Tensile strength of soil cores in relation to aggregate strength,soil fragmentation and pore characteristics
Lars J. Munkholma,*, Per Schjønninga, Bev D. Kayb
aDepartment of Crop Physiology and Soil Science, Danish Institute of Agricultural Sciences, Research Centre Foulum,
P.O. Box 50, DK-8830 Tjele, DenmarkbDepartment of Land Resource Science, University of Guelph, Guelph, Ont., Canada N1G 2W1
Abstract
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, ea, 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 ðPO ¼ Ka=eaÞ 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 frag-
mentation 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. # 2002 Elsevier Science B.V. All rights reserved.
Keywords: Tensile strength; Direct tension test; Soil fragmentation; Soil strength; Soil pore characteristics; Pore organization
1. Introduction
In seedbed preparation, tensile failure is the desired
mode of soil failure in which the soil breaks down in to
smaller fragments without disturbing soil microstruc-
ture. Methods of measuring tensile strength in a
compression test on single aggregate (e.g. Dexter
and Kroesbergen, 1985) or on soil cores (e.g. the
Brazilian method (Kirkham et al., 1959)) are well
known. However, it is difficult to measure tensile
strength in a compression test at soil water contents
similar to those in the field at tillage (i.e. close to field
Soil & Tillage Research 64 (2002) 125–135
* Corresponding author. Tel.: þ45-899-91768;
fax: þ45-899-91719.
E-mail address: [email protected] (L.J. Munkholm).
0167-1987/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 1 9 8 7 ( 0 1 ) 0 0 2 5 0 - 1
water capacity for sandy and loamy Danish soils).
Plastic deformation will to some extent occur in wet
soil in compression. At low confining stress the mode
of failure is expected to shift from pure tensile in dry
state to shearing and compression in wet state (Hatibu
and Hettiaratchi, 1993). Therefore, aggregate tensile
strength has up till now almost exclusively been deter-
mined on air-dry or even oven-dry aggregates. Extra-
polating these results to soil behaviour in the field may
be problematic. This is particularly a problem when
seedbed preparation is carried out at water contents
around field capacity, which usually is the case in
humid climates like that in Denmark. Methods to mea-
suresoil tensilestrengthinadirect tension testhavebeen
introduced by a number of authors (Gill, 1959; Farrell
et al., 1967; Nearing et al., 1988; Junge et al., 2000).
However, tensile strength was measured on remoulded
soil packed into cylinders or moulds in all cases.
Ease of soil fragmentation has been assessed by the
drop-shatter test suggested by Marshall and Quirk
(1950) and refined by Hadas and Wolf (1984). These
authors applied the test to relatively dry soil. Recently,
Schjønning et al. (2001) have applied the drop-shatter
test to field moist soil. Although a relation between
tensile strength and the ease of soil fragmentation is
implicitly assumed, only a few researchers have
related strength measurements to soil fragmentation
in the field (e.g. Macks et al., 1996).
Soil pore characteristics are expected to affect the
soil mechanical behaviour, especially when tensile
failure occurs. Tensile failure occurs due to the pro-
pagation of cracks in the stressed sample (e.g. Hallett
et al., 1995a). An applied stress is concentrated at
crack tips and the crack propagates if the stress
exceeds the strength in the crack tip, which may lead
to catastrophic failure of the sample. The stress con-
centration increases with increased length and narrow-
ness of the cracks tips. Therefore, stress concentration
is expected to increase with increased pore continuity
and to decrease with increased pore tortuousity. For a
moist soil, stress concentration will take place in the
air-filled cracks and pores. The water-filled pores will
show no stress concentration because the load is
uniformly borne by the pore water (Snyder and Miller,
1989). Experimental results have confirmed the strong
influence of air-filled cracks and pores on soil frag-
mentation and tensile strength. Hallett et al. (1995b)
found that dry natural soil blocks fragmented mainly
along pre-existing crack surfaces. Guerif (1990) found
a strong negative correlation between macroporosity
and tensile strength of dry soil.
The primary objective of this study was to evaluate
a method to measure tensile strength in a direct tension
test on undisturbed, field-sampled soil cores at matric
potentials around field capacity. An additional objec-
tive was to investigate the relation between tensile
strength or rupture energy of cores and aggregates on
one hand and soil fragmentation and soil pore char-
acteristics on the other hand.
2. Materials and methods
2.1. Soil type and field trial
The soils used in this study were obtained from a
field experiment conducted on a sandy loam located at
Rugballegard Experimental Station, Denmark. Some
basic soil characteristics are presented in Table 1. A
compacted soil, labelled (PAC) was compared with a
non-compacted reference soil, labelled (REF). The
experiment was established in 1997 and the treatments
were applied to plots in a randomized block design
with three blocks every year. In each experimental
year (1997–1999), the soil was mouldboard ploughed
to 20 cm depth at or above a water content of field
capacity. Immediately after ploughing, the PAC treat-
ment was applied by creating adjacent wheel tracks
using a 6–8 t tractor (inflation pressure 125 kPa).
