What Does Magnesium Do For Plants? (How to Give Them More)

Are you looking to find out what magnesium does for plants?  Do you want to find out how to give plants more of this important nutrient?  If so, you’re in the right place.

So, what does magnesium do for plants?  Plants need magnesium to make chlorophyll, which is used in photosynthesis to make energy for growth.  Magnesium also helps to carry phosphorus through plant tissues.

Magnesium is important for plant growth, so you need to get the right amount in your soil.  Both too little and too much magnesium will harm your plants.

We’ll start off by taking a closer look at what magnesium does for plants and why it is so important.  Then, we’ll look at ways to find out if your soil needs magnesium, and how to add it.

Let’s begin.

What Does Magnesium Do For Plants?

The most important role of magnesium is as the central atom in a chlorophyll molecule.  As you can see in the image below, there would be no chlorophyll without magnesium.

chlorophyll molecule
Magnesium is the central atom in a chlorophyll molecule, surrounded by 4 nitrogen atoms.

Chlorophyll is what makes plants green, but it does a lot more than that.  Chlorophyll is a vital part of photosynthesis.  You can learn more about photosynthesis on Wikipedia.

Photosynthesis allows plants to turn light, water, and carbon dioxide into energy (sugar) and oxygen.  Plants need energy in order to grow and produce flowers or fruit.

Without enough magnesium, a plant cannot produce enough energy.  Without energy, the plant will stop growing.

Magnesium also helps to carry phosphorus through plant tissues.  Phosphorus is one of the 3 primary nutrients for plants (NPK, or nitrogen, phosphorus, potassium).

According to the Mississippi State University Extension, magnesium is a secondary plant nutrient.  Most plants need less magnesium than nitrogen, phosphorus, or potassium.

However, it is still an important nutrient, and plants will suffer without enough of it.

How Do You Test for Magnesium Deficiency in Soil?

The easiest way to test for magnesium deficiency in soil is to do a soil test.  A soil test will tell you the pH and nutrient levels in your soil.

soil test kit
A home soil test kit can give you some information, but a University Extension lab test will give more accurate and detailed results.

Some home soil test kits are not very accurate.  Others will not indicate the level of magnesium in your soil.

A safer bet is to send a soil sample to your local agricultural extension for testing.  You can learn more about how to do a soil test in my article here.

Without enough magnesium in soil, your plants will suffer from magnesium deficiency.  One sign of magnesium deficiency in plants is yellow leaves (also called chlorosis).

According to the Michigan State University Extension, magnesium is a mobile nutrient.  This means that a plant can easily move magnesium through its tissues.

So, when a plant has a magnesium deficiency, it moves magnesium from lower leaves to upper leaves (since the new growth is more important).

Thus, the bottom leaves will turn yellow first if your plant has a deficiency of magnesium (or perhaps another mobile nutrient).

You can learn more about the symptoms and causes of magnesium deficiency in my article here.

What Causes Magnesium Deficiency in Plants?

A magnesium deficiency in plants may be caused by magnesium deficiency in soil.  This is more likely to happen in sandy soil (as opposed to clay soil, which tends to have more magnesium).

sandy soil
Magnesium deficiency is more likely in sandy soil.

However, there are other possible causes of magnesium deficiency in plants.  One common cause is a lack of water, so we’ll start there.

Lack of Water

A lack of water does more than just cause wilting leaves.  A lack of water can also cause nutrient deficiencies in plants.

Without water, magnesium cannot dissolve, and plant roots cannot absorb it from soil.

Plants need water to help them get the nutrients they need.  For example, after magnesium dissolves in water, plants absorb the water and magnesium solution through their roots.

Without enough water in the soil, plants will not be able to absorb magnesium.  This will lead to yellow leaves and other nutrient deficiency symptoms.

Soil pH Imbalance

A soil pH that is too low or too high will also cause a magnesium deficiency in plants.  As you can see in this chart from Research Gate, soil pH has a dramatic effect on nutrient availability in soil.

For example, according to the University of Minnesota Extension, magnesium deficiency in plants is more likely to occur when soil pH falls below 5.5 (somewhat acidic).

If your soil pH is too low, you can add lime (calcium carbonate) to raise the pH.  You should consider using dolomitic lime to avoid excessive calcium, which can cause magnesium deficiency (more on this later).

Nutrient Imbalance

An excessive amount of other nutrients in the soil can also cause a magnesium deficiency in plants.  When there is too much of one nutrient in the soil, it can prevent plants from absorbing other important nutrients.

For example, too much calcium in soil can prevent plants from absorbing magnesium.

calcium carbonate
Adding too much lime to your soil leads to excessive calcium, which can cause magnesium deficiency in plants.

