Abstract The need for an understanding of the mechanics of handling soil by the mouldboards of ploughs has been shown by the difficulties designers have experienced in developing ploughs that have an acceptable performance at speeds in excess of 5 mile/h. Earlier investigators on the action of mouldboards had difficulties in expressing accurately the shape of the mouldboard and the paths taken by the soil in flowing over it. These difficulties can be overcome with the aid of a digital computer. The path of the soil over the mouldboard was obtained experimentally from the scratch traces left by the soil as it flowed over the mouldboard, which had been coated with nitrocellulose lacquer. Polynomial curves were fitted to these scratch traces. By successive differentiation the velocities and accelerations along the curves were obtained. The mechanics of handling soil by the mouldboard were considered by regarding the furrow slice in contact with the mouldboard as composed of a number of elementary prisms. The work done by the mouldboard in accelerating each element and in overcoming the effects of gravity, friction and soil-metal adhesion was calculated. Summation of these terms for all the elements gave the work done by the mouldboard on the furrow slice. The reactions between the landside and the base of the plough, and the soil caused a parasitic drag due to friction and adhesion; the value of this drag was also calculated. The performance of two ploughs was examined in the speed range 2–8 mile/h. The validity of the analysis was established by the close agreement which was obtained between the values of draught that were predicted by the computer calculations and those that were obtained from measurements on one of the ploughs in a soil dynamometer. The forces acting on the share were separated from those on the plough body.