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Soil Friability- concept, assessment, and effects of soil properties and management. D.Sc. Thesis, Department of Agroecology, Aarhus University, Denmark.

Authors:
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Soil friability
concept, assessment and effects of soil properties and management
D.Sc. thesis by
Lars J. Munkholm
Ph.D., Senior Scientist, Faculty of Science and Technology,
Department of Agroecology, Aarhus University
3 April 2012
2
Preface
The thesis consists of experimental studies reported in scientific journals and reports and of
reviews appearing in scientific journals and book chapters. The following 14 publications form
the substance of the thesis.
1. Munkholm, L.J. (2000). The spade analysis - a modification of the qualitative spade
diagnosis for scientific use. DIAS-report No. 28 Plant Production, Danish Institute of
Agricultural Sciences.
2. Munkholm, L.J., Schjønning, P. & Petersen, C.T. (2001). Soil mechanical behaviour of
humid sandy loams: Case studies on long-term effects of fertilization and crop rotation.
Soil Use and Management 17, 269-277.
3. Munkholm, L.J., Schjønning, P. & Rasmussen, K.J. (2001). Non-inverting tillage effects
on soil mechanical properties of a humid sandy loam. Soil & Tillage Research 62, 1-14.
4. Munkholm, L.J., Schjønning, P. & Kay, B.D. (2002). Tensile strength of soil cores in
relation to aggregate strength, soil fragmentation and pore characteristics. Soil & Tillage
Research 64, 125-135.
5. Munkholm, L.J., Schjønning, P., Debosz, K., Jensen, H.E. & Christensen, B.T. (2002).
Aggregate strength and soil mechanical behaviour of a sandy loam under long-term
fertilization treatments. European Journal of Soil Science 53, 129-137.
6. Munkholm, L.J. & Kay, B.D. (2002). Effect of water regime on aggregate tensile
strength, rupture energy and friability. Soil Science Society of America Journal 66, 702-
709.
7. Munkholm, L.J., Schjønning, P. (2004). Structural vulnerability of a sandy loam
exposed to intensive tillage and traffic in wet conditions. Soil & Tillage Research 79, 79-
85.
8. Kay, B.D. & Munkholm, L.J. (2004). Management-induced soil structure degradation:
organic matter depletion and tillage. In: Schjønning, P, Christensen, B.T. & Elmholt, S.
(eds). Managing soil quality: challenges in modern agriculture. CABI Publishing,
Wallingford, UK, pp. 185-197.
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9. Munkholm, L.J., Perfect, E. (2005). Brittle fracture of soil aggregates: Weibull models
and methods of parameter estimation. Soil Science Society of America Journal 69, 1565-
1575.
10. Munkholm, L.J., Perfect, E. & Grove, J. (2007). Incorporation of water content into the
Weibull model for soil aggregate strength. Soil Science Society of America Journal 71,
682-691.
11. Munkholm, L.J. (2011). Soil friability: A review of the concept, assessment and effects
of soil properties and management. Geoderma, 167-168, 236-246.
12. Kay, B.D. & Munkholm, L.J. (2011). Managing the interactions between soil biota and
their physical habitat in agroecosystems. In: Ritz, K. & Young, I.M. (eds.). Architecture
and biology of soils: life in inner space. CABI Publishing, Wallingford, UK, pp. 170-195.
13. Munkholm, L.J., Heck, R. and Deen, B. (2012). Long-term effects of rotation and tillage
on visual evaluation of soil structure, soil physical properties and crop yield. Soil and
Tillage Research (In Press), doi: 10.1016/j.still.2012.02.007.
14. Munkholm, L.J., Heck, R. & Deen, B. (2012). Soil pore characteristics assessed from X-
ray micro-CT derived images and correlations to soil friability. Geoderma 181-182, 22-
29.
The thesis also draws upon five supplementary papers in which I have been partially involved.
These papers, although not a formal part of the thesis, provide significant complementary results.
Supplementary studies:
I. Schjønning, P., Elmholt, S., Munkholm, L.J. & Debosz, K. (2002) Soil quality aspects
of humid sandy loams as influenced by different long-term management. Agriculture,
Ecosystems & Environment 88, 215-234.
II. Ball, B.C., Batey, T. & Munkholm, L.J. (2007). Field assessment of soil structural
quality – a revision of the Peerlkamp test. Soil Use and Management, 23, 329-337.
III. Schjønning, P., Munkholm, L.J., Elmholt, S. & Olesen, J.E. (2007). Organic matter and
soil tilth in arable farming: Management makes a difference within 5-6 years.
Agriculture, Ecosystems & Environment 122, 157-172.
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IV. Elmholt, S., Schjønning, P., Munkholm, L.J. & Debosz, K. (2008). Soil management
effects on biological and abiotic stabilization of aggregates segregated from bulk soil in
friable condition. Geoderma, 144, 455-467.
V. Schjønning,P.S., de Jonge, L.W., Munkholm, L.J., Christensen, B.T. & Olesen, J.E.
(2012). Clay dispersibility and soil friability testing the soil clay-to-carbon saturation
concept. Vadose Zone Journal (In Press). Doi: 10.2136/vzj.2011.0067.
Publications 2-6 were included in a Ph.D. thesis
List of abbreviations
E: Rupture energy
FSD: Fragment size distribution
GMD: Geometric mean diameter
MWD: Mean weight diameter
S: Index of soil physical quality
SOM: Soil organic matter content
SWRC: Soil water retention curve
Y: Tensile strength
w: Gravimetric water content
θ: Volumetric water content
θDL: Dry limit volumetric water content for soil fragmentation
θINFL: Volumetric water content at inflation point
θOPT: Optimal limit volumetric water content for soil fragmentation
θPL: Plastic limit volumetric water content
θWL: Wet limit volumetric water content for soil fragmentation
ΔθRANGE: Range in soil water content for tillage
Ψ: Soil water matric potential
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Abstract
Soil friability is a key soil physical property yielding valuable information on the ease of
producing a favorable seed- and root beds during tillage operations. Therefore, soil friability is a
crucial soil property in relation to the ability of soil to support plant growth and to minimize the
energy required for tillage. The topic has interested farmers and soil scientists for centuries but it
was the paper by Utomo and Dexter (1981) that significantly put the topic on the soil science
agenda. The awareness of soil friability is growing, both in practice and in soil science. This
must be viewed in the light of the present renewed focus on global food security together with a
focus on fossil fuel consumption and greenhouse gas emissions in crop production. Certainly, the
demand for well-functioning arable soils is rising to meet the global challenges.
This thesis summarizes and discuss research on soil friability produced during the last three
decades and is supported by 14 affiliated publications. The objectives are to: 1. review the
methodologies used to assess soil friability, 2. describe effects of basic soil properties affecting
soil friability with special focus on the soil water regime, 3. evaluate the effects of soil
management, and 4. identify knowledge gaps.
A friable soil is characterized by an ease of fragmentation of undesirably large aggregates/clods and
a difficulty in fragmentation of minor aggregates into undesirable small elements. Soil friability has
been assessed using qualitative field methods as well as quantitative field and laboratory
methods at different scales of observation. The qualitative field methods are broadly used by
scientists, advisors and farmers, whereas the quantitative laboratory methods demand specialized
skills and more or less sophisticated equipment. Most methods address only one aspect of soil
friability, i.e. either the strength of unconfined soil or the fragment size distribution after
applying a stress. All methods have significant advantages and limitations. The use of a mixture
of qualitative and quantitative methods to get a comprehensive and adequate assessment of soil
friability is recommended. Poor friability can be experienced if soil is either too wet or too dry
and there is a range in water contents for optimal friability. There is a strong need to get more
detailed knowledge about effects of soil water content on soil friability and especially to be able
to quantify the least limiting water range for soil friability and therefore soil tillage. A strong
relationship between organic matter and friability has been found but it is not possible to identify
a specific lower critical level of organic matter across soil types. Sustainable management of soil
requires continuous and adequate inputs of organic matter to sustain or improve soil friability.
