It depends on what sort of fertilizer bed you are looking at.  There are many dual purpose beds which will spread both, as well as some which will also spread litter. 

A litter spreader will have a 30" or wider chain.  

A lime spreader will have a 20" or wider chain or belt.  

Anything narrower then that must be a fertilizer spreader.  

Wider belts or chains will work in place of narrower as long as you can get the rate low enough.  Trying to use narrower when you need wider or a belt when you need a chain will result in wear and tear on your shovels and the operator of the shovels.