soil & tillage research 65 (2002) 61–75
TRANSCRIPT
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Possibilities for modelling the effect of compressionon mechanical and physical properties
of various Dutch soil types
U.D. Perdok*, B. Kroesbergen, W.B. HoogmoedWageningen University, Soil Technology Group, P.O. Box 43, 6700 AA Wageningen, The Netherlands
Received 18 October 2000; received in revised form 28 September 2001; accepted 31 October 2001
Abstract
The state of compactness of the arable soil layer changes during the growing season as a result of tillage and traction. The aim
of this study was to assess and predict some soil mechanical and physical properties governing machine performance and crop
response. The following mechanical properties were studied: compressibility, workability and cone index, CI, the latter as
indicator of load-bearing capacity or root penetration resistance. Compressibility of the soil could be described as a semi-log
function of pressure versus air volume and moisture content, with texture-specific coefficients for three representative soils, in
the range of 635% air content. The wet workability limit for 16 Dutch soils was reached when the compaction process turned
from dry into wet at 408 kPa of pressure. Soil rebound after pressure release was taken into account and quantified. Semi-
log relations were found for CI versus porosity and moisture. Other physical properties were also studied and it was found thatthe nature of the pFcurve of three representative soils (for seven levels of bulk density) was highly affected by the initial seven
pressuremoisture combinations. The effectivity of the pore system, indicating the effect of tortuosity and discontinuity on
the oxygen diffusion rate, turned out to be proportional to air content in the range of 625%. Critical machine and plant related
limits for aeration and mechanical resistance, CI, are available from the literature. Aeration is associated with minimum values
for air volume and oxygen diffusion rate, respectively. Using this information, CI was associated with minimum values for load-
bearing capacity and maximum values for root penetration.
The applicability of the comprehensive laboratory approach is found in farming practices and evaluations of land management
systems. On the operational level, machine performance can be predicted more accurately under fluctuating soil conditions.
Also, the effects of modified equipment can be quantified more accurately in the case of unchanged field conditions. The same
holds true for the prediction of crop response, as it is influenced by aeration and mechanical limits for plant growth. It was
concluded that the approach of predicting the mechanical behaviour of soil, followed by the pF-derived determination of
physical properties, will do justice to the dynamic character of the soil structure related input parameters in the present and futuremodels and simulations for machine performance, crop production and soil conservation. # 2002 Elsevier Science B.V. All
rights reserved.
Keywords: Compaction; Cone resistance; Workability; Load-bearing capacity; pF curve; Aeration; The Netherlands
1. Introduction
Field management and crop growing are highly
dependent on the ever changing soil properties and
Soil & Tillage Research 65 (2002) 6175
* Corresponding author. Tel.: 31-317-48-2966;fax: 31-317-48-4819.E-mail address: [email protected] (U.D. Perdok).
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 7 7 - X
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qualities of the arable layer. After all, during the
agricultural production cycle, soil loosening by tillage
is alternated with soil compaction by transport and
traction devices. Machinesoil relations are governedby mechanical properties such as workability and
load-bearing capacity (for wheeled equipment)
throughout the entire growing season. On the other
hand, with respect to the growing environment for
plant production, the state of compactness of the tilth
resulting from tillage and traffic determines the value
of physical properties regarding water supply and
aeration. So, it is obvious that the evaluation of soil
structure and land management should be based on
complete machinesoilplant relations.
Tillage, traction and transport activities exert forces
on the arable soil layer. Soil compaction by wheeled
equipment can be estimated by analytical methods.
OSullivan et al. (1999) presented an easy-to-use
spreadsheet for that purpose. These machine-induced
compaction processes can in general also be simulated
in the laboratory by a uniaxial test procedure at
constant speed, as described by Dawidowski and
Lerink (1990), Lerink (1994) and Smith et al.
(1997). This approach differs from the one reported
by Hakansson and Lipiec (2000), who introduced a
degree of compactness, relative to a reference bulk
density resulting from a prolonged pressure of 200 kPaunder standardized moisture conditions.
