1. 1. Arvidsson, J and A. R. Dexter (2002). Tillage Course in agricultural soil mechanics, Swedish University of Soil Sciences, Uppsala, Sweden.
   2. ASAE S313.3 (2004). Soil Cone Penetrometer. The Society for engineering in agricultural, food, and biological systems 2950 Niles Rd., St. Joseph, MI 49085-9659.
   3. Abdulla, M. A and A. E. Mohamed (1998). Effect of conservation tillage on infiltration. University of Khartoum Journal of Agricultural Sciences., 6(2): 74- 84.
   4. Alnahas S. A. M. E (2003). Tillage implements performance and their effects on two Types of soil in khartoum area. M.Sc. thesis. Unpublished. University of Khartoum.
   5. Campbell, D. J. and J. K. Henshall, (1991). Bulk density. In: Smith, K. A. and Mullins, C. E. (Eds.),Soilanalysis. Physical methods. Marcel Dekker, New York, U.S.A, 329-366.
   6. Hettiaratchi, D.R.P (1988). Theoretical soil mechanics and implement design. Soil and Tillage Res., 11: 325-347.
   7. Kepner, R.A., R. Bainer and E. L. Barger (2005). Principles of farm machinery. CBS Publishers And Distributors. 136-157.
   8. Koolen, A. J. and H. Kuipers (1983). Agricultural soil mechanics. Springer-Verlag, Berlin. 241.
   9. Saber, N. and R. Mrabet (2002). Impact of tillage and crop sequence on selected soil qualityattributes of avertic calcixeroll soil in Morocco. Agronomy Journal.,22(5): 451-459.
   10. Oduma. O., D. C. Nnadi, C.S. Agu and J. E. Igwe (2017). Determination of the effect of tillage on soil resistance to penetration-a study of south-east agricultural soils. American Journal of Engineering Research., 6(7): 1-5