Depth-vs.-total length (A) and horizontal-vs.-vertical growth (B) of roots 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 fitting linear regression model for Groups Mix (see Methods for the mathematical formula), whereas the dashed red lines show the extrapolated values out of the data fitting region, and the dashed black lines indicate the confidence limits of the model prediction (90 % confidence). The dashed horizontal black line in B divides the plot into two regions: a lower region in which vertical growth is greater than horizontal growth and an upper region in which horizontal growth is greater than vertical growth.