In the summer there is no need for nitrogen.

List of necessary nutrients for grapes

Nitrogen fertilizers are responsible for the development of green mass in the leaves and stems of the plant. Therefore, it is especially necessary to serve them in early spring, when the vegetation period is just beginning. In the summer there is no need for nitrogen. With the onset of autumn days, nitrogen only harms grape bushes, since rapid growth interferes with the ripening of wood. Fertilize urea or add ammonium nitrate.