Well,
I have a link to an Austrian website, but it naturally is all in German. Basically what this guy writes is that you take modeling clay, form small spheres with a small hole in them (He says to push the end of hex key into the wet clay). When they are dry you can drip your standard liquid fertilizers into the little hole and the clay will suck them up.
In case anyone is interested, and proficient in German, here is the
web-site.
EDIT: Oh, I forgot, after applying the liquid fertilizer the hole is closed with some more clay...