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Magnesium For Plants: Deficiency, Toxicity, Sources, & More

Magnesium deficiencies are often present in soil and water, and it becomes necessary to supplement with additional magnesium amendments.

Magnesium For Plants: Deficiency, Toxicity, Sources, & More

If you are a new grower, you may not know just how important magnesium is for plants. You know all about nitrogen, phosphorus, and potassium - the big three.

But why do so many experienced growers swear by their cal-mag supplement?

In this nutrient guide, we're going to cover the importance of magnesium for plants. We'll explain why vital secondary nutrient is so necessary for your indoor or outdoor plants, along with the best sources of magnesium.

Then, we'll take a deep dive into how to identify and rectify magnesium deficiency in plants, along with toxicity, though this is less common.

What Does Magnesium Do For Plants?

Magnesium plays an important role in the health of our plants. It is one of the key components in how our plants convert the energy from its light sources into energy to grow and thrive.

Magnesium is one of the vital nutrients needed for optimal chlorophyll health.

Chlorophyll gives the green pigment found in all plant leaves, that enables the plant to absorb and convert energy from light into useful sugars, enzymes, and carbohydrates to grow.

In addition, it helps transfer phosphorus to various parts of the plant. It is necessary for phosphate metabolism and can also be used to stabilize the cell membrane and metabolize carbohydrates.

Magnesium helps the plant absorb and transport key nutrients in addition to phosphorus, too, such as iron. It can help your plant fruit and reach full maturity as well.

What Is A Good Source Of Magnesium For Plants?

You can add magnesium in several ways. Your garden will be the most successful if you are able to add magnesium to the soil or substrate prior to planting.

You can apply organic compost on a regular (annual) basis, which will help your soil retain both moisture and structure with a bit more ease. Well-structured soil will resist erosion and retain nutrients more easily, too.

Beyond that, you can use a chemical leaf spray to provide magnesium or sprinkle some Epsom salts in the garden.

Both of these should help your plants uptake nutrients more easily and improve the quality of magnesium-deficient soil.

Use A Cal-Mag Supplement

Many growers, particularly those who are gardening on a commercial scale, use Cal-Mag.

Cal-Mag supplements can be used both in hydroponic and soil-based gardens.

As you might be able to infer from the name alone, these supplements contain both calcium and magnesium, since plants are often deficient in both nutrients.

Cal-Mag supplements are particularly effective if your plants are deficient in both nutrients.

If you suspect a deficiency in magnesium, a Cal-Mag supplement is the way to go if you use treated water (which tends to not have much calcium or magnesium) or if you grow in an area that experiences high temperatures and humidity.

The Relationship Between pH & Magnesium Absorption For Plants

When you attempt to add magnesium to your soil or soilless setup, pay close attention to the pH.

If your pH is lower than 7.0, the magnesium you add can be easily absorbed.

If it’s very acidic, however, magnesium won’t be absorbed as readily by the plants - rendering your efforts futile. Therefore, testing your soil prior to administering any kind of treatment is essential.

Magnesium Deficiency In Plants

Magnesium deficiency is relatively common in plants. A deficiency often occurs when the soil is not rich in organic matter or is fast-draining (sandy soils, for example).

If heavy rainfall occurs, this can easily leach magnesium from the soil. There are several common causes of magnesium deficiency, including a wet, cold, or acidic environment.

When there are high levels of calcium, ammonia, and potassium, this can also cause magnesium deficiency.

Other stressors, like reduced evaporation, stress to the plant, and a limited root system, can also cause magnesium deficiency.

Normally, magnesium moves easily around a plant. Whenever there is a deficiency of magnesium, chloroplasts in leaves that are neither very young nor very old can become damaged.

As a result, young parts of the plant will have worsened chlorophyll production that will exhibit itself as brown or red blotches and chlorosis (or flecks) between the vines.

In older leaves, you won’t notice a magnesium deficiency quite as easily, as it attacks younger leaves first. It is already attached to organic substances here.

Therefore, a minor deficiency in magnesium won’t do much to the plant but one that is severe can occasionally affect more mature plants, typically in their ability to set and form fruit.

Unfortunately, magnesium deficiency is one of those problems that will keep getting worse until you address it. Here are the different stages.

The Progression Of Magnesium Deficiency In Plants

For the first few weeks of deficiency, you likely won’t notice any symptoms at all. However, as the plant gets older, typically around five or six weeks, you’ll notice necrosis appearing in middle-aged leaves.

