Magnesium deficiency
Yellowing between the veins of the OLDER, lower leaves while the veins themselves stay green. Magnesium is mobile within the plant, so it is withdrawn from old growth to supply new — which is exactly why the pattern starts at the bottom.
Described view
Two tomato leaves photographed flat side by side against a neutral ground: one showing magnesium deficiency, yellow between the veins with the vein tissue itself staying distinctly green in a herringbone pattern, and one showing nitrogen deficiency, faded uniformly pale from the whole leaf including the veins
What brings it on
Common in containers and grow bags, in light sandy soils, and where a high-potassium tomato feed has been used heavily — potassium and magnesium compete for uptake, so generous feeding can induce the deficiency rather than relieve it.
source
Sourced guidancehigh confidence
- Royal Horticultural Society: Magnesium deficiency
- University of Minnesota Extension — University of Minnesota: Nutrient deficiencies in vegetable crops
Checked 2026-08-13
Preventing it
Ordered by how much difference each measure actually makes, not by how often it is repeated.
- Do not exceed the feed rate on the label of a high-potassium fertiliser. More is not better and can cause this directly.
source
Sourced guidancehigh confidence
- Royal Horticultural Society: Feeding tomatoes
Checked 2026-08-13
- Use a fresh, good-quality growing medium for container crops, and refresh it annually rather than reusing exhausted compost.
source
Horticultural consensusmoderate confidence
- Royal Horticultural Society: Container growing — growing media
Checked 2026-08-13
Dealing with it once it is there
- A foliar spray of Epsom salts (magnesium sulfate) dissolved in water, applied to the affected foliage.
Traditional practiceDepends on conditionslow confidence
Applies when: Long-standing practice and widely recommended; the correction is temporary and treats the symptom rather than the cause, which is usually the feeding regime or the growing medium.
- Royal Horticultural Society: Magnesium deficiency — treatment
Checked 2026-08-13
- Confirm the pattern before acting. Yellowing that starts on the youngest leaves is not magnesium, and yellowing of whole lower leaves without the green veins is more likely nitrogen shortage or simply age.
source
Horticultural consensushigh confidence
- Royal Horticultural Society: Diagnosing nutrient deficiencies
Checked 2026-08-13
What it affects
Signs that lead here
Each of these lists the other things that produce the same sign, and how to tell them apart.
On sprays and treatments
GrowingHQ does not tell anyone what to apply, at what rate, or how often. Pesticide approvals differ by country, change without notice, and the product label is legally binding — it is also the document you are holding. What is described here is cultural and physical control, which is what a home grower can act on without a label in front of them.
Plant-specific guides
Where this is one of several causes that look alike on a particular plant, GrowingHQ publishes the differential rather than the general answer.