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Intensive tillage and traffic in unfavorable conditions threatens soil friability and may initiate a
vicious cycle where increasingly higher intensity of tillage is needed to produce a proper
seedbed.
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Dansk sammendrag
Jordens smuldreevne er en nøgleparameter der giver vigtig viden om dyrkningsjordens evne til at
danne et passende såbed i forbindelse med jordbearbejdning. Det er en vigtig egenskab set i
forhold til jorden som dyrkningsmedie og energiforbrug ved jordbearbejdning. Emnet har
interesseret landmænd og forskere igennem århundreder, men det var først med Utomo og
Dexter (1981) at det for alvor blev sat på den jord-videnskabelige dagsorden. Opmærksomheden
om jordens smuldreevne er voksende i både praksis og forskningen, hvilket skal ses i lyset af den
fornyede fokus på global fødevaremangel, forbruget af fossile brændstoffer samt udslippet af
drivhusgasser fra landbrugsjorden. Behovet for en velfungerende landbrugsjord er stigende set i
forhold til at kunne imødekomme de globale udfordringer i relation til bæredygtig
planteproduktion.
Den afhandling opsummerer og diskuterer forskningsresultater vedrørende jordens
smuldreevne på basis af 14 vedhæftede publikationer. Formålet er at: 1. evaluere metoder brugt
til at bestemme jorden smuldreevne, 2. evaluere betydningen af basale jordegenskaber som på
virker jorden smuldreevne med særlig fokus på jordvandets betydning, 3. evaluere betydningen
af dyrkningsfaktorer for jordens smuldreevne og 4. identificere forksningsbehov.
En smuldrende jord er karakteriseret ved at store uhensigtsmæssige knolde/aggregater
kan smuldre let, mens det er svært at bryde mindre hensigtsmæssige aggregater. Jorden
smuldreevne er blevet bestemt ved brug af kvalitative feltmetoder og kvantitative felt- og
laboratoriemetoder på forskellig fysisk skala. De kvalitative feltmetoder benyttes udbredt af
forskere, rådgivere og landmænd. Derimod kræver de kvantitative metoder specialiseret viden og
udstyr og benyttes derfor primært af forskere. De fleste metoder måler kun et aspekt af jordens
smuldreevne – enten jordstyrken eller størrelsesfordelingen af fragmenter (aggregater og knolde)
efter at jorden har været udsat for en fysisk påvirkning. Alle metoder har deres fordele og
ulemper. Det anbefales at anvende en kombination af kvalitative og kvantitative metoder for at
opnå en grundig og dækkende beskrivelse af en given jords smuldreevne.
En dårlig smuldreevne kan findes, hvor jorden er enten for tør eller for våd og der er et
område i vandindhold, hvor jorden har optimal smuldreevne. Der er et stort behov for større
viden om betydningen af jordens vandindhold og særligt at kunne kvantificere det såkaldte ”least
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limiting water range” (område i vandindhold) for jordens smuldreevne og dermed
jordbearbejdning.
Mange undersøgelser har vist en stærk korrelation mellem jordens indhold af organiske
stof og jordens smuldreevne. Det er dog ikke lykkedes at finde en generel nedre kritisk grænse
for indholdet af organisk stof i relation til jordens smuldreevne på tværs jordtyper og
klimaområder. Konklusionen er dog, at vedvarende og passende tilførsel af organisk stof til
jorden er en forudsætning for opretholdelse eller forbedring af jordens smuldreevne.
Intensiv jordbearbejdning og kørsel på jorden særligt under våde forhold forringer
jordens smuldreevne. Herved risikerer man at igangsætte en ond cirkel eller spiral, hvor stadig
stigende behov for intensiv jordbearbejdning er nødvendig for at danne et passende såbed.
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1. Introduction
Soil friability is a key soil physical property yielding valuable information on the ease of
producing a favorable seed- and root beds during tillage operations. Therefore, soil friability is a
crucial soil property in relation to the ability of soil to support plant growth and to minimize the
energy required for tillage.
The awareness of soil friability is growing, both in practice and in soil science. The topic has
interested soil scientists for centuries (e.g. Christensen, 1930), but it was the paper by Utomo and
Dexter (1981) that significantly put the topic on the soil science agenda. The interest in the topic
has recently escalated according to citations registered in the ISI Web of Science database
(Thomson Reuters). The increased interest must be viewed in the light of the present renewed
focus on global food security (FAO, 2009) together with a focus on fossil fuel consumption and
greenhouse gas emissions in crop production. Certainly, the demand for well-functioning arable
soils is rising to meet the global challenges. However, the threats to soil quality appear also to be
on the increase due to climate change and changes in soil management. In North-Western Europe
soil compaction, loss of organic matter and soil erosion are the main threats to soil quality and, in
particular, to soil friability. The soil organic matter content is expected to decrease with
increased temperatures and the expected higher frequency of intensive rainfall will increase the
risk of water erosion. In North-Western Europe, the expected increase in winter precipitation will
limit the window for timely traffic and tillage in the spring and thus increase the risk of severe
soil compaction.
Soil friability is related to brittle fracture of soil as described by Braunack et al. (1979) and
Dexter and Watts (2000). Brittle fracture results from the progressive development of cracks
ending with a crack opening and a sudden loss in strength (Hatibu and Hettiarachi, 1993). The
propagation of cracks in an unconfined stressed soil depends on the density and the morphology
(connectivity, orientation) of the air-filled pores and the strength at the crack tips as stated by
Hallett et al. (1995a,b). The occurrence and nature of cracks in the soil depends on basic soil
properties (texture, clay mineralogy), climate (cycles of wetting-drying and frost-thaw), soil
biological activity as well as tillage and traffic.
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1.2. Objectives
This thesis summarizes and discuss research on soil friability produced during the last three
decades and is supported by 14 affiliated publications. The supporting papers reflect the different
aspects of soil friability that I have addressed from year 2000 until today. There has been an
ongoing focus on methodology and on the influence of basic soil properties, water content and
management. The overall objectives of the thesis is to connect the results presented in the
individual papers, view them as a whole and synthesizes the most important take home
messages.
The specific objectives are to:
review the methodologies used to assess soil friability
describe effects of basic soil properties affecting soil friability with special focus on the
soil water regime
evaluate the effects of soil management
identify knowledge gaps
2. Soil friability the concept.
The term soil friability has been discussed, defined and redefined by soil scientists for decades.
Christensen (1930) defined it as “the ease of crushing, crumbling or rubbing apart of the particles
of which it is composed” and thus emphasized the tendency of unconfined soil to crumble and break
down. Utomo and Dexter (1981) elaborated on this definition and came up with the present widely
accepted definition of the concept: “Soil friability: the tendency of a mass of unconfined soil to
break down and crumble under applied stress into a particular size range of smaller fragments”.
Therefore, a friable soil is characterized by an ease of fragmentation of undesirably large
aggregates/clods and a difficulty in fragmentation of minor aggregates into undesirable small
elements (11). Excessively small aggregates (<0.5-1 mm) enhance soil erodibility and may impede
seedling emergence as they increase the risk of surface crusting. But what is the ideal size
distribution of soil aggregates in the seed and rooting bed? Karlen et al. (1990) stated that a soil
with a good soil tilth “usually is loose, friable and well granulated”. This qualitative perception of
a “crumb structure” as the optimal environment for plant growth is supported by empirical data.
