of all solid nitrogenous fertilizers in common use. Therefore, it has a low transportation cost per unit of nitrogen nutrient. The most common impurity of synthetic urea is biuret, which impairs plant growth. Urea breaks down in the soil to give ammonium. The ammonium is taken up by the plant. In some soils, the ammonium is oxidized by bacteria to give nitrate, which is also a plant nutrient. The loss of nitrogenous compounds to the atmosphere and runoff is both wasteful and environmentally damaging. For this reason, urea is sometimes pretreated or modified to enhance the efficiency of its agricultural use. One such technology is controlled-release fertilizers, which contain urea encapsulated in an inert sealant. Another technology is the conversion of urea into derivatives, such as formaldehyde, which degrades into ammonia at a pace matching plants' nutritional requirements.