Phosphorus (P) is used in proteins, metabolic transfer processes, ATP, ADP, photosynthesis and respiration. It is also a component of membranes (phospholipids).
A few sources of phosphorus are bone meal, rock phosphate, and superphosphate. Phosphorus is not considered a "mobile" nutrient so correct applications should be direct to where the plant roots are or where they will grow too.
Phosphorus deficiency can easily go by unnoticed for some time. Growth rate may be restricted and a light green or dull coloration might be noticed during low P availability. However, as the deficiency progresses, one may begin to notice a purple coloration on the underside of the leaves. This is usually in the viens but may progress to the intervienal areas as well. In severe deficiencies the plant will be stunted, leaves will turn upward and yellow on the upper side with purpling underneath.
P deficiencies will begin on the lower leaves and progress to newer, upper leaves. Leaf drop of the older leaves may occur in severe cases. Fruit and seed yield may be reduced