Secondary tillage and sowing was carried out later in
the spring using a rotary harrow and a traditional drill
at moisture contents around field capacity. Small grain
cereals were grown in all experimental years.
Table 1
Basic soil characteristics for the soil under investigation as averaged
for both tillage treatments (no significant difference)
Soil parameter
Organic matter (g 100 g�1) 3.1
Clay, <2 mm (g 100 g�1) 13
Silt, 2–20 mm (g 100 g�1) 14
Fine sand, 20–200 mm (g 100 g�1) 39
Coarse sand, 200–2000 mm (g 100 g�1) 31
pH (CaCl2) 5.9
CECa (mmolc kg�1) 111
a Cation exchange capacity (Kalra and Maynard, 1991).
126 L.J. Munkholm et al. / Soil & Tillage Research 64 (2002) 125–135
2.2. Sampling and soil preparation
Soil sampling was performed in May 1999 shortly
after plant emergence at water content slightly lower
than field water capacity (Table 2). Soil was sampled
from the 7 to 12 cm depth in order to collect only
plough-layer soil that had not been directly disturbed
by the seedbed preparation and sowing operations.
Within each plot, samples were taken in three ran-
domly located 0.5 m2 areas.
Two-piece steel cylinders were constructed. Each
cylinder had an inner and outer diameter of 4.45 and
5.09 cm, respectively, and a height of 2.5 cm. To
secure sampling of undisturbed soil, the edge of the
bottom cylinder was bevelled outwards. A strong PVC
tape was used to hold the cylinders together during
sampling and storage. Undisturbed soil cores were
taken using the unified two-piece cylinders described
above. These were driven horizontally into the walls of
carefully created soil pits by means of a hammer. The
two-piece cylinders were held in position by a special
flange ensuring a horizontal sideward movement into
the soil wall. After careful removal of the soil-filled
cylinder from the bulk soil, the end surfaces were
trimmed with a knife. The cylinders were protected
from physical disruption and evaporation during trans-
port and later storage by means of a plastic lid. Thirty
soil cores were retrieved in each plot. The soil cores
were stored at 2 8C before adjusting their matric
potential and testing for tensile strength. The samples
were saturated on sandboxes and adjusted to matric
potentials of �50, �100 or �300 hPa.
Additional soil cores (6.1 cm diameter, 3.4 cm
height, 100 cm3 volume) were collected at the 7–
12 cm depth. These cores were collected using metal
cylinders forced into the soil by means of a hammer.
The cylinders were held in position by a special flange
ensuring a vertical downward movement into the soil.
After careful removal of the soil-filled cylinder from
the bulk soil, the end surfaces were trimmed with a
knife and mounted with plastic caps. Two replicate
samples were collected at each of the three sampling
areas in all plots (i.e. 18 replicates per treatment). The
minimally disturbed soil cores were stored at 2 8Cuntil analysis for pore characteristics could take place.
2.3. Direct tension test
The direct tension test was performed in an auto-
matically operated mechanical press (Fig. 1). The
lower half of the two-piece cylinder was fixed in a
specially designed rigid frame by the use of three
screws horizontally driven against the cylinder wall. A
plastic cap was put on the upper half of the two-piece
cylinder and fixed similarly. The top cap was con-
nected to the pressure transducer by a steel bar of
adjustable length. Immediately before testing, the tape
that held the two-piece cylinder together was pulled
off as gently as possible. The cylinders were pulled
apart with a longitudinal strain rate of 2 mm min�1.
The force was measured by a pressure transducer
ð0�100 � 0:03 NÞ and recorded automatically on a
computer. After the tensile strength measurement, the
top cylinder with soil was weighed in order to account
for the force needed to counterbalance the weight of
the sample.
Problems with the soil cores sliding in the rings
were encountered in the tests. Therefore, an approxi-
mately 3 mm deep furrow was made with a knife at the
boundary to the metal ring. This was performed both
at the bottom and at the top of the two-piece cylinder.
Silicone glue was squeezed into the furrow to improve
the contact between the soil and the metal ring. This
worked out quite well for soil tested at �50 and
�100 hPa, but was not applicable for soil drained to
�300 hPa.
