The most important plant nutrients are nitrogen (N), phosphorous (P), and potassium (K).
Nitrogen helps above-ground leafy growth and gives dark green color to leaves.
Phosphorous encourages plant cell division. Without phosphorous, flowers and seeds could not form. Phosphorous also helps root growth and protects the plant from disease.
Like phosphorous, potassium increases the plant's resistance to disease and encourages root growth. Potassium is needed for the making of chlorophyll.