adding fertilizer to soil, everything you put into a pot stays right there. Over time, magnesium salts can build up in the soil, damaging the roots and causing chemical burns to the leaves.

I get around this by performing a flush of the soil once a season, to ‘reset’ the condition of the soil. All this involves is watering the plant deeply, until water flows abundantly from the drainage holes. If done for long enough, this removes excess fertilizer and keeps the soil fresh.

Can you add too much Cal-Mag?

You can always have too much of a good thing, and Cal-Mag is no different.

At best, it’s possible to use Cal-mag to treat disorders caused by totally unrelated deficiencies, or even bacteria or fungus. While in these cases the Cal-mag itself isn’t going to cause too many problems, they certainly aren’t going to fix your problem.

More critically, both calcium and magnesium can spell trouble in too high concentrations. Too much calcium in the soil can result in the uptake of too much of other minerals and not enough of others, a tricky thing to detect. Magnesium sickness is easier to spot, leaving browning on the tips of new growth.

I always stress its best to know for sure before you go applying any specialized nutritional supplement like Calmag. For a general boost, a broad-spectrum tonic like kelp meal is a safer option.

Foliar application vs. Root application

It’s possible to use Cal-Mag as both a spray that’s applied to the leaves – a foliar application – or to add it to the soil as a root application.

Foliar application is fast. It gets those micronutrients right into the leaves through the stoma, bypassing the plant’s vascular systems and getting straight to where it’s needed most.

How to use foliar spray

But it’s also quick on, quick off. While foliar application will cause an immediate improvement in any given plant, the true cause of the issue hasn’t been addressed. That soil is still deficient in calcium, magnesium and iron, and once the leaves have used their foliar dose they’ll soon be in need of more. Adding to the soil may be a slower solution, but it’s a more enduring one.

It can become more complicated if your soils already have an abundance of one mineral or another. Too much calcium can inhibit the uptake of other nutrients, and too much iron, especially at high pHs, can cause bronze speckle disorder in vulnerable species, like marigolds or geraniums.

Will Cal-Mag Raise pH?

Calmag does have a tendency to raise the pH of your growing medium. If you’ve got plants in garden beds or pots, the change is minimal and can be safely ignored.

But for hydro setups it’s worth checking. I personally prefer to apply Calmag as a foliar spray on my tomatoes and peppers, as a preventative to end rot. This gets the nutrients straight to the plant without the risk of knocking my solution’s pH off kilter.

Which plants benefit the most from Cal-mag?

tomato seedlings in eggshells 1
Tomato seedlings in eggshells

Heavy feeders are, without a doubt, the most viable candidates for a Cal-Mag treatment.

Tomatoes, peppers, eggplants and other nightshades do well with a dose as they begin to flower. Calcium is critical for the development of good, lush fruit, and without enough during formation you may well lose your whole crop to blossom end rot.

Heavy blooming plants that flower dramatically will also do well with a bit of Calmag in their soil. Ros

Alternative low nitrogen fertilizers essential to plant health


We’ve been applying manure to crops for as long as humans have been gardening. Its benefits have long been understood, and we’re increasingly aware of just how beneficial manures are to the garden.

Different animals produce manure with different levels of nitrogen. Cow manure is the lowest, starting at as little as 1% nitrogen. Once allowed to age, that can drop to effectively zilch, and even very high nitrogen composts like poultry manure will reach a nitrogen of zero if left long enough. It also loses a lot of its characteristic odor, and becomes simply another olfactory note in the garden.

It’s rich in phosphorous and potassium, as well as all the trace minerals key to plant development. That includes calcium and magnesium, though not as abundantly as other sources.

Kelp Meal

Growing Sea kelp
Growing Sea kelp

Kelp meal is probably my favorite of the low nitrogen fertilizers. It’s low in the big three – nitrogen, phosphorus and potassium, but provides an astonishing range of other micronutrients, as well as plant hormones that stimulate root growth.

It can be added to the soil loosely, or used to produce a nourishing tea. I write at great length about kelp meal here. Suffice it to say that it’s an excellent source of trace elements that are often overlooked in other fertilizers, with very few drawbacks.

Bone Meal

For calcium, you can’t go past bone meal. It’s exactly what you’d imagine – ground bone. Gruesome though it might sound, its actually a very sustainable soil amendment, and safe to use too. The bone is superheated to kill pathogens before being finely ground, resulting in a nutrient dense material that provides abundant calcium, as well as being a rich source of phosphorus. I go into more detail on how to use bone meal here.

Like most soil amendments, it pays to check you need it before adding it to soils. Too much phosphorus can be counterproductive, especially if your soil is quite acidic. It also has the alarming tendency to attract scavengers like racoons, keen on finding that nice juicy bone they can smell!

Final thoughts

I don’t expect Cal-mag to work miracles on my garden. But it is a valuable tool, and like any tool it can produce wonderful

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    1. Calmag supplements are typically used to correct calcium and magnesium deficiencies in plants. If your water has a low level of calcium and magnesium, you may need to use calmag every time you feed your plants. However, if your water already has sufficient levels of these nutrients, you may not need to use calmag every time you feed your plants. You should be able to get an idea of the water parts per million (PPM) from your municipality.

      It’s always a good idea to test your water and soil regularly to determine the nutrient levels and adjust your feeding schedule accordingly.

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