additive alphabet soup

[Joe Reiter <joe at spintronic_com>]

I'm sure this has been brought up before, but I couldn't quite dig up 
the right posts during my search.  Once again, I turn to this most 
gracious and knowledgable group of folks for assistance!

What exactly are you getting out of the common additives, such as those 
Tom Barr recommends?  For example(s):

potassium nitrate (KNO3) gives you nitrate, which gives the plants 
nitrogen, correct?

potassium sulphate (K2SO4) gives you potassium?

magnesium sulphate (MgSO4) gives you magnesium?

If my above assumptions are correct, then it seems using products like 
Seachem Flourish Potassium and Flourish Nitrogen could take the place of 
the KNO3 and K2SO4, right?  I'm only trying to fully understand how 
everything works.  I specifically mention the Seachem line of products 
because my LFS happens to carry them.  So in a pinch, if I can't find 
the cheaper alternatives it should be perfectly reasonable (price not 
withstanding) to pop down to the LFS and pick up the Flourish products, 
right?
-joe