Willows are large, beautiful trees that grow well in almost any soil as long as they are given sufficient water. Their root systems are large and require lots of water. They should be positioned well away from water and sewer lines, or any place where a thick, dense root system can cause trouble.
Dig a hole about twice as wide and deep as the root system of your sapling. Loosening the soil in this way promotes root growth. If you bought the tree in a pot, pull the roots out from the edge of the root ball to estimate their length.
While you are digging the hole, soak the roots in a bucket of water to make sure they are plump and moist. Even if they look damp, give them a 15-minute bath anyway since the tiniest roots, the root hairs, may need the extra moisture.
Set your tree in the hole so that the roots will be covered by soil to exactly the same depth that they were in the pot, or in the ground if the tree was sold bare-root. If you look on the trunk, you'll see a change in the color of the bark that marks the soil line.
Fill the hole about halfway, then water to settle the soil and remove air pockets. Add the rest of the soil and gently firm it around the roots, then water again.
Fertilize with 10-10-10 fertilizer, but keep it 4 or 5 inches away from the trunk, out where the smaller roots can use it.