Fertilizers can be made from natural or man-made materials. Natural fertilizers consist of animal manure, bird excrement, composted materials or plant residue. Human-made fertilizers are derived from chemicals, combining hydrogen and nitrogen to create ammonia, ammonia nitrate and urea.
The process of making chemical fertilizers is very energy-intensive, making it extremely dependent on the availability of energy sources like fossil fuels.
The process of using manure as fertilizer, both from animals and humans, has a long tradition. Natural fertilizer is beneficial because it returns nutrients and organic matter to the soil they were derived from. It is also less expensive, and less toxic to the environment.