Suitability of procedures for determination of substances in fertilizer

[acidbase]

Okay, I have a chemistry sac tomorrow, and there are a few things I need to know and I don't know where to find them
I need to know the suitability of different procedures to find the percentage of a substance in fertiliser
The procedures we used were:
Gravimetric determination of sulphur
Volumetric determination of Nitrogen by back titration
Volumetric determination of Iron content by redox titration
Colorimetric determination of phosphorus

I already know why we did a back titration, because ammonium is too weak a base to find the equivalence point with indicators. The rest I've got no clue why we tested the content differently