Braunack and Dexter (1989) concluded in a review that the optimal seedbed (i.e. the soil layer
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that has been tilled to a condition to promote seed germination and the emergence of seedlings)
is produced by 0.5-8 mm aggregates. Berntsen and Berre (1993) concluded that an optimal
seedbed for cereals is characterized by about 50% of the aggregates by weight being in the 0.5-6
mm fraction. A large fraction of small aggregates are not desired due to reasons stated above,
whereas a large fraction of aggregates >5-8mm is not wanted due to risk of rapid drying and
delayed emergence (Håkansson et al., 2011). Small seeded crops are normally more sensitive to
seedbed structure and may require a finer and more homogeneous structure (Braunack and
Dexter, 1989). Much less experimental work has been done on characterizing the optimal soil
structure below the seedbed. However, results by e.g. Misra et al. (1986) support the perception
that a crumb structure is desirable throughout the arable layer. Soil friability is not just relevant
in conventional tilled systems it is also of crucial importance in relation to successful seeding
and crop establishment under no-till farming as highlighted by Macks et al. (1996). To sum up,
soil friability concerns: 1. the strength of different sizes of unconfined soil and 2. the resulting
fragment size distribution after applying a stress.
3. Assessment of soil friability
Soil friability has been assessed using qualitative field methods as well as quantitative field and
laboratory methods at different scales of observation (figure 1). They measure different aspects
of friability and have been used for different purposes. The qualitative field methods are broadly
used by scientists, advisors and farmers, whereas the quantitative laboratory methods demand
specialized skills and more or less sophisticated equipment. Most methods address only one
aspect of soil friability, i.e. either the strength of unconfined soil or the fragment size distribution
after applying a stress. An overview of friability indices and their advantages and drawbacks is
shown in Table 1.
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Figure 1. Schematic illustration of methods used to assess soil friability in relation to scale and
quantitative/qualitative. Republished from (11).
3.1. Visual assessment of soil friability
These methods assess bulk soil friability at actual water contents in the field. They address both key
aspects of friability stated above. The assessment of soil friability at field level is intuitively integral
to the farmer’s visual evaluation of soil tilth in relation to tillage operations and crop growth. Many
combine pure visual assessment at distance with hands-on examination of soil sampled in the field.
The strength of soil can roughly be evaluated by crushing in the hand (unconfined, uniaxial
compression between thumb and finger) or by dropping minimally disturbed, unconfined soil from
an arbitrary height. The latter will also provide information on the fragment size distribution. Soil
friability assessment is integrated into the standard procedure of soil profile description, relating it to
soil consistency and grade of soil structure (1, FAO, 1990; Soil Survey Division Staff, 1993).
Tensile
strength
aggregates
Tensile
strength
Quantitative
S-index
km
cm
mm
Drop shatter
Visual
evaluation
VSSE
Visual
evaluation
farmer
Qualitative
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Table 1. List of methods used to assess soil friability. Republished and modified from (11).
Main group
Friability
index
Description
Aspect of friability
assessed
Advantages
Drawbacks
References
Visual
assessment
FI
1
FI
1
= descriptive characterization in
relation to soil profile description
Both strength and fragment
size distribution is evaluated
Fast, simple, evaluate soil
behavior in the field
Qualitative, operator
dependent, sensitive to water
content
1, FAO (1990); Soil
Survey Staff (1993)
FI
2
FI
2
=grading in Visual Soil Structure
Evaluation.
Both strength and fragment
size distribution is evaluated
Fast, simple, evaluate soil
behavior in the field,
reference chart, grading
Qualitative, operator
dependent, sensitive to water
content
II; 13
Drop shatter
FI
3
FI
3
= fragment size distribution after
drop shatter test; GMD, MWD or
grading from visual assessment.
Fragment size distribution
Simple, ease to perform,
field and laboratory test,
semi-quantitative
Semi-quantitative, sensitive to
water content
I; 14;Marshall and Quirk
(1950); Hadas and Wolf
(1984); Snyder et al.
(1995); Shepherd (2000)
Tensile
strength FI4
FI4=k1 ; Y=aVk1
Scaling of aggregate strength
with aggregate size Quantitative, strength with
scales, sensitive to
management differences
No information on FSD,
difficult to measure at high
water contents, disturbance
during soil separation, time
consuming
Utomo and Dexter (1981)
FI
5
; FI5=1/β
Variation in aggregate
strength for a specific size
fraction
Quantitative, sensitive to
management differences
As for FI
4
plus only strength
for one aggregate size
Perfect and Kay
(1994b);Watts and
Dexter (1998)
FI
6
Variation in aggregate
strength for a specific size
fraction
Quantitative, sensitive to
management differences
As for FI
4
and FI
5
. Normal
distribution of data is assumed
Watts and Dexter (1998)
Soil water
retention curve,
S theory
FI
7
Indirect assessment. Based
on the relationship between
structural porosity and
friability
Quantitative, undisturbed
soil, water contents
No direct information on either
soil strength or fragmentation,
time consuming
Dexter (2004); Dexter
and Birkás (2004); Keller
et al. (2007).
Structural
porosity
FI
8
Indirect assessment. Based
on the relationship between
structural porosity and
friability
Quantitative, undisturbed
soils, fast
Indirect measurement, i.e. no
direct information on either soil
strength or fragmentation.
4; 14; Guerif (1990)
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Examination of friability is carried out in relation to assessment of “moist consistency”. That is, at a
window in soil water content between plastic condition and “hard” condition. The evaluation of
moist consistency is related to the strength of aggregates/peds. Friability index, FI1, assigns
friability assessed during soil profile description. Visual assessment of soil friability is also an
integral part of other visual soil examination methods such as the revised Peerlkamp method, Visual
Soil Structure Evaluation (VSSE) (II). Soil aggregation is evaluated by describing the type and size
of the dominant soil structural units. The method is simple and quick and soil is described by
comparing a spadeful with a reference chart. The soil is graded from 1 to 5 where 1 is best,
FI2=VSSE score. The fragment size distribution after applying a stress can be evaluated in the field
by simply dropping a soil sample from a given height and visually assessing the results as described
by Munkholm (1). The advantage of the visual methods is that they are fast, simple and yield
valuable information on soil behaviour in the field in terms of both strength and fragmentation. The
disadvantages are that they are qualitative, operator-dependent and very sensitive to spatio-temporal
changes in soil conditions such as water content and texture.
3.2. Drop shatter tests
The drop shatter tests may be used to estimate bulk soil friability and address mainly the fragment
size distribution after applying a specific stress. Drop-shatter tests have been developed and refined
by e.g. I, 14, Marshall and Quirk (1950), Hadas and Wolf (1984) and Shepherd (2000) to evaluate
the fragment size distribution (FSD) after applying a specific input of energy. In principle, a soil
sample is supplied with a specific energy input by dropping it from a given height onto a hard
surface and subsequently determining the aggregate size distribution. Intact soil clods were used by
Marshall and Quirk (1950) and Hadas and Wolf (1984). Undisturbed monoliths were used by I, 14
and Shepherd (2000). The resulting fragment size distribution has been visually evaluated
(Shepherd, 2000) or expressed graphically as aggregate size distribution, or as calculated values in
the form of the geometric mean diameter (GMD) or the mean weight diameter (MWD) or as surface
area. GMD and surface energy (i.e. the ratio between fragmentation energy and the new surface
area produced) have been proposed as soil friability indices (Snyder et al., 1995). In the
following part of this paper FI3=FSDdrop shatter. The drop shatter tests are simple, easy to perform
and they yield semi-quantitative information. They are also flexible (i.e. both field and laboratory
methods) and they mimic conditions in practice rather well. The disadvantage is that they yield little
15
information on soil strength and are very sensitive to changes in water content. Sandy soils will
fragment under most conditions, whereas clayey soils may bounce without breaking up. Further, the
energy input in the soil drop test is low in comparison with the estimated energy input in
different tillage operations as highlighted by Munkholm et al. (4).
