Hiya,
Oh man, no wonder all my Diesels smoke, there's fuel oil in the combustion chambers.......

OK all kidding aside, It's engine temperature.
All Diesels depend on heat from the compression stroke to provide heat to burn the fuel as it's injected into the combustion chamber. When the engine is first started, all the components, (Piston, head, block, intake, etc.) are the same temp as the surrounding air. as the piston begins to compression stroke, the air is compressed and begins to heat, however, the cold components act as heat sinks chilling the air, when the fuel is injected, the now chilled air does not provide enough heat to fully combust the fuel. What burns is the "light" parts of the fuel, leaving the "heavier" oilier parts partially burnt. These heavier parts of the fuel cause the smoke your seeing. You will notice along with the smoke is a strong "smoking candle" smell, this is caused by paraffin contained in the fuel that is partially burnt because of lack of heat.
What you can do to reduce this smoking is either use a block heater to warm the engine before you start it. (easier on the tractor, not on your electric bill) or buy a cetain booster additive. The higher the cetain # or the fuel, the easier and smoother it combusts and with that, less smoke on start up and more BTU's released in the combustion chamber and not going out the exhaust pipe as either soot or unburnt fuel.
Now, another thing to keep in mind is that not all the unburnt fuel is going out the exhaust, some is getting blown past the rings. Engines no matter what make, fuel or vintage, have the most blow-by when they are cold. This blow-by contains various stuff that you really don't want swimming in your engine oil. So, what I have seen as the recommendations to minimize this is to lightly load the engine 30-60 seconds after startup to try to heat the cylinders quicker than just letting it idle for 5 minutes. That being said, running the engine with a small bit of throttle while driving out of the barn easily is better for you and the tractor as it gets up to operating temp quicker.
OK, as always, my 2 cents,
Tom