Well you could send the soil in for testing. A really cheap place is the University of Mass. Only 10 bucks for a basic test. See if it is worth using. My guess would be yes it is.
But the trees need to grow in the normal soil. Best only to use as a top dressing and not to try and amend soil. I have tried both ways. Natural and amended soil. The trees seem not to care, but I need years for full results. To see if the roots in the amended soil reach out, or not. I think they will seeking water myself. My soil right now is bone dry. That is a very good reason for them to spread and search for water.I myself believe that the current thinking not to use amended soil is incorrect. But I have no proof.

Muck soil is extremely high in organic matter because OM builds up in anaerobic conditions. It is highly colloidal which means it holds nutrients well and releases them gradually.

Generally I would say it is good for fruit trees but testing isn't a bad idea. Most likely it will just show you it is off the charts in various nutrients. However, fruit trees perform best in moderately rich soils and too much of a good thing leads to excessive vegetative growth.

If you are just trying to speed the establishment of young trees I agree with Drew. Either use it as a top dressing or if soil is poor you can work it into the top few inches of soil in 6' diameter circles to get young trees to plug in quickly.

Muck soil I've tested for pH has usually been close to neutral but that is something you should check on for sure.

With a clay substrate you will need to essentially develop a " raised bed " orchard to allow the non spring flood waters to drain.

Will the addition of the pond serve to keep the orchard dry during the spring flood?

From what I know wet feet are not happy fruity feet .

Mike

You could try and make a pond in that deep corner to hold water and drain the land.

Sorry that was a bit of a drive by... Ill elaborate a bit...

Rushes only will grow on constantly moist soil. This means the corner of your yard is much lower and already has a high water retention. Clay here will be a great help creating a natural seal and allowing the waters to slowly seep into the ground.

Wet lands are extremely productive ecosystems anyways. You can grow the cat tails as food as well as some lotus, cray fish or what floats your boat. You could also create mounds with the muck layering it with straw to create "finger beds" leading into the wetland. You can grow blue berries and cranberries on such mounds.

You could also try some big trees that love water. I dont know where youre located, but the first that comes to mind is the bald cyprus. Willow may also help pump out excess water...

You may also think about a contour swale. Basically its a 1 - 3 foot wide by 3 - 8 foot ditch on controur, which will hold and send water away. Even if its only filled in spring it should help move water away. Swales do much more if they are used in conjunction with a pond....

I think it is a good idea overall. However if digging it out with a backhoe it will take time for the muck to dry out, (maybe all year). So it will take some work. But i do like your plan of developing a nice pond and elevating the rest with what you dig out, (I I understand correctly)

Be careful working in the swamp, it is easy to get stuck in the muck.

Organic matter settles to low points but so does clay. All muck is not equal and if yours takes more than a few days to drain during dry weather it would be an impermeable clay indeed.

I used to mound up marsh muck regularly to grow herb and come back a couple weeks later to plant in the fertile black stuff. By then it would be very hard and dry but fairly easily broken up (unlike a pure clay soil). I could have grown fruit trees on those same mounds.

Mounded up, muck soil is very productive. Farmers here call it black dirt soil and drain fields of it to grow lettuce and onions. Of course, it is also often protected as wetland soil.

Have you considered growing cranberries in the swampy area?