Use of artificial fertilisers based on fossil fuels became common after World War II, significantly increasing yields, and is considered as one of the prerequisites for the global population growth we have seen in the last century. Nitrogen fertilisers are however energy intensive to produce.

The basic component in current industrial nitrogen fertiliser production is ammonia. Ammonia is formed in the Haber-Bosch process, the overall reaction being N2 + 3H2 à 2NH3. At present, the production of ammonia is most commonly based on steam reforming of natural gas, but gasification of coal and heavy oil also occurs. The fossil resource is needed to produce the hydrogen used, while the nitrogen is derived from normal air.

However, if the hydrogen can be produced from for example biomass, it would be possible to produce mineral nitrogen fertilisers based on renewable sources.

In the wake of the 1970s energy crisis, some research and demo was carried out to try the possibilities of producing nitrogen fertilisers without fossil fuel. These were based on electrolysis of water to produce the hydrogen needed for ammonia synthesis. A more recent project with water electrolysis is being carried out in Minnesota, US where a demonstration plant has been built with a windmill driving an electrolysis process that supplies hydrogen to a small-scale ammonia production plant Link.

I have done life cycle assessments on different ways of producing fertilisers based on renewable resources, in order to investigate the environmental impact of such systems. Below is the results for global warming. 
Impact on global warming when producing ammonium nitrate from different raw material for hydrogen production (from my PhD thesis, 2009 see publication list). In Salix and straw case, thermochemical gasification assumed. Maize and ley anaerobic digestion assumed to produce biogas which is then steam reformed. The fossil reference can actually vary, in older sites based on coal gasification up to 7.5 kg CO2-eq per kg N