1. FAO (United Nations Food and Agricultural Organization), FAO agricultural data bases, http://www.fao.org, 2004.
2. Greenland, D. J., Soil organic matter in relation to crop nutrition and management, in Proceedings of the International Conference on the Management and Fertilization of Upland Soils in the Tropics and Subtropics, Nanjing: Chinese Academy of Sciences, 1988, 85–89.
3. Boyer, E. W., Howarth, R. W., The nitrogen cycle at regional to global scales, Dordrecht: Kluwer Academic Publishers, 2002, 519.
4. Galloway, J. N., Cowling, E. B., Seitzinger, S.P. et al., Reactive nitrogen: Too much of a good thing? Ambio, 2002, 31: 60–63.
5. Clark, F. E., Rosswall, T., eds., Terrestrial nitrogen cycles, Processes, Ecosystem Strategies and Management Impact. Ecol. Bull., 1981, 33:163–178.