Preparing the Ground:

It had been suggested that we could use a water auger make the holes for planting out the pre-grafted trees.

This device is connected to pressurised water (e.g. a fire fighting pump, etc), twisted into the ground, the water turned on to loosen up the soil and leave a wet slurry that would work well for planting out and watering the trees.  However, our soil was (a) too heavy (clay loam with a high organic content) and (b) had too many roots from the pasture grasses for this to work. 

In the end, a post hole digger had to be used.

This produced a suitable hole with a pile of fine soil that could be put back around the soil. This soil from the pots was left undisturbed as possible. The trees were then watered with 8 to 10 litres of water each.

For most of the holes, the grass along the tree lines had been previously treated with herbicide. This made the job a lot easier and the result much cleaner. This photo is from one of some "extra" holes we had to make to accommodate more trees than we calculated for initially. 

Ideally, the soil should have been ripped along the tree lines as well as treated with herbicide. (While we would prefer to avoid the use of chemicals, some times their use is unavoidable, in the absence of that proverbial small army of workers who spend their whole time weeding.)