Answer: If the grapes were planted in the last couple of years, they won't produce a crop for at least another year, provided they're getting what they need. Hopefully the site is correct for them: full sun, rich, deep soil with a pH of 5.0-6.0, have good support and are properly pruned and trained. Grapes also need up to 4 gallons per square foot of root area per week. Root areas are 1.5-2 times the diameter of the crown. Slack off watering while the grapes are ripening or after harvest unless there's a drought. Mulch the area heavily with wood chips or cocoa bean hulls. Fertilize with 1.5 lbs of compost per square foot in late winter/early spring. Excess fertilizer can cause rampant vine growth at the expense of fruit.
Blackberries prefer slightly more alkaline soil - pH 6.5. Otherwise, they don't require more than the grapes unless they show signs of a specific nutrient deficiency. You can have your soil tested to find out it's lacking key nutrients. The extension service (ph# 217/543-3755) can help you with testing. Best of luck!
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