I'm planting a 2 month old avocado in soil that is sandy silt/silty sand. There isn't much(if any) visible organic matter in it and it drains fairly fast.

How large a hole should I dig for it, and what
should I add to this silty soil?

One idea I have is to make a 50/50 mix of the original soil and a potting soil I make up. The potting soil is 1/3 compost, 1/3 Earthgro ground cover bark sifted through a 1/2" screen, 1/3 old/used SuperSoil mix (mostly peat moss), . with a handful of gypsum added per 5 gal container size. The compost has a lot of oak leaves added to it, would this be a problem for the avocado tree?

Any suggestions?

John