Fixing Magnesium Deficiency in Plants: How Magnesium Affects Plant Growth


By: Susan Patterson, Master Gardener

Technically, magnesium is a metallic chemical element which is vital for human and plant life. Magnesium is one of thirteen mineral nutrients that come from soil, and when dissolved in water, is absorbed through the plant’s roots. Sometimes there are not enough mineral nutrients in soil and it is necessary to fertilize in order to replenish these elements and provide additional magnesium for plants.

How Do Plants Use Magnesium?

Magnesium is the powerhouse behind photosynthesis in plants. Without magnesium, chlorophyll cannot capture sun energy needed for photosynthesis. In short, magnesium is required to give leaves their green color. Magnesium in plants is located in the enzymes, in the heart of the chlorophyll molecule. Magnesium is also used by plants for the metabolism of carbohydrates and in the cell membrane stabilization.

Magnesium Deficiency in Plants

The role of magnesium is vital to plant growth and health. Magnesium deficiency in plants is common where soil is not rich in organic matter or is very light.

Heavy rains can cause a deficiency to occur by leaching magnesium out of sandy or acidic soil. In addition, if soil contains high amounts of potassium, plants may absorb this instead of magnesium, leading to a deficiency.

Plants that are suffering from a lack of magnesium will display identifiable characteristics. Magnesium deficiency appears on older leaves first as they become yellow between the veins and around the edges. Purple, red, or brown may also appear on the leaves. Eventually, if left unchecked, the leaf and the plant will die.

Providing Magnesium for Plants

Providing magnesium for plants begins with annual applications of rich, organic compost. Compost conserves moisture and helps keep nutrients form leaching out during heavy rainfall. Organic compost is also rich in magnesium and will provide an abundant source for plants.

Chemical leaf sprays are also used as a temporary solution to provide magnesium.

Some people have also found success with using Epsom salts in the garden to help plants take up nutrients easier and improve magnesium deficient soil.

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Magnesium

Magnesium is essential for healthy plants and is deemed a secondary macronutrient. It is a constituent of chlorophyll so is required for photosynthesis. It is also a component of many plant enzymes and aids in their function. Magnesium helps plants move phosphorus to where it is needed and to use iron. It is important for the uptake of a variety of nutrients and for nitrogen fixation by bacteria associated with with legumes.

The availability of magnesium in the soil is affected by:

  • pH - low soil pH reduces the availability of magnesium, high pH increases it
  • Manganese - excess manganese decreases magnesium uptake
  • Cation exchange capacity - soil that is high in organic matter and clay will maintain higher levels of magnesium (such soil will absorb magnesium easily and will prevent it from leaching) though if the soil contains little magnesium, it will be harder for plants to take it up
  • Other cations - excess levels of other cations, potassium in particular, will prevent the uptake of magnesium
  • Soil temperature - low soil temperature reduces magnesium uptake

Ideally, for healthy and productive soil you should aim for a magnesium concentration of at least 1.6 meq/100g (milliequivalents - this is a special term used to describe the amount of some elements in soil).


Garden plants love magnesium

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Since February is considered the start of spring in Southern California gardens, with the first daffodils blooming and cold-weather food crops coming along, it’s a good time for an occasional discussion about Epsom salt.

I get more mail about Epsom salt than anything else.

It’s too bad that Epsom salt has the word “salt” in the name. Epsom salt has nothing to do with table salt and more to do with the chemical structure of this form of magnesium sulfate.

Magnesium in the garden is a vital plant nutrient that helps seeds germinate better, prompts plants to produce flower and fruiting buds, roses to grow new canes, and promotes overall health of most plants by aiding in the absorption of other nutrients.

If your African violets refuse to bloom after many months, a warm, Epsom salt drench will get them going.

Epsom salt is named for the region where it was discovered in the 1600s – Epsom, England – by a dairy farmer who found that his cows didn’t like it much, but that it worked wonders for his health. A spa town was born.

Today Epsom salt is easy to find at your favorite pharmacy. Buy one big bag for you, and one for the garden.

There are two ways to use Epsom salt in the landscape. Dissolve some in a bucket of water and use as a drench, or sprinkle on top of the soil and let the watering or rain move it downward.

The Epsom Salt Council recommends these amounts, although I use it haphazardly, since most gardeners believe you can’t overdo it.

•Houseplants – 2 tablespoons to a gallon of water. Apply once a month.

•Tomatoes and peppers – 1 tablespoon to a gallon of water. Apply every two weeks.

•Roses – 1 dry tablespoon to each hole at planting time. Half a cup sprinkled dry around the root zone of established roses in spring.

•Vegetable garden – 1 dry cup to 100 square feet, dug under before planting.

I used to be a drencher, but since hauling around a gallon-size bucket of water is heavy, I’ve turned into a sprinkler lately. I sprinkle dry Epsom salt everywhere flowers grow, in the same fashion as salting food, and let the rains water it in.

You can consume magnesium sulfate in the form of Epsom salt, and the packaging even tells you how. But expect some serious laxative results, in the same way Milk of Magnesia works.

I take a long, hot bath in Epsom salt about once a week. One cup to a tub full of water and I stay in as long as I can — at least 12 minutes is recommended.

