I bought some acreage in which most was old farmland, last farmed 4 years ago. This Spring I noticed lots of little trees popping up. I'd like to dig up and put a 100 or so of these into pots to transplant into other areas of land. Any advice

What kind of trees are they? I look at your picture and see a big patch of goldenrod...
My experience with potting trees... They tend to out grow the pots before I ever do anything with them.... Can't tell you how many trees I have with nursery pots around their trunks...
Suggest instead flagging trees and in MN, dig in the early spring before leafing out... plant into permanent location... In my area, it would be done in the Autumn.