The importance of magnesium for your plants

We are used to working with NPKs, nitrogen-phosphorus-potassium and we are well acquainted with micronutrients. The first thing that comes to mind when we see a yellowish plant is that it has iron chlorosis . However, many other times we do not know that it is magnesium that also intervenes in giving greenness to the crop. Today we talk about its importance.


Within all forms of magnesium, the plant only absorbs the Mg2 + ion. This assimilation can be done both in irrigation (fertigation) and subsequent root absorption or in foliar input , through penetration via the epidermis through the leaves.

A medium crop performs a magnesium extraction ranging from 20 to 80 kg / ha. It has a clear and key physiological role for the development of any plant. In this that we comment below, this element participates.

  • Basic element in the chlorophyll molecule. It invests in the greenness of the plant. It represents 2.7% of the total weight, but it is essential.
  • It is involved in the synthesis and formation of proteins. Carotenes and Xanthophylls (color formation in fruits) need magnesium to fulfill certain basic metabolisms of the plant.
  • Reduces the transfer of carbohydrates from the leaves and stems to the root. In root crops such as potatoes, beets or others, it must be taken into account.
  • Great sensitivity to light appears. In summer, necrosis can be seen on the leaves due to a lack of photoregulation of the crop.


Although magnesium has greater mobility than calcium, there are times that either due to the presence of other antagonist elements (potassium, sodium, calcium, etc.) or due to the lack of magnesium itself in the soil, the plant may present symptoms of deficiency.

Since magnesium is involved in photosynthesis and in the chlorophyll molecule, the first thing we can think of is that a magnesium deficiency induces chlorosis in the crop.

Magnesium mobility is high in the plant, therefore, unlike with iron , magnesium deficiency usually appears at the bottom of any crop. That is, on the old leaves .

Lack of magnesium in pepper. Photography: Houses

Although we are correctly fertilized with continuous magnesium contributions, it may be that magnesium chlorosis appears in the plant. This is mainly due to large contributions of potassium in the fruit ripening phase, reducing the assimilation of magnesium.

As an example, this would be the order, for ease of absorption, of these cations:

Na+ > K+ > Mg2+ > Ca2+

It is determined by the size of the ions and by the electrical charge . Smaller size and less charge of the cation will have greater ease of absorption


Depending on the area where we are, on many occasions and depending on the conductivity of the water, we can provide a large amount of calcium and magnesium for free.

You have to think that this contribution is not always absorbable by the plant, since it is often blocked and is closely related to the amount between these two elements.

There is a rule that establishes that for there to be a perfect absorption of calcium and magnesium from irrigation water, it must have a ratio of 2 to 1 onwards. That is, twice as much calcium as magnesium. And from then on.


If we take a laboratory water analysis , we can obtain a large amount of relevant information to save money on the contribution of fertilizers.

In this case, we have to know how to measure the units with which we work in these analyzes. Meq / L, ppm or mmol / L.

If we go to a typical water in many areas of Spain, with a pH of 8.5 and a conductivity of 1.2 mS / cm , we can have a great contribution of calcium and magnesium, among others. In this example, the quantity would be as follows:

  • Calcium: 200 mg / 50 = 10 meq / 50 = 5 mmol / 50
  • Magnesio: 100 mg/L = 8,23 meq/L = 4,11 mmoles/L

With this, we would practically cover the needs of the calcium and magnesium crop, so it would not be necessary to provide these nutrients. We can imagine the great savings that we would obtain with it.

However, the rule discussed above tells us that there must be practically a difference or ratio of 2 for maximum absorption of the two nutrients to occur, so it could be appropriate, in the fruit setting and fattening phase, to provide an extra calcium (calcium nitrate, for example).

Let’s say that the water we have is from the tap and has a lower number of dissolved salts. A case with a practically neutral pH (7.5) and an electrical conductivity of 0.45 mS / cm.

These are the values ​​that we can see in the water analysis.

  • Calcium: 22 mg / 50 = 1.1 meq / 50 = 0,55 mmol / 50
  • Magnesio: 3 mg/L = 0,25 meq/L = 0,12 mmoles/L

In this case, with a “flat” water, it is necessary to add calcium and magnesium, so that we complete the recommended standards in fertigation. Up to 10 meq / L of calcium and 4 meq / L of magnesium.


We can find many forms of magnesium supply , either in a chelated, complexed form or without absorption-enhancing additives.

The best known (and most economical) magnesium supply formulas are magnesium nitrate and  magnesium sulfate. Each one with a different richness of this element.

  • Magnesium nitrate: 10.5% N (nitrogen) and 15% MgO.
  • Magnesium sulfate: 16.6% MgO and 32% SO3 (sulfur).

We can also find, as we have said, chelated forms or magnesium complexed with organic acids .

Chelation forms can be heptagluconates, EDTA chelation, hexahydroxy capric acid, etc.

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