how does slow release plant food work?

ok so if the bag says , slow release plant food? this is bad for my weed? they will overfeed or what? but how big of a deal is it really?

cause its hard to find a potting mix with all the hard ingredients

just wanted to know how big of a deal it really would be to start my seeds off in this dirt here :
http://www.canadiantire.ca/browse/product_detail.jsp?FOLDER%3C%3Efolder_id=1408474396673840&PRODUCT%3C%3Eprd_id=845524443302667&bmUID=1230597241051&assortment=primary&fromSearch=true

here is one i have my eye on more
http://www.canadiantire.ca/browse/p...0597241068&assortment=primary&fromSearch=true

so if i buy the soil with out the plant food, do i have to put ferts in the soil or would they be fine for the first couple of weeks?