i just planted mine in some normal compost/mixed in with the earth i had dug out - did it in november last year - and they've come roaring on.
I added some fruit plant feed in the spring when they showed signs on growth starting.
the plants tend to be fairly hardy!! And just to highlight that problem, i'll tell you a wee story
I ordered 15 plants last year, but only 10 got delivered!! I planted the 10 and contacted the supplier - who apologised and sent out the remaining 5.
However, i had got some extras from a friend, so before the 5 arrived, i had planted 5 more. So when the 5 turned up, i chucked them in the shed and forgot all about them.
they stayed wrapped up in straw and polythene all winter and in early spring, when i was clearing the shed out, i found them. i opened them up and they looked a sorry state. but i thought i'd try save them. Stuck them in a bucket of water and moved outside, after a few days, i put them into a pot of compost.
a few days later, leaves started to appear. So i gave them some feed and low and behold the bloody things took
by mid summer they were looking as healthy as the original 15 plants that had been planted out.
so, i now have 20 rasps growing away.