I came across this article and I find that it goes against everything that's said about soil amendment before planting. I've copied bits of it down below.
You are planting a new tree, and you want to do everything that you can to make it grow well. You dig a hole and examine your soil. It might be very sandy, or it might contain a lot of clay. You decide to add organic matter to ‘condition the soil’. You complete the planting process, and water well. What happens?
You have created a big hole in the ground. Around the outside of the hole you have your normal native soil. Inside the hole you now have a different kind of soil – it contains more organic matter and is your amended soil. You have created the same condition we talked about in the previous post, namely two types of soil in contact with each other. We know that water has difficulty moving between two types of soils. You have created one of two problems depending on the type of native soil you have:
1) you have created a hole that retains water. Excess water sits in the hole and does not move away, drowning the tree roots.
2) you water the area but water tends to stay in the native soil and does not enter the hole. Your tree roots are dry.
Neither is good for your tree. So just replace the soil. Don't amend it.