Urea is easy to get and you can get coated urea if your concerned about disapation due to time constraints and mother nature, although it will raise your sulfer levels a bit. Total disapation will occur after 72 hours if not coated . Urea may be a little strong for seedlings and being that it is 46-0-0 there is no P or K to support the root systems eestablishment. Can’t go wrong with triple 17 as starter fert, generaly 15 to 18 bucks for 50 #s. I have had some visable leaf burn this year on brassicas under a foot when top dressing urea hours before heavy rains but am getting huge growth rates on the other patchs that were over 12 inchs when top dressed with the urea.