Table 2
Water retention, density and porosity of the investigated soils
(7–12 cm depth)
Treatment
PAC REF
Water content (sampling) (m3 100 m�3) 30.0 aa 29.2 a
Water content (�100 hPa) (m3 100 m�3) 34.3 b 32.7 a
Water content (�300 hPa) (m3 100 m�3) 30.5 b 28.1 a
Particle density (Mg m�3)b 2.613 a 2.609 a
Bulk density (Mg m�3) 1.57 b 1.41 a
Porosity (m3 100 m�3) 39.7 a 46.2 b
Density, 2–4 mm aggregates (Mg m�3)c 1.83 b 1.76 a
Density, 4–8 mm aggregates (Mg m�3)c 1.71 b 1.63 a
Density, 8–16 mm aggregates (Mg m�3)c 1.67 b 1.61 a
a Figures followed by the same letter are not significantly
different at the P < 0:05 level.b Determined from the pycnometer method (Blake and Hartge,
1986).c Data from Munkholm and Kay (2002).
L.J. Munkholm et al. / Soil & Tillage Research 64 (2002) 125–135 127
As the soil cores were kept inside the cylinders
during the tension test, some soil–metal friction could
not be avoided, i.e. the cores did not always break at
the interface between the two cylinders. The cores that
broke at the very top or bottom were rejected. To be
able to take account of the soil–metal friction, the
height of the broken halves of non-rejected cores was
measured after the test. The deviation from a break
exactly at the interface between the cylinders (labelled
breakpoint deviation) was calculated as the mean
height of soil measured above the lower half cylinder
plus the mean height of soil above the upper half
cylinder. The height of the broken soil core above the
steel cylinder was measured at three points for both
halves. A number of 30 soil cores were tested for each
combination of treatment and matric potential.
The tensile strength of the soil cores (Ycore) was
calculated as
Ycore ¼Fmax � Flift
Acore
(1)
where Fmax (N) is the maximum longitudinal force
measured, Flift (N) the force needed counterbalance
the weight of the sample and Acore the sectional area of
the soil core.
2.4. Pore size distribution
The soil cores were capillary wetted to saturation in
sandboxes and then drained progressively to matric
potentials of �4, �10, �16, �30, �50, �100, �160,
�300 and �1000 hPa. Finally, they were oven-dried
at 105 8C for 24 h. The weight of each sample was
recorded at each matric potential and after oven
drying. Soil porosity was estimated from bulk soil
density and particle density (Table 2). The volumetric
water content at each potential was calculated from
the weight loss on oven drying. Air-filled porosity,
ea, at a given matric potential was calculated as the
difference between total porosity and the water
retained at the specific potential. An estimate of water
retention at the �1.5 MPa, W�1.5 MPa, matric poten-
tial (permanent wilting point) was calculated from
the pedo-transfer function obtained by Hansen
(1976).
2.5. Air permeability
Air permeability, Ka, was measured by the steady-
state method recently described by Iversen et al.
(2001) on cores that had been equilibrated at the
�30, �100 and �300 hPa matric potentials. The flow
of air through the sample was controlled by a pressure
regulator and measured by a precision flow meter.
The flow was recorded at a pressure difference of
5 hPa as measured by a water manometer. Prior to
permeability measurements, gentle pressing the soil
at the very edge of the metal ring minimized the risk
of air leaking along the ring (Ball and Schjønning,
2002). An empirical index of pore continuity/pore
organization (PO, mm2) was calculated (Groenevelt
et al., 1984):
PO ¼ Ka
ea
(2)
High values of PO express a high capacity of a given
air-filled pore volume as to conduct air, i.e., a high
continuity or a low tortuousity.
Fig. 1. The experimental set-up of the direct tension test on soil
cores: (A) adjustable steel bar connected to the pressure transducer;
(B) plastic cap attached to the upper half of the two-piece cylinder;
(C) two-piece cylinder enclosing the undisturbed soil core; (D)
rigid frame to which the lower half of the two-piece cylinder is
fastened.
128 L.J. Munkholm et al. / Soil & Tillage Research 64 (2002) 125–135
2.6. Soil fragmentation
The ease of soil fragmentation was determined
in the field just after plant emergence in May 1999
using the soil drop test described by Schjønning et al.
(2001). The soil drop test is a modification of the drop-
shatter fragmentation method described by Hadas
and Wolf (1984). An undisturbed cubic soil sample
ð7:0 cm � 8:0 cm � 11:5 cmÞ was collected from a
depth of 7–14 cm with a specially designed metal
shovel (Schjønning et al., 2001). The field moist soil
cube was then dropped from a height of 75 cm into a
metal box and the aggregate size distribution deter-
mined. The field moist soil was passed through a nest
of sieves with openings of 32, 16, 8, 4 and 2 mm and
each size fraction was recorded by weighing the mass
of soil. Non-dropped cubic samples were also sieved
and the size distribution recorded. The fragmentation
was expressed as thegeometric mean diameter for drop-
ped (GMDD, mm) and non-dropped samples (GMDN,
mm). The relative change in GMD from non-dropped
to dropped DGMD/GMDN was also calculated. Nine
replicate measurements were carried out for dropped
as well as non-dropped samples (i.e. three per plot).
