There are quite a few provisions.
http://www.atf.gov/publications/download/p/atf-p-5400-7.pdf Also there may be state and local laws prohibiting or regulating, so make sure it's legal where you are. The letter I got from the ATF stated under 50 lbs and no resale. If over 50 a permitted, approved storage magazine was needed. And to resale a manufacturers license was needed. As long as you are using it for a legal use and can legally have it, not barred in 845(a) it is legal on the Federal level. I am not a lawyer so would suggest you contact the ATF with a letter describing your specific intent of use and make sure. It's always better to have an ATF letter saying it is legal, instead of trying to explain on the spot to a local LEO that it is legal when he doesn't have a copy of US code. Nor the ability to interpret the vagueness they love to leave so every case can be open to interpretation.
For the salt peter
http://www.pyroguide.com/index.php?title=Black_Powder_Manufacture_-_Questions_page Kpower is a local brand of fertilizer. I would imagine it changes by region. You could call around your area and see who stocks it. If that is to much, strawberry fertilizer is usually kno3 and sold in little boxes. Asian markets sell it as pickling salt. Butcher stores sell it as a meat preservative. And livestock places sale it as a diuretic. I have never used it but the specter or specrta(spelling) brand at walmart is supposed to be the best source from the stump removers.