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Cal-Mag for Plants: What It Is & Why, When, and How To Use It

If you’re looking for a supplement to feed your plants, you need to know about Cal-Mag before you buy just any old supplement with a few positive reviews.

Cal-Mag is a highly recommended supplement for plants that includes crucial nutrients necessary for achieving optimal growth and high yields.

What is Cal-Mag used for? Cal-Mag is a liquid supplement that provides much-needed calcium, magnesium, and iron to both hydroponic and soil-based plants. The micronutrients in Cal-Mag are primary ingredients needed by all plants but especially by plants that bloom or produce fruits or vegetables.

That said, there’s far more to Cal-Mag than most common supplements for plants have to offer.

Read on below and discover exactly why you should already be using it on your plants.

The Importance of Calcium & Magnesium for Plants

Calcium and magnesium are two of the most essential nutrients needed by all plants.

Both calcium and magnesium also help raise the pH level of the growing medium in which plants grow, be it water or soil. 

Calcium is critical because it aids in the creation of new cells in plants, therefore greatly contributing to crop yields.

A lack of calcium results in slower growth and fewer (or smaller) fruits, vegetables, or blooms.

Magnesium on the other hand is the central core of chlorophyll molecules in the tissue of plants.

That means it helps capture the energy of the sun and convert it into energy for growth. A major lack of magnesium results in slow or stunted growth in plants.

How To Identify a Cal-Mag Deficiency

The most obvious signs of a Cal-Mag deficiency are slow and stunted growth. Localized rot on buds, new leaves, fruits, and blooms are also a sure sign of a Cal-Mag deficiency.

Even though you may not be able to see them as easily as the other signs, rotting and/or slow-growing root systems are another sign of a Cal-Mag deficiency.

Any one of the above issues, if left untreated, eventually leads to further damage, stunted growth, or even the slow death of your plants.

What Does Cal-Mag Do for Plants?

Cal-Mag is designed for reducing damage to plants due to lack of nutrients and preventing tip burn and root and blossom rotting.

Further, it is meant for generally maximizing overall plant health, including growth states and yield sizes.

What Are the Benefits of Cal-Mag?

The main benefit of giving plants Cal-Mag is the marked increase in their general health and optimal yields.

When you feed your plants Cal-Mag, you know that they are getting all the secondary nutrients they need. 

Another benefit is the lack of root rot, burned leaves, and generally poor-looking and poor-performing plants.

These benefits are possible due to the calcium, magnesium, and iron provided by the Cal-Mag supplement.

Do You Need Cal-Mag in Hydroponics?

Cal-Mag is extremely beneficial in hydroponics, though not technically absolutely necessary.

But, as hydroponic plants rely on liquid nutrients for their very survival, let alone to thrive, Cal-Mag can make a world of difference on your plants’ health. 

Unlike soil-based plants that leach nutrients, hydroponic plants can not grow without you introducing nutrients to their system.

Cal-Mag is particularly useful in hydroponics because it provides your plants with healthy levels of much-needed secondary nutrients that other products sometimes minimize.

How Do You Use Cal-Mag in Hydroponics? 

A young woman tending to her tiered hydroponic setup.

You can add Cal-Mag straight to the main reservoir in your hydroponic grow system every time you change the water.

You can also add it directly to the reservoir each time you top off water levels. All you need to do is follow the instructions on the Cal-Mag product.

Typically, adding Cal-Mag to your system simply consists of mixing a measured amount into a water container, shaking, and pouring it into your reservoir.

Just keep in mind that you may need to adjust the pH levels of your system before and/or after using Cal-Mag in your hydroponic system. 

How Much Cal-Mag Should I Use?

Cal-Mag is really easy to use. All you have to do is add a teaspoon of powder, or 5 milliliters of liquid, for every  gallon of water in your hydroponic system.

For application with soil-based growing, add the same amount into each gallon of water you put into your watering can.

For example, for tomatoes growing in a hydroponic growing system, with a 20-gallon tank, you need to add 100 milliliters into the tank whenever the water is changed out.

Also, adding just a half of a teaspoon to a full teaspoon into a gallon jug of water for topping the tank in between changes.