2.7. Shear strength
In situ shear strength, t, was determined at the time
of sampling when the matric potential was close to
�300 hPa. A 10.0 cm diameter torsional shear box
operated at five normal stresses, P, ranging from 7.3 to
32.3 kPa was placed on the soil surface after removal
of the 0–7 cm soil layer and forced to a depth of
about 12 cm below the original surface (Payne and
Fountaine, 1952). The soil surrounding the metal box
was carefully removed before shearing. For all soils,
one series of determinations (five normal stresses) was
performed at two of the three sampling areas within
each plot, i.e. six determinations for each combination
of normal load and traffic treatment.
2.8. Statistical analysis
All parameters were analysed for normal distribu-
tion and transformed to yield normality if necessary.
Averages were calculated for each sampling area and
used in the F-tests, taking the inter-sampling area
variation as the residual error.
3. Results and discussion
3.1. Tensile strength of soil cores and aggregates
The suggested method was applicable at high
matric potentials (�50 and �100 hPa), while it failed
to work at �300 hPa. To be able to measure tensile
strength at lower matric potentials the approach of
Farrell et al. (1967) may be applicable. They used epoxy
resin, Araldite, to seal the ends of the soil cores to end
plates.
The heavily trafficked soil had significantly higher
tensilestrengthatbothmatricpotentials tested(Table3).
The difference in strength between the matric poten-
tials tested was rather small and the measured strengths
were also small. According to Farrell et al. (1967),
tensile strength increases only slightly with decreasing
potential at matric potentials around �100 hPa. More-
over, it was more difficult to measure tensile strength at
�100 hPa than at �50 hPa. At the lower matric poten-
tial, more problems with soil sliding in the metal cylin-
ders occurred to a greater extent and the deviation from
a break at the cylinder interface (breakpoint deviation)
increased. Although the PAC treated soil had a rather
largedeviationfromabreakat the interfaceat�100 hPa
matric potential, the effect of breakpoint deviation was
not significant.
The range of tensile strengths measured in this
study (2.0–3.2 kPa) agrees with results reported by
Nearing et al. (1988). They measured tensile strength
on repacked soil cores (3.88 cm in diameter) at matric
potentials comparable to those applied in this study.
On the other hand, Farrell et al. (1967) measured
tensile strength between 10 and 15 kPa on repacked
soil cores (3.8 cm in diameter) at matric potentials
between �100 and �500 hPa. The latter study used
soil with approximately the same textural composi-
tion as the soil used in this investigation, however,
remoulded and packed to a bulk density as high as
1.7 Mg m�3.
The results from the direct tensile strength mea-
surements agree well with the aggregate tensile
strengths determined by Munkholm and Kay (2002)
for the same treatments (Table 3). In both cases, the
PAC soil showed the largest tensile strength. The
tensile strengths of the soil cores were markedly lower
than those of the soil aggregates measured at the same
matric potential (�100 hPa, Table 3). In general,
L.J. Munkholm et al. / Soil & Tillage Research 64 (2002) 125–135 129
tensile strength has been found to scale linearly with
sample size on a log–log scale (Utomo and Dexter,
1981). As illustrated in Fig. 2, the predicted tensile
strength (from regression line) of soil units with a
cross-sectional area of 1555 mm2 is close to the tensile
strength measured on soil cores with the same cross-
sectional area. This finding thus indicates an excellent
correlation between the results obtained by the direct
and the indirect tension tests.
