When planting these trees, plant in a square shaped hole, not round. Loosen the soil to the same depth as the pot, you can loosen the soil deeper but the plant will settle more and may sink a bit over the years if you don’t compensate by planting higher.
If you have good to decent soil, there is no need to amend the soil, backfill with 100% native soil. If you want to give it a little extra love, you can add some organic tropical Fruit tree fertilizer and some beneficial soil microbes/mycorrhizal fungi. I also like to give my trees frequent dilute feedings of kelp emulsion during the establishment period.
You can top dress with a good compost and definitely mulch the tree with at least 3-4 inches of mulch. Mulch decomposes fast and needs to be replenished every 3-6 months.
If your soil is heavy clay, you should plant on a small to medium mound to allow for adequate drainage. Many new Mango growers make the mistake of adding too much organic amendments to the soil but this is a big mistake because the organic matter holds too much water and it will decompose so your tree will sink after a few years.
Think about it this way, if you add 30% organic amendments to your planting hole, the tree will likely sink by approximately 30% when that organic matter decomposes. In real life, the tree won’t sink exactly that much because the roots will grow out and anchor it a bit but it will sink a lot and can be extremely detrimental to your trees health.
I have heavy clay soil at my place and I plant on small mounds and backfill with about 50% native soil, 40% Pumice and about ten percent organic matter to get the tree going. To compensate for the loosening of the soil and for the incorporated organic matter, I plant my tree about 10% higher. It’s already on a mound but remember that the soil will settle.
For more detailed information, see this thread.
http://tropicalfruitforum.com/index.php?topic=23124.0Some of my suggestions may have changed over the years as new information comes in. I have to go through the entire thread to make sure I have updated with the latest techniques that work best for growers in SoCal.
Simon