Thanks for the pictures Sam, the NDM looks really healthy on the Manilla rootstock. My situation for amended vs non amended planting holes is different than most other growers because I plant on mini mounds. If you plant in an amended hole, the organic amendments become food for microbial and insect life which eventually breaks down and your amended hole will sink and settle a bit. If you over amend with organics, the soil will become very heavy and stay wet for long periods of time but my holes are elevated to help with drainage.
For my newer plantings, I loosen the soil in the square shaped hole and backfill with the native soil but I amend with about 20-30% topsoil mixed in with my native heavy clay soil. I also seed my backfill soil with a handful of actinomycetes from bagged soil and I also add beneficial bacteria and mycorrhizal fungi. All my amendments really go on as a top dressing and do not go into the backfill soil.
I feel that too many people over amend with organic matter which can cause root issues several years later. If my soil were a bit better, I would backfill with 100% native soil but my soil is so bad it repels water like rain off a ducks back.
Several of my trees have been in the ground for about 4 years now including several citrus and they are all really healthy and hold more fruit than I can use.
Knlim000, if your tree was purchased from mail order from Florida or it has a Pine Island Nursery tag on it, it is likely on Florida Turpentine rootstock. Lavern Manilla is an excellent proven rootstock here in California and I highly recommend it. Almost any seedlings from store bought mangos have made great rootstock so far but this is much too early to make any conclusions.
Seedling mango trees planted directly into the ground can turn into huge trees. Leo tells me that seedlings planted into pots and then planted into the ground can also turn into huge trees if you ensure the roots are not restricted and pot bound. Take a look at these huge mango trees in California.
http://tropicalfruitforum.com/index.php?topic=1835.0Simon