3.2. Soil shear strength and fragmentation
The data obtained in the laboratory correspond well
with the observed soil behaviour in the field. The PAC
soil had significantly higher shear strength at all
normal loads (Fig. 3). The treatments differed signifi-
cantly in apparent soil cohesion (i.e. intercept with the
y-axis, Fig. 3), but not in internal friction (slope of
lines). It is remarkable that both treatments showed
approximately 10 times higher apparent soil cohesion
than tensile strength of soil cores even though the
measurements were performed at fairly similar matric
potentials. The torsional shear test operated at low
normal loads (<32 kPa) allows the soil to fail along
‘‘natural’’ weak planes, i.e. as a result of crack pro-
pagation, the same as for the tension test. However,
torsion loading is less effective than tension loading to
induce crack propagation (Anderson, 1995) and inter-
nal friction may to some extent have been included in
the estimate of apparent soil cohesion. Inhomogeneity
Table 3
Tensile strength, Y, measured on undisturbed soil cores in a direct tension test at �50 and �100 hPa (geometric mean values), and deviation
from a break exact at the interface (breakpoint deviation) between the cylinders (geometric mean values)a
Matric potential (hPa) Treatment
PAC REF
Tensile strength (soil cores) (kPa) �50 3.1 bb 2.1 a
Tensile strength (soil cores) (kPa) �100 3.2 b 2.0 a
Breakpoint deviation (mm) �50 3.0 a 3.3 a
Breakpoint deviation (mm) �100 6.4 b 3.9 a
Tensile strength (2–4 mm aggregates) (kPa)c �100 16.9 a 15.5 a
Tensile strength (4–8 mm aggregates) (kPa)c �100 11.7 b 9.6 a
Tensile strength (8–16 mm aggregates) (kPa)c �100 7.1 a 5.9 a
Rupture energy (2–4 mm aggregates) (J kg�1)c �100 1.7 a 1.8 a
Rupture energy (4–8 mm aggregates) (J kg�1)c �100 0.7 a 0.8 a
Rupture energy (8–16 mm aggregates) (J kg�1) �100 0.4 a 0.3 a
Soil fragmentation, non-dropped (GMDN, mm) Sampling 44.1 b 30.2 a
Soil fragmentation, dropped (GMDD, mm) Sampling 38.7 b 14.2 a
Soil fragmentation, DGMD/GMDN (%)d Sampling 13 a 53 b
a Tensile strength, Y, measured on single aggregates in an indirect tension test (geometric mean values) and specific rupture energy
(geometric mean values). Soil drop test results—GMD for dropped and non-dropped samples (field test at sampling).b Figures with the same letter are not significantly different at P ¼ 0:05 level.c Data from Munkholm and Kay (2002).d DGMD ¼ GMDN � GMDD.
Fig. 2. The relationship between tensile strength, Y (kPa) and
sample cross-sectional area, A (mm2) on a log–log scale. Lines
indicate linear regression for log aggregate tensile strength vs. log
aggregate cross-sectional area (i.e., direct tension test results not
included). All measurements were performed at �100 hPa matric
potential: (—) PAC, (- - -) REF; bars indicate �standard error of
the mean.
130 L.J. Munkholm et al. / Soil & Tillage Research 64 (2002) 125–135
in crack and pore characteristics may also explain
some of the difference as the soil was forced to fail
along the horizontal and the vertical plane, respec-
tively, for the torsional shear and tension tests.
The PAC soil fragmented poorly when performing
the soil drop test (Table 3 and Fig. 4). Both with and
without dropping, the PAC soil fragmented into large
clods, whereas the REF soil fragmented into signifi-
cantly smaller aggregates. The relative change in
GMD from non-dropped to dropped (DGMD/GMDN)
may be considered as an empirical index of ease of
fragmentation and showed a strong treatment effect
(i.e. DGMD=GMDN ¼ 53 and 13% for REF and PAC,
respectively).
The extent of correlation between the tensile
strength of soil cores and GMDD or DGMD/GMDN
varied with matric potential and compaction treatment.
Within each treatment no significant correlation was
found between DGMD/GMDN and tensile strength of
the soil cores (data not shown). For the REF soil, a
significant positive correlation ðR2 ¼ 0:65��Þ was
found between GMDD and tensile strength of soil
cores measured at �50 hPa matric potential but not
at �100 hPa. Such a correlation could not be found
for the PAC treated soil, which exhibited little frag-
mentation in the soil drop test (data not shown).
Obviously, the energy input in the soil drop test was
too low to induce substantial fragmentation of the
PAC treated soil. The energy input in the soil drop test
is approximately 7.4 J kg�1 field moist soil, which
corresponds approximately to 8.9 J kg�1 dry soil when
assuming a wet and dry bulk density of 1.7 and
1.4 g cm�3, respectively. This level is low in compar-
ison with the estimated energy input in different tillage
operations. For instance, Patterson et al. (1980) esti-
mated that a secondary tine cultivation on clay loam
required an energy input of 24 MJ ha�1 at the imple-
ment connection. When assuming a dry bulk density
of 1.4 g cm�3 and a tillage depth of 5 cm this corre-
sponds approximately to an energy input of 34 J kg�1
dry soil. However, the energy input in the soil drop test
supposedly is realistic as a substantial part of the
Fig. 3. Peak shear strength, t, related to normal load, P, as
determined by the torsional shear box method in the field at field
capacity: (—) PAC, (- - -) REF; bars indicate �standard error of the
mean.
Fig. 4. Aggregate size distribution for the soils when subjected to the soil drop test at approximately �300 hPa in the field: (—) PAC, (- - -)
REF; bars indicate þ1 standard error of the mean.
L.J. Munkholm et al. / Soil & Tillage Research 64 (2002) 125–135 131
energy input in tillage is lost in friction, and transfer
and re-orientation of the structural units (Hadas and
Wolf, 1983).
