Abstract When a plant root meets an interface within soil, either it is deflected or it penetrates the interface and enters the new medium. If the root has just grown across a void, it may buckle when it meets the interface and hence be deflected. If a number of roots have grown through a weak soil medium and meet a stronger soil medium, the proportion penetrating depends on the strengths of the two media and on the angle of incidence of the roots with the interface. An equation for the proportion of roots penetrating a soil interface is developed which accounts for these effects. It is fitted to some new experimental results for wheat roots and to some results of Greacen, Barley and Farrell for pea roots. The functional form of the response is similar for both plant species. Data are presented which enable the deflection behaviour of nine plant species to be estimated. The results indicate the significance of soil structure and strength factors on the distribution and morphology of roots.