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Phosphorus deficiency: how to recognise and treat it

Deficiency

Phosphorus deficiency — Photo: عمرو بن كلثوم, CC BY-SA 2.5, Wikimedia Commons

Symptoms

Stunted growth with purple discolouration on the older, lower leaves; later these leaves turn dull yellow.
Purplish veins or a purple tint on the leaf undersides; leaves may curl, stay small or drop early.
Often seen on young tomatoes, sweet potatoes and other tender plants set out while the soil is still cold.

Organic treatment

In the ground, mulch with garden compost to improve soil structure and drainage, which helps roots take up phosphorus.
As a short-term fix, a general fertiliser such as fish, blood and bone at the label rate.
Use phosphorus-rich bone meal only if a soil test shows a shortage — true phosphorus shortage in garden soil is rare.

Chemical treatment

In pots, a general-purpose liquid feed; in the ground, a general fertiliser, or ammonium phosphate where a soil test confirms a shortage — at the label rate.

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

Prevention

Delay planting out tender crops until the soil is warmer and drier; cold, wet or waterlogged soil hinders phosphorus uptake.
Do not overwater plants in pots, especially in cool weather.
Very acidic or very alkaline soil makes phosphorus less available — test the soil pH.
Add phosphorus only on the advice of a soil test; it washes off with surface run-off and pollutes water.

Sources

Figures checked against these sources.