Root Growth: Pressure at the Soil—Root Interface

Abstract

Plant roots apply force to make cylindrical cavities in soil, which allow them to grow and elongate. However, in certain situations, roots need to apply only a small amount of force, for instance, when a root tip reaches an existing channel (made by either earthworms, roots of a previous crop, or cracks from shrinkage of soil). Studies in homogeneous soils of uniform strength usually show development of localized zones of compressed soil around roots. A root growing in soil exerts force within the zone of elongation behind the root cap (Fig. 1) until the elongation zone reaches maturity and the force shifts to a new elongation zone developed through cell division. Roots apply force in axial and radial directions. As roots have a cylindrical shape (some departure from cylindrical shape is expected for roots in very strong soils) they apply force over a cylindrical area. Therefore, root growth pressure is an estimated quantity from separate measurements of force and area. For axial pressure, area over which the force is exerted is circular, which can be estimated from the measurement of diameter in the elongation zone of the root. For radial pressure, force is exerted over a cylindrical surface; so, both diameter and length of the root are important. At the soil-root interface, axial and radial root growth forces are expected to vary with time, and so does the area over which these forces are exerted.