Asparagus plants planted as crowns are best planted in mid to later winter in frost free conditions and then for the first 4 to 6 weeks covered with a black plastic sheeting covered with straw of old carpet. This will act as frost protection and help warm up the soil which in tern with help the Asparagus crowns to start to establish. Then in mid March replace the black plastic with a clear plastic or glass cloche.
With regards to the soil Asparagus is a seaside or maritime plant and requires slightly salty and acid soil conditions so dig in lots of well rotted farm yard manure and add some table salt over the soil a hand full per meter once the crowns are planted. Then in the first season feed with liquid tomato feed every 2 weeks through the season, this contains lots of potash and will help to create a strong crown that will in turn help to produce lots of asparagus spears every spring