1. Understanding soil health;Acton,1995
2. The Economics of Production;Beattie,1985
3. Evaluation of selected soil physical, chemical and biological parameters as indicators of soil productivity;Bowman,1990
4. Bowman, R., Petersen, M., 1996. Soil organic matter levels in the Central Great Plains. Conservation tillage fact sheet #1–96. USDA-ARS and USDA-NRCS, Washington.
5. Gams User Guide Release 2.25;Brooke,1992