Just sprinkle ammonium sulfate over the top of the bark and water it in. Or if you are using a water soluble fert just water over the bark, either way it will run down through the bark without any problem

The method I use on our blueberry shrubs is to mix one tablespoon of water soluble fertilizer with 4 gallons of tap water, and spread this around the base of the shrub. This is done once a month, in March, April, May, and June. I am using Schultz Plant Food-acidic formula, but in past years I have used Miracle Gro-acidic formula. This is a minimal amount of fertilizer, but the plants do well. We live near Lake Monona, here in Madison, Wisconsin, and I do not want any excess fertilizer to run-off and get into the lake.