We plant cover crops to soak up excess nutrients in the fall, store them in the plant and release them for the crops in the spring when we cultivate them into the soil. We sow some seeds like vetch, peas and beans which harbor nitrogen fixing bacteri on their roots. This adds fertility from the atmosphere to the soil.
Table grapes, unlike their wine producing friends, like nitrogen. Wine grapes are often times starved to get the great flavors the wine grower and maker desire. So in the spring, after we have mowed this emerging cover crop, we will spread compost to add more fertility. The compost contains a miriad of bacteria which will help breakdown the cover crop and release its nutrients.