Root growth through strong soils is necessarily
inaccessible; this has led to a paucity of direct
observations and a great deal of experimentation
in artificial soil systems and ballotini. This
approach has been vindicated because we are
now quite confident that roots penetrate hard
soils by a combination of cylindrical stress and
axial extension. This mechanism is founded in
impeccable physics and contrasts with the axial
resistance estimated from blunt, steel probes. The
energy saving embodied in this mechanism of root
growth is further improved by low root-soil friction
and compression of the growth zone during
impedance. Roots have evolved to be sophisticated
biological probes. However, in spite of
the subtlety of root behaviour in hard soils, pen-