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Potassium deficiency: how to recognise and treat it

Deficiency

Potassium deficiency — Photo: Alandmanson, CC BY-SA 4.0, Wikimedia Commons

Symptoms

Browning that starts at the edges and tips of older leaves, often with yellow or purple-red tints.
Older leaves are affected first and may crinkle, curl or drop early.
Poor flowering and fruiting; tomatoes may ripen unevenly (blotchy ripening, yellow shoulders).

Organic treatment

For the long term, mulch with garden compost.
In the ground, potassium-rich comfrey pellets or fish, blood and bone at the label rate.
For pot plants and fruiting crops, a seaweed-based or organic tomato liquid feed.

Chemical treatment

Sulphate of potash at the label rate; on very sandy soils, where it washes out quickly, slow-release (coated) potassium fertilisers last longer.

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

Prevention

Light, sandy or chalky soils lose potassium by leaching, while clay soils hold it — test and feed light soils.
Do not overdo potassium; too much blocks magnesium uptake and causes magnesium deficiency, especially in tomatoes.
Plants in containers run short sooner and may need regular potassium-rich feeding.

Sources

Figures checked against these sources.