People use fertilizer to supply lawns and plants with the nutrients they need—primarily nitrogen, phosphorus, and potassium. It is typically added to the soil when planting seeds or young plants to help them get established. Some people add fertilizer to their yards and gardens several times during the growing season to help keep them green and growing strong. But over-fertilizing can be a bad thing. Too much fertilizer can actually kill the plant and excess fertilizer can runoff into streams and lakes causing toxic algal blooms that are harmful to aquatic life and even people and their pets.