A magnesium deficiency is characterized by yellowing of the leaves between the veins.
Before studying the causes and looking at how to fix them, let’s start by understanding the importance of magnesium for plants.
Magnesium and plants
Magnesium (Mg) plays an essential role in:
- the formation of chlorophyll
- synthesis of proteins, carbohydrates and RNA
- activation of various plant enzymes
- the assimilation of phosphorus (P) by the plant
In summary, this element is essential for the ripening of fruits and the quality of vegetables.
In this article we take the example of the tomato.
But a magnesium deficiency can have particular consequences (non-exhaustive list):
- corn or soybean
- various vegetables: potato, beetroot, cauliflower
- fruit: raspberry, grape, apple
- and some ornamental plants such as rhododendron or rose bush
A plant with a magnesium deficiency accumulates starch in the leaves. This results in a reduction in root development and a weakening of the plant (which becomes more susceptible to diseases or animal attacks).
When a tomato plant (or other fruiting vegetable) is heavily loaded with fruit, the magnesium will be monopolized by the latter at the expense of the leaf.
In general, this shortage has no significant impact on the harvest.
In my literature it is often mentioned that magnesium deficiency mainly occurs in greenhouse tomatoes, but rarely in outdoor cultivation… This seems to be less and less the case with global warming (you will understand why below).
Symptoms of magnesium deficiency
Symptoms of magnesium deficiency in tomatoes normally appear quite late in the season (plant full of fruit) in plants whose leaf tissues are magnesium deficient (we are talking percentages less than 0.35 to 0.50%).
The leaves turn very yellow between the veins, while they remain green.
In particularly severe deficiencies, the discoloration can turn purple or even white.
Affected leaves become brittle.
In general, the deficiency is mainly noticeable on the old leaves (i.e. in the lower part of the plant for the tomato, for example).
Also note that a magnesium deficiency can impair root development.
Conditions Conducive to Magnesium Deficiency
Without getting into complicated scientific explanations, let’s sum things up by saying that a magnesium deficiency is the result of poor mineralization at the soil level or faulty chlorophyll synthesis (the 2 may be related).
So it’s a nutritional problem.
A magnesium deficiency can have many causes:
- high soil acidity, especially in sandy soil
- a soil rich in potassium but poor in magnesium (this is often the case in sandy soil)
- excess water
- a lack of water
- too much calcium
- excess ammonium nitrogen
- too much fruit
- an insufficiently developed root system
To these causes I would like to add one, which itself may be the cause of some of the above…global warming!
Indeed, at high temperatures, life is greatly slowed down, both at the soil and plant level (chlorophyll synthesis).
And as a result, the mineral elements, so magnesium, are poorly released.
Also, as we have already seen in previous articles, it is now an option to consider shading certain crops in the summer more and more seriously…
What to do in case of magnesium deficiency?
We will distinguish the different causes here, in order to remedy them in the right way.
Shortage due to too much water
With strong heat, and despite the watering restrictions in many regions, the temptation is great to water a lot…
Sometimes too much!
On the one hand, the mineral elements run the risk of being washed out.
And on the other hand, the root system may rot…
In either case (and even more so if you have both symptoms) the mineral elements, including magnesium, will no longer be available to the plant.
What must we do ?
- water less (at home, during heat waves and in light soils I only water the tomato plants once a week in the greenhouse… and every two weeks outside. Of course the soil is mulched)
- remove the affected leaves to prevent them from contracting certain diseases (we have seen them become more vulnerable) that could infect the rest of the plant
Shortage due to lack of water
Conversely, a lack of water has the same consequences.
The mineral elements are then not (or badly) released, not because of too much water, but because of lack…
It’s also very common today with the ongoing heatwaves we’re experiencing this year (and the next…). At least for those who respect the watering restrictions.
What do you think should be done then?
- mulch if you haven’t already
- water a little more often, or increase the amounts with each watering
Make changes rich in magnesium
If the deficiency is directly related to an excess or a lack of water, it goes without saying that the first thing to do is to correct this by reducing or increasing the waterings … Changes are then normally not useful.
But in case of a real deficiency (actually a blockage) at the soil level for some of the reasons mentioned above, you should be aware that there are additives rich in magnesium.
Choose your amendment according to the PH of your soil (very simple tests are presented in My Natural Vegetable Garden):
- in calcareous soil choose magnesium sulphate (kieserite or Epsom salts)
- in acidic soils, dolomite (or carbonate of lime and magnesium), well supplied with magnesium but also rich in calcium, is preferred
However, keep in mind that you are only correcting an imbalance … And the problem will reappear if you do not address the cause.
At home, for example, I don’t notice a magnesium deficiency… Why?
- annual additions of compost and other natural additives (in the form of mulch) contribute to a living and therefore naturally balanced soil
- From the start of cultivation I get used to watering the plants in a spread (but abundant) way, which encourages them to develop a nice root system and therefore makes them more independent and less dependent on watering
In short, I simply apply the advice I give in Mon Potager au Naturel.
I am also at your disposal, in personal guidance, to analyze together with you the causes of a possible deficiency (it is not only magnesium) and to remedy it in an appropriate manner.