Causes of Magnesium Deficiency in Plants, and How to Fix It


Magnesium deficiency is a common problem for plants, and it has some nasty side effects.  Luckily, there are ways to reverse the condition and save your plants.

So, how do you treat magnesium deficiency in plants?  One method is to spray the leaves of the plant with a solution containing magnesium, such as Epsom salt dissolved in water.  Another method is to mix Epsom salt or compost directly into the soil.

Before you go and try these methods, it is important to make sure that your plants really do have a magnesium deficiency.  Otherwise, you could end up with too much magnesium, which can also cause problems for your plant.

We’ll start off with an explanation of why magnesium is important for plants, along with symptoms and causes of magnesium deficiency.  Then, we’ll go into a little more detail about how to add magnesium to your soil.

Why is Magnesium Important for Plants?

Magnesium plays a key role in plant growth and development.  For one thing, magnesium (Mg) is the central atom in a chlorophyll molecule, as illustrated in the picture below.

chlorophyll molecule
A chlorophyll molecule – note the central magnesium (Mg) atom.

Chlorophyll is what makes plants green.  Chlorophyll is necessary for plant photosynthesis, a process where energy is produced from water, light, and carbon dioxide.

Magnesium also plays a role in carbohydrate metabolism, meaning that it helps plants to use energy.  In short, magnesium is essential for the survival of a plant.

Symptoms of Magnesium Deficiency in Plants

If your plant is lacking magnesium, there are a few ways to tell.  Check for these symptoms to diagnose the problem.

Interveinal Chlorosis

Perhaps the most obvious symptom is interveinal chlorosis, where the plant’s leaves turn yellow, but the veins stay green.  You can see the appearance of such a leaf in the picture below.

interveinal chlorosis
Leaves displaying interveinal chlorosis – the veins are green, but the space between is yellow.

The older leaves (which are found lower on the plant) tend to turn yellow first.  This is because magnesium is a mobile nutrient, meaning that plants can move the nutrient easily through its tissues.

When a plant suffers from a magnesium shortage, it takes magnesium from older leaves and sends it to the new leaves higher up on the plant.  In essence, the plant is sacrificing the older leaves to ensure the health of the new leaves.

This makes sense, especially since leaves that are higher up on the plant have a better chance of absorbing sunlight to turn into energy.

In some plants, the edges of the leaves will also curl upward in response to magnesium deficiency.

Some plants where magnesium deficiency is common:

  • Apple
  • Currant
  • Potato
  • Raspberries
  • Rhododendron
  • Tomatoes

Necrosis

If your plant suffers from a severe magnesium deficiency, then you may see brown or black spots appear on the plant.  You may also see wilting or drooping of the plant (it looks like the plant is tired, and would like to lie down and take a nap).  These are signs of necrosis, caused by dying plant cells.

Stunted Growth

You may also notice that your plants suffer from stunted growth.  The roots will be shorter and the shoots will be smaller than normal.  The plant will also be more susceptible to disease.

If it does survive, you will notice reduced yield for fruit and vegetables.  This all goes back to lack of magnesium.  Without enough magnesium, plants cannot produce enough chlorophyll.  Without enough chlorophyll, plants cannot produce enough energy to produce fruit, to grow, or even to survive.

Causes of Magnesium Deficiency in Plants

There are many possible causes of magnesium deficiency in plants.  Many of them can cause a problem even when there is plenty of magnesium in the soil.

Incorrect Soil pH

Every nutrient, including magnesium, has an ideal soil pH range where the nutrient is readily available for plants to absorb through their roots.  In general, a good guideline is a soil pH of 5.5 to 6.5 (slightly acidic).

If your soil pH is too far below 5.5, the magnesium in the soil will start to become less available for uptake by your plants.  Similarly, a soil pH that is too high will also make magnesium less available for your plants.

For more information, check out this chart on Research Gage that illustrates nutrient availability by pH.

To find out what your soil pH is, you should buy a soil test kit from a garden center or online.  Make sure to test your pH and adjust it to a proper level before you add supplements and fertilizers for a magnesium deficiency! To learn more, check out my article on testing your soil.

Note: adding lime to your soil will increase the pH, but it also adds calcium to your soil, which can be a problem – more on that next.

Nutrient Imbalance

Gardening is all about balance, and I’m not trying to go all “Zen” on you.  It’s true.  Not only do you need to have the right amount of each nutrient in your soil, but you also need to have the right ratios of nutrients.

For instance, high levels of potassium can interfere with a plant’s ability to absorb magnesium.  Basically, the potassium is “competing” with magnesium to be absorbed by the plant’s roots.  To avoid this, make sure to use fertilizer without excessive potassium.