Modelling and simulation of the above machine
and plant related processes is increasingly being used
for better understanding and predicting the entire
system. However, these models require an accurate
description of the soil input parameters and coeffi-
cients.
In the study presented here, a laboratory approach
was taken by testing a number of relevant mechanical
properties, followed by measuring and predicting a
number of physical properties derived from pFcurves. The objective was to present the above soil
data as equations or graphs as functions of compres-
sion in order to facilitate a more realistic simulation
of the whole sequence of machinesoilplant
interactions.
As far as mechanical properties were concerned,
there was concentration on three major items, namely
compressibility, workability and penetration resis-
tance, CI, as an indicator for wheel load-bearing
capacity and for plant root penetration resistance.
These items were studied using the uniaxial test
procedure.
At the start of the uniaxial test, the matrix of loose
soil consists of a three-phase system of solid parti-cles, water and air. Pressure is applied, causing the
volume to reduce, at first affecting the air fraction but
ultimately also affecting the water component. If
only the air is removed, the soil aggregates and
derived properties remain largely intact. At the end
of the rapid compaction process in the test, almost all
the air has gone, with roughly a mere 6% remaining
entrapped (Koolen and Kuipers, 1983). From that
point onwards, the wet compaction process of knead-
ing starts because the entrapped air exerts tri-axial
pressure on the soil.
Workability and timing are clearly very important
for efficient field management and farming profit-
ability. Perdok and Kroesbergen (1996) already
proved the usefulness of such a compressibility test
for detecting the wet workability limit during second-
ary tillage. After compression, some soil rebound will
always occur. In the tests, this phenomenon was taken
into account.
For soil and field management during the agricul-
tural production cycle, there must also be a range of
optimal structural soil strengths causing the soil to be
sufficiently loose to be penetrated by plant root sys-tems, yet stable enough for manipulation by tillage
tools and firm enough for field traffic. Cone penetra-
tion resistance is a widely accepted indicator of soil
strength and should therefore be quantifiable and
predictable (Perdok and Kroesbergen, 1999). Criteria
for upper and lower critical mechanical limits for
vegetation and vehicles are available from literature,
e.g. Boone (1988) and Dwyer et al. (1976).
As far as physical properties were concerned, the
study concentrated on how the shape of the pF curve
was modified, as a result of the use of farm machinery.In models for prediction of soil water behaviour at
regional and national scale (functional models; Con-
nolly, 1998), the required pF curves are generally
based on broad relationships with easy-to-obtain soil
survey data such as soil texture classes as used in soil
maps (Wosten et al., 2001). More elaborate models
used on field and plot scale (mechanistic models;
Connolly, 1998) need pF curves which are also a
function of soil structure and density. Thus, a link
should be established between typical densities
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associated with machinery use, and the density effect
on pF curves as they are used in the calculation of
water retention and aeration status.
Airless soil is a poor growing medium for plants.Threshold levels of the volume and rate of diffusion of
air in the soil were determined by Boone and Veen
(1994). It was for this reason that relevant pF curves
were determined for various soil densities and com-
pression histories, i.e., various initial combinations of
pressure and moisture.
These curves can be used as benchmarks for the
mechanical resistance and aeration limits that must be
respected to ensure unimpaired plant growth and
efficient vehicle performance and as such will provide
information on actual and potential plant growing
and soil working conditions. Not only crop growth
models and machine performance models can benefit
from the above approach where dynamic instead
of static mechanical and physical properties are used
as soil input parameters. The same holds true for
models on soil erosion and soil crusting as e.g. stated
by Hoogmoed (1999) in a study on tillage for soil and
water conservation.
So, the overall objective here was to supply devel-
opers and users of the models with adequate soil
structure related input parameters, including their
variation in time due to soil management activitiesand soil mechanical interventions.
2. Materials and methods
2.1. Soil types and preparation
Compressibility and workability tests were carried
out on 16 soils from The Netherlands (their texture
ranging from light to heavy, see Table 1). Compressi-
bility, cone index (CI) and pF were determined in
three soils: sand, loam and clay (Table 2). The sandy
soil was frictional, with a relatively high organic matter
content, and some cohesive and elastic binding forces.
