Nitrate of Soda is sodium nitrate. Personally I wouldn't use it because of the extra sodium that would be added. Potassium Nitrate is much much better because of the extra potassium being beneficial rather than problematic as sodium could be.
The Superphosphate is okay, but a much better alternative is the "Ready-to-use Fleet disposable enema. It's already mixed and only requires a few drops. A bottle costs about $2.00 and is probably a several years supply. If you need dosing information let me know.