I hate it all the potting soils contain fertilizer now - you can't fertilize according to the plant without fear of burning the plants. I use Dr Earth potting soil (hard to find here) and blend it with vermiculite and perlite so it drains better and I add a little fertilizer (Dr. Earth) to the mixture. Depends what you are growing whether you should add more fertilizer or not (and what type of fertilizer)

Unless it specifies (No feeding for X months), the fertilizer charge potting soils come with are not very long lasting. You can pretty much ignore them. Even the ones that say "No need to feed for x months", are usually not going to provide enough nutrients to satisfy your planting over the course of an entire growth period.

For media that don't contain CRFs, you should operate as though YOU are the plant's sole source of nutrition - that you can't depend on the soil to supply any nutrients. Basically, plants use the same nutrients in the same ratios (with minor variation) - some plants just use more nutrients than others (grow faster), so we can throw out the thought that we need all these 'specialty' fertilizers. They're better for the fertilizer industry's bottom line than they are for your plants.

Al

How long fertilizers last in potting mix was recently discussed.

http://forums2.gardenweb.com/forums/load/contain/msg0519290010970.html?5

You might find these Frequently Asked Questions on the Fafard site interesting.

http://www.fafard.com/?p=115

Scroll down to "How long does the fertilizer starter charge last and do they contain micronutrients?"

The other FAQ's on this Fafard site contain a wealth of information frequently discussed on the Container Forum--
ie pH of mixes, nitrogen depletion, alkalinity of irrigation water, etc, etc.

Here is a link that might be useful: Fafard FAQ

I'm with nancyanne on this one. I do use Miracle Grow but much prefer to add it myself than have it in the soil.