Magnesium deficiency
Magnesium sits at the centre of chlorophyll. When it runs short, older leaves yellow between their veins, sometimes with red or brown blotches, while new growth stays green. Heavy potash feeding makes it worse.
Soil & nutrients
What it looks like
The veins stay green while the tissue between them fades.
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Striped older leaves Yellow bands between green veins on the lower leaves. -
Brown blotches Later, the yellow areas turn brown or reddish and die. -
Upward spread The pattern climbs the plant through the season. -
Green young leaves New growth at the top stays normal.
How to confirm it
Where it starts is the key.
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Older leaves first
Interveinal yellowing starting low on the plant means magnesium; starting at the tips means iron.
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Check your feeding
Weekly high-potash tomato feed ties up magnesium.
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Spray test
A foliar spray of Epsom salts greens new leaves within a couple of weeks if magnesium was short.
What to do
Give a quick fix to the leaves and a lasting one to the soil.
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Foliar spray
Dissolve 20 g of Epsom salts per litre of water and spray the leaves every two weeks through summer.
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Balance the feed
Alternate tomato feed with a balanced one, or choose a feed with added magnesium.
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Lime acid soils
Where soil is acid, dolomitic limestone adds magnesium as it raises the pH.
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Leave affected leaves
Keep them unless they die; they still feed the plant.
Prevent it next time
- Use feeds that include magnesium for tomatoes and grapes.
- Add compost to sandy soils, which lose magnesium in rain.
- Avoid overusing high-potash feeds.
What not to do
Do not keep increasing tomato feed to cure yellow leaves; more potassium makes magnesium harder to take up.
Sources
- Nutrient deficiencies, Royal Horticultural Society
- Tomatoes: leaf problems, Royal Horticultural Society