Posted: 03/02/2017 at 11:34
The GW magazine & in particular Garden Which do regular trials and results can be very variable even different size bags from same manufacturer! B&Q own brand has come out quite well from Which trials recently but not all sizes of pack. Some quite well known brands do not always fare that well, but it really does seem that multipurpose is not necessarily best at everything (obvious when you think about it!) AYM280 remember John Innes is a RECIPE ie x parts loam y parts Peat (yes though it's considered soil based it tends to have peat or sometimes now peat substitute) and various amounts of sand,grit,& nutrients etc depending on which formula it is i.e. 1,2,or 3. So it really depends on who makes it and where they get their base ingredients from.
I tend to buy as big a pack as I can handle, better value, but you should not buy too much at once as it does go stale if you can't use it up within one season. When I did the RHS course the commercial grade composts we used for the practicals were a way different grade to the stuff you buy at the DIY centre, much finer more even texture best germination rates I have ever seen!