Hornbeam hedge and planting medium

Hi and help

I am about to take delivery of 75 hornbeam bareroot trees in early March to plant around the perimeter of my garden.  I need to order top soil to to plant the hornbeams into (approx 8 tom!) should I mix the new top soild with compost (I live in Dorset on chalk) to ensure some water retention and nutrients. 

I was planning to purchase the compost from the local authroity in bulk.