In general, grape vines are not heavy feeders and do not need much fertilizer after they are well-established. Too much fertilizer can stimulate wood and leaf growth rather than fruit production. Moderate amounts of fertilizer during the first and second years after planting will help stimulate young plants to size up for earlier fruit production.
First, test the soil pH and nutrient deficiencies...
Grape vines prefer an acidic soil with a pH ranging from 5.5 to 6.5. It's a good idea to test or have your soil tested to determine pH and nutrient deficiencies. If your local nursery and garden center does not have soil testing kits available, you can buy a soil test kit online here. Or check with your local Extension Service to see if they provide soil testing services. Soil testing kits will includes complete instructions for adjusting soil conditions.
What type of fertilizer...
At planting time - Incorporate a well-balanced organic fertilizer or compost into the soil at time of planting. Compost is especially beneficial to condition heavy clay soil that might retain too much water during the wet season and become hard as a brick during the dry season. Thoroughly mix the composted organic matter with the native clay at a 50/50 ratio.
Established vines - In general, grapevines respond well to a well-balanced fertilizer such as 10-10-10. Or, if you would rather go the organic route like I do, well rotten manure or compost spread as a mulch works equally as well if not better. Organic vineyards choose organic methods to obtain the strongest and richest grapes possible, with the fewest detrimental effects on the environment, and their grapes reflect that dedication to quality. If you're a wine drinker, try organic wine and you won't get the headache in the morning!
When to fertilize...
Fertilize grape vines in early spring and again about a month later, but do not fertilize past mid summer. Fertilizing too late in the season will promote new, tender growth and your vines need to enter dormancy at a normal time in fall or they will receive greater than normal winter damage.