According to the Epsom Salt Council, we don’t readily absorb magnesium from our food, although it is easily absorbed through the skin.

Magnesium not only helps sore muscles recover faster, it provides a boost of energy the next day. And just like with plants, magnesium sulfate helps us absorb other nutrients better.

More isn’t better when it comes to your health. After all, magnesium is a laxative, and you can also get that result by soaking. Use it once or twice a week at the most, and check with your doctor if you have health concerns.

I’ve discovered a new use for Epsom salt: floral arrangements.

Fill a clear glass bowl with Epsom salt, and then poke pretty bare branches into it. These wintry-looking arrangements lend a sort of sparkly look for tabletops. Recycle into the garden, later.

To get an in-depth look at Epsom salt, visit epsomsaltcouncil.org.


Deficiency symptoms

Magnesium is a mobile element in the plant and deficiency symptoms will occur first in the oldest leaves.

The loss of a healthy green color can be the first indication of a Mg deficiency. Color loss reflects the shortage of chlorophyll in the plant. As the deficiency becomes more severe, the area between the veins of the leaves becomes yellow while the veins stay green. In corn, there is a definite striping the full length of the leaf, appearing first on the lower leaves (see Figure 2).

Potatoes

In potatoes, the loss of the green color begins on the tips of the lower leaves when there is a mild Mg deficiency. When the deficiency is more serious, the yellowing progresses between the veins toward the center of the leaf. In the advanced stages of Mg deficiency, leaf areas between the veins show small brown dead spots (see Figure 3). Diseases, herbicide damage, and environmental factors also cause leaves to die prematurely. So, care should be taken in identifying a Mg deficiency. Use plant analysis to be sure.


What Is A Good Source Of Magnesium For Plants? (How To Add Magnesium To 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 sources add magnesium to the soil, and others add itdirectly to the plant, but both methods will work.

Plant Magnesium Source #1: 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.

Either way, you can use Epsom salt to help address the problem.

How To Add Epsom Salt To Soil

To treat a magnesium deficiency in your soil, add some Epsom salt (magnesium sulfate), along with any other fertilizers you use (such as compost, manure, etc.)

To add magnesium before planting, use a shovel to turn the mixture into the soil and blend it in evenly. Then, plant your seeds or move your transplants into the improved soil.

Epsom salt (magnesium sulfate) provides magnesium for plants, either by adding to soil or spraying on the leaves after dissolving in water.

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 by their leaves.

How To Use Epsom Salt As A Foliar Spray

First, add one tablespoon of Epsom salt to one gallon of water. Mix them together thoroughly until the salt is completely dissolved.

Next, pour some Epsom salt water solution from the gallon into a spray bottle.

Use a spray bottle to apply an Epsom salt solution in water to the leaves of plants (foliar feeding).

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: regardless of which method you use, be careful about adding too much Epsom salt all 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.

Plant Magnesium Source #2: Dolomite Lime

Dolomite lime (or dolomitic lime) is another source of magnesium for your soil to help cure a deficiency. Dolomitic lime, or calcium magnesium carbonate, is a type of natural limestone.

Dolomitic limestone will supplement both calcium and magnesium. This is helpful if a soil test indicates that your soil lacks both of these nutrients.

Plant Magnesium Source #3: Sulfate of Potash Magnesia

Sulfate of Potash Magnesia (also called Sul-Po-Mag or K-Mag) is another soil supplement that can provide magnesium if your garden is lacking.

This supplement also provides sulfur and potassium, which is helpful if one or both of these other nutrients are also deficient in your soil.

Plant Magnesium Source #4: Compost or Manure

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.

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. At the very least, it will allow you to use a little bit less of the magnesium supplements mentioned earlier.

Healthy garden soil should always start with natural supplements, such as compost or manure. These will restore nutrients to the soil and replace organic material in the soil, which improves drainage.

Plant Magnesium Source #5: 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 to consider the nutrient imbalances we talked about earlier.

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

Fertilizer can help to provide nutrients for your plants, but don’t use too much at once or you will burn them! Follow the instructions on the package.

Finally, be careful about over fertilizing. After all, too much of a good thing can cause a bad thing, such burning your plants with too much nitrogen.


How To Take A Soil Sample

In order to extract a soil sample for analysis, you will need a garden trowel and clean plastic (not metal) containers. Gallon storage bags work well for this purpose.

Take samples when the soil is fairly dry as overly wet soil can produce false readings. Although garden soil can be tested any time of the year, early spring or fall testing is preferred because it allows gardeners time to make soil adjustments before planting.

USDA NRCS South Dakota / Flickr (Creative Commons)

Here’s how to collect your soil sample.

  1. Make sure the trowel is clean and wiped down with rubbing alcohol to remove contaminants.
  2. Dig a hole 3 to 6 inches deep, removing a generous scoop of soil from the sidewall of the hole.
  3. Place the soil sample in a plastic bag and label it to indicate the location in the landscape and the type of plant or plants presently or previously cultivated in that location.
  4. When testing larger areas for field crops, collect soil samples from different areas that will be growing similar plants. Samples can be co-mingled in a clean plastic bucket.
  5. Spread the mixed soil samples on sheets of newspaper to dry. When the soil is dry, gather about a 1/2 gallon of the mixture to submit for testing.



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