Urea is an inexpensive form of nitrogen fertilizer with an NPK (nitrogen-phosphorus-potassium) ratio of 46-0-0. Although urea is naturally produced in humans and animals, synthetic urea is manufactured with anhydrous ammonia. Although urea often offers gardeners the most nitrogen for the lowest price on the market, special steps must be taken when applying urea to the soil to prevent the loss of nitrogen through a chemical reaction.