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SOIL PHYSICS

Mechanical Stresses in Soils Assessed from Bulk-Density and Penetration-Resistance Data Sets

^a Institute of Plant Nutrition and Soil Science, Kiel Univ. Olshausenstr. 40, Kiel, 24118 Germany
^b Habichtshorst 9, Garbsen 30823, Germany
^c Univ. of Hannover, Herrenhaeuserstr. 2, Hannover, 30419 Germany
^d Dep. of Agronomy, 2004 Throckmorton Hall, Kansas State Univ., Manhattan, KS 66506-5501

^* Corresponding author (rhorn{at}soils.uni-kiel.de).

Measurement of soil strength with sophisticated parameters is tedious and expensive. Therefore, we developed two straightforward methods to determine this parameter down to about 80 cm, based on the classical measurements of bulk density and penetration resistance as a function of depth. They were applied to three profiles of arable Luvisols, all developed from glacial till. For each method, a procedure was worked out that allows expression of the results in terms of a normal (NC) or precompacted (PC) state. We defined the NC state as that observed in packing characteristics of virgin soils like forests and meadows, and the PC state as the packing characteristics that exist in the topsoil of agricultural soils and intensely grazed areas. Bulk density data were used to examine the packing characteristics and overburden pressures with the assumption that the horizon was in a NC state below 80 cm. For penetration resistance, we assumed a linear increase in penetration resistance with depth to represent the hydrostatic stress distribution in the NC state and deviations of measured values from this line as the PC state. The upper approximately 60 cm of all three soils were compacted, which is proofed both for the penetration resistance and for the bulk density data. For both approaches, the dimensionless coefficient of "stresses at rest," K₀, was calculated following the line of thought used in engineering soil mechanics (K₀ = _x/_z, where _x and _z are the horizontal and vertical stresses, respectively). The K₀ values are highest in the precompacted soil horizons and decrease with depth.