1. 1- Fertilizer Manual, Edited by the United Nations Industrial Development Organization (UNIDO) and the International Fertilizer Development Center (IFDC), in Kluwer Academic Publishers (The Netherlands, 1998).
2. 2- International Fertilizer Industry Association (IFA), Phosphate rock statistics (2002).
3. 3- The World Phosphate Institute, Strategic Plan of Action 2001-2010 (2000).
4. 4- IMPHOS Phosphate Newsletter, Phosphorus Availability in the 21st century (issue n°14/2001).
5. 5- The World Phosphate Institute (IMPHOS, Unpublished data (2002).