Last fall and early winter quite a few cases of potassium (K) deficient poinsettia plants were observed. Both stock plants and panned cuttings exhibited the deficiency symptoms. Nitrogen insufficiency was also seen. The potassium symptoms were normally observed on the lower, older leaves. Symptom expression appeared as a yellowing and mottling of the leaf margin. If the deficiency was allowed to continue, browning and death (necrosis) of the entire leaf margin followed. Several of these severe cases were verified by a complete soil test.
Source: • Connecticut Greenhouse Newsletter #101