Magnesium deficiency

Yellowing between the veins of older, lower leaves while the veins themselves stay green — common on tomatoes and after heavy feeding.

Disorder Disfiguring Doesn't spread

What you'll see

Yellow or bronzed patches between the veins, with the veins remaining green
The oldest, lowest leaves show it first, working upward as it spreads
Affected areas may turn orange, red or purple then brown and die
Fruit and overall growth are little affected at first, unlike nitrogen shortage

Why it happens

Magnesium is central to chlorophyll, and the plant robs its old leaves to feed new growth when supply runs short — which is why the lowest leaves yellow first. Heavy potassium feeding is a common trigger, as potassium competes with magnesium at the roots and blocks its uptake.

Common on tomatoes and pot plants given repeated high-potash tomato feed, on light sandy or acid soils where magnesium washes out, and after a wet winter leaches it away. It shows up more in a fast-growing crop under glass.

Treating it

Spray leaves with a dilute Epsom salts solution for a quick top-up on visibly affected plants
Ease back on high-potash feeds until the plant recovers
Apply Epsom salts to the soil around long-term plants in spring
Do not expect Epsom salts to fix unrelated problems — it only helps a genuine magnesium shortage

Keeping it away

Balance feeding rather than relying heavily on high-potash tomato feed
Add magnesium-containing ground rock or garden lime to acid soils that are prone to it
Improve light sandy soils with organic matter to hold nutrients against leaching
Keep watering even, since drought slows uptake of magnesium as well as calcium

Who gets it

Tomatoes are the usual sufferers, along with apples, grapes, roses and potted plants on a high-potash diet.