Root elongation rate (A) and diameter (B) with respect to soil bulk density, texture and soil structure for primary maize roots. Mean values are shown ±s.d. for each group. Solid red lines represent the linear regression model for Groups Mix (see Methods for mathematical formula), whereas the dashed red lines show the extrapolated values that are out of the data fitting region, and the dashed black lines indicate the confidence limits of the model prediction (90 % confidence). The red X indicates the estimated bulk density and root diameter for root growth arrest according to the linear models of root growth responses in the mixed sand of Groups Mix.