Tensile strength of soil cores in relation to aggregate strength, soil fragmentation and pore characteristics

Summary

Tensile failure of soil is desirable in tillage. Soil tensile strength estimates most often are obtained from compression tests of dry aggregates. As tillage is performed under moist conditions, it would be highly relevant to measure tensile strength at high water contents.
Plough-layer soil was sampled in a compacted soil (PAC) and in a non-compacted reference soil (REF). Tensile strength was measured in a new direct tension test using undisturbed soil cores (4.45 cm in diameter and 5.00 cm in height) adjusted to either -50 or -100 hPa matric potential. The air-filled pore space, a, was determined from water retention measurements. Air permeability, Ka, was determined at -30, -100 and -300 hPa matric potentials and from these measurements an index of pore organization (PO=Ka/a) was calculated. Soil behaviour in the field was evaluated at approximately -300 hPa matric potential by measuring soil strength using the torsional shear box method and soil fragmentation using a simple soil drop test.
The direct tensile strength results showed that the PAC soil had significantly higher tensile strength than the REF soil (e.g. 3.2 and 2.0 kPa, respectively at -100 hPa matric potential). This finding was in accordance with the aggregate tensile strength results and also agreed well with soil fragmentation in the field (i.e. geometric mean diameter (GMD) equal to 38.7 and 14.2 mm, respectively, for PAC and REF samples dropped from 75 cm height). The tensile strengths of the soil cores were close to the predicted values determined from the aggregate tensile strength results. The energy input in the soil drop test (i.e. approximately 8.9 J kg-1 dry soil) was low in comparison with the energy input in tillage but high compared with the specific rupture energy of soil aggregates (e.g. 0.4 and 0.3 J kg-1, respectively for PAC and REF aggregates adjusted to -100 hPa matric potential). The relatively poor fragmentation in the soil drop test indicated that a substantial amount of the energy input was stored as volumetric strain energy and/or lost to processes such as plastic deformation. The tensile strength of soil cores was negatively correlated to the macroporosity of the soil, whereas the ease of soil fragmentation was positively correlated to PO.