Given the need for fertilizer, its costs in nonrenewable resources and energy, and the consequences of the CO2 emissions required to make it, people have begun to explore ways to make fertilizer manufacture and use more sustainable. The US EPA defined sustainability as "[the ability to] create and maintain the conditions under which humans and nature can exist in productive harmony to support present and future generations". An alternative formulation of sustainable fertilizer and agriculture are the 4-Rs: Reduce, Reuse, Recycle, and Recover, which constitute guiding principles that can be used for sustainable agricultural management of fertilizers and animal and other wastes. Here recovery is used for processes which directly generate useful waste, while recycling processes convert wastes to useful products. Reducing the resource, energy, and environmental cost of fertilizer manufacture could help if fertilizer quality and usage is maintained. While phosphorus and potassium deposits are concentrated geographically and their mining and manufacture requires limited resources, the manufacture of nitrogen fertilizers can be made more sustainable by replacing the use of natural gas with renewable energy for hydrogen generation and developing better catalysts to reduce the temperatures and pressures needed to generate ammonia. Methods to generate ammonia electrochemically or photochemically would facilitate the use of renewable and non-CO2-generating energy to make nitrogen fertilizers.