3.3 Assessment based on tensile strength of unconfined soil
Soil friability has been quantified extensively from tensile strength measurements as proposed in
the Utomo and Dexter (1981) paper. Tensile strength measurements yield, on their own, essential
information on key aspects of soil friability, i.e. soil strength of unconfined soil, and a range of
friability indices has been developed based on tensile strength measurements. The water content
at measurement is adjusted in many cases. Air-dry condition has been the standard condition for
the classical procedure by Utomo and Dexter (1981).
3.3.1. Tensile strength of soil cores
Methods of measuring tensile strength in a compression test on soil cores (e.g. the Brazilian
method, are well known. Methods for measuring soil tensile strength on remoulded and repacked
samples soil (Ø= 19-45 mm and l=50-140 mm) in a direct tension test have been introduced by a
number of authors (Gill, 1959; Farrell et al., 1967; Nearing et al., 1988; Junge et al., 2000).
Munkholm et al. (4) developed a method to measure tensile strength on undisturbed, field-
sampled soil cores (Ø=44.5 mm, l=50.0 mm) at water contents around field capacity. Soil
friability has been quantified from tensile strength measurements on soil cores. Christensen
(1930) proposed the ratio between unit deformation (relative axial strain) and the work of
deformation to the yield point as a friability index. These methods yield information on “bulk”
soil strength.
3.3.2. Tensile strength and rupture energy of aggregates
Tensile strength may be determined from the force needed to crack an individual aggregate
between two flat parallel plates (Rogowski, 1964; Rogowski and Kirkham, 1976; Braunack et
al., 1979; Dexter and Kroesbergen, 1985):
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(1)
Where Y= tensile strength; F= polar force of failure; d= diameter of spherical particle;
c=constant.
Perfect and Kay (1994b) suggested calculating the specific rupture energy instead of tensile
strength, avoiding assumptions having to be taken regarding the exact mode of loading by which
the soil fails. The rupture energy, E, was derived by calculating the area under the stress-strain
curve:
(2)
Where F(si) is the mean force at the ith subinterval and i
s
is the displacement length of the ith
subinterval. The specific rupture energy was estimated on the gravimetric basis, Esp:
(3)
Methods for quantifying friability based on aggregate tensile strength measurements have been
reviewed by Watts and Dexter (1998) and Dexter and Watts (2000). In the “classical” procedure
described by Utomo and Dexter (1981) the friability index, denoted, FI4, was determined as the
scaling of aggregate tensile strength, Y, with aggregate volume, V:
; FI4= k1 (4)
where α is a fitting parameter that is an extrapolated estimate of the strength of a 1m3 bulk soil
sample. The parameter k1 was proposed by Utomo and Dexter (1981) as a friability index. The
larger the strength of the small aggregates in relation to the large aggregates, the higher the
friability index. When log-transforming Eq. 4 the relationship between Y and V will be shown as
a straight line with log a as the intercept on the y axis and k1 as the slope of the line. The scaling
17
effect of aggregate tensile strength may be explained by increased possibility and probability of
long cracks with size according to the Griffith theory (Rogowski, 1964, Dexter, 1975). Others
have applied Weibull statistical theory (9; Freudenthal, 1968; Braunack et al., 1979) or a
Weibull-fractal cube theory (Perfect and Kay, 1995) to describe the relationship between
aggregate tensile strength and size. When applying the Weibull statistical theory, k1 is an
estimate of the average spread of strength for the differently sized aggregates used. Based on the
FI4 friability index results, Utomo and Dexter (1981) classified soils into 5 broad classes:
FI4<0.05 not friable, FI4= 0.05-0.10 slightly friable, FI4 = 0.10-0.25 friable, FI4 =0.25-0.40 very
friable and FI4>0.40 mechanically unstable.
For a specific size of aggregates, i, the cumulative frequency distribution of Y can be expressed
as:
; FI5=1/β (5)
where P(y≤Y) is the probability of failure (cumulative relative frequency for Y),
α
is a constant
and β is a constant indicating the spread of strength. The 1/β parameter has also been used as an
index of soil friability by e.g. Perfect and Kay (1994b) and Watts and Dexter (1998).
The friability indices FI4 and FI5 would be identical if all the assumptions of the Weibull
weakest link theory of brittle fracture are fulfilled. For various reasons, FI5 is larger than FI4 in
many cases as discussed by Watts and Dexter (1998). They also suggested using simply the
standard deviation from aggregate tensile strength measurements on a single size of aggregates
(13.2-19.0 mm used) as an index of soil friability.
(6)
Perfect and Kay (1994b) suggested using specific rupture energy rather than tensile strength to
determine the friability index in order to avoid making assumptions on the exact mode of failure.
Assessment of soil friability from tensile strength measurements is a well-established and
commonly used procedure to quantify soil friability. There are good reasons for the widespread
use of this approach, i.e. quantitative information is obtained for an essential aspect of soil
18
friability and the approach is more objective than visual evaluation and the drop shatter test. It
has also proven to be sensitive to soil management, i.e. quantifying effects of tillage, compaction
and organic matter. However, there are also a number of drawbacks related to the use of tensile
strength to quantify soil friability. Firstly, little information is acquired regarding the FSD after
applying a stress. Secondly, it is rather difficult to determine tensile strength at water contents
similar to those in the field at soil tillage. Tensile strength is for practical reasons normally
determined in a compression test. Plastic deformation will occur in wet soil and the mode of
failure shifts from pure tensile to shearing and compression. Therefore, most researchers have
determined tensile strength on air-dry or even oven dry soil. This drawback can to some extent
be overcome by measuring fracture energy rather than tensile strength as suggested by Perfect
and Kay (1994b). Thirdly, if tensile strength is determined solely on aggregates, it may be
difficult to upscale the results to the behaviour of bulk soil in the field as brittle fracture of bulk
soil is expected to be strongly related to the structural porosity, i.e. properties of the inter-
aggregate pore space (Aluko and Koolen, 2000). Measuring tensile strength for different size
fractions will to some extent overcome the problem. This problem relates primarily to soils with
a hierarchical structure. Fourthly, extracting minimally disturbed aggregates from the soil is not
easy. There are considerable risks of mechanical disturbance during separation if the soil is
separated when moist. Air-drying before separation reduces this risk, but on the other hand
increases the risk of inflicting drying stress greater than any previously experienced by the soil.
3.4 Assessment based on soil water retention characteristics
As friability depends on the presence of flaws and microcracks, Dexter (2004) suggested using soil
pore data deduced from the soil water retention curve (SWRC) to characterize soil friability. He
suggested using the slope at the inflection point, labelled the index of physical quality, S, as a
measure of soil friability.
(7)
Where θ is the gravimetric water content and h is the matric potential.
19
The S index scale follows that of structural porosity and therefore of the occurrence of microcracks
in the soil. Dexter (2004) found good correlations between S and the friability index FI6 (R2=0.88
and p=0.002) in a study including a sandy loam, a clay loam, a silt loam and a clay. The S index has
also successfully been used to predict the FSD resulting from tillage for specific soils (Dexter and
Birkás, 2004; Keller et al., 2007). Please note, that the S index is an indirect measure of soil
friability as it does not directly yield information on either soil strength or soil fragmentation. The
applicability of the index builds on a strong relationship between these parameters and soil pore
characteristics. The method is promising as friability is assessed on undisturbed soil cores and at
water contents similar to those in the field at soil tillage. However, there are also a number of
drawbacks that need to be explored. Firstly, S, a measure of structural porosity, depends not just on
microcracks but also on voids and biopores that are not expected to play a major role in soil
fragmentation. Secondly, S, does not encompass the effect of bonding strength at the crack tips. This
will be highly dependent on clay and carbon content, clay mineralogy, pH, and sodicity. Guérif
(1990) showed that tensile strength for soils compacted to similar bulk densities displayed a strong
increase with increasing clay content and this dependence was labelled textural strength. Therefore,
the influence of texture etc. on soil strength and friability may limit the use of S as a universal
friability index across soil types. Thirdly, if the correlation between S and friability only reflects
the influence of structural porosity, then this parameter could be assessed more simply by
measuring water content at only one matric potential rather than the complete SWRC. Many
have used air-filled porosity at field capacity (typically at -100 hPa matric potential) as a
measure of structural porosity. This may not always yield an appropriate estimate of structural
porosity as the change between textural and structural porosity may occur over a wide range of
matric potentials for different soils (Dexter and Richard, 2009).
