Urine contains significant levels of nitrogen, as well as phosphorous and potassium. The relative ratios are typically around 11 parts nitrogen to 1 part phosphorus to 2.5 parts potassium. Phosphorus is difficult to obtain from natural sources, which is really the only reason I can see for using it. Urea is very high in nitrogen and can burn or kill any type of seedlings if applied heavily, and it should only be applied to the field at least 2-3 weeks before planting. The small amounts contained in fertilizers like Miracle Grow are not a threat to seedlings. I have used it for years on everything I grow.
The same is true of chlorides. Using a fertilizer containing potassium chloride on seedlings will not affect the burn rate of tobacco later on. But using it in the field can. I only use fertilizers containing potassium nitrate after planting. Potassium chloride is a salt and is abundant in nature, which is why it is commonly used as a source of potassium in fertilizers. But the natural decay of potassium chloride releases Radon gas, which is another reason to avoid using it on tobacco.
