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This article presents results obtained by the author in an attempt to introduce new parameters that influence the working process of agricultural soil working machines, in the classical formulas that mathematically model the tillage draft force. This article naturally introduces soil moisture. To introduce soil moisture into the calculation formulas of the tillage draft force, other physical parameters affected by soil moisture were used: density, cohesion, adhesion, etc. In this article, is explicitly describes, only the variation of soil density with its moisture. Considering some phenomena such as soil swelling or shrinkage, required the development of an adequate mathematical model of the expression of the density of soil-water mixture, concerning the concentration of water and the densities of dry soil and water. The final formulas obtained are tested to experimental results from the literature. Finally, the author sets out his opinion regarding the use of the results in design calculations and regarding the elevation to the rank of physical law of a mathematical model represented by a formula with a large number of parameters, which in turn are still dependent on many others parameters. [ABSTRACT FROM AUTHOR]