(8)
Guérif (1990) and (4) have found significant correlations between soil strength and
macroporosity. The latter authors found a negative linear correlation between the strength of soil
cores and the volume of pores>60µm (R2=0.54***) and volume of pores>30µm (R2=0.35*). The
use of FI8 as a friability index has the same limitation as described above for using S as a
20
friability index, i.e. structural pores includes both cracks and biopores and structural porosity
does not in itself yield information on bonding strength at the crack tips.
Significant correlation between drop shatter fragment size distribution and soil pore
characteristics has also been found (14). The pore characteristics were determined using X-ray
micro-CT imagery (60 µm voxel size) and traditional methods. Munkholm et al. (14) found
strongest correlations with total porosity, surface area and number of junctions per cm3. At best,
the CT pore characteristics explained 48% of the variation, which was at the same level as for
bulk soil air-filled porosity but poorer than for total porosity (R2=0.66). The authors concluded
that there was a need for further studies to improve the correlation between CT soil
characteristics and soil friability.
4. Effects of basic soil properties
4.1 Clay and exchangeable cations
The strength of the unconfined soil will in general increase with clay content as outlined by e.g.
Hatibu and Hettiarachi (1993) and Guérif (1990) for soil cores and e.g. 6, 10, V, Guérif (1990)
and Barzegar et al. (1994, 1995) for soil aggregates. The rate of increase in tensile strength of dry
soil with clay content depends on clay mineralogy as shown by Barzegar et al. (1995). They
found that tensile strength increased more strongly with clay content for smectite dominated than
for illite and especially kaolinite dominated soils. In other studies, Dexter and Chan (1991) and
Barzegar et al. (1994) showed that tensile strength also depends on the composition of
exchangeable cations. The strength of remoulded dry soils saturated with different exchangeable
cations decreased in the order Na>Na-Mg>Na-Ca>Na-Ca>Mg>Ca-Mg>Ca (Barzegar et al.,
1994). That is, the lowest strength was observed for soils saturated with the exchangeable cations
having the greatest flocculation capacity. The tensile strength increased with dispersible clay
and this was especially the case for sodic soils where clay dispersion is promoted. Several other
studies have shown that a high amount of readily dispersible clay increases tensile strength of
dry aggregates (IV; V and Watts and Dexter, 1997a) and reduces soil friability (V;
Shanmuganathan and Oades, 1982; Macks et al., 1996) (Figure 2). Acidity and base saturation
also have crucial roles in soil structure formation (Bronick and Lal, 2005). Low pH levels and
base saturation is expected to yield high strength in dry conditions (Kim et al., 2007) and poor
21
friability. It is an effect of great importance in humid areas but very little work has been carried
out to quantify this in details for natural soils.
Friability, k
Y
0.00 0.05 0.10 0.15 0.20 0.25
Dispersible clay (g kg
-1
soil)
5
10
15
20
25
30
35
40
45
Figure 2. Dispersible clay from field-moist soil (cubes) plotted vs. index of friability, kY (FI4),
for each individual experimental plot in 2007 and 2008. The regression line only reflects
differences between years (no significant trends within each individual year). Republished from
(V).
4.2. Water content
In this thesis, there will be special focus on the effects of water content as this topic has been
explored in detail in recent years. There will be focus on the effect of water content on the two
fundamental aspects of friability as outlined in section 2, i.e. the strength of unconfined soil and
soil fragmentation.
4.2.1 Soil strength
22
The increase in soil strength with decreasing water content can be ascribed to an increase in
cohesive forces of capillary-bound water (Bishop, 1961) and to increased effectiveness of
cementing materials Caron and Kay (1992) (e.g. drying and hardening of dispersed clay).
Mullins and Panayiotopoulos (1984) proposed the following relationship between tensile
strength, Y, and matric potential, ψ:
Yc= − χψ
(9)
where c is cohesion and χ is a factor related to the degree of saturation. In the absence of
externally applied stress, the χψ term describes the effective stress experienced by the soil.
Mullins et al. (1992) showed a linear relationship between Y and ψ for matric potentials in the
range 0 to 100 kPa in a study using remolded core samples from a hardsetting Australian soil.
For drier soil, the effective stress theory tends to overpredict Y (Mullins et al., 1992; Aluko and
Koolen, 2000). Differences in the geometry of pores involved with tensile failure and the
creation of internal shrinkage cracks at low water contents have been proposed as possible
mechanisms explaining non-linearity (Mullins et al., 1992). Empirical correlations between
aggregate tensile strength, Y or rupture energy, E, and volumetric water content, θ, or ψ have
been found in a number of studies (6, Guérif, 1988; Causarano, 1993; Chan, 1995). The latter
authors expressed the relationship by a power function:
n
Yq=−ψ
(10)
Where q and n are fitting parameters. It is difficult to measure θ and ψ on individual aggregates.
Therefore (10) related rupture energy of aggregates to the gravimetric water content:
(11)
where α0 is the characteristic rupture energy for the air-dry condition and γ is a water content
scaling factor. This function excellently fitted data from individual size fractions (10). The water
content had little effect on the spread of strength as expressed by the Weibull distribution
23
parameter β (Eq. 5). It was concluded in (10) that potentially only four parameters would be
needed to model the effect of water content on the rupture energy of aggregates across a large
range of water contents and a range of size fractions:
( )
( )
( )
1 exp D
ij
PE E xx E β

≤=− − α


(12)
where x is the length of the aggregate, xj is the average length of the particle “building blocks,”
and D is a size scaling factor,
4.2.2 Soil fragmentation
An optimum in water content for soil fragmentation (θOPT) has been defined as the water content
where the production of small fragments is highest relative to the production of large elements
(Dexter and Bird, 2001). This is also stated as the optimum water content for tillage. The value
of θOPT has been found to be at a water content just below the water content at the plastic limit
(0.90 θPL) (Dexter and Bird, 2001 and references therein). Recent studies indicate that the
optimum water content occurs at the water content at the inflection point, θINFL, on the soil water
release curve (Dexter and Bird, 2001; Keller et al., 2007). However, Mueller et al. (2003)
concluded that θINFL in many cases was larger than θOPT in an extensive study on 80 different
soils. They proposed a water content at 0.7*gravimetric water content at -5kPa as a safer limit for
maximum water content for optimal tillage. It has to be mentioned that Mueller et al. (2003) did
not carry out tillage experiments in the field. The range in water content for tillage was assessed
using a field scoring method.
Soil fragmentation is not expected to show a distinct peak at a specific water content, but
rather a gradual change with water content and to display satisfactory levels (seen from a tillage
point of view) over a range of water contents (Figure 3). This calls for working with the concept
of a water range for satisfactory fragmentation (ΔθRANGE) rather than just focussing on the
optimum water content for fragmentation. Dexter and Bird (2001) proposed a method to quantify
the concept for tillage based on the water retention curve as outlined below. The concept is
equivalent to the least limiting water range (LLWR) concept that has been proven to be very
useful for characterizing the effect of water content on other crucial soil functions such as root
growth (da Silva et al., 1994) and nitrogen mineralization (Drury et al., 2003). In a recent review
24
(12) the authors argued for a broader use of the concept in relation to other vital soil functions.
