Phosphorus deficiency on tomato
A physiological disorder that mostly shows up on young plants in early spring. It usually is not a lack of phosphorus in the soil but a lock-up — in cold soil (below 12-14°C) roots cannot take it up even when there is plenty of it.
Symptoms
- Purple-violet discoloration on the leaf underside, especially along the veins
- Dark green, dull leaves with a bluish cast
- Symptoms first on the older, lower leaves
- Severely stunted growth — dwarfed plants with a stiff habit
- A poorly developed root system
- Delayed flowering and ripening, and poor fruit set
What it does to the crop
- Root and whole-plant development held back right at the start
- Harvest delayed by as much as 2-3 weeks
- Lower yield and poorer fruit coloring
What encourages it
- Cold spring soil (below 12-14°C) blocking phosphorus uptake
- Acidic soils (pH below 5.5) or strongly alkaline ones (pH above 7.5)
- Heavy, compacted soils with poor structure that limit rooting
- Excess iron, aluminium or calcium locking phosphorus into unavailable forms
- Planting out too early, before the soil has warmed
Prevention
- Plant out only once the soil has warmed — cold-induced symptoms clear up on their own
- Keep soil pH at 6.0-6.8, the best range for phosphorus uptake
- Place a starter phosphorus dressing close to the transplant roots
- Foliar feed with phosphorus if symptoms appear in early spring
- Mulch and use fleece to warm the soil
⚠️ Before you reach for a spray
This atlas is informational and still growing — different diseases can look almost identical. Confirm the diagnosis and read the product label before applying any plant protection product. If you are unsure, ask your local extension or advisory service.
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