Loam and clay were cohesive soils of fluvial origin,
with a relatively lowangleof internal friction,especially
at high moisture contents.
All the tests were carried out on prepared soil
samples consisting of aggregates of 2.03.4 mm,
Table 1
Wet workability limits of 16 Dutch soils, according to three tests procedures: Atterberg lower plastic limit, permeability test and compressiontest
Origin and soil types Clay content,
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achieved by careful crumbling and sieving dry soil,
moistening to pF 2 and then drying or moistening to
seven moisture levels to a precision of 1.01.5% mass
of water to mass of dry soil.
2.2. Compression
To assess the compression and workability of the
soil, 100 cm3 steel cylinders of 50 mm diameter and
51 mm height were used. An electrically driven and
computer controlled Zwick pressure device com-
pressed 80 g of soil at a speed of 40 mm/min, to a
maximum of 1 MPa for compaction, and 408 kPa for
workability. Force and displacement were measured at
intervals of 0.02 mm of displacement.
Load and sinkage were monitored and converted
into pressure versus air volume curves. Pistonwall
friction turned out to be negligible because of thefavourable height-to-diameter ratio, ranging from 1 to
0.5 (Perdok and Kroesbergen, 1996). Sand particles
occasionally became trapped between the piston and
the cylinder wall, causing aberrations. These peaks in
load were excluded from the analysis.
2.3. Rebound
For soil rebound, soil samples with a range of seven
different moisture contents were compressed at five
increasing pressure levels, ranging from 10 to1000 kPa, on a logarithmic scale. Piston position
and sample height were recorded under pressure,
and after pressure release, respectively. The difference
in height, i.e. soil rebound, was expressed as a per-
centage of final height under pressure.
2.4. Cone penetration
For each of the three soil types, seven soil samples
of different moisture contents were compressed in
order to reach seven different density levels, yielding
a total of 49 samples per soil type for penetration
tests. The cone penetration resistance was measured
in at least four replicates by monitoring the strength
during penetration in identical 100 cm3 confined
samples (51 mm high), using a narrow cone, 2 mm
in diameter with a tip angle of 308, at a speed of
40 mm/min. Readings from the top and bottom
10 mm were excluded from the calculation of the
average values because of possible initial and end
effects in front of the small cone. Lateral cone effects
were not encountered, thanks to the relatively large
distance to the sides of at least five times the cone
diameter.
2.5. Retention curves
On a series of samples similar to those for the CImeasurements pF curves were determined, based on
six suction values in the range of pF1.02.7 (1.0, 1.3,
1.7, 2.0, 2.3 and 2.7).
2.6. Oxygen diffusion
The sample rings used for the determination of
diffusion rate were 75.7 mm in diameter and 50 mm
in height. A range of seven densities was created at one
moisture level, approximately 2% below the wet
workability gravimetric moisture content limit. Next,these samples were exposed to the same six pFvalues
as mentioned above. The diffusion rate was deter-
mined by standard procedures based on replacement
of pure nitrogen by atmospheric oxygen (Boone et al.,
1994). For this purpose, the top of each soil sample
was connected to a closed chamber filled with pure
nitrogen. Eventually, after the base had been exposed
to the atmosphere containing 21% O2, equilibrium
was reached. The increasing O2 content in the cham-
ber was monitored by an electrode. Because sample
Table 2
Granular composition and wet workability limit of sand, loam and clay
Particle density
(g/cm
3
)
Organic matter
(g kg
1
)
Particle size distribution (g kg) Workability
limit (%, w/w)50 mm
Sand 2.59 36 40 70 890 18.2
Loam 2.68 16 130 290 580 17.2
Clay 2.69 23 360 470 170 21.7
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and chamber dimensions were known, the O2 diffu-
sion rate could be calculated.
3. Results and discussion
3.1. Compressibility
The pressuremoistureair volume diagrams were
determined for the three soils. The diagram for loam
soil is presented in Fig. 1. The loading process of the
initially very loose soil samples was accurately
described by the following equation:
logp a0 a1Fa wa2 a3Fa (1)
a0a3 are soil coefficients, see Table 3, for sand, loam
and clay. The log(pressure), p, was inversely related to
air volumeFa and gravimetric water content w (Fig. 2).
