Fertilizers of all sorts require energy and resources to make and transport and their correct usage can still have negative consequences. There is also significant uncertainty in the evaluation of the CO2 emissions encompassed in the use of organic and inorganic fertilizers. Making fertilizers sustainable is an important aspect of sustainable agriculture. Nitrogen in synthetic fertilizers is primarily derived from the Haber-Bosch process, in which nitrogen from the air is reduced with hydrogen to yield ammonia in the presence of an iron-based catalyst at high temperature. Roughly 70% of the hydrogen needed to manufacture ammonia comes from natural gas, with further energy required to heat the mixture of gases to the reaction temperatures necessary. As a result, synthesis of ammonia by the Haber-Bosch process (not all of which is converted to fertilizer) uses 1.8% of global CO2 emissions and 1.8% of total energy generation. In addition, the use of ammonia-based fertilizers can lead to the formation of nitrous oxide, a potent greenhouse gas; per kilogram of ammonia used in fertilizer, roughly 3 kilograms of CO2 equivalent are emitted. Phosphates for fertilizer come from mining and reaction of the phosphate rock with sulfuric or phosphoric acids to yield phosphate or triple superphosphate. Phosphorus is a nonrenewable resource, with supplies estimated to last between 40 and 400 years; in addition, most of the estimated reserves of phosphorus are contained in Morocco, Western Sahara, Iraq, China, Algeria, and Syria making the supply subject to geopolitical vagaries. Nitrogen and phosphorus that are not used by plants are likely to enter runoff, which may also lead to high local concentrations in water and cause eutrophication. According to the United States Geological Service: "No substitutes exist for potassium as an essential plant nutrient and as an essential nutritional requirement for animals and humans."; making it essential for agriculture. Potassium is mined from underground brines and recovery received significantly less attention since its supply is still sufficient. Belarus, Canada, and Russia are the largest producers of potash (potassium oxide), making its supply also subject to geopolitical vagaries similar to phosphate. For example, Belarussian potash is subject to sanctions from the United States and other nations. Political tensions with Russia can also cause issues with supplies, though US regulations in particular do not ban imports of potash from Russia. Finally, nitrogen, phosphorus, and potassium sources all require significant energy to produce or mine, much of which currently comes from fossil fuels (natural gas and oil) contributing to climate-change inducing greenhouse gas emissions. Measures being taken to reduce greenhouse gas-induced climate change may thus affect the costs and productivity of the above traditional sources of these nutrients. Defined as "fertilizers derived from animal products and plant residues containing sufficient nitrogen." , organic fertilizers have a different set of concerns. In particular, the bulk of most of the animal and plant residues or wastes that are sources for organic fertilizers makes it impractical and costly to transport them over significant distances. Reducing the organic materials transport costs requires it to be either generated on site or nearby, reducing the ability to scale its production and thus to make it economical. The largest sources of organic fertilizers are likely to be residues and meals derived from crops and animals, manure from livestock, and wastes from water and sewage treatment plants. Particularly for the wastes from water and sewage treatment, pharmaceutical and heavy metal contamination are likely, limiting their economic use, and require testing and mitigation to avoid transferring contaminants to farmland. In addition, pathogens may be transferred from sewage and water treatment wastes to soil and can be taken up by crops, making the people that consume the crops sick or making the crops unsaleable. Mitigation of these issues requires energy and resources.