building on black dirt (organic soil)

I want to build a 100' x 60' building on my property. The dirt is black and is great for growing vegetables and crops. The biggest problem I assume would be the foundation. The black dirt is more than 6' deep and is very soft. I had a mobile home on the property for over a year, that was sitting on just tires and a couple of blocks and it didn't settle to much. I have used a floor jack to get the blocks under the mobile home, so the dirt can sustain some load. When I use a backhoe and put the pans down, you can feel the ground shake, just to give you an idea what the soil is like. Any insight on this would be greatly appreciated. Thank you

Welcome to the forum.
I'd suggest you follow the guidelines of local builders. Not sure what you mean by 'foundation' but buildings are built on 'black dirt' all the time. Granted, those buildings include removing any unstable dirt, putting a foundation on footings down to solid ground, or hard pan, and below frost line. If you plan to do this, you should have no problem. Local building codes are usually what need to be followed, unless you don't have to go by them in your area.
Some buildings, depending on size, can be built on a floating slab of concrete. Your building size is pretty big, and I would suspect you should have an engineering firm involved in the specs for your foundation.
From the sounds of your question, you have no experience in building. Care to clue us in to what you do have? No offense menat by that. Just trying to be of some help to you.