One thing I use the water soluble fertilizer on is the container plantings at my church. I mixed in a little slow release fertilizer at planting, and I fertilize with a half-strength solution about every week.
I had fun trying to choose color combos for the planters.
For the black planters I used coleus (a variety that is a little more sun tolerant), begonia, and licorice plant.
I love to look at different combinations that people choose for container gardening. Keep them fed and watered, and don't throw away the soupy fertilizer.