The occurrence of a least limiting range in water content for soil fragmentation can be explained
by opposite pulling processes with increasing water content. On the one hand, soil strength
increases with decreasing water content as outlined above. On the other hand, the likelihood of
cracks occurring where the fragmentation process can begin also increases with decreasing water
content. When exceeding the boundaries of ΔθRANGE, soil fragmentation is unsatisfactory in wet
conditions due to smearing and compression and in dry conditions due to too high strength.
Figure 3. Schematic illustration of the least limiting water range concept applied for soil
friability. ΘDL = lower water content for tillage (=plastic limit), θOPT = optimal water content for
tillage, θWL= upper water content for tillage. Republished from (11).
Empirical evidence suggests that the wet limit for soil fragmentation θWL is at water content
just above the lower plastic limit (i.e. lower Atterberg limit) (Ojeniyi and Dexter, 1979; Mueller
et al., 2003, Barzegar et al., 2004). Dexter and Bird (2001) applied the water content at the
plastic limit as θWL for tillage. However, it is not ideal to use a property determined on moulded
soil as a measure of θWL. At θ
WL, fragmentation is expected to be limited by density and the
morphology of cracks and therefore it would, ideally, have to be an index that is linked to these
25
properties. The easiest way would be to follow the approach for LLWRroot, where a certain value
of air-filled pore space (10%) is used as the standard wet limit (da Silva et al., 1994). So far, the
best documented estimate of θWL is the one proposed by Mueller et al. (2003). In an extensive
study covering 80 different soils from Germany and North and Central USA, they found that θWL
could best be expressed as the water content at maximum Proctor density or 0.7*water content at
-5kPa matric potential. The lower limit, θDL, is at water content where soil strength limits soil
fragmentation. This is comparable to the LLWRroot where soil strength limits root growth. Dexter
and Bird (2001) assessed θDL arbitrarily, and in a rather crude way, as the water content at which
aggregate strength is twice the strength at θWL. It would be more appropriate to use a fixed value
of soil strength. This could be the water content at which the tensile strength for a standard soil
core or standard aggregate size (e.g. 8-16 mm) exceeds a specific value of tensile strength. This
would be comparable to the concept of LLWRroots where a penetration resistance of 2 MPa is
used as the critical dry limit for root growth.
4.2.3. Water potential optima
Evidence suggests that a range of soils exhibits maximum friability between approximately -300
and -1000 hPa. The maximum value of the friability indices based on tensile strength
measurements was found to be between -300 and -1000 hPa pressure potential (Figure 4) (4; 6,
Utomo and Dexter, 1981; Causarano, 1993). Optimal friability in this maximum value range
corresponds with soil fragmentation results by Snyder et al. (1995). They found maximum soil
fragmentation at -400 to -700 hPa for a silty clay loam.
26
Pressure Potential (kPa)
Friability index, FI4
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
abPAC
REF
DFG
CCC
-10
-1
-10
0
-10
1
-10
2
-10
3
-10
4
-10
6
-10
5
-10
-1
-10
0
-10
1
-10
2
-10
3
-10
4
-10
5
Figure 4. Friability indices, FI4 as a function of pressure potential for the two cropping system
soils (a) and two traffic treatments (b). Bars indicate +/- 1 standard error of mean. Republished
from (6).
4.3. Soil organic matter content
Soil organic matter (SOM) acts as a crucial bonding material in soil structure formation (Oades,
1984). SOM forms complexes with primary mineral particles and secondary structural units. In
this way, interaggregate pores are formed and the result is a general reduction in bulk density
(Bronick and Lal, 2005). The consequence of low SOM content is severe as outlined by (8) and
Schjønning et al. (2009). Poor soil structure formation may lead to low strength in wet conditions
and a large risk of clay dispersion. Upon drying the dispersed clay will harden resulting in a
harder soil at low SOM content compared with a soil with high SOM content (5; 6) (Figure 5).
This phenomenon is also known as hardsetting (Mullins et al., 1987). SOM not only affects soil
strength but also tends to increase θOPT and expand ΔθRANGE (5, Dexter and Bird, 2001). This
indicates that depletion in SOM content decreases the optimum water content for tillage and,
more importantly, the range in water contents for tillage. The important role of SOM for soil
friability is also reflected in the positive relationship between SOM and friability index (FI4)
27
found by Watts and Dexter (1997a) and Macks et al. (1996), and the significant negative
relationship between aggregate tensile strength and SOM found by Abid and Lal (2009). The
study of Watts and Dexter (1997a) was based on soil taken from the Rothamsted long term
experiment at Highfield in the UK and the soil organic carbon (SOC) values varied from 1.1 to
3.2 g 100g-1. They found a strong linear correlation (R2=0.97) between friability (FI6 estimates)
and SOC. The occurrence of a critical SOM content for vital soil functions has been discussed by
Schjønning et al. (III, 2009) and (8) and the authors concluded that there is no unique minimum
level for SOM across soil types discussed. The ratio between clay and SOC (n) may be a useful
index for some soils as proposed by Dexter et al. (2008) in a study including a range of French
and Polish soils. Their results indicated that a ratio n of 10 was optimal. With increasing n ratios
>10 (i.e. decreasing relatively SOM levels), an increasing proportion of the clay was assumed
uncomplexed, resulting in higher bulk density and levels of readily-dispersible clay. Both these
factors have been associated with low soil friability due to effects on soil strength and soil pore
characteristics. It has to be mentioned that silt sized particles also has the capacity to form
complexes with SOC and it was concluded in a recent paper (V) that soil particles <20 µm may
better predict adsorption of SOM than clay. More work is needed to explore the applicability of
the ratio between clay or clay + fine silt and SOC in relation to soil friability. It is important to
keep in mind that the ratio cannot stand alone. Other factors such as clay mineralogy, CEC and
cation composition play a key role in relation to soil structural stability, levels of dispersible clay
and soil friability as discussed above.
28
Water content/ kg 100 kg
-1
0 5 10 15 20 25
AM
UNF
NPK
0 5 10 15 20 25 30
AM
UNF
NPK
B
2
B
4
θ
LL
θ
WL
θ
OPT
θ
DL
θ
LL
θ
WL
θ
OPT
θ
DL
Figure 5. Water contents for tillage. ΘDL = lower water content for tillage (=plastic limit), θOPT =
optimal water content for tillage, θWL= upper water content for tillage and θLL = liquid limit.
Republished from (5).
4.4. Bulk density
High bulk density levels can result from poor structural stability (i.e. collapse of soil structure
after wetting) or from compaction by animals or farm machinery. Soil friability decreases, in
general, with increasing bulk density (5; 6; Watts and Dexter, 1998). Soil tensile strength
increases with bulk density as shown by numerous authors (e.g. Guérif, 1990). The strength
increases due to an increase in cohesive forces of capillary bound water and the increased
effectiveness of cementing materials (e.g. drying and hardening of formerly dispersed clay).
Further, increased density is usually associated with the decrease in mainly structural porosity
(Guérif, 1990), which plays a vital role in brittle failure. Soil fragmentation may be even more
sensitive to soil compaction than tensile strength. In (6) the authors showed that compaction of a
sandy loam in wet conditions increased tensile strength by 20 and 60% for 8-16 mm aggregates
soil cores (Ø=4.45 cm), respectively, whereas the geometric mean diameter in a drop-shatter test
increased by 172%. High bulk density can be a result of direct impact from heavy traffic or a
consequence of poor structural stability caused by e.g. low SOM content.
