tillage effects on soil strength and solute transport

Tillage effects on soil strength and solute transport

Iris Vogeler a,*, Rainer Horn b, Holger Wetzel b, Julia Krummelbein b

a HortResearch, Private Bag 11030, Palmerston North, New Zealandb Christian Albrechts Universitat, Kiel, Germany

Received 6 March 2005; received in revised form 18 May 2005; accepted 27 May 2005

Abstract

To study the effect of different soil tillage practices and the consequences of soil deformation on the functioning of the pore

system, we performed unsaturated leaching experiments (by applying a suction of �10 kPa) on undisturbed soil columns from

the Ap-horizon of a luvisol derived from glacial till (agricultural site at Hohenschulen, North Germany). We compared two

different tillage practices (conventionally tilled to 30 cm depth, and conservational chiselled to a depth of 8 cm-Horsch system)

with respect to soil strength, pore connectivity and their effect on the fate of surface-applied fertilisers. The soil strength was

measured by determining the precompression stress value (PCV). The conventionally tilled topsoil had a PCVof 21 kPa at a pore

water potential of �6 kPa, while the conservation treatment resulted in a slightly higher PCV of 28 kPa, suggesting a slowly

increasing soil strength induced by the formation of aggregates under reduced tillage practice.

The leaching experiments were modelled using the convection dispersion equation (CDE) and a modified version of the

mobile–immobile approach (MIM), which included three water fractions: mobile, immobile and totally immobile water. From

the CDE mobile water fractions (um) ranging from 47 to 67% were found, and um was slightly higher in the ploughed seedbed

compared to the conservation-tilled one. This could be due to higher aggregation in the latter one. Dispersivities were relatively

large, ranging from 44 to 360 mm, but no difference was found for the treatments. The MIM could simulate the drop in

concentration when leaching was interrupted, but overall did not improve the simulation, despite the larger number of fitting

parameters.

Compacting the soil with loads of 70 kPa prior to the leaching experiment did not affect solute transport in the conservational

tilled soil. In the conventional-tilled soil, however, the dispersivity decreased and the mobile water content increased compared

to the non-compacted soil, suggesting that the former one is less prone to deformation by mechanical loads.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Precompression stress value; Leaching experiments; Modelling

www.elsevier.com/locate/still

Soil & Tillage Research 88 (2006) 193–204

Abbreviations: BTC, breakthrough curve; CDE, convection

dispersion equation; MIM, mobile–immobile concept; PCV, pre-

compression value

* Corresponding author. Tel.: +64 6 3568080; fax: +64 6 3546731.

E-mail address: [email protected] (I. Vogeler).

0167-1987/$ – see front matter # 2005 Elsevier B.V. All rights reserved

doi:10.1016/j.still.2005.05.009

1. Introduction

Intensive agriculture with heavy machinery can

cause soil deformation by compaction and shearing

which results in changes in soil structure, pore size

distribution and the connectivity of the pore

.

I. Vogeler et al. / Soil & Tillage Research 88 (2006) 193–204194

List of symbols

Cf flux-averaged solute concentration (kg

or mol m�3)

Ci resident soil solution conc. in immobile

phase (kg or mol m�3)

Cm resident soil solution conc. in mobile

phase (kg or mol m�3)

C0(t) time-dependent input solution concen-

tration (kg or mol m�3)

D hydrodynamic dispersion coefficient

(m2 s�1)

Di diffusion coefficient in soil (m2 s�1)

Do diffusion coefficient in water (m2 s�1)

e void ratio

er residual void ratio

es initial (maximum) void ratio

KI air permeability coefficient

Ks saturated hydraulic conductivity

(m s�1)

l column length (m)

L length (m)

m constant

n constant

p liquid filled pore volume

pa air pressure (Pa)

qa air flux (m s�1)

qs solute flux density (kg or mol m�2 s�1)

qw water flux density (m s�1)

t time (s)

z depth (m)

Greek letters

a diffusional mass transfer coefficient

(s�1)

b dispersivity in mobile water phase (m)

g constant

h air viscosity (Pa s)

u volumetric water content (m3 m�3)

ui immobile water fraction (m3 m�3)

um mobile or effective water fraction

(m3 m�3)

ux excluded water fraction (m3 m�3)

l dispersivity (m)

rb soil bulk density (kg m�3)

s applied stress (Pa)

network. These changes affect the flow processes of

water, nutrients and gas in the soil, their availability

to plants and microorganisms and thus the quality of

a soil for agricultural production.

