Grapes usually require very little nitrogen fertilizer. Getting a soil test is the best way to determine accurately the amount of fertilizer to apply to your grapes. Ideally this would be done before the soil is prepared for planting. In lieu of that, you will find that general recommendations vary somewhat depending upon the source. It is important to determine what nutrients and pH constitute your soil. Have the soil tested every three to five years. The best method to determine nutritional needs when vines are mature is with tissue sampling. See more on tissue sampling of grape vines here.