I am very surprised some of you are suggesting filling with water. Do you realize what your doing when you do that?
The inside of the wheel WILL rust. After a few years, the rust will make it's way to the outer edge, where the bead of the tire seats. Once corrosion takes place there, tires usually leak between the bead and the wheel. Sorry, but I will never fill a tire with pure water.
I should have said this before, but my ground is VERY soft for about 8 months of the year. Even with no weight but the machine itself, right now I am still leaving some imprints and making tracks. I just wish John Deere didn't jack up their prices so high. What they want for the click-n-go system and the rear weight bracket is insane! Now I guess the next question is, how much weight is enough? I guess the real answer is you can never have too much weight, but let's be realistic. I want to push some fairly heavy material, so I am worried that 6 suitcase weights and chains may not even be enough. I guess at that point my only option is to add wheel weights WITH rear suitcase weights.