In general, the above mathematical model fits well for
all soil types, light to heavy, as encountered in earlier
compression and workability studies, e.g., Sohne
(1952) and Tijink (1988). If iso-moisture lines are
fully parallel, a3 was zero. If, as happened in a few
exceptional cases, these lines are not exactly straight,
then curve fitting was slightly improved by using a
binomial function. In general, however, Eq. (1) fits the
data fairly well, within a range from 6 to 35% of airvolume. After all the free air has been removed, only
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situation intrinsic air permeability and diffusion rate
are very low.
3.2. Workability test
A workability test was developed by Perdok and
Hendrikse (1982), based on the pressurepermeability
relation. The soils tested turned out to be workable if
air permeability remained at least 1 mm2
after a pres-sure of 408 kPa (4 bar) was released. It was found that
the associated moisture content seemed to supply the
correct and minimum level of mechanical stability
required in order to call a soil workable. Table 1
shows these moisture contents, together with the
conventional Atterberg lower plastic consistency limit
per soil type. The situation of 1 mm2 permeability after
408 kPa of pressure was always accompanied by
approximately 10% of air volume. Table 1 also shows
the moisture level associated with the inflection
point found in the compression curves of Fig. 1,
i.e., 6% air left under 408 kPa pressure. Fig. 3 showsthat the moisture contents associated with the perme-
ability test on the one hand and with the compression
test on the other hand, correlate very well with
Fig. 2. Pressureair content relations for loam at various moisture contents in the range from 6 to 25%.
Fig. 3. Relation between moisture contents according to permeability test and compression test for the wet workability limit of 16 Dutch soils.
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r2 0:965 allowing the conclusion that this is asuitable alternative test for a wet workability limit.
As mentioned above, the respective associated air
volumes were about 10% after release of 408 kPaand about 6% entrapped air under 408 kPa. This
implies that under these pressure conditions the max-
imum soil rebound (or swelling), is approximately 4%,
due to air volume expansion.
3.3. Rebound or swelling
Compressibility was derived from loadsinkage
curves. After pressure release, the sample will rebound
or swell. An accurate estimation of rebound of the soil
matrix is needed if further measurements are to be
made on samples. Fig. 4 shows that rebound (R, %)
was roughly related to pressurep and moisture level w,
according to the following equation:
R a0 a1 logp wa2 a3 logp (2)
a0a3 are soil coefficients summarized in Table 3.
Eqs. (1) and (2), respectively, give a good estima-
tion of the air and pore volume of the soil sample
during the loading and after the rebound (swelling)
process. High pressure and moisture levels yield max-
imum rebound levels of about 3% for all three soil
types.Rebound or swelling is generally associated with
the pre-compaction stress of initially loose soil. If there
is re-loading beyond this stress, further compaction
will occur along the virgin compression line; see, for
example, Vermeulen and Perdok (1994) and Lebert
(1989).
3.4. Cone index
CI (MPa) could only be determined after piston
removal causing pressure release and soil rebound.
The CI data for sand, loam and clay are shown in
Fig. 5. CI is a semi-logarithmic and inverse function of
porosity (F) and moisture content w, according tothe following equation:
log CI a0 a1F wa2 a3F (3)
The soil coefficients (a0a3) relevant for sand, loam
and clay, are also summarized in Table 3. The above
equation fits very well for all three soil types. The
character of these laboratory curves coincides with
that of field soils, as reported by Boone et al. (1980).
As stated before (Boone and Veen, 1994), for
unhampered crop growth, CI should be interpreted
in view of the lower critical mechanical limit (LCML)
and upper critical mechanical limit (UCML) (1.5 and
3.0 MPa, respectively). Below 1.5 MPa, plant growth
is hardly affected, and beyond 3.0 MPa plant growth is
almost impossible. For good vehicle performance, a
minimum load-bearing capacity is required. As statedby Dwyer et al. (1976) the aim should be at least
0.5 MPa, dependent on wheel equipment, inflation
pressure, etc.
