Save yourself some time and energy and be good to your grass by leaving grass clippings on the lawn. Short clippings do not cause thatch. They add nutrients, organic matter and moisture to the soil as they break down. Leaving the clippings on your lawn all season is equal to applying one pound of actual nitrogen fertilizer (or 10 pounds of a 10 percent fertilizer) per 1000 square feet. That’s one less fertilizer application you will need to make.