Now that you have planted your selections, what are the feeding requirements for your new blackberries? You don’t begin fertilizing blackberry plants until 3-4 weeks after the setting of new plants. Fertilize after growth starts. Use a complete fertilizer, like 10-10-10, in the amount of 5 pounds per 100 linear feet or 3-4 ounces around the base of each blackberry. Use either a complete 10-10-10 food as fertilizer for your blackberries or use compost, manure or another organic fertilizer. Apply 50 pounds of organic fertilizer per 100 feet in the late fall prior to the first frost. As growth starts to appear in early spring, spread inorganic fertilizer over the top of the soil in each row in the amount as above of 5 pounds of 10-10-10 per 100 feet. Some folks say to fertilize three times a year and some say once in the spring and once in the late fall before the first frost. The blackberries will let you know if you need a supplemental feeding. Look at their leaves and determine if the plant is fruiting and growing well. If so, no fertilizing the blackberry plants is necessary. Read more at Gardening Know How: Fertilizing Blackberry Plants – Learn When To Fertilize Blackberry Bushes http://www.gardeningknowhow.com/edible/fruits/blackberries/fertilizing-blackberry-plants.htm