The size and shape of the hole are important. We want to encourage roots to grow out horizontally, where they will find the resources they want. The ideal planting hole is only as deep as the root system, measured from the top of the highest root to the bottom of the root ball. The hole should be much wider, however – two to three times as wide as the root ball (see figure below). Carefully place and align the tree in the hole, and backfill with the same soil you took out to make the hole. When the hole is halfway backfilled with soil, add enough water to fill the hole, and let it settle in before adding the rest of the soil. Flatten the soil surface, making a retention berm around the edge of the hole, and flood that basin. This gives the tree a very thorough initial watering.
Most soils will not need any amendments (like compost) added. Trees don’t need rich soils. However, if the native soil on your site is very sandy (like on the west side of Albuquerque) or a heavy, tight clay, adding a small amount of compost can help. Add no more than 10% by volume, and mix it well into the backfill soil before backfilling the hole. Do not add sand to clay soils, or clay to sand soils – it is not easy or practical to change the natural soil texture, and you are more likely to create additional problems than to improve things.