Forces acting on the sugar beet combine plough are caused not only by physical properties of soil (e.g. its compactness) but also by physical properties of the harvested crop (i.e. sugar beets). A current theory on rooting out sugar beets has included only physical properties of soil along with physical and geometrical parameters of the ploughing mechanism. The article presents a mathematical model of mechanical influence of the soil layer together with the mass of sugar beets roots on the beets ploughing element. Mathematical formulas of the developed theory were verified with exemplary calculations for the accepted working parameters of the sugar beets harvesting combine (inter alia working speed, working depth).