Symptoms
Due to strongly restricted growth, and delayed heading, or no heading at all (rosetting), phosphorus deficiency causes high yield losses.
The leaves become dark green and can look crinkled. Sometimes reddish discoloration appears. Older leaves show paper-like necrotic areas and finally die off.
Reasons
Phosphorus deficiency (right) compared to optimum supply
Symptoms may be confused with excessive nitrogen supply. Excess nitrogen leads also to dark green lettuce plants, but in this case the leaves are more rounded and the underside of the leaves does not turn purple.
Low temperatures may inhibit the uptake of P.