The intensity of the changes in the pore system

depends on internal soil strength properties, which are

influenced by texture, organic matter content, aggre-

gation, pore water pressure and the chemical

composition. Thus, agricultural practices influence

the strength of soils. Conventional ploughing has been

shown to decrease the strength in the topsoil, and thus

makes the soil more susceptible to deformation and

compaction by heavy loads. Horn (2004) has shown

that conversion from conventional to conservation

tillage can increase the mechanical strength and the

pore functioning; it resulted in increases of air and

water conductivity of a soil to greater depth over a

longer period of time.

To quantify the internal soil strength the pre-

compression value (PCV) of a soil is often used. The

PCV value quantifies the stresses either mechanical or

hydraulic, which a soil has been exposed to in the past,

and can be exposed to without any irreversible changes

in pore system and its functioning. At larger loads

plastic soil deformation changes in the pore system

and its connectivity occur (Horn and Fleige, 2003).

The fraction of coarse macropores decreases, as they

are transformed into smaller pores. This results in a

decreased saturated hydraulic conductivity and air

permeability.

While the effect of soil deformation on physical

soil properties, such as changes in the pore

system, hydraulic conductivity and air permeability,

has been demonstrated in several studies, the effect

of mechanical stress and soil deformation on

chemical transport has not received much attention.

Mooney and Nipattasuk (2003) looked at the effect

of soil compaction on solute transport by means

of a dye tracer and image analysis, and quantified

the extent of preferential flow for different soil

types.

The most commonly used model to predict solute

transport in soil is the convection dispersion equation

(CDE). However, the CDE does not satisfactorily

describe solute transport when preferential flow

occurs. In order to improve the prediction of solute

transport various dual-porosity, dual-permeability,

multi-porosity and multi-permeability models have

I. Vogeler et al. / Soil & Tillage Research 88 (2006) 193–204 195

been developed (Simunek et al., 2003). Both, dual-

porosity and dual-permeability divide the soil into

two regions. While dual-porosity models, such as the

mobile immobile version of the CDE, MIM, by Van

Genuchten and Wierenga (1986) assume that the

water in the intra-aggregate pores is stagnant and

water movement occurs only in the inter-aggregate

region, dual-permeability models, such as the models

by Skopp et al. (1981), Gerke et al. (1993) and Jarvis

et al. (1991) assume that both regions have a non-zero

pore water velocity. The dual-permeability models

are meant to be an improvement over the dual-

porosity models, because they allow movement of

water in the micropores. However, they have the

same limitations as the dual-porosity models, as it is

not possible to discriminate between the two flow

regions. The increase in accuracy obtained by

considering two water flow velocities introduces

the new difficulty of determining these two water

velocities as well as the two dispersion coefficients.

Thus, in the current study we only consider the CDE

and the MIM.

Models developed for simulating water and

chemical transport through compacted soils consider

the effects of compaction by changing the bulk

density, the penetration resistance and the hydraulic

properties of the soil (Lipiec et al., 2003), but ignore

the effect of compaction on solute transport para-

meters.

The objectives of the current study were to (i)

determine the effect of tillage practices on the

strength of the soil, (ii) elucidate the role of the

altered soil structure due to tillage practices for

the movement of water and solutes, (iii) validate

and parameterize solute transport models for non-

reactive tracers and (iv) to look at the effect of

mechanical loads in solute transport and model

parameters.

2. Theory

2.1. Precompression value

The precompression value can be calculated from

the stress strain curve obtained from a soil sample

compacted with different static loads. It describes

the relation between the volume of a specific mass

of soil and the vertical load, or stress applied. Water

content, time of loading and kind of stress applica-

tion also influence the stress strain behaviour.