As a booster, you may add 10 to 15 milliliters to a gallon of water and serve it to your plants. This works for both hydroponic and soil-based growing.

Can I Mix Cal-Mag With Nutrients?

Cal-Mag can be used in tandem with nutrients. It can also be added before or after feeding your plants with nutrients.

When used with proper nutrients, Cal-Mag acts as an amplifying agent, greatly increasing the nutrient absorption rate of plants.  

What Is the Best Cal-Mag Supplement for Plants?

Cal-Mag products are all specialized supplements for plants, making them no-nonsense purchases for gardeners that want to go the extra step in maintaining their plants and producing extraordinary yields.

The best overall Cal-Mag Supplement for plants is by far Cal-Mag Plus.

Below, we take a closer look at Cal-Mag Plus and why it’s the favorite supplement for plants of gardeners all around the world.

Botanicare Cal-Mag Plus

Cal-Mag Plus is a specially formulated solution designed to fortify plants. It is used effectively for preventing deficiency commonly found in plants lacking micronutrients. 

The Cal-Mag Plus formula corrects imbalances in plants caused by low calcium, magnesium, and/or iron.

It works both for regular watering regimens and/or as a micronutrient booster for your plants.

Even more impressive, Cal-Mag Plus works well in just about any sort of growing medium or system, including hydroponics, soil, LECA, coco coir, and more. It has an NPK of 2-0-0.


  • For use in water and/or spray.
  • Provides calcium, magnesium, and iron.
  • Works with most any plant types.
  • For use in containers, soil, coco coir, and hydroponics.


  • Needs to be mixed with water before application.

Should I Use Cal-Mag Every Watering?

You can use Cal-Mag with every watering, but there is really no need unless your plants are absolutely suffering from nutrient deficiency.

In the case that your plants are obviously unhealthy and seem to be deficient in nutrients, you can add some Cal-Mag to the water you use for regular watering. 

Otherwise, after a preliminary treatment with Cal-Mag, your plants won’t need Cal-Mag with every single watering. 

Also, if you’re watering more than once per week, you should avoid using Cal-Mag for more than one of those weekly watering sessions.

Should I Use Cal-Mag During Flowering?

Whether or not you use Cal-Mag during the flowering stage of your plant’s growth cycles is entirely up to your own opinion and better judgment.

Although it may not be necessary for the general wellness of your plant, if they are healthy during their flowering, it may still be used in order to achieve extraordinary blooms and/or fruit.

Can Cal-Mag Be Used for a Foliar Spray?

A sprayer attachment hooked onto a hose used for foliar feeding of plants.

Cal-Mag, specially Cal-Mag Plus, works great as a foliar spray. Simply mix it with water and use it to spray as you typically would. 

That said, if you use Cal-Mag as a foliar spray, avoid applying other/similar products before or after as it may lead to poor results including molding or mildew.

Should You Use Cal-Mag for Tomatoes?

Calcium and magnesium are a couple of the most crucial nutrients for growing amazing tomatoes.

That’s why so many gardeners feed their tomatoes these two specific nutrients in any way possible, like constantly feeding their soil with eggshells.

Forget about grinding messy eggshells and constantly scooping them out around your tomato plants.

Instead, add a teaspoon of Cal-Mag to your water. That way, you take the guesswork out of whether or not your tomatoes will be fat and juicy.

Is Cal-Mag Good for All Plants?

Cal-Mag may not be necessary for plants that are super healthy and growing in an amazingly well-cared-for environment, but for just about all other plants, it is definitely helpful.

One of the best parts about feeding plants with Cal-Mag is that it works well for nearly any species you can think of and it’s hard to overdo it.

Will Cal-Mag Raise pH?

Depending on the version of Cal-Mag you’re using, it may slightly affect pH levels one way or the other.

Make sure to read the label of your Cal-Mag product regarding its effect on pH levels.

When using Cal-Mag in hydroponic systems, it’s highly recommended to adjust pH levels accordingly before use with Cal-Mag (anticipating its regular effect on the pH of your system) as well as testing and adjusting properly afterward. 

Having an accurate pH monitor, like this American-made pH tester, on hand will eliminate any guesswork and ensure you’re maintaining proper levels for your plants.

