This is a very common challenge, especially in recently developed subdivisions where often the topsoil has been scraped off leaving a very hard, compact clay soil. 

The worst thing you can do is to dig a small hole large enough for the plant to fit in and then back fill with clay or compost. This is a very restricted growing environment for the plants roots and often contributes to its death. 
Therefore, it is essential to break up your soil first with a sharp spade or a trenching spade to at least twice the width of the planter bag that the plant came in. Without digging too deep, a shallow scoop is preferable to a single hole, mix compost and Gypsum Clay Breaker into the area of clay – mixing well. 

At the time of planting, mix the previously prepared ‘chopped up clay’ in with Garden Time™ Compost or Daltons Premium Garden Mix. Daltons Blood and Bone can also be added to encourage beneficial soil organisms in the soil and to provide nutrients for the new tree.

The final planting level of the tree should be slightly raised above the existing ground level to stop water pooling around the trunk.