How big is the hole you want to dig?
How many do you need?
How often do you need to dig them?

This is going to be a really low-BS comment. If I had a paycheck riding on this working I wouldn't show up for the job. Dewalt/Milwaukee/Ridgid aren't as good as they say they are, and their half inch drills do not have enough power to mix drywall mud or thinset (with an eggbeater mixer) for more than half a minute without overheating to the point of smoke. In dirt? No way. There's also the problem of securely mounting the two things together.

Quality gasoline engines do not have startup troubles like the crap from HF and they sell nothing that isn't in the throwaway category. Nothing.

Buy Honda, Stihl, Husqvarna.

I am Bokashing so need to drill holes arounf the trees 4" for that but would like an 8" to use with just burying the bikashi, Basically 4" and 8"

You suggestion about Honda, Stihl, Husqvarna. is great but I cannot justify investing 600+ for 6 trees sub tropical trees

So dig the holes by hand ... or just spread the material under the trees and le tit trickle down.

My husband did something similar when we planted a mesquite tree. He wanted a way to deliver water deeply. He dug the holes by hand using one of those heavy iron bars that has a pointed end and a blade end. It was arduous for sure. He did about 5 holes 4' deep, then inserted a pvc pipe that has holes drilled all over it. He put a removable cap on the end to keep debris out. Worked really well but it *was* a lot of work.

I use one of those "weaker" drills and a large auger to turn compost and to dig holes like you are talking about. I lost the original auger I was using in a move but substituted one used for mixing paint in large buckets (available in any big box store) and even that works OK.

I volunteered with the county on plantings at a new park and they used a Husqyarna with a 6" auger and dug holes for gallon sized plants in about 15 seconds. It was amazing but those augers are pricey. If I was young and had a lot of gardening years ahead of me I would invest in one.

It is pretty hard to dig holes using those single people gasoline powered auger. The two man work fairly well to handle the torque, but even they can struggle with it. What chance does your wrist have?

Buy a post hole digger. Best $50 I ever spent. If you need to keep filling the holes with water to soften the soil, let it sit 2 days and continue digging (less if you don't mind cleaning mud off).

I already own a post hole digger but it is ardous work. Being a single dad it almost gets impossible to to do extra stuff in the garden.
Digging by hand you would do more damage to the feeder roots of a plant. feeding tube is great idea however most plants have feeding roots withing the first 18 inches of the ground. Lots of discussions about deep root feeding

@scpalmetto
Thank you for pitching in. How did you make a adapter for the auger bit?. I was planning to get a 1" wide shaft. Have a machinist drill a hole through the sides and use a nutbolt to connect the auger bit.

Try using one of these to get a start and softening soil. It will dig out pretty easy afterwards using a post hole digger trowel or spade. I agree the auger on a drill sounds good but I never got it to work well for all the above mentioned. They have big motors or gas engines for a reason and a stalled bit is a great recipe for bashed wrists and broken hand parts.Bokashi is anaerobic and putting wet dirt back on top will help exclude air and get it going faster. it's also great to have around for the occasional ultra fast deep watering or aerating.

Here is a link that might be useful: Say hello to my little friend

well. I ordered the 4" one from ebay, inexpensive 29.99(American made) shipped. Planning to try on 5380-21 1/2-Inch 9-Amp heavy duty hammer drill see if that works. Will update with my experiences