29
5. Managing soil friability
Soil management affects soil friability in multiple ways. An overview of the effect of different
soil management types is shown in Table 2.
Table 2. Overview of management impact on soil friability. Reproduced and modified from (11).
Management type
Management
option
Overall
trend
Reason
References
Crop rotation
Cereal monoculture
Low OM inputs and
others
I; 2; 13, Reganold (1988); Chan
and Heenan (1996); Watts et al.
( 1996); Watts and Dexter (
1997a); Mueller et al. (2009)
Fertilization
Manure application
Increased OM input
5; Dexter and Bird (2001)
Residue
management
Residue removal
Decreased OM
input
Blanco-Canqui and Lal
(2007,2008)
Tillage
Reduced primary
tillage
Accumulation of
SOM in surface
layers, low
disturbance
3; Chan (1989); Macks et al.
(1996); Perfect and Kay,
1994a; Perfect et al. (1998);
Arvidsson and Feiza (1998);
Blanco-Canqui et al. (2005);
Blanco-Canqui and Lal (2007);
Abid and Lal (2009).
Intensive secondary
tillage
High degree of
disturbance,
kneading in wet
condition
7; Watts et al. (1996,1997b)
Traffic
Heavy traffic
Increased density
6; 7; Watts and Dexter (1998)
5.1 Cropping systems
Monocultural cereal growing systems have been shown to result in poorer soil friability
compared with more diverse cropping systems (Reganold, 1988; Chan and Heenan, 1996; Watts
et al., 1996a; Watts and Dexter, 1997a; Mueller et al., 2009). This was confirmed in a recent
study (13) where a positive effect of diverse rotation (including cover crop) on soil friability (FI2,
FI3) was found and this was especially under no tillage. In an on-farm study (2), Munkholm et
al. compared a continuously cash-cropped and mineral-fertilized soil (>20 years) (CCC) with an
animal-manured soil with a diversified crop rotation (DFG(2)) and assessed soil friability using a
range of methods (visual assessment, drop shatter and aggregate tensile strength). The series of
30
measurements unambiguously showed that DFG(2) had a better tilth and was more friable than
CCC. The DFG(2) soil had a more crumbly structure than the cloddy and rather massive and
dense CCC soil. The difference between CCC and DFG(2) could be explained by a difference in
SOM content resulting from differences in both crop rotation and fertilization. For another field
pair including animal manured soils (2), surprisingly, found poorer soil friability for a soil with a
diverse crop rotation (forages included), DFG(1), than for its arable counterpart, DFA. Both soils
were healthy soils with high biological activity and had similar bulk densities. However, the
DFG(1) soil had markedly higher biological activity than DFA (I, IV), which means a higher
concentration for biological binding and bonding material. This may explain the lower friability
observed for the DFG(1) soil compared with the DFA soil. Other studies have also shown that
e.g. high earthworm activity increases aggregate tensile strength (Schrader and Zhang, 1997) and
thus tends to reduce friability in the short term.
5.2 Fertilization
The effect of contrasting fertilization on soil friability has been studied using the Askov long-
term trials (>110 years) located on a sandy loam in Denmark (5). The long-term unfertilized soil
(UNF), low in SOM, tended to show the lowest strength in wet soil (i.e. pressure potentials
around -100 hPa) and the highest strength in moist and dry soil compared with a mineral
fertilized (NPK) and an animal manured (AM) treatment. The optimum water content for
fragmentation, θOPT, and the range in water content for fragmentation, ΔθRANGE, decreased in the
order AM>NPK>UNF. Results from the Rothamsted trial showed a similar trend (Dexter and
Bird, 2001) although they included a much broader range in management practices, i.e. from
permanent bare fallow to permanent grass.
5.3. Residue management
Blanco-Canqui and Lal (2007, 2008) showed that 3 or 10 years of continuous crop residue
removal decreased aggregate tensile strength for silt loam to clay loam soils in Ohio, USA.
Therefore, the results are in agreement with (2) as outlined above (i.e. DFG(1) vs. DFA). This
was explained by a promotion of aggregate formation by residue soil organic matter input. For
31
the 10 year long-term study, bulk density increased with crop residue removal. This is expected
to have counteracted the weakening effect of lower concentration of biological binding and
bonding material. Continued removal of residues resulting in lower SOM content is expected
over time - to cause increased aggregate tensile strength with decreasing SOM content as found
in a number of long-term studies as outlined above.
5.4 Tillage and traffic effects
Soil tillage disturbs the soil, i.e. causes loosening and fragmentation, but also affects the
conditions for soil aggregation, e.g. through the spatial distribution and the mineralization of soil
organic matter and via the effect on soil wetting and drying. No-till has been shown to increase
soil friability for hardsetting Australian soils (Chan, 1989; Macks et al., 1996) in comparison
with conventional tillage. They found that no-till was able to maintain friability at approximately
the same level as for permanent pasture or woodland, whereas cultivation resulted in a strong
decrease. For example, friability (FI4 estimates) for soil taken at 0-10 cm depth dropped from c.
0.8 (i.e. mechanical unstable) for no-till to c. 0.08 (slightly friable) for traditional tillage for two
potentially hardsetting soils (Macks et al., 1996). The drop in friability could to a large extent be
related to a loss of SOM and problems derived from this in terms of poor structural stability.
Similar effects have also been found in some cases for less degraded soils (Perfect et al., 1998;
Blanco-Canqui et al., 2005; Abid and Lal, 2009) but are lacking in others (13; Perfect and Kay,
1994a; Blanco-Canqui and Lal, 2007). The effect of chisel versus conventional mouldboard
ploughing on a sandy loam has been studied soil using a range of methods to assess soil friability
(3). The visual evaluation revealed no substantial differences between the treatments at the 0-20
cm soil depth. On the other hand, quantitative field and laboratory measurements showed poorest
soil friability for the chiselled soil. There was a significant difference in soil fragmentation when
using a field drop shatter test and aggregates from the chiselled soil had higher tensile strength.
The soil friability index (FI4) was also lowest for the chiselled soil for autumn sampled soil. In
another Scandinavian study, Arvidsson and Feiza (1998) found no clear difference in aggregate
tensile strength between a chiselled and mouldboard ploughed silty clay soil.
Intensive secondary tillage may result in decreased soil friability, especially if it is carried out in
wet soil. Watts et al. (1996b) and Watts and Dexter (1997b) showed that aggregate tensile
strength increased significantly after rotary cultivation, indicating decreased friability. Likewise,
32
(7) showed that intensive rotavation of a wet sandy loam increased aggregate tensile strength. In
fact the intensive tillage had a similar negative effect on density and strength of aggregates as
soil compaction induced by heavy traffic. The negative effect persisted for at least six months,
but was not discernible the following year, i.e. after a Northern European winter and a spring
ploughing operation. Surprisingly, the soil friability index, FI4, was not sensitive to effects of the
rough tillage and traffic treatments (FI4 varied from 0.21 to 0.34). This indicates that the
treatments affected the structure of the smallest aggregates (1-2 mm) equally as much as the
largest aggregates (8-16 mm). Aggregate density from the trial confirms that this was the case
(6). This example shows that FI4 cannot stand alone in all cases when evaluating friability.
6. Research needs
A lot of data have been produced during the last 30 years yielding valuable knowledge on
friability and how to manage soils to optimize this soil property. The results have also made it
possible to identify and clarify the knowledge gaps in understanding and managing friability.
