Nitrogen fertiliser. Polarised light micrograph of a crystal of urea, also known as carbamide, which is the commonest nitrogen fertiliser in use across the world. It is water soluble, and is quickly broken down in soil by an enzyme, urease. The ammonia released is quickly converted to nitrites, then oxidised to give nitrates, by various species of bacteria. Urea occurs naturally in urine and was the first organic compound to be synthesised from inorganic materials on an industrial scale (by Woehler in 1828). The modern process reacts carbon dioxide with ammonia to form ammonium carbamate; this is then dehydrated to give urea. Magnification: x10 at 35mm size.