They look like slow release fertiliser granules,to me,Hank..which will help feed your plants over the season..no need to break them down..I think these will be the added enriched part of your compost...You can buy these seperately at all GC and other outlets,where there is a gardening section..I add some to my compost every year,when potting tubs..

Yeah, definately slow released fertilizer.

If they are as densly spread through the bag as in the picture then there is far too much fertiliser in the compost and it should be weakend with sand or non-enriched compost.

This is pricey stuff but if you don't want to bother or have the means of making something better this would be the way to go. I'm with Bulbaholic. I would get a bag of regular compost and if you wish to enrich, buy a small container of slow release fertilizer nuggets and sprinkle on SPARINGLY and turn it into the compost. Tried that slow release stuff and found the weeds loved it when added to the garden bed. Use it the way Bloomer does and you will get good results. As far as loosening up potting soil you could use easy to get builders sand, which I prefer( no salt in that) or vermiculite or perlite.

Thanks guys. No, there are not as many as shown here, I've picked these out thinking they were something other than fertiliser. Just shows how much I know !
Will return them to the bag.