Urea is a synthetic protein and is the best choice of all the synthetic fertilizers.  Microbes accept urea the same as they do urine. It contains 45 percent nitrogen. Natural urea is a waste product from animals. Synthetic urea is the only manmade fertilizer that contains carbon. However, sugar or molasses and humic acid needs to be applied with it to maximize its efficiency. It is made synthetically by reacting ammonia and carbon dioxide under high pressure and temperature. Urea, when used at low levels and in conjunction with organic matter and sugar, is an effective and organically acceptable fertilizer. It helps jump start the microbes. Humates are a good organic matter choice for this use. Large amounts of urea can damage seeds and vegetation can be toxic to aquatic life because of ammonia buildup.