You need to treat the trade-in value of the old tractor as a gain on sale of it, since it was fully depreciated. Then, you can add the new tractor as a farm asset and depreciate the cost of the new tractor.
You will start by entering the Business section of TurboTax and accessing your farm income and expenses entries. On the screen that says Your Farming Income and Expenses, choose the Farm Assets option, and then Assets depreciated in TurboTax associated with this farm. Update that section and say "Yes" to the option of going directly to your asset summary.
There you will see the tractor you traded in. Edit that section and when you come to the screen that says Tell Us More About This Farm Asset, check the box that says the item was sold. Work through the rest of that section to enter the information regarding the sale of the tractor, and enter the trade in value as the sale price.
To add the new Tractor, go through the same steps but indicate that you are adding a new asset.
@ckrieg
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