One reason for this is that magnesium and calcium have similar chemical properties (they are in the same column on the periodic table, so they behave in similar ways).

Basically, calcium and magnesium “compete” for uptake by a plant’s roots.  When there is too much calcium in the soil, magnesium loses this competition.

Thus, the plant ends up with a magnesium deficiency.  This can happen even when there is plenty of magnesium in the soil.

If you use too much lime (calcium carbonate) or other calcium-rich soil amendments, then you can end up with too much calcium in your soil.

According to the South Dakota State University Extension, adding too much potassium or ammonium nitrate (a high-nitrogen fertilizer) to your soil will also cause a magnesium deficiency in your plants.

What is a Good Source of Magnesium for Plants?

If you need to add magnesium to your soil, there are several good sources you can use.  However, as mentioned earlier, it is a good idea to do a soil test before adding amendments to your soil.

Otherwise, you could add too much magnesium to your soil, causing another problem entirely (more on this later).


Adding compost to your garden soil is a smart decision, even if your plants don’t have a nutrient deficiency.  Compost adds nutrients to soil, but it also adds organic material.

compost bin
Compost adds nutrients and organic material to your garden soil.

This organic material breaks down over time to provide slow-release nutrients to plants over many years.  Organic material also attracts organisms, such as earthworms and beneficial bacteria, to help improve your garden soil.

Depending on the materials you use, compost can contain quite a bit of magnesium and other nutrients.  For example, bananas contain lots of magnesium, so using the peels for your compost pile can help to add magnesium to your garden.

If you want to learn how to make your own compost using kitchen scraps and yard waste, read my article all about it here.

Dolomitic Lime

Dolomitic lime is basically lime (calcium carbonate) that also contains magnesium.  Usually, dolomitic lime contains around 10% magnesium by weight, and around 20% calcium by weight.

Dolomitic lime is useful for adding magnesium to your soil if you also want to add calcium or raise the soil pH.  You should not use dolomitic lime to add magnesium to your soil in these cases:

  • If you already have enough (or too much) calcium in your soil
  • If your soil pH is already at the right level (or is too high)

Instead, try one of the other options below to add magnesium to your soil.

Epsom Salt

Epsom salt is a common supplement used to add magnesium or sulfur to garden soil.  Epsom salt contains 10% magnesium by weight and 14% sulfur by weight.

magnesium sulfate
Epsom salt, or magnesium sulfate, adds both magnesium and sulfur to your garden soil. Just be careful not to use too much at once!

However, it is sometimes overused, or used for the wrong reasons.  For example, it will not help with blossom end rot of tomatoes, or with a calcium deficiency in plants.

One way to use Epsom salt is to mix it into the soil before planting your crops.  Just don’t leave a high concentration of Epsom salt (or any other salt or fertilizer) in the soil.

Doing so can burn your plants with excessive nutrients.  Make sure to mix fertilizer evenly into the soil before planting.

Another way to use Epsom salt is to dissolve it in water, put it in a spray bottle, and spray it onto the leaves of your plant.  The plant will absorb the solution through its leaves and get a boost of magnesium and sulfur.

You should not use Epsom salt to add magnesium to your soil if:

  • You already have plenty of sulfur in your soil

Sulfate of Potash Magnesia

Sulfate of Potash Magnesia, also called Sul-Po-Mag, contains 11.2% magnesium, 22% sulfur, and 22% K2O (potassium) by weight.  This is a good option if you want to add potassium to your soil in addition to magnesium and sulfur.

Magnesium Oxide

Magnesium Oxide, also called magnesia, contains 55% magnesium by weight.  This is easily the highest percentage by weight of magnesium in any supplement on this list.

Due to the high magnesium content, magnesia should only be used if you have a severe magnesium deficiency in your soil.  Otherwise, you will end up with excessive magnesium in the soil, which will cause other problems in your garden.

What Does Too Much Magnesium Do To Plants?

Too much magnesium in your garden soil causes the following problems for plants:

  • Calcium deficiency – as mentioned earlier, magnesium competes with calcium for uptake by a plant’s roots.  Too much magnesium in soil means plants cannot uptake calcium properly.
  • Potassium deficiency – too much magnesium in soil can also cause potassium deficiency.

Too much of any nutrient is bad for plants, and magnesium is no exception.  Do a soil test before adding any supplement to your soil, and always follow the instructions on the label!

Go easy at first when adding supplements to your soil.  You can always add more supplements later on, but it’s almost impossible to take them back out if you overdo it.


Now you know what magnesium does for plants, and why it is important to have enough but not too much.  You also know how to provide extra magnesium to your soil and plants if need.

I hope you found this article helpful.  If so, please share it with someone who can use the information.

If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here.  Enjoy!


Jon M

Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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