Fig. 4. Pressurerebound relations for loam at various moisture contents. Rebound in % of final height under pressure.
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Fig. 5. CIporosity relations for sand (a), loam (b) and clay (c) at various moisture contents.
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3.5. pF curves
In the field, weather and soil moisture conditions
will fluctuate in the short term. For this purpose, pFcurves were determined. The pF curve resulting from
a specific pore size distribution is therefore not only
density related, but also dependent on the initial
compaction process, and thus can be regarded as
the product of pedo-transfer functions. For this reason
seven densities per soil type were used at seven
moisture contents. This meant, there were 49 pF
curves per soil type. As an example, some of those
curves for sand, loam and clay are presented in Fig. 6
for one intermediate density of approximately 49%
porosity.
Dry compression conditions resulted in high mois-
ture retention levels at low pF ranges and lowerretention levels at high pF range compared to wet
compression conditions. As far as the effect of actual
variations in density are concerned (not shown), it was
found that at pFvalues lower than 2.0, the more loose
the soil, the wetter it was. At pF> 2:5, the role of soil
structure, and thus density is minimal. For this reason,
only the range of pF values that could be determined
by suction equipment (up to pF 2.7) was tested and
presented here. Note that plant-available water is
Fig. 6. pFcurves for sand (a), loam (b) and clay (c) at one intermediate level of density (47, 49 and 51% porosity, respectively) resulting from
different moisture contents at compression.
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accounted for from pF 2.0, field capacity, to pF 4.2,
wilting point. Low pFvalues, i.e., pF1.0, near satura-
tion, to pF 2 should be avoided because of expected
aeration problems.
3.6. Aeration and diffusion
Air volumes during compression were calculated
from sinkage data and constant gravimetric moisturecontent. Under field conditions, the weather condi-
tions and soil moisture status fluctuate, so that air
volume, air permeability and gaseous diffusion rate
will also change over time.
The oxygen diffusion rate in air is defined by
D0 2:08 105 m2 s1 at 20 8C. Theoretically,
the oxygen diffusion rate in soil, Dst, with air-filled
porosity Fa, would amount to D0Fa (m2 s1). In
practice, the diffusion rate is lower, due to the tortu-
osity and discontinuity of the air-filled pore system. To
take account of this, the effectivity factor (E) of the
pore system was introduced 0 < E< 1:
Dsa D0FaE (4)
With a known D0 and a measured Dsa and Fa, Ecould
be derived. Fig. 7 shows the linear relationship of E
Fig. 6. (Continued).
Fig. 7. Effectivity of the pore system for O2 diffusion related to air content, for sand, loam and clay, and the LCAL and UCAL.
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and Fa, expressed as
E a bFa (5)
where a and b are coefficients, presented in Table 4,
together with the associated r2 values.
The above linear functions for sand, loam and clay
are valid for air volumes less than 25%. The two
lowest pF values were omitted for all three soils in
order to get rid of scatter in this unrealistic low p F
range. Sand depicted the lowest r2, due to deviating
values for pF 1.7 (not shown).
The upper critical aeration limit (UCAL) and lower
critical aeration limit (LCAL) expressed as Ds 3 107 and 1:5 108 m2 s1, respectively, are alsopresented in Fig. 7. Aeration problems for plant
growth will not at all occur at diffusion rates beyondthe UCAL, but are very serious below the LCAL.
In Table 4, air volumes related to these limits are
presented. Loam and clay required roughly 78% of
air for the LCAL and about 14% for the UCAL. Sand
needed approximately 9 and 16%, respectively. The
work of Boone (1988) confirms this observation.
Hakansson and Lipiec (2000) used a critical limit
of 10% air-filled porosity for all soil types, but for
an adequate description of the true oxygen stress of
plants, they indicated and preferred a higher limit for
sandy soils. The air volume recorded at pF 2 showsthat the effectivity and thus diffusion rate at field
capacity might be problematic. Note that entrapped
air content was predicted here under the condition
of E 0, and ranged from 5.7 to 8.8%, close to thereference 6%.
