Previously published studies of the mechanical impedance of granular media to root growth have shown that the rate of elongation of barley roots was halved by a lateral cell pressure of 0.02 MPa, applied externally to the growing media. It was incorrectly assumed that this lateral pressure was always the pressure on the growing root. In this paper, the stress distribution around a growing root was modelled both theoretically and experimentally by a thin cylindrical rubber tube, which was expanded radially in two granular materials in a modified triaxial cell. The theoretical model which simulated the deformation of the granular material around the expanding rubber root analogue was controlled by the elastic stiffness parameters, bulk and shear modulus in the early stages and later, at larger diameters, by the plastic yield parameters, cohesion and friction angle. This theoretical stress-strain model was validated experimentally by the good agreement with the results obtained using the thin rubber tube in the two granular materials. Using the theoretical model with root diameters similar to those for barley roots, it predicted soil pressures on the 'root' surface of 0.2-0.3 MPa for lateral triaxial pressures of the order of 0.02 MPa. This result was similar to the predictions made using earlier analytical models. The model also predicts lower mechanical impedance for finer roots, and strongly suggests that cylindrical expansion of the root behind the tip is effective in relieving soil pressure ahead of the elongating root.