Based on the rupture energy of the aggregates,
much higher soil fragmentation would have been
expected in the soil drop test, i.e. the rupture energy
was <2 J kg�1 dry soil for all aggregate sizes at
�100 hPa (Table 3). The relatively low fragmentation
implies that a substantial amount of the energy input in
the soil drop test also did not dissipate into soil
fragmentation. It may have been stored as volumetric
strain energy (Chancellor et al., 1969) and/or lost as
fragment rebound and heat evolution related to plas-
tic deformation. It is also noteworthy that there was
no difference between the treatments in the specific
rupture energy of soil aggregates. This was evident at
all potentials in the range �100 hPa to �166 MPa
(Munkholm and Kay, 2002). The results imply that
caution must be taken in predicting soil fragmenta-
tion in tillage from measurements of rupture energy of
single aggregates.
3.3. Soil pore characteristics
The matric potential at sampling and when perform-
ing field measurement was close to �300 hPa, as indi-
cated by the water retention results in Table 2. The PAC
soil contained significantly more water at all the matric
potentials. The compaction treatment resulted in lower
total porosity (i.e. 40 and 46 m3 100 m�3 for PAC and
REF, respectively) (Table2).Likewise thedensityof the
aggregates was also highest for the PAC treatment (e.g.
1.71 and 1.63 Mg m�3 for 4–8 mm aggregates from the
PAC and REF, respectively). The compaction treatment
caused a decrease in macroporosity ðpores > 30 mmÞ(6 and 13 m3 100 m�3 pores > 30 mmforPACandREF,
respectively) (Fig. 5). The compaction treatment also
significantly reduced the volume fraction of medium
sized pores (10–30 mm) (Fig. 5). The influence of soil
compaction on the large pores agrees with other studies
(Rasmussen, 1985; McAfee et al., 1989).
The compaction treatment also caused a significant
reduction in the ability of the soil to transport gas by
convection (i.e., a lower air permeability, Ka) (Table 4).
Moreover, the significantly lower index of pore orga-
nization (PO) found for the PAC soil at �30 and
�100 hPa indicate a relatively poorer continuity
of the macro-pores in the PAC treated soil. At
�300 hPa there was no significant difference between
the soils in pore continuity. The negative effect of soil
traffic on gas transport characteristics and PO found in
this study corresponds with the results by Ball et al.
(1988) and the results from simulated traffic in the
laboratory by O’Sullivan et al. (1999).
3.4. Influence of pore characteristics on tensile
strength and soil fragmentation
The tensile strength of the soil cores was signifi-
cantly negatively related to macroporosity. At a matric
potential of �50 hPa, the tensile strength of soil cores
Fig. 5. Fractions of pores of selected size classes as derived from
water retention measurements, assuming the approximate relation
d ¼ �3000=Cm, where d is the tube equivalent pore diameter in
mm and Cm the matric potential in hPa. Fractions labelled with
different letters (a/b) are significantly different ðP ¼ 0:05Þ.
Table 4
Air-filled pore space, ea, air permeability, Ka (geometric meanvalues)
and PO ðPO ¼ Ka=eaÞ (geometric mean values) for soil sampled
vertically at the 7–12 cm depth
�30 hPa �100 hPa �300 hPa
PAC REF PAC REF PAC REF
ea (m3 100 m3) 0.03 aa 0.08 b 0.06 a 0.13 b 0.09 a 0.18 b
Ka (mm2) 0.28 a 1.18 b 0.68 a 2.82 b 2.05 a 6.91 ab
PO (mm2) 7.1 a 21.2 b 16.0 a 33.2 b 28.6 a 45.1 ab
a Values in the same row and at the same matric potential with
the same letter are not significantly different at the P < 0:05 level.b Significant interaction between treatment and block. When
testing against treatment � block variation no significant difference
between treatments.
132 L.J. Munkholm et al. / Soil & Tillage Research 64 (2002) 125–135
showed a linear decrease with increasing volume
fraction of >60 mm pores ðR2 ¼ 0:54���Þ (Fig. 6, left).
At �100 hPa, the tensile strength decreased linearly
with volume fraction of >30 mm pores (Fig. 6, right).
The observed significant influence of macroporosity
on tensile strength is in accordance with studies by
Guerif (1990) and Hallett et al. (1995b). The correla-
tion between tensile strength and pore characteristics
were not improved by including the index of PO in the
statistical model (data not shown). In itself, there was
no significant correlation between PO and tensile
strength within the treatments (data not shown). The
poor correlation may be due to a large small-scale
variability of air permeability, Ka (e.g. Koszinski et al.,
1995; Iversen et al., 2001). Therefore, the PO in the
failure zone in the cores used for tensile strength
measurements may be distinctly different than in those
taken for measurement of pore characteristics.