3.7. Critical limits for CI and aeration
The actual water status in the field is the result of
rainfall, evaporation and drainage, with the movement
and redistribution of water in the soil governed by the
pF curve applicable at that moment. Once the moist-
ure content and porosity are known, the actual air
content can be determined so that the O2 diffusion rate
can be calculated with help of Eqs. (4) and (5). Next
the condition of aeration can be evaluated with
reference to both critical limits LCAL and UCAL
in Fig. 7.
As far as mechanical resistance is concerned,
Eq. (3) provides the true and actual CI value resulting
from the given water retention. Both plant growth
limiting factors (CI and aeration) are included in
Fig. 8ac, for sand, loam and clay, respectively, at an
intermediate initial pressure level of 408 kPa at
increasing moisture contents at compression, result-
ing in various densities. It seems that 408 kPa is toolow for the upper critical mechanical level to be
reached.
Soil compaction below the wet workability limit
was well described by Eq. (1). Beyond the workability
limit W, as shown in Table 2 and Fig. 8, Eq. (1) is not
valid. The decrease of porosity here was restricted by
entrapped air, causing soil structure to deteriorate and
water content to increase at constant pF (see Figs. 1
and 6).
3.7.1. Loam and clayIn general, mechanical resistance increases with
drier soil and higher density (Fig. 5). At constant
compression and increasing water content below the
wet workability limit, the CI is mainly governed by
density. But beyond the workability limit, it is mainly
governed by increasing water contents at constant pF.
This leads to a minimum pF required for constant CI
for cohesive soils, such as loam and clay, see Fig. 8b
and c. Mechanical limits are maximum values, so
these iso-lines should not be exceeded.
Table 4
Coefficients a and b and goodness offit, r2, for sand, loam and clay. Air contents for oxygen diffusion rate Dsa, at 0 and at LCAL and UCAL,
respectively. Air contents at pF 2.0
Soil type Coefficients (Eq. (5)) Air content (%, v/v) for Dsa at Air content (%, v/v)at pF 2.0
a b r2 0 LCAL UCAL
Sand 0.1142 1.294 0.907 8.8 9.4 15.8 14.5Loam 0.0728 1.272 0.974 5.7 6.6 13.9 11.5Clay 0.1130 1.531 0.987 7.4 8.0 14.1 9.5
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The aeration limits for loam and clay are repre-
sented as minimum values. Poorer aeration conditions
due to denser soil or soil with poor geometry is caused
by wetter initial compression conditions after 408 kPapressure. Under these conditions, higher pFvalues are
required to keep aeration above the critical level
UCAL. Drier initial compression conditions bring
about a looser soil with better aeration, which permits
a lower pF value.
As shown in Fig. 8b and c, for loam and clay,
varying the pFvalue had an opposite effect on meeting
both mechanical and aeration limits. As a result of this,
an intermediate pF range may be valued as safe and
appropriate in land use. It should be noted here that the
appropriate pF values in the field are difficult to
control, being highly dependent on weather conditions
and drainage. Comparing loam and clay at 408 kPa,Fig. 8b and c, it is obvious that clay reacts strongest to
changes in initial moisture content and pF.
3.7.2. Sand
Because sand is a frictional soil, the mechanical and
aeration limits were hardly influenced by initial moist-
ure content at compression (Fig. 8a). However, in the
higher moisture range (>18%), the UCAL required an
increased pFvalue >2, while LCML had already been
Fig. 8. Effect of pF in situ and water content at compression with 4 bar on LCAL and UCAL and LCML, for sand (a), loam (b) and clay (c).
Within the area, indicated as safe, both mechanical and aeration limits are met and plant growth is unimpaired. W indicates the soil
workability limit.
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reached at a pFvalue less than 2.5. Fig. 8a shows that
sand behaviour is almost unaffected over a wide range
of initial moisture contents below the wet workability
limit, but the safe zone as far as land use is concerned
is found within a relatively small pFrange above field
capacity.
