Do I need fertilizer?
If your soils good and its not a permanent location, you might not even need to fertilize. I think some breeds of cannabis have better tasting buds if they have had to struggle a bit for survival. I can almost taste the buds that have been "spoiled" by an over-protective gardener.
However, if your growing in the same location year after year or if your soil is poor to begin with, you will need to add amendments. If an acre of hemp is burned the ash could contain as much as 25 to 30 kilos of pottasium, so youll have to add some mass. Some organic amendments will improve the texture and drainage as well as supply nutrients, and they have the advantage of not building up into a toxic condition.