Fertilizer is food for your grass, shrubs and trees. There are two main types of fertilizer: Chemical and Organic. Organic fertilizer is steer manure and compost. You can usually purchase compost at your local re-use center or nursery and steer manure at your nursery. When purchasing steer manure, make sure it is filtered for weeds.
Chemical fertilizer is man made and is a combination of nitrogen, phosphorus and potassium. These three chemicals make up the combination of numbers on the store bought bags of fertilizers. There may also be secondary elements such as calcium, magnesium, and sulfur. The minor elements that may be present are zinc, iron, manganese, copper, molybdenum and boron.
Nitrogen creates greening, phosphorous promotes root growth and potassium protects against drought.
Most soils don't have the right nutrients to help grow thick, thriving grass. Its up to the home owner to make sure grass has the nutrients. Lawns need essential nutrients and fertilizing is the way to provide these nutrients.