Root elongation is restricted by axial but not by radial pressures: so what happens in field soil?

Root distribution determines largely the zone of soil that roots have access to for water and nutrient uptake, and is of great importance especially if water and fertilizer input is restricted. Mechanical impedance is the major limitation to root elongation in many field soils. Until now, experiments have focused largely on the axial resistance to root growth. In a fascinating study of the radial forces exerted by the roots of chickpea, root extension, diameter change, and the radial forces that axially unimpeded roots exert are reported: Kolb et al. (this volume) record radial stresses of about 0.3 MPa that are broadly consistent with cell turgor pressures, but, interestingly, find no restriction to axial elongation. This result is in marked contrast to large decreases in elongation of pea radicles resulting from much smaller axial pressures reported elsewhere in the literature (e.g., an 85 % decrease in root elongation in response to axial pressures of < 0.1 MPa). The situation is different also from that in homogeneous soil, where root penetration resistance pressures of 0.4-1.0 MPa are typically required to halt root elongation. Soil structure and strength properties will determine the balance of axial and radial pressures on an individual root tip, and hence the root elongation response. It appears that a degree of radial confinement may help roots to extend axially into hard soil. This result also complements recent findings that in strong field soils the availability of soil macropores has a large influence on regulating the root-elongation rates of seedlings.