Getting Enough of the Right Stuff: Calcium and Magnesium

By Jennifer Casey
Published: December 1, 2016 | Last updated: May 4, 2021 10:19:04
Key Takeaways

Calcium and magnesium are the building blocks of plants, but they need to come in specific forms and amounts in order to do any good.

Like humans and other animals, plants (particularly young plants) require the appropriate amounts of calcium and magnesium in order to grow and thrive.


Calcium is important to the cellular structure of your plants. It is also a messenger that controls nutrient uptake, and it acts as a catalyst and activator for enzyme activity. Magnesium is the central molecule of chlorophyll and is integral to photosynthesis.

Deficiencies in calcium and magnesium can lead to problems like yellowing of leaves between veins, leaving a marbled appearance.


They can also lead to stunted plants and fruit, blossom end rot in tomatoes, bitter fruit in apples and tip burn in cabbage and Brussels sprouts.

So, care should be taken to ensure that your plants are getting the right amount of calcium and magnesium they need to thrive, but also that they are getting these minerals in the form they need. If present, unusable forms of these elements can interfere with the absorption of nutrients.

Calcium in Your Water Supply

Calcium might be the fifth most abundant element in the Earth's crust, but unlike other elements like gold or silver, it isn't found in its elemental form in the environment because it easily bonds to other elements and compounds like oxygen and water.


One of the most common compounds formed is calcium carbonate, which combines calcium with two of the other most abundant elements on earth—carbon and oxygen.

Calcium carbonate is the form of calcium found in most water. Small amounts give water a crisp, clean taste.


Unfortunately, calcium carbonate is not always the most viable source of calcium for plants—instead, most nutrient companies use one or a combination of the following: calcium nitrate, calcium chloride, calcium gluconate, calcium acetate, magnesium nitrate and magnesium carbonate, among others.

Each of these forms of calcium and magnesium have different absorption properties, and some of the better chelated forms can be absorbed by diffusion of water through the cell membrane, enabling the fastest and most thorough absorption.

It is a common misconception that water, especially hard water, provides ample mineral content for plants and humans.

After all, water hardness refers to high mineral content. (Those minerals can include many different compounds, but are primarily composed of calcium and magnesium.) Seems convenient, right? So, what's the problem?

The problem is threefold: size, form, and quantity. Calcium and magnesium enter our water through underground deposits that leach into aquifers.

This leaching is purely physical—basically, tiny little pieces of calcium and magnesium are sloughing off into the water and the force of the water's motion is breaking some of it into smaller pieces.

However, if calcium and magnesium particles are too big, they can't be absorbed through root hairs, and often don't enter the plant at all. In that case, the plant isn't getting the nutrients it needs because the minerals are the wrong size—and the minerals can even bind up on root hairs, preventing the absorption of other nutrients.

The best way to get rid of of unusable calcium and magnesium is by using reverse osmosis filtration (RO). This system will remove 96 to 99% of the contaminants in your water, including—but not limited to—unusable calcium and magnesium.

Water softeners are an excellent way to get unwanted minerals out of your home's water supply. Unfortunately, however, softeners exchange calcium carbonate for sodium or potassium chloride, which aren't any better for your plants than the unusable minerals!

The good news is softeners work wonderfully as a pre-filter for your RO systems. Reverse osmosis removes the potassium or sodium chloride the softener leaves behind, and using the softened water in your RO system will extend the life of your RO membranes.

Using RO to remove unusable calcium and magnesium and other contaminants is standard practice for most indoor gardeners. This pure water helps stabilize pH levels and ensures a healthy plant by removing toxic contaminates, as well as preventing nutrient lock-out from those unusable calcium and magnesium particles. (Read More: Perfecting the pH in Your Hydroponic Nutrient Solutions)

It is important, however, to replace that unusable calcium and magnesium with a form more readily absorbed (your RO water will have no mineral content).

When purchasing a calcium-magnesium product, look for a higher Ca:Mg ratio above 3:1 and chose a buffered form of calcium, like calcium carbonate, to reduce pH fluctuations.

When feeding later in the flowering cycle, use a calcium-magnesium product, which are typically called cal-mag supplements, that has a low nitrogen concentration. Ensuring the proper calcium and magnesium levels in your water, and therefore your plants, will help lead to a successful and healthy crop.


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Written by Jennifer Casey

Profile Picture of Jennifer Casey
Jennifer Casey loves her two dogs, her husband, the amazing garden he grows, and living in the country. In that order. Crafting, reading, and cooking food she picks from the garden are her favorite ways to spend her time.

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