Your lawn takes nutrients out of the soil that it is bedded in and uses these nutrients to aid its growth. If your soil lacks these nutrients your lawn will ultimately suffer. So while fertilizing makes your lawn greener it also provides a top of any nutrients that your soil is lacking helping grass to grow. When the nutrients are available, the lawn develops healthier roots which invariably grow deeper allowing the lawn to draw upon water and nutrients in the soil. Furthermore, deep roots reduce thatch and browning. Finally, fertilizer contains nutrients that grass needs to resist disease and drought.
Fertilizers also offer the additional benefit of including chemicals to inhibit or kill weeds. The three primary nutrients in a fertilizer are Nitrogen, Phosphorous and Potassium. Each element has its own affect upon grass. Nitrogen stimulates grass growth and greening. Phosphorous stimulates the development of roots and seedlings. Potassium promotes disease and drought resistance.
As the weather changes, so too does your lawn have changing needs. In the heat of the summer, your lawn needs water and little else. In order to survive the cold weather, your lawn needs to have healthy roots and so a fertilizer to promote root growth should be applied before the cold weather sets in. At other times of year your lawn needs nutrients to enable growth and greening. Weeds also have their own growing seasons so you should fertilize at the beginning of the various weed growth seasons.
Where you live, the type of grass you have, the condition of your soil and the weather all affect your choice of fertilizer and when it should be applied. You also have a choice of organic fertilizer, liquid fertilizer, which is sprayed on, or time released granules which are distributed with a spreader. All of these factors result in the many choices of fertilizer. And of course you can always purchase your fertilizer via a wholesaler online, meaning that the more you buy the cheaper the deal.