Does Cal-Mag Raise EC?

Cal-Mag products may raise EC levels by as much as 0.5, which is quite a jump. Most hydroponic systems shoot for an EC of around 1.0.

Using Cal-Mag without testing EC levels with an accurate tester like this, could be damaging to your plants, as an improper EC level makes it hard for plants to absorb water, nutrients, and oxygen.

Is Cal-Mag Organic?

Not all Cal-Mag is completely organic. FoxFarm Bush Doctor, TPS Cal-Mag OAC, and Roots Organics Cal-Mag are, however, completely organic sources of calcium and magnesium.

Natural Cal-Mag products don’t include nitrogen, phosphate, or potassium.

Does Cal-Mag Have Nitrogen in It?

The older versions of Cal-Mag products were mainly made with nitrogen, and some of the products do still have nitrogen in them.

That said, there are numerous organic Cal-Mag products that have zero nitrogen in them.

What Is Cal-Mag Made Of?

The recipe for Cal-Mag is simple yet potent: calcium nitrate and magnesium nitrate, though many contain iron as well.

Some products may contain additional micronutrients, such as zinc, copper, boron, manganese, sulfur, and fulvic acid.

Does Cal-Mag Kill Microbes?

Cal-Mag doesn’t kill microbes. That said, it is theoretically possible that if used in copious amounts, the iron EDTA levels could affect microbes.

The issue with this idea is that the iron EDTA levels would kill the plants long before they affected present microbes.

Can You Use Cal-Mag in Soil?

A lovely cottage garden overflowing with colorful blooms.

Cal-Mag can absolutely and should be used in soil.

That’s because it doesn’t affect the main level of macronutrients (or NPK) of plants but greatly aids the micronutrients present in the soil for plants to feed on.

Calcium and magnesium are the main two supplements plants need to grow and remain healthy, besides the main nutrients nitrogen, phosphorus, and potassium.

Cal-Mag is the only source of these micronutrients that your plants will need. 

How To Use Cal-Mag With Soil-Based Plants

Cal-Mag is very easy to use with soil-based plants. Just mix in one teaspoon of Cal-Mag to every gallon of water.

Apply the solution to your garden just as you would normally water it.

To use Cal-Mag as a booster, accelerate your plant life by adding an extra teaspoon to each gallon of water (two teaspoons total) before feeding your plants. 

How Much Is a Bottle of Cal-Mag?

Depending on the size of the Cal-Mag product and the retailer you shop with, Cal-Mag costs around $18 to $20 for one quart and between $35 and $40 for one gallon.

That said, there is a range of Cal-Mag products from which to choose, and prices may vary.

How Do You Make Homemade Cal-Mag?

Cal-Mag can be made at home, but there is no guaranteeing how successful it will be in comparison to actual Cal-Mag products.

To make a homemade Cal-Mag solution, mix roughly two parts calcium to one part magnesium.

A general rule of thumb for measuring homemade Cal-Mag is a single tablespoon of calcium phosphate mixed with a single tablespoon of Epsom salt into a single gallon of water. 

Pro Tip: A single tablespoon of molasses may be used in the absence of Epsom salt.

Does Cal-Mag Go Bad?

It is unclear exactly how long Cal-Mag lasts, but the general opinion is that it takes a very long time for the product to go bad.

Many gardeners claim Cal-Mag works even after it is several years old. 

At any rate, if you use it within the first few years after purchasing it, you’ll not have to worry about it going bad.

When Should I Stop Using Cal-Mag?

You should stop using Cal-Mag on your plants as soon as they visibly seem to have recovered from the nutrient deficiency.

If your plants seem happy and healthy, there is no need to continue adding Cal-Mag with every watering.

A Final Word About Cal-Mag for Plants

Cal-Mag is a great supplement for plants that are suffering from a nutrient deficiency or simply need a little boost.

Cal-Mag doesn’t just help your plants recover, it helps prevent future deficiencies as well as the common issues that come along with a lack of crucial micronutrients calcium, magnesium, and iron.

Whether you’re looking for a good and safe product to give your plants a little pick-me-up, bigger blooms, and larger fruit or you need to save your plants from root rot, Cal-Mag may be a great solution for you to explore.