FIG. 3. (A) Root impact map illustrating soil exploration by roots in compacted soil horizons in which cracks represent paths of least resistance preferentially explored by roots (Tardieu and Katerji, 1991). (B) A model for relative root elongation rate as a function of matric potential, at different levels of soil strength Qp (MPa), measured by a penetrometer (Dexter, 1987): as soil strength increases and soil is drier, relative root elongation (R/R_max) decreases.