Root : soil adhesion in the maize rhizosphere: the rheological approach

This study was designed to investigate the strength of attachment of plant seedling roots to the soil in which they were grown. The study also assesse d the effects of differing soil textures and differing soil matric potentia ls upon the strength of the root:soil attachment. A device for growing root s upon a soil surface was designed, and was used to produce roots which wer e attached to the soil. In order to quantify root:soil adhesion, roots of m aize seedlings, grown on the soil surface, were subsequently peeled off usi ng a universal test machine, in conjunction with simultaneous time-lapse vi deo observation. To clarify the partitioning of energy in the root:soil pee ling test, separate mechanical tests on roots, and on two adherent remoulde d topsoil balls were also carried out. The seedling root was characterised by a low bending stiffness. The energy stored in bending was negligible, co mpared to the root:soil adhesion energy. The mechanical properties of two a dherent remoulded topsoil balls were a decrease of the soil:soil adhesion e nergy as the soil:soil plastic energy increased. These two parameters were therefore interdependent. Using a video-camera system, it was possible to s eparate the different processes occurring during the root:soil peeling test , in particular, the seed:soil adhesion and the root:soil soil adhesion. An interpretation of the complex and variable force:displacement curves was t hus possible, enabling calculation of the root:soil interfacial rupture ene rgy. At a given suction (10 kPa), the results of the peeling test showed a clear soil texture effect on the value of the root:soil interfacial rupture energy. In contrast, for the same silty topsoil, the effect of the soil wa ter suction on the value of the interfacial rupture energy was very moderat e. The root:soil interfacial rupture energy was controlled mainly by a prod uct of microscopic soil specific surface area and the macroscopic contact s urface area between the root and the soil. Biological and physical interact ions contributing to root:soil adhesion such as root:soil interlocking mech anics were also analysed and discussed.