Estimating soil frictional resistance to metal probes and its relevance to the penetration of soil by roots

Summary Friction accounts for a large proportion of the resistance to a penetrometer probe, often much more so than to a plant root. The contribution of frictional resistance to penetrometer resistance was investigated in five soils with texture ranging from sandy loam to silty clay. The effect on penetration resistance of rotating the conical tip of the probe was studied in both intact cores of undisturbed field soil, and in cores remoulded from sieved soil. Rotation altered the orientation of the vector of frictional resistance towards a direction perpendicular to the probe axis, and so decreased the component of frictional resistance that opposed the axial penetration of the probe. The decrease in friction was greater for probes with a semiangle of 5° than those with a 30° semiangle and was more than half of the total resistance to a nonrotating probe in 15 out of the 16 cases studied. A theoretical treatment of the effect of rotation period on probe resistance showed good agreement with the experimental results. The penetration resistance of a metal probe is related to that of a root in terms of frictional resistance and factors such as the penetration rate. When all frictional resistance was subtracted from the resistance measured to the penetration of a 5 semiangle probe, the remaining resistance was similar to that measured for roots growing in the same soil.