Summary
Previous work is reviewed in which the ratio of the pressures required for soil penetration by roots and penetrometers are compared. It appears that this ratio can vary from about 2 to 8 depending on conditions. However, there is very little experimental evidence and most of the work has been inferential.
Direct measurements are reported for the stresses exerted by a 1 mm diameter penetrometer probe and by the roots of pea seedlings when penetrating Urrbrae fine sandy loam. Six soil conditions were used: (non-weathered remoulded soil cores + artificially weathered remoulded soil cores + undisturbed field clods) × (confined + unconfined cores or clods). The confinement treatment was to test for any effects of additional restraint to cylindrical root expansion. The weathering and field clod treatments were to test the hypothesis that root elongation is facilitated by tensile failure ahead of the root tip.
The principal conclusions are as follows. The laboratory weathering treatment reduced the soil tensile strength by 25%. This resulted in a small but significant reduction in the pressure for root penetration into confined cores. Compared with remoulded non-weathered cores, field clods had a 2 to 3 fold greater penetrometer resistance and a 50% lower tensile strength. The force required for root penetration into unconfined field clods was only 10% greater than for unconfined non-weathered cores. For the former (which is closest to field conditions) the penetrometer had to exert a pressure 5.1 times greater than a root tip in order to penetrate the soil. Penetrometer penetration pressure was independent of probe diameter in the 1–2 mm range in the soil used. Core confinement restricts root radial expansion and modifies the penetration force of metal probes and plant roots.
On the basis of the new results it is tentatively concluded that soil tensile failure can facilitate penetration by roots.
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References
Abdalla, A. M., Hettiaratchi, D. R. P. and Reece, A. R. 1969 The mechanics of root growth in granular media. J. Agric. Engng. Res.14, 236–248.
Barley, K. P. 1962 The effects of mechanical stress on the growth of roots. J. Exp. Bot.13, 95–110.
Barley, K. P. and Greacen, E. L. 1967 Mechanical resistance as a soil factor influencing the growth of roots and underground shoots. Adv. Agron.19, 2–43.
Blanchar, R. W., Edmonds, C. R. and Bradford, J. M. 1978 Root growth in cores formed from fragipan and B2 horizons of Hobson soil. Soil Sci. Soc. Am. J.42 437–440.
Bradford, J. M. 1980 The penetration resistance of a soil with well defined structural units. Soil Sci. Soc. Am. J.44, 601–606.
Bradford, J. M., Farrell, D. A. and Larson, W. E. 1971 Effect of soil overburden pressure on penetration of fine metal probes. Soil Sci. Soc. Am. Proc.35, 12–15.
Braunack, M. V., Hewitt, J. S. and Dexter, A. R. 1979 Brittle fracture of soil aggregates and the compaction of aggregate beds. J. Soil Sci.30, 653–667.
Cockroft, B., Barley, K. P. and Greacen, E. L. 1969 The penetration of clays by fine probes and root tips. Aust. J. Soil Res.7, 333–348.
Dexter, A. R. 1975 Uniaxial compression of ideal brittle tilths. J. Terramechanics12, 3–14.
Dexter, A. R. 1978 A stochastic model for the growth of roots in tilled soils. J. Soil Sci.29, 102–116.
Dexter, A. R. and Tanner, D. W. 1973 The force on spheres penetrating soil. J. Terramechanics9, 31–39.
Eavis, B. W. 1967 Mechanical impedance and root growth. Agric. Engng. Symp. (Silsoe, U.K.). Paper4/F/39, 1–11.
Eavis, B. W. and Payne, D. 1969 Soil physical conditions and root growth.In Root Growth. Ed. W. J. Whittington, Butterworths, London, pp 315–336.
Eavis, B. W., Ratliff, L. F. and Taylor, H. M. 1969 Use of a dead load technique to determine axial root growth pressure. Agron. J.61, 640–643.
Farrell, D. A. and Greacen, E. L. 1966 Resistance to penetration of fine probes in compressible soil. Aust. J. Soil Res.4, 1–17.
Gill, W. R. and Bolt, G. H. 1955 Pfeffer's studies on root growth pressures exerted by plants. Agron. J.47, 166–168.
Gooderham, P. T. 1973 Soil physical conditions and plant growth. Ph.D. thesis, University of Reading.
Greacen, E. L., Barley K. P. and Farrell, D. A. 1969 The mechanics of root growth in soils with particular reference to the implications for root distribution.In Root Growth. Ed. W. J. Whittington, Butterworths, London, pp 256–268.
Greacen, E. L., Farrell, D. A. and Cockroft, B. 1968 Soil resistance to metal probes and plant roots. Trans. 9th Cong. Int. Soil Sci. Soc., Adelaide, South Australia, 769–779.
Hewitt, J. S. and Dexter, A. R. 1979 An improved model of root growth in structured soil. Plant and Soil52, 325–343.
Kirkham, D., De Boodt, M. F. and De Leenheer, L. 1959 Modulus of rupture determination on undisturbed soil core samples. Soil Sci.87, 141–144.
Mirreh, H. F. and Ketcheson, J. W. 1973 Influence of soil water matric potential and resistance to penetration on corn root elongation. Can. J. Soil Sci.53, 383–388.
Mulqueen, J., Stafford, J. V. and Tanner, D. W. 1977 Evaluation of penetrometers for measuring soil strength. J. Terramechanics14, 137–151.
Pfeffer, W. 1893 Druck und Arbeitsleistung durch wachsende Pflanzen. Abh. Sächs. Akad. Wiss.33, 235–474.
Russell, E. W. 1973 Soil Conditions and Plant Growth. Longmans, Green and Co., London, 849 p.
Stolzy, L. H. and Barley, K. P. 1968 Mechanical resistance encountered by roots entering compact soils. Soil Sci.105, 297–301.
Taylor, H. M. and Ratliff, L. F. 1969 Root growth pressures of cotton, peas and peanuts. Agron. J.61, 398–402.
Taylor, H. M. and Ratliff, L. F. 1969 Root elongation rates of cotton and peanuts as a function of soil strength and water content. Soil Sci.108, 113–119.
Utomo, W. H. 1981 The effects of wetting and drying on soil physical properties. Ph.D. Thesis, University of Adelaide.
Utomo, W. H. and Dexter, A. R. 1981 Age hardening of agricultural top soils. J. Soil Sci.32,In press.
Utomo, W. H. and Dexter, A. R. 1981 Tilth mellowing. J. Soil Sci.32,In press.
Utomo, W. H. and Dexter, A. R. 1981 Soil friability. J. Soil Sci.32,In press.
Voorhees, W. B., Farrell, D. A. and Larson, W. E., 1975 Soil strength and aeration effects on root elongation. Soil Sci. Soc. Am. Proc.39, 948–953.
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Whiteley, G.M., Utomo, W.H. & Dexter, A.R. A comparison of penetrometer pressures and the pressures exerted by roots. Plant Soil 61, 351–364 (1981). https://doi.org/10.1007/BF02182016
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DOI: https://doi.org/10.1007/BF02182016