Soil Mechanical Resistance and Root Growth and Function

The effects of soil strength on root growth impact plant growth and crop yield, particularly in the context of soils that are structurally degraded and subjected to changing climate where rainfall may be intense, sometimes with long periods between rainfall events. Even relatively loose soils may become sufficiently strong to limit root extension when drying, with restricted rooting volumes often decreasing water and nutrient uptake by roots. Root elongation in strong soils may be limited by the biophysical characteristics of the root, which are often under physiological control. Roots can exert large root growth pressures generated by cell turgor to penetrate soil and yet there is negligible evidence for genotypic variation in the maximum root growth pressure. Root diameter, root frictional properties, the stiffness of root tips and the stiffness of cell walls in expanding root cells all contribute to control root growth patterns and extension rates in strong soils. Many of these biophysical properties are under physiological control, with the role of ethylene, auxin and ABA of particular interest in compacted soils. This chapter aims to introduce the physical basis for soil penetration by roots, the factors controlling soil strength and the process of root penetration. Experimental approaches to studying root interactions in strong soils and the physiological factors underlying root responses are discussed. In the context of the practical application of this science to crop production systems, both the selection of appropriate plant genetic resources for adverse soil physical conditions and the sustainable management of the soil structural environment by enhancing soil organic matter contents are of equal importance.