Avocado trees like loose, decomposed granite or sandy loam best. However when planting in the ground you really want to mix your native soil into the hole with admendment as it has to get use to your normal soil. You can also try planting it raised above your soil line with a mound if your soil is sub par. Here is a great video on how to pant an avocado tree. There are bunch of videos on this page that should help you.