The use of penetrometers for estimating mechanical impedance to root growth
Soil resistance to root elongation is between about 2 and 8 times greater than soil resistance to a metal probe, or penetrometer. Much of this difference might be accounted for if penetrometers experience greater frictional resistance than plant roots. To reduce the vertical component of soil frictional resistance, a penetrometer with a rotating tip was built and tested on cores of both undisturbed and remouled soils. Penetrometer resistance frequently dropped to less than half of its original value upon rotation, and for sharp (5o semi-angle) probes, this drop in resistance agreed reasonably with predictions made using the rotation speed and independent measurements of the coefficient of soil-metal friction. In cohesive soil it is possible that soil bodies formed in front of the blunt (30o semi-angle) probes and these bodies were disrupted upon rotation, while in air-dry sand, the rotation of sand grains may have reduced penetrometer resistance. Direct comparisons between soil resistance to maize root growth and penetrometer resistance were made in two sandy loam soils. In both soils, resistance to roots was comparable to the resistance to a 5o probe after subtracting the frictional component of resistance. In addition to measurement of penetrometer resistance, the pressure required to inflate a 'root analogue' (3 mm diameter tubing) was measured in order to estimate the resistance to root elongation within flexible-sided cells of ballotini or sand. Results suggested that the resistance to root growth within these cells probably exceeds the externally applied confining pressure by a factor of more than five times. This suggests that it is necessary to re-interpret the results of many root growth experiments designed to quantify the effects of mechanical impedance on root growth rates and performed in such cells.