Believe the book.

The guy at the nursery said to dig a hole just slightly larger than the root ball and plant the tree with 50/50 native soil and compost. Bad idea, because you just created a "flower pot" of very loose soil in the middle of the native dirt. I've done this without the compost and it worked well, but only for native plants.

"Extreme Gardening" says to dig a hole as deep as the root ball and 4-5 times as wide, plant the tree in native soil, then topdress with compost, not allowing it to touch the trunk. Way better. Make sure the graft is above the dirt line.

ALSO! After you have dug this nice wide hole, fill it with water, let it soak in, then fill it again and let it soak in. This makes a large supply of water under the plant.

Around the edge of the hole, make a temporary ridge of dirt to make a basin. When you water, slowly trickle water in until the basin is full. That ensures that there is water outside the root ball to encourage roots to grow outwards.

Compost against the trunk can provide save harbor for fungi and bark-eating insects.

If you are going to stake the tree - and only stake it if it's floppy - pound the stakes in at the edges of the basin and use something slightly flexible like old panty hose to lightly hold the tree upright. The trunk neeeds to flex a bit to encourage it to get stronger.

This year, don't prune any branchlets anywhere on the tree. If it gets a crop of apples, thin them out real early or they'll break the tree.

Thank you very much for all the advice! I guess I'd better go out there and widen the holes.

I agree with what lazygardens said with the exception of heading/pruning back any wood. I'd suggest removing 30% of what you want in your main structure and removing any unstructured or unwanted wood.
Spray your tree late spring or early summer with Anti-Stress to stop sunburn.

Here is a link that might be useful: Arizona Grown Apple Trees

The debate about soil conditioning continues! One thing I noticed is that the studies suggesting that amending the soil is unnecessary were all done in sandy-silty Tucson, Yuma, or Coachella Valley soil. As far as I can tell, none were done in central Valley of the Sun adobe clay. Here's my experience.

If your soil is sandy, follow the book. If your soil is clay, amend it, but with modifications:
Use 30% compost and 70% native dirt. Adding a cup or so each of soil sulfur and high phosphate fertilizer is a boon to any non-native plant, too. Be sure that the amendments are thoroughly mixed in--impossible to do in the hole, itself--because lumps or layers of compost left underground will kill roots when the temps rise in summer.
Make the hole square (or even star-shaped): roots that would otherwise circle will strike out at the corners.
Before planting, scratch a half inch of amended soil into the bottom of the hole to make a transition layer: any sudden change in soil texture is going to stop drainage at least a little.

Soil conditioning is far more important with plants from containers, since you need to coax those roots out of all that nice potting soil, but the sulfur and fertilizer are good no matter how a plant was packaged.

Remember to check your soil first!
Kevin : )

I really appreciate all the help! Planting in the desert sure is a lot of work, isn't it?

First of all, moisten the soil in the area you plan to dig a few days prior - it will make it much easier. It isn't necessary to remove all the dirt it the "wide" hole. You can loosen it with a shovel or spade. This will allow the new roots to easily grow in to the surrounding soil after planting.

Don't add mulch/compost to the backfill. Here in the Southwest, organic material quickly decomposes (within a year or two) which can leave voids in the soil. Many times this will cause the soil around the rootball to sink over time and you can't replenish the organic material in the rootzone of a perennial plant. Just use native soil for the backfill and your tree(s) will be fine.

aztreelvr
Certified Arborist

Here is a link that might be useful: Planting Guidelines

Nice publication!

That sinking problem is why you always have a few strong roots resting on undisturbed soil, and you only use 30% or less organic matter. The reason I recommend using any organic matter at all is that pure "adobe" clay can pack down within a matter of days after planting, preventing even the start of root growth in bare roots, or effectively confining containerized plants to their original rootball. The "sinkhole" that may form around the rootball is easily disguised with a coarse mulch, such as ground bark or gravel.

Kevin : )