Melvin Wei's video: Growing a Mango Seedling in a Tube Days 0-52

This is my fourth posted attempt at germinating and growing mango seeds. I've had offline attempts that never made it too. In my previous attempts the mango seedlings never made it past the initial set of 6 leaves. Some of my viewers have attested to failing at that stage as well. Since my seventh leaf is growing and my 6 starter leaves are bigger than ever before, I figure I might actually be on my way to long term success as of day 52! In previous mango growing attempts I had my seedlings growing in shallow pots, so the taproot would hit the bottom of the pot in short order, followed by stagnation in leaf development. I remembered hearing that the taproot needs to go down about a yard/a meter for a mango tree to thrive? There are plants that send taproots down way deeper than that in the literature, and I've heard experts say that roots can grow only as deep as oxygen can reach (two somewhat conflicting concepts, since roots going down 50 feet for water are completely lacking in fresh oxygen down that deep). But I've also seen various cultivars of mango saplings growing in large pots too, so I'm not sure what to think in the end. Perhaps the 50% sand/50% clay soil mixture is the key to success? Many people insist that mixing sand and clay is disastrous as it dries out and cures like cement. So far my half-and-half mix appears to be very loose and well aerated with air spaces naturally forming deep within the tube. I would tend to think that a composition that's nearly all clay would end up like sun-dried mud bricks. The same has been said for mixing in organic materials such as hay to break up and aerate gardens of almost pure clay, but there are also those who say that adding lots of organics into their clay soil year after year eventually made for phenomenally fertile soil. Myth busted?