The relationship is generally described via a semi-

logarithmic plot of the void ratio versus the normal

stress. The resulting curve can often be divided

into two parts, a precompression curve at the lower

stress range, and a virgin compression curve at

higher stresses (Casagrande, 1936). The point of

the transition from the re-compression to the virgin

compression line is the point of highest curvature,

or the inflection point, and presents the precompres-

sion value (further information can be obtained

amongst others in Horn et al., 2000; Pagliai and

Jones, 2002).

The inflection point can be calculated using

Casagrande’s method based on van Genuchten’s

model (Baumgartl, 2002):

e� er

es � er

¼ ð1þ ðgsÞnÞ�m(1)

where e is the actual void ratio, es and er the initial

(and maximum) and residual void ratios, respectively,

s the applied stress (Pa) and g, n and m are the

parameters describing the shape of the soil deforma-

tion function.

2.2. Solute transport

The most widely used approach for modelling

solute transport in soils is the convection dispersion

equation. Under steady-state one-dimensional water

flow the CDE for inert solutes is given by:

u@Cf

@t¼ D

@2Cf

@z2� qw

@Cf

@z(2)

where Cf is the flux concentration in the soil solution

(mol m�3), u the volumetric water content (m3 m�3), t

the time (s), z the depth (m), D the dispersion coeffi-

cient (m2 s�1) and qw is the Darcy flux density

(m s�1).

We assume the soil to be initially free of the solute

of interest, and when t equals zero the flux

concentration changes to C = C0 in the infiltrating

solution. At depth L we are interested in the flux soil

solution concentration as a function of liquid filled

pore volumes p, where p is defined as qt/uL. The

solution of the CDE in terms of z, p and l, where l (m)


Table 1

Selected physical and chemical properties of the two different

treatments (rb is the bulk density, es is the void ratio, Ks is the

saturated hydraulic conductivity and OM is the organic matter

content)

Treatment rb

(g/cm3)

es Ks

(cm/d)

OM

(%)

pH

A, conservative 1.39 0.91 311 2.1 6.63

B, conventional 1.45 0.82 76 1.7 6.46

is the dispersivity defined as D/(q/u) is given by (Van

Genuchten and Wierenga, 1986):

CfðL; pÞC0

¼ 1

2erfc

�1� p

2ðl p=LÞ1=2

�

þ 1

2exp

�1þ p

2ðl p=LÞ1=2

�: (3)

2.3. Mobile–immobile approach

The second model considered here is the mobile–

immobile modification (MIM) of the CDE (Van

Genuchten and Wierenga, 1986). In this model,

the pore space is divided into two conceptually

different domains, a dynamic domain, which con-

tains mobile water (um), and a stagnant domain,

which contains immobile water (ui). Convective–

dispersive transport occurs in the mobile water

fraction only. For non-reactive solutes the transport

of solutes in the mobile phase is given by (Vogeler

et al., 1998),

um

@Cm

@t¼ qb

@2Cm

@z2þ Dium

@Cm

@z2� q

@Cm

@z

� auiðCm � CiÞ (4)

where Cm and Ci denote the resident soil solution

concentration in the mobile and immobile phase

(ml m�3), b the dispersivity in the mobile phase

(m), a the diffusional transfer coefficient for solute

exchange between mobile and immobile regions (s�1)

and Di is the molecular diffusion coefficient of the

solute in the soil (m2 s�1), given by Di = 3.5u2Do,

where Do is the diffusion coefficient in the bulk

solution, estimated as 7.2 � 10�6 m2 h�1 for a CaBr2

solution.

The solute transport equation for the immobile

region is given by:

@Ci

@t¼ Di

@2Ci

@z2þ aðCm � CiÞ (5)

Eqs. (4) and (5) for the mobile–immobile approach

were solved numerically as described by Tillman et al.

(1991).

3. Methods and materials

For the experiments topsoil from the Research

Station Hohenschulen, near Kiel, North Germany

was used. The soil is a stagnic Luvisol derived

from Weichselian glacial till. The soil consists of

10% clay, 33% silt and 57% sand. The site was

originally conventionally ploughed. In 1991 the site

was divided into two different treatments, one

remained under conventional tillage (tilled to

30 cm depth) and the other was changed to

conservation tillage (chiselled to 80 mm depth-

Horsch system). A summary of different physical

and chemical properties of the two sites is provided

in Table 1.