6.1. Methodology
A wide range of methods are available to assess friability qualitatively and quantitatively. They
measure different aspects of friability and all have advantages and drawbacks. The field-based
methods are in general qualitative or semi-quantitative and are to a great extent operator-
dependent and very sensitive to spatio-temporal changes in soil conditions. On the other hand, the
quantitative laboratory typically yields information on only one of the two main aspects of soil
friability, i.e. either soil strength or the fragmentation pattern. Further, soil friability has in many
cases been assessed at low water contents, on physically disturbed soil and at meso-scale
(aggregates). This may altogether result in insufficient information on soil behaviour in the field at
relevant scales and water contents. There is a strong need to explore the correlations between
different measures of friability in particular the relations between qualitative field and quantitative
laboratory methods. This would help in selecting the best possible combination of methods to yield
comprehensive information on friability and to take account of spatio-temporal variations in e.g.
water content. At the moment a combination of the classical procedure (i.e. based on tensile strength
of air-dry aggregates, FI4), ∆θRANGE and drop shatter test or visual evaluation in the field appear to
33
cover the different key aspects of friability. Standardization is needed for some of the methods in
particular the field methods. For instance, with the drop shatter test, there is a need to standardise
sample size, optimum time and water content for measurement and input energy (i.e. drop height).
There is also a need to develop new methods to quantify soil friability at water contents relevant for
soil tillage. Novel non-invasive computer tomographic methods appear promising (14). They would
allow assessment of friability on undisturbed soil cores taken from the field and adjusted to relevant
soil water contents. Hopefully, these methods can be used to obtain information about both the
strength of soil and the fragmentation pattern when exposed to external stress. In the field there is a
need to develop new methods and sensors for fast and reliable assessment of soil friability, i.e.
methods that can be used to assess the need for soil disturbance on the go in a tillage operation.
6.2 Effect of soil water content
The soil water content strongly affects soil friability and there is a need for much more research
with focus on the relationship between soil water content and soil strength/soil fragmentation. It has
been found (10) that only four parameters may be needed to model the effect of water content on
the rupture energy of aggregates. They based their conclusion on three different soils from
Denmark and USA. Future studies are needed to validate this statement. There is also a strong
need to focus on determining the window of suitable water contents for tillage rather than the
optimum water content for tillage. This would be very useful for farmers who are in need of
knowledge to help in decisions on when to work the soil. The concept of a least-limiting water
range appears a promising concept also in this context. There are rather well-documented
estimations of the wet limit (θWL) in the literature (Mueller et al. 2003), whereas there is more
uncertainty regarding the dry limit.
6.3 Effect of soil organic matter content and other basic soil properties
Soil organic matter is a key player in soil structure formation and therefore greatly affects
friability. Clear linear relationships between SOM and friability have been for found for some
soils but we need to explore whether these relationships are applicable for other soils. The key
question to be answered is what is the critical SOM level for a given soil? There is also a need
for more focus on the temporal changes in soil friability after changing organic matter input rates
as present results indicates significant temporal dynamics. To fully understand SOM effects on
34
soil friability we need improved fundamental understanding of the interactions between SOM
and clay, CEC and exchangeable cations in relation to aggregation and clay stabilization. The
role of clay mineralogy, CEC and exchangeable cation has mainly been studied for soils
developed in a warm dry climate, where sodicity is a major issue. More studies are needed in this
respect but it is also important to focus on the role of these factors for temperate humid
conditions where acidification is a major issue.
6.4 Managing soil friability
Soil management affects soil friability in multiple ways and we need to continuously address the
central question: How do we manage soil to improve or maintain appropriate soil friability? In
that respect, there is a need to focus strongly on organic matter input and tillage and traffic. With
growing demand for food and fuel there is increased pressure on removing as much biomass
(crop and residues) from the soil as possible. Blanco-Canqui and Lal (2009), Blanco-Canqui
(2010) and Lal (2010) have raised special concern over the potential negative effects of using
crop residues for bio-energy production. In relation to tillage and traffic, there is a need for
further evaluation of the effect of different tillage systems. Friability is of key importance to the
quality of seeding and tillage for all types of tillage systems. Intensive tillage and traffic on wet
soil was shown to have a negative effect on soil fragmentation and friability properties. Larger
tractors and improved tyres have made it possible to traffic soil that is wetter than ever, with dire
potential consequences for the soil. Apparently, there is risk of entering a “vicious circle” (8)
where intensive tillage and traffic on wet soil leads to stronger and less friable soil, which again
increases the demand for more intensive tillage. Future research must evaluate the consequences
of this development and the persistence of poor friability.
6.5. Future perspectives
Soil friability is a soil parameter of vital importance for crop production and the impact of crop
production on the environment. Thirty years of focussed research in this area has yielded
significant results but serious knowledge gaps remain. Dexter (1979) stated more than 30 years
ago that “one of the principal aims of tillage research is to be able to predict what will be the
effects on the soil of using a given tillage implement in given soil conditions.” This still remains
a valid and timely objective in soil science and more knowledge on soil friability is needed to
35
address this objective. This is especially relevant as soil quality and thus soil friability is
threatened by a decrease in organic matter content, erosion and compaction.
7. Conclusion
It is concluded that the Utomo and Dexter (1981) definition of the concept: “Soil friability:
the tendency of a mass of unconfined soil to break down and crumble under applied stress
into a particular size range of smaller fragments” is still valid. It is, however, suggested to
add that a friable soil is characterized by an ease of fragmentation of undesirably large
aggregates/clods and a difficulty in fragmentation of minor aggregates into undesirable
small elements.
A range of methods is available to assess soil friability at different scales. They measure
different aspects of friability (i.e. typically either strength of unconfined soil or soil
fragmentation) and serve different purposes. All methods have advantages and
limitations. The use of a mixture of qualitative and quantitative methods to get a
comprehensive and adequate assessment of soil friability is recommended.
Water content strongly affects soil friability. Poor friability can be experienced if soil is
either too wet or too dry, i.e. there is a range in water contents for sufficient tillage for all
soils. The use the least limiting water range concept - expressed as ∆ θRANGE for soil
fragmentation - is highly recommended.
A strong relationship between organic matter and friability has been found in a number of
cases. No unique lower critical level of organic matter has been found across soil types.
Sustainable management of soil requires continuous and adequate inputs of organic
matter to sustain or improve soil friability.
Intensive tillage and traffic in unfavorable conditions threatens soil friability and may
initiate a vicious cycle where increasingly higher intensity of tillage is needed to produce
a proper seedbed.
Acknowledgements
This thesis work was mainly carried out on a sabbattical leave at University of Guelph, Canada
that was partly funded by the OECD Co-operative Research Programme. This thesis synthesizes
14 year of reasearch work initiated in 1998 at the start of my Ph.D. project. Many fellow
36
researchers and technicians have supported the work over the years. Many thanks for your help
and assistance.
My sincere thanks to all my coauthors, who made this work possible through their valuable
inputs, critical reflections and not the least their enthusiasm, open mindedness, passion for soil
science and awareness of the crucial role of soil quality in relation to sustainable
agroecosystems. A special thanks to Per Schjønning, Aarhus University who introduced me to
soil friability, suported my first steps in the research field and always has had an open door for
discussion on all kinds of aspects related to soil friability. Also, a special thank to the late Bev
Kay who hosted me during my first fruitful visit at University of Guelph in 2001 and who has
inspired me in many ways both as a scientist and as a person. Indirectly, Bev introduced me to
Ed Perfect, University of Tennessee who I visited in 2004. Thanks to Ed for a very rewarding
stay. I also like to express my gratitude to Richard Heck and Bill Deen who hosted me during my
successful visit at University of Guelph in 2010.
I strongly appreciate the technical support from the staff at Department of Agroecology
including staff at the research stations and in the laboratory. In particular, I want to thank the
technicians in the Soil Physics and Soil Resources research groupStig T. Rasmussen, Bodil B.
Christensen, Michael Koppelgaard, Jørgen M. Nielsen and Palle Jørgensen for highly skilled
technical assistance.
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