The concept of structural stiffness signifies the level of resistance a body of agricultural soil develops so as to withstand any form of further deformation from subsequent external loading. In order to verify the significant effects of tillage tool geometry on the soil structural stiffness, field clay loam and sandy loam soil were tilled with a chisel shaped tine at different tillage geometries. Samples of the tilled soils were sheared in a 'uniform strain' direct shear test box to determine the shear strength parameters from which the structural stiffness was calculated. The effects of the width of the tillage blade, the rake angle, the depth of tillage and their interactions on the clay loam and the sandy loam structural stiffnesses were significant at the 5 % level or higher. From this study, it was possible to predict from the structural stiffness concept the shear resistance behaviour of a soil that has been tilled with a tillage tool of known geometry.