Lowbatts, sounds like what you need is a drill and det cord!

Seriously, it appears there is not going to be any one tool to do it all. Some digs are going to be so difficult it is going to take special equipment. What I had in mind was a trowel type instrument that could get you down at least a foot in different types of soil. I was put off to see one such instrument that listed for $69.00 which was nothing more than a long knife blade with a big handle on it! It also appears that I am not the only one who has been thinking along these lines. So, since there are already those ahead of me in the planning, perhaps it would be better for me to share some of my ideas?
1. Material: The blade itself will have to be made of some material that will resist rust and corrosion such as stainless steel, titanium or a plastic polymer. Stainless steel is fairly expensive and often hard to work with depnding on the alloy used. Titanium is very expensive and light weight although quite strong. It is more maleable than steel however and perhaps would not retain its shape or hold an edge if sharpening were required. Plastics are greatly improved and perhaps may be the ultimate answer? It should be of a characteristic that if bent it would spring back or retain its shape after use.
2. Size: It shouldn't be over a foot long for transportation purposes or it becomes unweildy to carry around yet be handy for use. Maybe something could be fabricated that would extend the length as needed? ( interchangeable blades of various lengths, etc)? It shouldn't be too heavy to wear on a belt or carry in the free hand or the problem of transportation and handiness will also arise.
3. The hilt or handle should have a rounded end so pressure can be applied by pushing on it with the palm of the hand and not hurting the hand. (Try pushing a screwdriver through gravel by hammering on the end with the palm of your hand and you will see what I mean)!
4. The blade should be capable of being easily sharpened or touched up in the field. For instance cutting through heavy grass roots or tree roots, cutting a round plug out of grassy soil, etc.
5. It should come with a carrying device or belt scabbard not only to make transportation easier, but to protect the blade of the instrument and the user from the sharp blade.
6. I believe the use of a battery powered cutter/diggerr could be explored such as the concept of the cordless drill only designed to cut or dig. Something along the line of the electric knife perhaps only a little stronger to use an 18 or 20 V battery? This would raise the cost of the instrument but it would probably then be worth it if it worked well.
These are just some of my ideas off the cuff kinda' brainstorming as it were. I like #6 especially and think a hammer drill or similar tool could be used as a basis for the digging/cutting tool?
While I am at it I was wodering what kind of gloves anyone wears? I have noticed that the minerals and grit in some of the soils I have been digging in have a tendency to really tear up my hands.