plant tablets (terrestrial stuff)

    after purchasing some plant tabs from tfp mail order i read the 
ingrediants and then went to a plant shop and found that they were 
almost identical.  the only difference is that the terrestrial stuff 
has different concentrations of phosphorus, nitrogen and phosphoric 
these are the three main ingrediants of any plant fertilizer.  

    now what i was wondering is if anyone would have a reason that 
this would not be a good idea to use instead of the plant tabs from 
tfp, because for about half the cost you can get almost 3 to 4 times 
as much stuff.

    if anyone would like to know the exact percentages of each i 
would be happy to post them, but as i said they are pretty close and 
i am sure that if you searched enough you could get even closer.