Influence of soil strength on root growth: experiments and analysis using a critical‐state model

Abstract

Summary Roots grow thicker in compacted soil, even though it requires greater force for a large object to penetrate soil than it does for a small one. We examined the advantage of thickening in terms of the stresses around a root penetrating with constant shape, rather than the stresses around an expanding cylinder or sphere, as has been studied previously. We combined experiments and simulations of the stresses around roots growing in compacted soils. We measured the diameter of pea roots growing in sandy loam and clay loam at four different densities, and the critical‐state properties of the soils. At a penetration resistance of about 1 MPa the diameter of the roots in the sandy loam was about 40% greater than that at 0.7 MPa, and at 2 MPa it was about 60% greater. In the clay loam, there was less thickening – about 10% greater at 1 MPa and about 20% greater at 1.5 MPa. The maximum axial stresses were predicted using a critical‐state finite‐element model to be at the very tip of the root cap. When there was friction between the root and the soil, shear stresses were predicted with smaller values at the tip than just behind the tip. When the interface between the soil and the root was assumed to be frictionless, there were by definition no shear stresses. In the frictionless case the advantage of root thickening on relieving peak stress at the root tip was diminished. The axial and shear stresses were predicted to be smaller in the clay loam than in the sandy loam and may explain why the roots did not thicken in this soil although its resistance to penetration was similar. Our results suggest that the local values of axial and shear stresses experienced by the root near its tip may be as important in constraining root growth as the total penetration resistance.

Quantitative 3D Analysis of Plant Roots Growing in Soil Using Magnetic Resonance Imaging

“…The missing root fraction in the MRI data is relatively well defined, and thus poses no problem for data interpretation. Root diameters have been found to depend on soil strength (Kirby and Bengough, 2002), soil water content (Kuchenbuch et al, 2006), or nutrition levels (Zobel et al, 2007), and thus the ability to accurately determine diameters may be of importance for many studies. It is difficult to say what the precise accuracy is of our diameter estimates, as we do not have a coregistration of the magnetic resonance images with the WinRHIZO scans.…”

“…The soil strength was 40 kPa at full water holding capacity, increasing to 500 kPa when dried as measured using a penetrometer (Eijkelkamp, Giesbeek, The Netherlands). This is sufficiently low for good root growth (Kirby and Bengough, 2002) even at the relatively high bulk densities.…”

Open-mode discontinuities in soils

“…Root growth into granular materials resembles the propagation of an open-mode axisymmetric discontinuity, so one may wonder whether plants use similar strategies to optimally invade soils. Data show that the rate of elongation and the rate of radial expansion are related to mechanical conditions at the tip of the root: difficult invasion conditions promote radial expansion of the root before the root tip advances further into the 'weakened' soil mass ahead of the tip (Hettiaratchi et al, 1990;Kirby & Bengough, 2002;Gregory, 2006 A particle-level analysis and a macroscale effective stress numerical simulation follow.…”

Root Tip Shape Governs Root Elongation Rate under Increased Soil Strength

“…This adjustment of roots to increased soil strength reduces the risk of root buckling and decreases the mechanical stress acting on the root during penetration (Materechera et al, 1992;Kirby and Bengough, 2002;Chimungu et al, 2015). Root thickening in response to increased soil strength has been observed in a wide range of species under field and laboratory conditions and often coincides with increased cortical area (Atwell, 1990a;Materechera et al, 1992;Grzesiak et al, 2013;Siczek et al, 2013;Chen et al, 2014;Hernandez-Ramirez et al, 2014;Colombi and Walter, 2016).…”

“…Root thickening in response to increased soil strength has been observed in a wide range of species under field and laboratory conditions and often coincides with increased cortical area (Atwell, 1990a;Materechera et al, 1992;Grzesiak et al, 2013;Siczek et al, 2013;Chen et al, 2014;Hernandez-Ramirez et al, 2014;Colombi and Walter, 2016). Since root thickening decreases penetration stress and stabilizes roots, thick roots are likely to be an advantage in soils with increased mechanical impedance (Materechera et al, 1992;Kirby and Bengough, 2002;Chimungu et al, 2015). A recent study showed that the genotypic cortical thickness in maize is related to bending strength and, hence, to the risk of root buckling (Chimungu et al, 2015).…”