Long Island's ground is glacier-made and sandy, and by the third week of August a Smithtown lawn goes pale no matter how faithfully the sprinklers run. SL Synthetic Lawn Turf Inc. answers that with synthetic lawn turf, laid over ground that has been graded and drained first, so a yard holds one steady green from the first hot week to the last. Picture a Nesconset backyard at noon: a dog running the same wide arc it runs every afternoon, and the arc never wearing down to bare dirt. Play yards, pet runs, putting areas, the narrow shaded strip beside the house where nothing has ever agreed to grow — each is measured, seamed and edged to lie flat and read as grass. The pleasure of it is the second August, when the whole street has gone straw-colored and one yard has not.