BIG plant pots

Use a soil based compost, something heavy, as it is a tree that you are planting and it will stay in the pot for some years presumably.

Not sure if you need an acidic based one but I think not for magnolia's.

Beautiful stunning plant. I was admiring these in the garden centre a couple of days ago and nearly bought one myself to go in a big pot in back garden, but hesitated as I've killed a few trees in pots over the years (I've murdered 4 Acers and a Cotinus). Might see if I can persuade OH to go back later today, as I now find myself yearning for this Magnolia like a lovesick teenager 

I wouldn't have a problem putting well rotted manure / home made compost / some loam (stacked and rotted old turf) in the bottom.  Needs to drain reasonably well, so adding some sharp sand ... old Tomato bags from last year, that type of thing.

Peat / manure / compost will tend to dry out, but I think it will be reasonably OK at the bottom, if the top half-ish is soil-based.

Is J.I. No 3 much more expensive than standard multipurpose compost? I only buy a couple of bags a year, whereas I must buy nearly a dozen of m ulti purpose and I can't say I've every looked closely at the price of J.I. compost 

I have at last filled it!!! 6 bucketloads of big cobbles from the brook - 4 bucketloads of pebbles saved from a neighbours garden - 4 barrowloads of home mix soil and compost - 120 litres ericacious compost.  I just hope the magnoloia appreciates all my hard work.  Thanks for the advice.