Phosphorus - Phosphorus is a component of ATP and hence is needed for the conversion of light energy to chemical energy (ATP) during photosynthesis. Since ATP can be used for the biosynthesis of many plant biomolecules, phosphorus is important for plant growth and flower/seed formation. Phosphate esters make up DNA, RNA, and phospholipids. Phosphorus can also be used to modify the activity of various enzymes by phosphorylation, and can be used for cell signaling.
Nitrogen - Nitrogen is an essential component of all proteins. Most of the nitrogen taken up by plants is from the soil in the forms of NO3. Amino acids and proteins can only be built from NH4+ so NO3 must be reduced.
Magnesium - Magnesium is an important part of chlorophyll, a critical plant pigment important in photosynthesis. It is important in the production of ATP through its role as an enzyme cofactor
Iron - Iron is necessary for photosynthesis and is present as an enzyme cofactor in plants. Iron is not the structural part of chlorophyll but very much essential for its synthesis.