One of the problems that a poinsettia can get growing outdoors is a….

Magnesium deficiency. Characterized by symptoms of yellowing leaves, marginal scorch (black edges) or tip burn starting at base of shoots, advancing to tip. Usually prevalent in sandy soils. An application of magnesium sulfate (epsom salt) twice a year will correct the problem. Apply once in February-March and June-July – 3 tablespoons per square yard.