You don't have to buy blackberries in the grocery store; you can raise your own blackberry plants in your backyard. Blackberry shrubs are relatively low maintenance, but do require annual fertilization. Feeding the plants encourages proper plant development and helps ensure an optimal fruit harvest later in the summer. The specific fertilizer you use varies according to your soil condition.
Test the soil around your blackberry shrubs to determine the dirt's pH level. You can often find pH testing kits at many garden stores or your regional cooperative extension office (see Resources).
Spread fertilizer around the roots of the blackberry shrubs in the early spring season immediately after the plants emerge from dormancy and begin producing new growth. Texas A&M University recommends using a 21-0-0 fertilizer if your soil pH is above 7.0, and a 15-5-10 fertilizer if your soil pH is lower than 7.0. Apply the fertilizer at a rate of 2.5 lbs. for every 50 feet of shrubbery row.
Water the blackberry shrubs lightly to moisten the soil to a depth of 6 to 8 inches. This helps carry the fertilizer nutrients down to the plants' root level for immediate feeding.
Feed the blackberry shrubs again in early summer. This application is optional but will encourage more vigorous growth. For the second feeding, use a 21-0-0 product at a rate of 2.5 lbs. per 50 feet of shrubs, according to Texas A&M University.