The field with the quad trac in it is too wet.  End of story, at least in our soils. 

Belts compared to tires are a bit of a quandry.  Belts are better in most ways, in my opinion at least, but there are certain conditions where they seem to get you in trouble.  I think it is at least partially the fact that you can get away with so much more on belts, so you tend to do things which you know better.....I've threatened to put the 2wd 150 hp tractor back on planter duty....it would pretty well eliminate our "planted too wet" problems.  You can't plant when you are stuck.