Almost all plants prefer rich but well-draining soil. Clay is rich in nutrients but terrible at draining. Sand is great at draining but very low on nutrients. In the middle is loam to varying degrees.
The goal when we plant in our good ole red clay is primarily to improve drainage to help the roots expand and breathe. We don’t need to worry too much about nutrients with the exception of nitrogen, particularly with annuals and vegetables.
The key to good root growth is the quality and quantity of your topsoil. Often we have no topsoil at all. Topsoil is created naturally from organic matter: Leaves, dead branches, and other decaying material.
Amendments like mulch, manure, and compost break down into topsoil over time. Just top dressing with organic matter like these every year can be enough to soften the clay and give plants time to get established while adding to the layer of topsoil as it breaks down.
If the clay you are planting in isn’t too dense (you can dig the hole with a shovel without a hernia), topdressing is enough for most trees and shrubs.
A faster and more luxurious solution is to bring in topsoil before you plant. If you can hardly get a shovel in the ground, consider bringing in topsoil to lay on top of the clay and plant in. You’ll still want to mulch annually, at least for the first couple of years.
What about adding amendments to the hole?
I’m of the school of thought that the roots should be exposed to as much native soil as possible, so I shy away from adding soil amendments into the hole itself. If you need a pickaxe to dig, try adding a bit of bagged soil conditioner as you break up the chunks of clay. This is essentially composted mulch and will help loosen things up a bit.
Usually I just throw a little triple ground bark mulch in the hole when nobody is looking.
Another way to loosen things up is to till, either with machines or by hand. Tilling with a shovel feels a little too much like work, but it is worth it particularly if you need to use the space that season. Since it needs to be done annually it’s primary use is for your vegetable garden, but it can be helpful with perennials if you are prepping a large area to plant in.