How do I turn soil into gunpowder?

I am a high school Chemistry student and for half of our final lab we must extract calcium nitrate from soil, convert it into potassium nitrate, and allow that to crystalize until it is gunpowder. So far the internet has given me not much help. I need answers ASAP! Thanks.How do I turn soil into gunpowder?
Internet probably wont help too much. Ca(NO3)2 is relatively cheap, so noone stays up at night trying to extract it from soil. Once you do get to KNO3, you need to add granular charcoal and sulfur.