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When to Use Cal-Mag in Soil? | Best Time for Supplementation!

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You must have heard that nitrogen, potassium, and phosphorus are essential nutrients for plant growth.

However, these are not the only vital nutrients. Some other “lesser-known” nutrients play a significant role in keeping plants healthy.

Calcium and magnesium, or as they are collectively known, Cal-Mag, are two vital nutrients to keep plants in best form.

If you are new to applying Cal-Mag fertilizer, you might be confused about its usage.

One of the most confusing questions for gardeners is: “When to use Cal-Mag in soil?” I guess that’s your case as well.

If you have been in a similar dilemma, keep reading as we explore more about the Cal-Mag fertilizer.

Cal Mag fertilizer—when to use Cal Mag in soil?
When to Use Cal-Mag in Soil? – Image via Amazon.

What Is the Role of Cal-Mag in Plant Growth?

Before you learn when to use Cal-Mag in soil, let us see why these nutrients are essential and why you must use them for your plants.

  • Calcium plays an integral role in humans by keeping our bones healthy and helping us with the overall development process of our body. Similarly, calcium helps support the plant’s structure and skeleton in perfect condition.
  • The significant difference between the cells of animals and plants is that there is an extra cell wall in plant cells—calcium helps develop the cell wall, which is a central part of a plant structure. A healthy cell wall ensures that the plant stays sturdy and firm.
  • Moreover, calcium also makes the transfer of nutrients to and from the cell easy. That ensures that the plant is using all the necessary nutrients needed for the growth of plants and fruits.
  • Cell division and new cell formation are two other critical functions that require the presence of calcium.

The plants, leaves, stems, and fruits having inadequate nutrients will show stunted growth and signs of damage. 

  • Magnesium also has a central role in one of the essential functions of the plant.
  • Chlorophyll is the leaf’s green substance and is necessary for the plant to make food. Without the presence of chlorophyll, the plant won’t be able to manufacture food and will eventually die.
  • Magnesium is a core component of chlorophyll.
  • If magnesium is not present in suitable amounts, leaf growth and food production of the plant will diminish.
  • Like calcium, a lack of magnesium causes stunted growth in plants.
  • Both magnesium and calcium also play a secondary role in producing certain enzymes in plants.

Given this considerable involvement of Cal-Mag in plants, your soil in the garden must have a proper amount of both these nutrients.

Related article: What is 18-18-18 Fertilizer Used for? — Growgardener Blog

But, when to use Cal-Mag in soil?

Let’s find out!

When to Use Cal-Mag in Soil?

So, when to use Cal-Mag in soil? Should you add it in with every watering session?

The answer here is that it depends upon the soil and watering conditions.

Not every soil condition needs the addition of Cal-Mag to it. You only need to use it in extraordinary situations, especially when these nutrients are deficient, and the plants clearly show the symptoms.

When hard or untreated water is used for plants, you won’t need to use Cal-Mag as it is abundantly present in that water.

For example: If you are using normal plant water, Cal-Mag deficiency shouldn’t be an issue, as tap water will fulfill the need for these nutrients.

You may need to add Cal-Mag supplementation if you use soft or treated water like distilled water for your plants.

  • A condition in which Cal-Mag may become necessary is high humidity and temperatures, as the soil becomes deficient in nutrients.
  • In soils with low pH levels, the plants will likely need an added dose of Cal-Mag too.

Besides these specific conditions and a few more, the soil will probably already have the amount of Cal-Mag needed by the plants.

One important point to remember here is that Cal-Mag won’t affect your plants until they are deficient in these nutrients.

So, ensure that your flora needs the fertilizer before adding it.

You can look for signs like yellowing of the leaves, stunted growth, and blackened tips of the leaves.

How Much Cal-Mag Should You Use in Soil?

Cal-Mag deficiency can occur when you use treated water or grow plants in sandy and coarse soil—adding potassium in a high amount can also disbalance the mentioned nutrients.

The fact that fertilizers can promote growth may compel you to add as much supplementation as possible.

However, this is not a good idea.

It would be best to use Cal-Mag in only specific amounts.

  • If you water your plants only once a week, you can add Cal-Mag and use it in every watering session.
  • If you water the plants more than once a week, add the fertilizer separately, diluted in water.

The ideal amount of Cal-Mag is one teaspoon or 5 ml of the supplement in one gallon of water you will use to hydrate the plants.

If you notice signs of deficiency or intend to boost the flowers, you may occasionally add 10–15 ml of the supplement into one gallon of water. Most people use it in the initial stages of the plant development after sowing the seed or when it starts to grow a bit.

Adding Cal-Mag during the flowering conditions is also an excellent idea to promote growth.

When to Use Cal-Mag in Soil? | Best Time to Give Cal Mag – YouTube


I hope this post answers your question about when to use Cal-Mag in soil.

Cal-Mag is a necessary supplement for optimal plant growth, and you should take immediate action if the plant shows signs of stunted growth.

Cal-Mag is a great addition to give a boost to floral production.

Being mindful of the deficiency of Cal-Mag is essential as it will help prevent growth issues and nutrient deficiencies in the later stages of the plant.

However, be careful about the amount you add to the plants as excessive Cal-Mag can be a bit hazardous too.

Keep the quantity in control, stay ahead of the symptoms, and have a healthy plant in your backyard.

I hope you got what you needed!

Happy gardening!