Almost all fertilizer N is synthesized from nitrogen and hydrogen gas by the Haber-Bosch process. About 2% of domestic natural gas consumption is used to manufacture fertilizer N, and of the energy required to produce the 3 major fertilizer nutrients, 87% is consumed in ammonia synthesis. Fertilizer N may represent one-third of the fossil-derived energy input for corn production in the U.S. (Hardy et al, 1975).

If farmers were to stop using fertilizer now, crop yields would drop during some years to a very low level, and many countries which today use substantial amounts of fertilizers and export agricultural produce would barely be in a position to feed their own population (Mengel, 1992).

The demand for fertilizer N cannot be completely covered by rotation with nitrogen-fixing leguminous species (such as clover or soybean) unless more than 40% of arable land was cultivated with leguminous species; thus, a substantial proportion of arable land would be used for the production of fertilizer N. This would be more expensive than industrial production (Mengel, 1992).

Hardy RWF, Filner P, Hageman RH 1975 Nitrogen input. In (AWA Brown, TC Byerly, M Gibbs, A San Pietro eds) "Crop Productivity - Research Imperatives", Michigan-Kettering, pp. 133-176.

Mengel K 1992 Nitrogen: agricultural productivity and environmental problems. In (K Mengel, DL Pilbeam eds) "Nitrogen Metabolism of Plants", Clarendon Press, Oxford, pp. 1-15.