What Does 13-13-13 Mean on Fertilizer? | nitrogen-phosphorus-potassium | The first number represents nitrogen, the element that helps plants to grow by aiding in the formation of chlorophyll. Chlorophyll allows plants to absorb energy from light. Phosphorus is the second number. This element stimulates root growth and development. The third number represents potassium, which helps plants survive drought and other stressors that affect proper growth.