I am not sure about tractor mounted augers but auger attachments for a skid steer you can just get extensions for them and dig about 10' deep I would guess the problem is going to lack of downward pressure
My standard 3-point auger (mounted on a '48 Ferguson TO20) routinely goes down to 38" the way it's set up; I could probably get a few more inches out of it if I really wanted to.
I don't see getting 4 or 5 feet from an auger "as is". I have seen "extender shafts" for augers though, usually in the 18" length range. It seems like it would be simple enough to make your own to whatever length you desire. Of course, you'll be somewhat limited by the amount of travel/digging depth capabilty of your 3-point unit when you add or remove an extender, but that problem isn't impossible to deal with. After all, look at the principle involved when drilling a well with a "Hydra-Drill" ... you'd just be working with a larger tractor-mounted version.
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