Soil & Tillage Research 65 (2002) 61–75

8/14/2019 Soil & Tillage Research 65 (2002) 6175

1/15

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


2/15

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

62 U.D. Perdok et al. / Soil & Tillage Research 65 (2002) 6175


3/15

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,


4/15

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



5/15

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


6/15

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.



7/15

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.

U.D. Perdok et al. / Soil & Tillage Research 65 (2002) 6175 67


8/15

Fig. 5. CIporosity relations for sand (a), loam (b) and clay (c) at various moisture contents.



9/15

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.



10/15

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.



11/15

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



12/15

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.



13/15

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).



14/15

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.

References

Boone, F.R., 1988. Weather and other environmental factors

influencing crop responses to tillage and traffic. Soil Till.

Res. 11, 283324.

Boone, F.R., Veen, B.W., 1994. Mechanisms of crop responses to

soil compaction. In: Soane, B.D., van Ouwerkerk, C. (Eds.),

Soil Compaction in Crop Production. Elsevier, Amsterdam,

pp. 237264.

Boone, F.R., van Ouwerkerk, C., Kroesbergen, B., Pot, M., Boers,

A., 1980. Soil structure. In: Experiences with Three Soil Tillage

Systems on a Marine Loam Soil, Vol. II: 19761979. Agric.

Res. Report 925. Pudoc, Wageningen, The Netherlands, pp. 24

46.

Boone, F.R., Vermeulen, G.D., Kroesbergen, B., 1994. The effect

of mechanical impedance and soil aeration as affected by

surface loading on the growth of peas. Soil Till. Res. 32, 237

251.

Connolly, R.D., 1998. Modelling effects of soil structure on the

water balance of soilcrop systems: a review. Soil Till. Res. 48,

119.

Dawidowski, J.B., Lerink, P., 1990. Laboratory simulation of theeffects of traffic during seedbed preparation on soil physical

properties using a quick uni-axial compression test. Soil Till.

Res. 17, 3145.

Dwyer, M.J., Everden, D.W., McAllister, M., 1976. Handbook of

Agricultural Tyre Performance, 2nd Edition. NIAE Report No.

18. Silsoe, UK.

Hakansson, I., Lipiec, J., 2000. A review of the usefulness of

relative bulk density values in studies of soil structure and

compaction. Soil Till. Res. 53, 7185.

Hoogmoed, W.B., 1999. Tillage for soil and water conservation in

the semi-arid tropics. Ph.D. Thesis. Wageningen University,

The Netherlands, 184 pp.



15/15

Koolen, A.J., Kuipers, H., 1983. Agricultural Soil Mechanics.

Springer, Berlin, pp. 3941.

Lebert, M., 1989. Beurteilung und Vorhersage der mechanischen

Belastbarkeit von Ackerboden (Judgement and prediction of

the resistance of arable land to mechanical loading). BayreutherBodenkunde Berichte, Band 12, 131 pp.

Lerink, P., 1994. Prediction of the immediate effect of traffic on

field soil qualities. Ph.D. Thesis. Wageningen University, The

Netherlands, 221 pp.

OSullivan, M.F., Henshall, J.H., Dickson, J.W., 1999. A simpli fied

method for estimating soil compaction. Soil Till. Res. 49, 325

335.

Perdok, U.D., Hendrikse, L.M., 1982. Workability test pro-

cedure for arable land. In: Proceedings of the Ninth Interna-

tional Conference of ISTRO, Osijek, Yugoslavia, pp. 511

519.

Perdok, U.D., Kroesbergen, B., 1996. Compressibility and work-

ability of soils under uni-axial load as influenced by air

volume and water content. In: Proceedings of the 12th Inter-

national Conference of ISTVS, October 711, 1996, pp. 66

72.

Perdok, U.D., Kroesbergen, B., 1999. Cone index as a function of

soil moisture and pore volume. In: Proceedings of the 13th

International Conference of ISTVS, October 1417, 1999,

pp. 512.

Smith, C.W., Johnston, S.M., Lorentz, S., 1997. Assessing thecompaction susceptibility of South African forestry soils. I. The

effect of soil type, water content and applied pressure on uni-

axial compaction. Soil Till. Res. 41, 5373.

Sohne, W., 1952. Die Verformbarkeit des Ackerbodens (Deforma-

tion of arable land). Grundlagen Landtechnik 3, 5159.

Tijink, F.G.J., 1988. Load-bearing processes in agricultural wheel

soil systems. Ph.D. Thesis. Wageningen University, The

Netherlands, 173 pp.

Vermeulen, G.D., Perdok, U.D., 1994. Benefits of low ground

pressure tyre equipment. In: Soane, B.D., van Ouwerkerk, C.

(Eds.), Soil Compaction in Crop Production. Elsevier, Am-

sterdam, pp. 447478.

Wosten, J.H.M., Pachepsky, Ya.A., Rawls, W.J., 2001. Pedotransfer

functions: bridging the gap between available basic soil data

and missing soil hydraulic characteristics. J. Hydrol. 251, 123

150.


Documents

Soil & Tillage Research 65 (2002) 61–75