It seems every gardener has his or her favorite way to fertilize the garden. Rotted manures add organic matter and some fertilizers, and can therefore be beneficial. Because my soil is already a good quality garden loam, I use bagged fertilizers because they are inexpensive and easy to use.
I have successfully used a balanced fertilizer (nitrogen-phosphorous-potassium, or NPK ratio of 12-12-12) but prefer a fertilizer with more nitrogen to support growth of all those leaves. One commonly-used fertilizer in the nursery trade (Osmocote brand of controlled release fertilizer) has NPK of 19-6-12, which is a good ratio for hostas and many other garden plants. I make my own by blending 2 parts of a lawn fertilizer with NPK of 20-4-8 (no added herbicides!) with one part of a balanced garden fertilizer with NPK of 12-12-12; this yields for the blend NPK of about 17-7-9.