Interestingly, the soil fragmentation data showed a
stronger correlation to PO than to total or air-filled
porosity. Only a weak and insignificant relation was
found between soil fragmentation and total or air-
filled porosity, although GMDD tended to decrease
with total porosity and air-filled porosity, ea, at �50
and �100 hPa. In contrast, the GMDD decreased
linearly with log PO at �100 hPa for the REF treat-
ment ðR2 ¼ 0:76��Þ (Fig. 7), whereas DGMD/GMDN
increased significantly with log PO at �100 hPa when
including all data ðR2 ¼ 0:78��Þ (data not shown). The
results imply that soil fragmentation depends on the
geometrical characteristics of the air-filled pores and
cracks, which is in accordance with brittle fracture
mechanics (Hallett et al., 1995a). A high PO indicates
that the air-filled pore space to a large extent consisted
of long straight pores and cracks, i.e. pores and cracks
where crack propagation most likely will start.
4. Conclusions
Tensile strength of moist soil can be measured using
direct tension methods without making assumptions
about the mode of failure. The suggested method
worked well at high matric potentials (�50 and
Fig. 6. Tensile strength of soil cores measured at �50 and �100 hPa as related to the volume fraction of pores >60 and >30 mm, respectively.
The symbols represent a mean value for each sampling area. Lines represent linear regressions.
Fig. 7. GMD of dropped samples, GMDD, as related to the pore
continuity, PO of air-filled pores at �100 hPa matric potential.
L.J. Munkholm et al. / Soil & Tillage Research 64 (2002) 125–135 133
�100 hPa), while it failed to work at �300 hPa. Modi-
fications of the method may make it possible to
measure tensile strength at lower matric potentials.
The heavily trafficked soil had a significantly higher
tensile strength than the REF soil in the direct tension
test. The results corresponded well with the predicted
values determined from indirect measurements of
aggregate tensile strength.
The tensile strength results were in accordance with
soil behaviour in the field as estimated by shear
strength and soil fragmentation techniques.
The energy input in the soil drop test was low in
comparison with the energy input in tillage but high
compared with the specific rupture energy of soil
aggregates. For the latter, no difference was found
between the treatments. The relatively low fragmenta-
tion of the soils in the soil drop test indicated that a
considerable amount of the energy input was stored as
volumetric strain energy and/or lost to processes such
as plastic deformation. Furthermore, the findings in
this study imply that caution must be taken in pre-
dicting soil fragmentation in tillage solely from mea-
surements of rupture energy of single aggregates.
The tensile strength of the soil cores was negatively
correlated to the macroporosity of the soil, whereas the
ease of soil fragmentation was positively correlated to
PO.
Acknowledgements
The technical assistance of Bodil B. Christensen
and Michael Koppelgaard is gratefully acknowledged.
This work was financed by the Danish Environmental
Research Programme and was performed in the con-
textof theDanishResearchCenterforOrganicFarming.
References
Anderson, T.L., 1995. Fracture Mechanics—Fundamentals and
Applications, 2nd Edition. CRC Press, Boca Raton, FL, 688 pp.
Ball, B.C., Schjønning, P., 2002. Air permeability. In: Dane, J.H.,
Topp, G.C. (Eds.), Methods of Soil Analysis, Part 1, 3rd
Edition. Agronomy Monograph. ASA and SSSA, Madison, WI,
in press.
Ball, B.C., O’Sullivan, M.F., Hunter, R., 1988. Gas diffusion, fluid
flow and derived pore continuity indices in relation to vehicle
traffic and tillage. J. Soil Sci. 39, 327–339.
Blake, G.R., Hartge, K.H., 1986. Particle density. In: Klute, A.
(Ed.), Methods of Soil Analysis. Part 1. Physical and
Mineralogical Methods. American Society of Agronomy,
Madison, WI, pp. 377–382.
Chancellor, W.J., Vomocil, J.A., Aref, K.S., 1969. Energy
disposition in compression of three agricultural soils. Trans.
ASAE 12, 524–528.
Dexter, A.R., Kroesbergen, B., 1985. Methodology for determina-
tion of tensile strength of soil aggregates. J. Agric. Eng. Res.
31, 139–147.
Farrell, D.A., Greacen, E.L., Larson, W.E., 1967. The effect of
water content on axial strain in a loam soil under tension and
compression. Soil Sci. Soc. Am. Proc. 31, 445–450.
Gill, W.R., 1959. The effects of drying on mechanical strength of
Lloyd clay. Soil Sci. Soc. Am. Proc. 3, 255–257.
Groenevelt, P.H., Kay, B.D., Grant, C.D., 1984. Physical assess-
ment of a soil with respect to rooting potential. Geoderma 34,
101–114.
Guerif, J., 1990. Factors influencing compaction-induced increases
in soil strength. Soil Till. Res. 16, 167–178.
