Phosphorus Deficiency
Risk near you, next 48h
See whether your local forecast favours this disease right now.
When it strikes
Fix the cause, not the leaf
No spray will help here. The trigger is soil, water supply, nutrition or temperature, so change the growing condition and judge the result on new growth, not on damaged leaves.
Not contagious
Nothing spreads from plant to plant here. Neighbouring plants showing the same signs are reacting to the same soil, water or weather, so fix the cause rather than isolating plants.
Safe to compost
No pathogen carries over on this material, so it can go on an ordinary heap with the rest of the garden waste.
When and where to look
Scout the planting once temperatures drop below 12°C.
The signs that lead here
About
Phosphorus deficiency is usually a temperature problem rather than a supply problem. Roots cannot take phosphorus up efficiently below about 12°C, so plants set out too early in cold soil show classic symptoms even in a well-fed bed, then grow out of it as the soil warms.
Symptoms
Dull, dark blue-green leaves that later develop a distinctly purple or reddish cast, starting on the oldest leaves and the undersides. Growth is stunted, stems are thin, flowering and fruiting are delayed, and roots are sparse.
Organic Treatment
Warm the soil and wait — cover the bed with fleece or cloches and the purple usually disappears within two weeks. Where the soil is genuinely low, work in bone meal, rock phosphate or well-rotted manure, all of which release slowly.
Chemical Treatment
A high-phosphorus liquid feed gives the quickest correction in containers. In open ground, superphosphate applied at planting is more effective than a foliar feed, since phosphorus moves poorly through leaves.
Prevention
Do not plant tender crops into cold soil, keep soil pH between 6.0 and 7.0 where phosphorus is most available, and build organic matter — mycorrhizal fungi in a healthy soil deliver far more phosphorus to roots than fertiliser alone.
Related problems
Phosphorus Deficiency risk by location
Each location has a month-by-month profile computed from the weather archive: how many hours a year actually meet this disease’s thresholds.