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Magnesium Deficiency: how to recognise and treat it

Deficiency

Magnesium Deficiency — Photo: Goldlocki, CC BY-SA 3.0, Wikimedia Commons

Symptoms

Interveinal chlorosis: the veins stay green while the tissue between them turns yellow, sometimes with reddish-brown or purple tints.
It starts on the older, lower leaves, because the plant moves magnesium to new growth.
Later the yellowed areas turn brown and die, and older leaves drop early; common on tomatoes (especially under cover with a heavy fruit load), apples, plums, grapes, raspberries, roses and rhododendrons.

Organic treatment

Short-term: in summer spray or water the leaves with Epsom salts (magnesium sulphate) at 20 g per litre of water; repeat two or three times.
On acid soils, spread dolomite limestone at 100 g per m² — but not around acid-loving plants such as rhododendrons or blueberries.

Chemical treatment

Long-term on light, free-draining soils: magnesium sulphate (Epsom salts or kieserite) at 30 g per m² applied to the soil; apply only when symptoms confirm the deficiency.

Reference information, not a guarantee. Read and follow the label of any product: dose, safety and the days to wait before harvest.

Prevention

Do not overuse potassium-rich feeds such as tomato feed — plants take up potassium in preference to magnesium.
Light, sandy soils and very acid or alkaline soils are most prone; check the soil pH with a test kit.
Watch greenhouse tomatoes from mid-season, when a heavy fruit load makes the deficiency most likely.

Sources

Figures checked against these sources.