I use a mix of regular soil that you buy in the nurseries, and horse/cow manure, my plants love it.
Change soil is a little bit tricky with my amaryllis, because even after 2 years of growing in the pot, there is no more soil in the pot, only very packed root system. You can only add more soil by increasing the pot size.
What I do is adding granules of urea (as nitrogen fertilizer) and a water it with manure extract.