Fertilizers are usually labelled with three numbers, representing NPK (nitrogen, phosphorus, and potassium) by weight.  A 10-10-10 mix contains 10% nitrogen, 10% phosphorus, and 10% potassium by weight.

Just check the bag you buy, and make sure the potassium content is not too high. For more information, check out my article about NPK ratios on fertilizer packages.

Similarly, excessive calcium can interfere with a plant’s ability to absorb magnesium.  Adding bone meal or lime to your soil will increase calcium levels, so be careful about adding too much of these at one time.

To read more about how excess calcium or potassium can affect magnesium availability, check out this article on Science Direct.

Poor Soil

Poor soil conditions can also cause a magnesium deficiency.  For one thing, heavy rainfall may leach magnesium out of the soil, carrying it away so that your soil becomes deficient.

dry soil
This is definitely poor soil – and yet something is still growing in it!

Cold soil will also decrease a plant’s ability to absorb magnesium through its roots.  If you plant too early in the year, when the soil is still cold, you may encounter this problem.  A late frost or a prolonged period of unseasonably cold weather could cause the same problem.

Lack of Magnesium in Soil

If none of the above conditions apply to your garden, then you may in fact have a lack of magnesium in your soil.  This can happen for a couple of different reasons.  One possible explanation is that there was never enough magnesium in the soil to begin with.

Another possibility is that the magnesium that was in your soil has been depleted.  This can happen if you plant the same crop in the same place in your garden every year.  To avoid some of this depletion, use crop rotation (plant a crop in different parts of your garden each year).

Whatever the cause of magnesium deficient soil, you will want to know for sure if this is the problem, and how severe it is.  The best way to find out is to use a soil test kit, which you can buy at a garden center or online.

In addition to magnesium levels, a soil test kit can tell you the levels of other nutrients in your soil.

How to Add Magnesium to Garden Soil

If you are reading this, then you have decided that the magnesium levels in your soil are low.  Luckily, there are a few ways to give your plants the magnesium they need.  Some of these add magnesium to the soil, and others add them directly to the plant, but both methods will work.

Epsom Salt (magnesium sulfate)

Maybe you have not planted anything in your garden yet, but a soil test showed a magnesium deficiency.  Or, maybe you saw signs of magnesium deficiency in last year’s crop, and want to avoid the same problem this year.

To improve your garden this year, add some Epsom salt (magnesium sulfate) directly to your soil, along with any other solid fertilizers you use (compost, manure, etc.)

magnesium sulfate
Epsom salt (magnesium sulfate)

If you have already planted this year and your plants are showing signs of magnesium deficiency, don’t worry.  You can also make a spray to treat your plants directly.

Add one tablespoon of Epsom salt into a gallon of water, and mix thoroughly.  Next, pour from the gallon into a spray bottle.

Then, spray the leaves of the plants suffering from magnesium deficiency.  This is known as foliar feeding, and the plant will absorb the solution directly through its leaves.

Note: be careful about adding too much Epsom salt at once! Although it can be helpful for curing magnesium deficiency, it is not a cure-all to be whipped out for any and all reasons. For more information, check out my article on Epsom salt for growing tomatoes.

Compost

You can create your own mix to maintain magnesium and other nutrient levels in your garden.  Adding leaves, grass clippings, and food scraps (not meat!) to your compost pile will help to restore these nutrients to the soil.

compost
With a little more time, this compost will be ready for the garden.

As long as your leaves and grass are not magnesium deficient, this method should help to keep magnesium levels stable.

For more information, check out my article on how to make your own compost.

Fertilizer

You can also choose a fertilizer off the shelf at the store, as long as it has some magnesium included in the mix.  However, remember the nutrient imbalances we talked about earlier.

Be careful about adding fertilizer with high potassium or calcium levels, since these nutrients can interfere with a plant’s uptake of magnesium.

Also be careful about over fertilizing, since too much of a good thing can cause a bad thing, such burning your plants with too much nitrogen. For more information, check out my article on over fertilizing.

A Few Words About Excessive Magnesium

With all this talk about magnesium deficiency, it seems impossible to have too much magnesium in your garden soil.  However, it can happen.  For this reason, take my advice and check your pH and other nutrient levels before adding magnesium to your soil!

Remember what we talked about earlier, with regards to balance between nutrients.  Too much magnesium will turn the tables and inhibit a plant’s ability to absorb calcium and potassium, leading to deficiencies in these nutrients.

Conclusion

At this point, you have a good idea of how to identify magnesium deficiency and what the cause may be.  Hopefully you also know how to solve the problem, and how to prevent it in the future.

I hope this article was helpful.  If you have any questions, please leave them in the comments below.

jonathon.david.madore

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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