The last couple comments above is where I would start. With the tractor on a paved surface, look at the outermost edges of the rear tire bars (both inside and outside of each tire) that are in contact with the ground. The tires should be inflated (or deflated) so those bars are in full contact and carrying weight all the way to the outer edges. Then check the pressure and make note of it so you can inflate/deflate to this sweet spot easily by checking pressure in the future. For very soft soil, I deflate a little beyond that point. Carrying heavy loads might require more pressure, but I prefer to have my tires squat somewhat to insure good traction. This probably seems like a minor thing but it makes a huge difference especially in 2WD. You might still want to add weights, but I'd start with checking the tires since most people inflate to max. I lower the pressure before winter for better snow traction, and leave it there through the spring when I'm working soil. When I switch over to the rotary cutter I bump up the pressure closer to max.