The N, of course, is nitrogen. So, the K is potassium. That leaves the P for phosphorous. Technically, potassium is involved in cell metabolism, protein synthesis, water use and retention, and movement of plant foods through the plant. Three necessary elements needed in lesser quantities are sulfur, calcium and magnesium.
Sulfur is used in developing amino acids used in protein synthesis.
Calcium is an essential part of cell wall construction and must be present to form new cells. Magnesium promotes chlorophyll formation and activates plant growth enzymes. Calcium and magnesium are also used in the leaves to neutralize toxic acids which form as a by-product of metabolism. As we clean up air pollution and change to other fungicides, sulfur deficiency has become more of a concern. Calcium and sulfur can also be provided through gypsum.