The number and size of rusty brown patches will gradually extend themselves over the leaf and the leaf will become more yellowed.After time, the magnesium deficiency will result in symptoms that increase over the plant.

The plant leaves can fall off (younger leaves will begin to be affected, too) and the plant’s overall production can take a serious nosedive.

To treat a known magnesium deficiency, you’ll want to check the temperatures, pH, humidity, and EC of your substrate or soil.

Ideally, you will have guarded against magnesium deficiency at the outset, using a magnesium-rich fertilizer (Epsom salts are a good choice).

However, after you've confirmed a deficiency, the best thing you can do is to spray with a solution of Epsom salts.

You can also use synthetic fertilizers to balance out the levels of magnesium. Before treating your plants, make sure all other conditions are stable.

Maintain the pot temperature and keep things dry. In most cases, providing a little extra magnesium won’t be problematic.

Identifying Magnesium Deficiency In Plants

Now that you know the importance and various sources of magnesium for plants, let's cover the most common issue you'll experience with this nutrient - magnesium deficiency in plants.

A magnesium deficiency can easily be diagnosed by examining our plant leaves.

The great news is, a magnesium deficiency is easily identifiable. When plants aren’t receiving sufficient amounts of magnesium at their roots, they will begin to degrade the chlorophyll in the oldest leaves first.

This is a condition called chlorosis, in which the leaves begin to look yellow, yellow-white, pale, and have a marbled, striped appearance between the leaf veins.

This process will often start in the lower regions of the plant first. As deficiencies persist, leaves will begin to curl under and die off.

  • Yellowing of older leaves first between veins
  • Leaf curl over
  • Leaf necrosis

Examples Of Magnesium Deficiency

There is evident chlorosis in the lower left image here, a tell-tale sign of magnesium deficiency.

On the right, you can see another example of magnesium deficiency. This one is a bit less developed, which is important to note as catching the issue early will be crucial.

magnesium for plants

Magnesium is a mobile nutrient, wherein in can move around, and it is at this point that the plant is breaking down the chlorophyll in the old leaves, in order to transport it to the fresh, young leaves.

If left unchecked, the plant will become completely depleted of its magnesium reserves and will eventually die.

How To Correct Magnesium Deficiency

One of the ways to provide plants the magnesium they need is through the use of organic compost mulch and teas.

When compost solutions are applied at the roots, the much needed magnesium in the compost will be taken up by the plant’s roots to be put to work where it’s needed.

Another way gardeners and growers improve the magnesium content is through the use of Epsom salt. Epsom salt is made up of magnesium and sulfur, and it helps to improve the creation of chlorophyll, which again is essential to a plant’s health.

Dilute Epsom salt in water, about 2 tsp/gallon, and apply it to the plants. Apply at both at the roots and as a foliar spray. For best results, spray the leaves with a 2% solution of Epsom salt. Continue this process until all signs of deficiency disappear.

Another solution is to apply dolomite limestone. Dolomite limestone is made up of calcium and magnesium, and provides much of the same benefits as Epsom salt.

Adjusting soil pH above 6.5 or hydroponic water pH above 5.5 while reducing EC for up to a week will also improve nutrient uptake.

magnesium deficiency or toxicity in plants

Oftentimes signs of a magnesium deficiency can signify deficiencies in other minerals and nutrients. Many gardeners and growers like to have a Cal Mag supplement on hand in case any of these deficiencies appear.

All-in-one combined solutions give plants the added mineral and nutrition boost that they need. This will result in enhanced foliage growth and healthier fruits and blooms.

Magnesium Toxicity In Plants

Magnesium toxicity in plants is rare and very difficult to see with the naked eye.

In extreme cases, overfeeding often creates a conflict with other ions such as calcium. This is especially true in hydroponic solutions. Toxic buildup in soil is very uncommon.

Now that you know the benefits of monitoring the magnesium content in your soil and hydroponic growing systems, and what to do when you spot a deficiency, you can be sure to have the necessary tools on hand to keep your plants at their best.

Read our entire nutrient deficiency series to learn more about the different signs of nutrient deficiencies.

After reading, you'll be able to identify any deficiency you encounter in the grow room or garden.

Final Thoughts On Magnesium For Plants

Plants can be finicky creatures! Without the proper balance of nutrients, it will be difficult for your plants to produce healthy, productive yields.

Magnesium may not be as commonly discussed as other nutrients, like nitrogen, phosphorus, or potassium, but it still plays a valuable role in your garden.

Pay attention to the role of magnesium for plants, as well as signs of potential toxicity and deficiency, so that you can grow successfully without any kind of limitations.

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