Fig. 1. Root length density at anthesis of maize as a function of soil depth and tillage system [no-tillage (NT) and conventional tillage (CT)] in Zollikofen and Schafisheim in 1997 and 1999. *, **, and *** indicate significance at P < 0.05, 0.01, and 0.001 probability levels, respectively; blank is not significant.