Revisiting penetrometer models for estimating root elongation

Mechanical impedance has been reported as a major factor reducing root elongation. Penetrometer is the main tool for diagnosing mechanical soil conditions regarding root growth; however, soil mechanics processes influence root cavity expansion, friction and adhesion at the soil-metal interface which can induce root-related measurement overestimations. Models based on penetrometers have been used to estimate root elongation and assign penetration resistance thresholds, which have been used to determine soil physical limitation for plant development. In this paper, we revisited soil mechanical aspects modeling considering root-soil and penetrometer-soil interfaces, including calculation examples. Moreover, we revisited the application of penetration resistance threshold in soil integrated physical indices for root and plant growth. Our calculations showed that friction is a major factor inducing overestimates at penetrometer-soil interfaces. However, current mathematical models enable estimating normal stress for cavity expansion by removing the effect of soil adhesion and friction, and reducing the impact of penetrometer cone tip angle on soil-metal friction. Additionally, we estimated root elongation rate for a series of plant species as a function of penetrometer resistance which could be applied to soil physical indices for estimating limit plant growth threshold.