Hostas require moist soil with good aeration.  With the larger plants, I have seen roots extending down 2 or more feet!

Mushroom compost, compost, peat moss, or any other organic additive that improves water retention, and allows water to seep down to the roots easily is highly recommended.  A two foot in diameter whole to a foot down will do your hostas right.

Hostas are quite hardy and if one does not do anything special to the site, the hosta most likely will look OK.  Giving your investment the best start with preparing the site will enable the hosta to flourish to it's highest potential.  Good soil will also enable the hostas to look it's best during the adverse time like drought on those hot summer days.