In TX they use expanded shale to loosen up and aerate heavy clay soils. Is that available in CA? Don't know if that would help mango trees, however.

i was told to use gypsum to loosen up the clay soil.

Instead of digging down like you have done...Plant the tree where you have amended the soil. Keep a nice 4ft-6ft diameter 6" thick mulch year round to keep mellowing and moderating your clay. Throw in some worms. You want to encourage the the mango roots to spread out, not down. The downward roots will grow too, just struggling through the heavy clay at their own speed

As far as NPK ...straight P (phosphate) can be added to help root development if you want to. Or just rely on the P that a good NPK with minors will give you

I would say (having experience in some TX clay) add some expanded shale and vermiculite to increase soil drainage/aeration. This is beneficial to mango trees. Also add some nutrients to the soil, because clay to mango trees is like cream of wheat to humans. It's bland. Mango trees like light, draining, nutrient soil. Just fix up that clay and your mango trees will love it!

Hey guys, thanks for all the inputs.
My concern is that whatever you amend the soil with, be it compost, vermiculate, etcs. It is gonna create a discrepency in the porosity and structure of the planting hole and the surrounding soil. If I can't amend the whole yard, the roots will eventually grow out of the amended area and get shocked when grow into an unfamiliar environment and the worst scenario will be to decline and probably die after a while.

Yeah, I would think that in order to accommodate a tree you'd have to amend a whole lot of soil, but it could be done. Something like expanded shale is supposed to be a permanent amendment that doesn't break down. Good luck, and God bless your trees!