Magnesium deficiency in tomatoes is a fairly common problem, particularly in plants grown in pots, greenhouses, or soils that are frequently fertilised. It is relatively easy to recognise because it produces quite characteristic symptoms on the leaves.
The symptoms of magnesium deficiency initially appear on the lower leaves, which are the oldest leaves on the plant.
How to recognise magnesium deficiency
Magnesium is a mobile nutrient within the plant. When its availability becomes insufficient, the plant can move it from older leaves to younger leaves and actively growing organs.
For this reason, the first symptoms generally appear on the older leaves in the lower part of the plant (see photo above).
The most characteristic symptom is interveinal chlorosis: the leaf tissue between the veins gradually loses its green colour and turns yellow, while the veins remain green (see photo below).
In the early stages, lighter areas may appear between the veins. As the deficiency progresses, the yellowing becomes increasingly evident and can affect a large part of the leaf. In severe cases, necrotic areas may also develop, and the most severely affected leaves may eventually dry out.
Warning: it is not always a true magnesium deficiency
There is an important aspect to consider when looking at the lower leaves of a tomato plant.
Over time, older leaves gradually become less important to the plant. After carrying out photosynthesis for an extended period, they naturally enter a phase of senescence and the plant progressively begins to shed them.
Before this happens, however, the plant tries to recover some of the nutrients still present in these tissues. Because magnesium is a mobile nutrient, it can be moved from older leaves to younger leaves, shoots, and actively growing organs.
Consequently, yellowing of the oldest leaves does not automatically mean that magnesium is lacking in the soil or growing medium. In some cases, what we are observing is simply part of the plant’s natural renewal process, with magnesium being remobilised from leaves that are already undergoing senescence.
The situation is different when interveinal chlorosis continues to progress and gradually affects additional leaves. In this case, it is advisable to assess the plant’s nutrition and determine whether there is a genuine deficiency or a problem with nutrient uptake.
Why can tomato plants develop magnesium deficiency?
The appearance of symptoms does not necessarily mean that magnesium is absent from the soil. Very often, the problem is related to low magnesium availability or reduced uptake by the roots.
An unbalanced fertilisation programme can contribute to the problem. In particular, high concentrations of potassium, calcium, or ammonium can interfere with magnesium uptake.
The characteristics of the soil or growing medium, pH, problems affecting the root system, and incorrect fertigation management can also contribute to the development of magnesium deficiency.
The problem may also become more evident when the plant enters full production and its demand for nutrients increases.
Should old tomato leaves be removed?
In normal tomato crop management, particularly in professional production, older, yellowing, or poorly functioning lower leaves are progressively removed.
Leaf removal helps keep the lower part of the plant cleaner and improves air circulation within the canopy. Removing old or damaged leaves can also contribute to pest and disease management by reducing plant tissue that may harbour pests such as spider mites (see here) or act as a source of inoculum for foliar diseases, including erly blight (see here).
Leaf removal should, of course, be gradual. Removing too many leaves that are still active unnecessarily reduces the plant’s photosynthetic area.
How to correct magnesium deficiency
Before adding magnesium, it is important to assess the fertilisation already applied and, above all, observe which leaves are showing symptoms and how those symptoms develop over time.
If yellowing affects only a few of the oldest leaves at the base of the plant, it may simply be normal leaf senescence. However, if the characteristic interveinal chlorosis continues to appear and progressively affects other leaves, the situation should be investigated further.
When there is a genuine magnesium deficiency, magnesium-containing fertilisers can be used, selecting the product and application rate according to the type of cultivation and the instructions on the product label.
One of the best-known products is magnesium sulphate, which, depending on the formulation, can be applied to the soil, through fertigation, or as a foliar treatment. Formulations suitable for use in organic farming are also available.
Magnesium nitrate can also be used, although particular care should be taken when applying it during the fruit-ripening stage because it also supplies nitrate nitrogen. Products containing both calcium and magnesium are also available and, depending on the formulation, can be applied either through the roots or as foliar treatments to optimise the supply of both nutrients.
Foliar application can be useful for rapidly supplying magnesium to the plant, but it is equally important to identify and correct any underlying factor limiting magnesium uptake.
Before applying additional fertiliser, therefore, look at the plant as a whole: a few old yellowing leaves at the base do not necessarily mean that your tomato plant needs more magnesium.
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Useful products
- Magnesium sulphate: https://amzn.to/44TKtsw
- Magnesium nitrate: https://amzn.to/45uA7iU
- Calcium and magnesium: https://amzn.to/4fvFWBe