1. Abt R.C. (1987) An analysis of regional factor demand in the U.S. lumber industry, Forest Science 33, 164–173.
2. Amemiya T. (1985) Advanced Econometrics, Harvard University Press, Cambridge MA.
3. American Society of Agricultural Engineers, 1985. Erosion and Soil Productivity. American Society of Agricultural Engineers, New Orleans, Louisiana, 277 pp.
4. Ayres, R. U. 1978. Resources, Environment, and Economics: Applications of the Materialsl Energy Balance Principle. John Wiley and Sons, New York.
5. Ball, V. E., Bureau, J.-C., Nehring, R. and Somwaru, A. (1995). Agricultural Productivity Revisited, United States Department of Agriculture, Washington DC.