Many London garden centres will sell topsoil, but when you’re planting your trees and shrubs, you should generally use whatever your existing soil is, rather than carting it away and using topsoil.
The roots are fairly quickly going to go much wider than the planting hole – averaging about 2-3 times further than the drip line – so they’re going to have to get used to your soil anyway.
But still, I always incorporate some compost when planting, in the backfill and over the whole bed, often as much as 6 inches. In nature, the soil would have some organic matter, so we’re trying our best to mimic that.
Proper watering is even more important than amending the soil for trees, so be sure to water when you plant and then regularly for the first year, and still occasionally in subsequent years.
When your tree is in the hole, make sure it is straight by viewing it from at least 2 angles. Fill the hole halfway with your backfill. Use a shovel to lightly tamp down the soil to remove air pockets.
First I slice down with the shovel and then I may use the other end. Very gently. A hose helps to get rid of air, too, and this is about the time I want to add 10-20 gallons of water anyway, which takes about 3 minutes.
Fill the rest of the hole with backfill and tamp and water to settle it, but don’t cover the top of the rootball with soil. You can put a very thin layer of mulch for aesthetics, but especially for the first year, it’s best to leave it fairly bare to allow moisture and air exchange.
Mulch can cause problems with the trunk and bark, which needs to be exposed to the air, not a moist environment. Mulch too close to the trunk can also promote rodent damage and encourage formation of stem girdling roots.
The rest of the area outside the root ball should be mulched with leaves or straw 2 to 4 inches deep. Even wood chips could be used here, preferably deciduous chips for deciduous trees and coniferous chips for coniferous trees. But I prefer leaves or straw.
No berm is necessary unless it’s on a slope and needs something to hold water.