Proper gardeners will tell you to use different mixes for different things and, decidedly, if you grow something in a pot which says it needs ericaceous compost then it truly does if you want it to flourish. ( azaleas, heathers, blueberries, camellias spring to mind)
You should use different mixes for seeds and shrubs and so on. In truth I don't bother. For growing almost everything that I grow in a pot I just use the cheapest bags of compost I can find. They are generally the ones from B & Q when they do five bags for five pounds or whatever: but keep your eyes open for cheaper, this year Aldi did a better price.
If you just want to step it up a notch then it is more sensible to pot up anything shrubby or perrenial with a good top soil mix, especially as it will be outside for probably, several years. Top soil is richer in nutrients, holds water better and gives weight to the pot so it won't blow over. Sadly, I have to confess to not ever bothering.
This year is the first year since 'forever' that I have bought in top soil. My gardener (Clean and Green) got me some at a good price - £3 a bag. He said they are 25 litres but they look like more than that to me.
|left: top soil, right: compost|
I will be mixing these bag for bag with cheap compost and filling in the major dips. If there is anything left it will make a great top dressing for all round the garden, mixed with a couple of handfuls of 7-7-7.