Phosphorus deficiency or excess
Symptoms
Phosphorus is necessary for cell growth, flower and fruit formation. When deficient — young plant leaves darken, acquire a purple hue; the growth of an older plant stops, the underside of the leaves and veins are purple. In case of excess — fruits become bitter.
Control and prevention
Supplement with phosphorus fertilizers (in case of deficiency).
Related problems
Similar symptoms might indicate a different cause — compare them.
Nitrogen deficiency or excess
When deficient — growth slows down, old leaves lose color, newly growing leaves are yellow-green. Nitrogen is quickly broken down or washed out by water. In case of excess — tomatoes grow many leaves, but bloom little and set fruit sparsely.
Potassium deficiency
Potassium is responsible for metabolism in the plant. When deficient — leaves begin to wither, deform, yellow, the stem weakens, fruits fall prematurely. When a young plant is deficient — it grows deformed, the edges of older leaves dry out.
Blossom end rot
A large rot appears on the bottom of the fruit. This happens due to a lack of calcium in the fruit. The deficiency can manifest not only due to its absence in the soil, but also due to irregular watering.