To remove sulfur, a metal compound, etc., from a crude oil and use the residue as a fertilizer source.

A crude oil is desalted, brought into contact with a large amount of ammonia to transfer sulfur compounds and metal compounds to an ammonia side as ammonium salts and a complex salt mixture. In the operation, an iron- based or a luthenium-based catalyst is used as the catalyst, the catalyst is arbitrarily used since a carbon steel and an alloy are used for a heat exchanger, a reactor, pipings, etc. The separation of the sulfur compounds and the metal compounds from the crude oil is carried out by a water washing column and impurities are introduced to an ammonia water side. In the separation, the impurities eventually contain ammonia, Fe, water, minerals, sulfur and heavy hydrocarbons, made into an excellent fertilizer source, conveniently laid under the ground so as to green a desert and are effectively mixed with grass peat, etc.