ABSTRACT
Mechanical stress is inherent to root growth in nature. In order to push their way through soils, roots need to generate a force that can overcome the mechanical resistance of soil aggregates to displacement and deformation. This resistance, commonly referred to as soil strength, is influenced by a number of parameters; some are permanent characteristics of the soil, such as texture, while others, such as soil density and water content, vary with management and stochastic climatic variations, in rainfall and temperature especially. The drier or denser a soil is, the higher its strength (Taylor et al., 1969a). Depending on the length and intensity of soil-drying events and on the spatial heterogeneity in soil structure and compaction, mechanical impedance to root penetration may be a local, transient stress at the root tip or a chronic condition, relevant to a large fraction of the root system.