3.7.3. Effect of pressure level
Apart from the 4 bar series, corresponding testswere also prepared for the other pressures tested of
1, 2 and 8 bar (not shown). Increasing the pressure on
sand moves the parallel aeration limits upwards and
the LCML line downwards. It was also found that
increasing the pressure for the cohesive soils (loam
and clay) caused the iso-limit lines to shift to the left
for both critical factors. Moreover, the aeration lines
rose slightly and the mechanical limits fell slightly.
4. The applicability of the study findings
4.1. Mechanical properties: workability and
load-bearing capacity
The procedures and criteria presented here can
improve the understanding of soil workability and
help optimize crop growth and vehicle performance.
They offer a way of overcoming the problem of
interpretation of the outcomes of single field trials
and case studies as they are relevant only for the given
machinery and soil conditions. The empirical infor-
mation from such field studies can now be placed in
well-defined context by pressuremoistureair volume
diagrams, as expressed in Eq. (1), indicating the
potential compacting effect at different levels of
moisture and pressure. With help of these curves it
would be possible, for example, to predict the soil
behaviour under different tyre pressures or at different
intensities of drainage and soil moisture contents.These diagrams also show the wet workability limit
as a reference value against which the actual field
conditions can be judged in terms of workability. With
the help of such information, one can balance the
risks of soil structure deterioration against the costs
of delaying tillage.
Contact pressure brings about an increased load-
bearing capacity indicated by increased CI values.
After rebound (Eq. (2)) CI values can be calculated
according to Eq. (3) and predicted. This enables
comparisons to be made with the critical mechanicallimits relevant to plants, 0.5 MPa.
4.2. Physical properties
The soil properties affecting crop growth and crop
response should also include water and air supply
within the soil matrix. It was for this reason that we
established pF curves resulting from various initial
conditions of pressure and moisture. This enabled us
Fig. 8. (Continued).
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to adopt and define critical aeration limits, based
on minimum diffusion rates, i.e. 1:5 108 and3:0 107 m2 s1. Accordingly, Fig. 8 shows the
soil-specific and comprehensive diagrams with thecritical mechanical and aeration limits that must
be met. Applying the above knowledge will help to
optimize the quality of soil structure related input
parameters and coefficients in models for machine
use, soil water status and crop production, including
soil conservation aspects (Hoogmoed, 1999).
4.3. Restrictions
In temperate, humid regions, improved field condi-
tions can be achieved in practice with the help of better
drainage and with patience while waiting for evapora-
tion. In drier regions of the world soil might be too dry
and, therefore, too hard for tools and plant roots. This
will be accompanied by a high level of moisture stress,
impeding transpiration and plant production. Such
situations will call for a different approach.
Another restriction of this study is the concentration
on the soil compaction process, thus excluding the soil
loosening process.
However, when complying to these restrictions the
laboratory results should be applicable to the arable
layer in the field. It should be kept in mind that afterall, an arable layer under conventional tillage practices
is probably just as artificial in structure formation
than soil prepared in the laboratory.
5. Conclusions
The main objective of this study was to supply
developers and users of soil management models with
soil structure related input parameters, being variable
in time. Results of the tests showed that the immediatecompressibility of various types of loose soil was well
predictable. The phenomenon of soil rebound after
pressure release could rather well be estimated. Cone
penetration resistance CI as an indicator of mechanical
strength of the pre-compacted soil was also well
predictable.
The compaction process at the same time, modified
the character of the pFcurve which could be expressed
as a pedo-transfer function. Graphical presentation
clearly showed its dependence on bulk density and
pore geometry, though the relationships could not be
placed in analytical functions.
Oxygen diffusion rate, as a quality indicator for
plant growth and crop response, was also influencedby geometry and porosity of the pore system, as
expressed by the effectivity factor E. This factor
was found to be linearly related to air-filled porosity
over a wide range (625%) of air content for many pF
values.
Finally, comprehensive diagrams could be pro-
duced showing the mechanical and aeration limits
for crop growth to be met for sand, loam and clay.
This study only addressed limitations in land use
caused by high soil moisture contents, associated with
low air contents. It represents the common Dutch
situation with a surplus of rainfall during the growing
season.
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