Posted: 12/04/2013 at 12:22
Seed compost has hardly any fertiliser because some seeds germinate less well when fertilizer is present. Potting compost has a full dose of fertilizers. Multipurpose compost is a sort of half way house. I doubt if the strength of fertilzer will make much difference for most types of seed.
The composition of the base material (peat or peat substitute) varies a great deal, and manufacturers have struggled to find an alternative to peat that works as well for seed germination. Apparently the difference is not so important for potting on, but there seems to be quite a variation when it comes to seedlings.
I know that last year there were lots of complaints about the quality of various composts (I know Wickes was one) where you could see bits of recycled plastic and other rubbish) so goodness knows what else was being chucked in, such as shreddings of wood chips from wood that had been preservative treated).
Trouble is, if you expect to pay £4 or less for 60 litres of a bulky material that costs a lot to handle, ship and store, then you should be wary about what you are getting. I pay extra for a good quality seed compost. It may cost quite a bit more, but when you think of the extra cost per seed trayful and compare it to the cost of seed and your time, and the end result when successful, the extra cost is peanuts. For potting on I am less fussy.