1. L. E. Apodaca , Mineral Commodity Summaries 2020 , U. S. Geological Survey , Reston, VA , 2020 , pp. 116–117
2. M. Appl , Ullmann's Encyclopedia of Industrial Chemistry , Wiley-VCH , Weinheim , 7th edn, 2011 , vol. 3, pp. 107–261
3. Mapping Global Flows of Chemicals: From Fossil Fuel Feedstocks to Chemical Products
4. The global nitrogen cycle in the twenty-first century