Hadas, A., Wolf, D., 1983. Energy efficiency in tilling dry clod-
forming soils. Soil Till. Res. 3, 47–59.
Hadas, A., Wolf, D., 1984. Refinement and re-evaluation of the
drop-shatter soil fragmentation method. Soil Till. Res. 4, 237–
249.
Hallett, P.D., Dexter, A.R., Seville, J.P.K., 1995a. The application
of fracture mechanics to crack propagation in dry soil. Eur. J.
Soil Sci. 46, 591–599.
Hallett, P.D., Dexter, A.R., Seville, J.P.K., 1995b. Identification of
pre-existing cracks on soil fracture surfaces using dye. Soil Till.
Res. 33, 163–184.
Hansen, L., 1976. Jordtyper ved statens forsøgsstationer. Tidsskr.
Planteavl. 80, 742–758.
Hatibu, N., Hettiaratchi, D.R.P., 1993. The transition from ductile
flow to brittle failure in unsaturated soils. J. Agric. Eng. Res.
54, 319–328.
Iversen, B.V., Schjønning, P., Poulsen, T.G., Moldrup, P., 2001. In
situ, on-site and laboratory measurements of soil air perme-
ability: boundary conditions and measurement scale. Soil Sci.
166, 97–106.
Junge, T., Grasle, W., Gisela, B., Horn, R., 2000. Effect of pore
water pressure on tensile strength. J. Plant Nutr. Soil Sci. 163,
21–26.
Kalra, Y.P., Maynard, D.G., 1991. Methods Manual for Forest Soil
and Plant Analysis. Information Report No. R-x-319. Forestry
Canada, Northwest Region, Northern Forestry Centre, Canada.
Kirkham, D., De Boodt, M.F., De Leenheer, L., 1959. Modulus of
rupture determination on undisturbed soil core samples. Soil
Sci. 87, 141–144.
Koszinski, S., Wendroth, O., Lehfeldt, J., 1995. Field scale
heterogeneity of soil structural properties in a moraine land-
scape of northeastern Germany. Int. Agrophys. 9, 201–210.
Macks, S.P., Murphy, B.W., Cresswell, H.P., Koen, T.B., 1996. Soil
friability in relation to management history and suitability for
direct drilling. Aust. J. Soil Res. 34, 343–360.
Marshall, T.J., Quirk, J.P., 1950. Stability of structural aggregates
of dry soil. Aust. J. Soil Res. 1, 266–275.
134 L.J. Munkholm et al. / Soil & Tillage Research 64 (2002) 125–135
McAfee, M., Lindstrom, J., Johansson, W., 1989. Effects of pre-
sowing compaction on soil physical properties, soil atmosphere
and growth of oats on a clay soil. J. Soil Sci. 40, 707–717.
Munkholm, L.J., Kay, B.D., 2002. Effect of water regime on
aggregate tensile strength, rupture energy and friability. Soil
Sci. Soc. Am. J., in press.
Nearing, M.A., West, L.T., Bradford, J.M., 1988. Consolidation of
an unsaturated illitic clay soil. Soil Sci. Soc. Am. J. 52, 929–934.
O’Sullivan, M.F., Robertson, E.A.G., Henshall, J.K., 1999. Shear
effects on gas transport in soil. Soil Till. Res. 50, 73–83.
Patterson, D.E., Chamen, W.C.T., Richardson, C.D., 1980. Long-
term experiments with tillage systems to improve the economy
of cultivation for cereals. J. Agric. Eng. Res. 25, 1–35.
Payne, P.C.J., Fountaine, E.R., 1952. A field method of measuring
the shear strength of soils. J. Soil Sci. 3, 136–144.
Rasmussen, K.J., 1985. Jordpakning ved forskellig belastning (Soil
compaction with different surface pressures). Tidsskr. Plan-
teavl. 89, 31–45.
Schjønning, P., Elmholt, S.E., Munkholm, L.J., Debosz, K., 2001.
Soil quality aspects of humid sandy loams as influenced by
organic and conventional long-term management. Agric.
Ecosyst. Environ., in press.
Snyder, V.A., Miller, R.D., 1989. Soil deformation and fracture
under tensile forces. In: Larson, W.E., Blake, G.R., Allmaras,
R.R., Voorhees, W.B., Gupta, S.C. (Eds.), Mechanics and
Related Processes in Structured Agricultural Soils. NATO ASI
Series E, Vol. 172. Kluwer Academic Publishers, Dordrecht,
Netherlands, pp. 23–35.
Utomo, W.H., Dexter, A.R., 1981. Soil friability. J. Soil Sci. 32,
203–213.
L.J. Munkholm et al. / Soil & Tillage Research 64 (2002) 125–135 135