At the time of sampling the sites were planted

with wheat (Triticum aestivum L). Intact soil cores of

two different sizes, large and small, were taken from

the topsoil, from both the conventionally and

conservation plots, in June 2004. The large samples,

200 mm in diameter and 100 mm high, were used

for the measurement of the precompression stress

value, and the changes in air permeability due to

mechanical stress. The small samples, 56 mm in

diameter and 40 mm high, were used for the

measurement of the water retention curve, and the

saturated hydraulic conductivity. The bulk density,

which also provides the void ratio, was determined

for all samples. The various measurements allowed

the determination of the effect of conventional versus

conservation tillage on physical soil parameters and

soil structure. Additionally, disturbed soil samples

were collected for the determination of organic

matter content and pH.

All large undisturbed soil samples were taken into

the laboratory, saturated, and drained to a soil-

moisture tension of �6 kPa, which corresponds to

soil moisture content at field capacity.


3.1. Air permeability

The air permeability was measured before and after

stress application, using the modified KI apparatus

(Hartge and Horn, 1992), with an air pressure

difference of 1 cm water column, corresponding to

0.1 kPa. The air permeability coefficient (KI) is then

calculated using:

KI ¼ qalh

pa

(6)

where qa is the air passing through the soil area

(mm s�1), l the soil height (mm), pa the air pressure

(0.1 kPa) and h is the air viscosity at 20 8C(1.821 � 10�8 Pa s).

The saturated hydraulic conductivity was measured

using the permeameter method of Hartge (1966), with

five replicates per treatment. The water retention curve

and pore size distribution was determined using a

combination of tension plates and pressure chambers

(Hartge and Horn, 1992) and three replicates.

3.2. Precompression stress and settlement

Time and stress-dependent soil settlement, and the

precompression stress were determined by the

uniaxial confined compression ‘‘Multi Step Soil

Compression Test’’ device as described by Horn

et al. (2004) (see Fig. 1). Loads of 20, 40, 60, 80, 100,

Fig. 1. The ‘‘multi step compression device’’.

150, 200 and 400 kPa were applied stepwise, without

any stress release between the loads, to the soil

samples. Each pressure was applied for 2 h, and the

settlement, which also provides the total porosity, was

monitored by strain sensors. At the end, the load was

released and the elastic rebound recorded for 2 h.

During compaction draining conditions occur, since

water can leak through the porous sintered metal

plates. The cores were weighed before and after load

application to calculate the soil water loss during

compaction. For each treatment four replicates were

used.

3.3. Leaching experiments

Leaching experiments were carried out in the

laboratory under controlled flow conditions using

undisturbed soil columns, 10 cm in diameter and

9 cm high, taken from the field after removing the

above ground part of the wheat plants. For each

treatment two columns were used to study the effect

of tillage system on the transport of inert solutes. Two

further columns from each treatment were compacted

with a load of 70 kPa for 2 h using a load frame

(INSTRON 5569; Instron Corporation, Norwood,

MA). The leaching was performed under unsaturated

conditions by placing the columns in a leaching

apparatus consisting of porous plates at the top and

the bottom. Unsaturated, gravity driven flow was

achieved by applying a suction of�10 kPa to both the

reservoir on top, containing the solution, and the

bottom of the columns. All the columns were

saturated and then placed into the leaching apparatus.

The columns were then preleached with a weak

solution of CaSO4 (0.0025 M), until steady-state flow

was achieved. Then, the input solution was changed

to 0.025 M CaBr2. After infiltration of about

one liquid-filled pore volume ( p) infiltration was

stopped for 12 h to look at the effect of molecular

diffusion on solute transport. After infiltration of

another 2p of 0.025 M CaBr2, the input solution was

again changed, to 0.1 M MgCl2. After an infiltration

of about 6p leaching was interrupted for 1 week, and

then another p of MgCl2 was applied. The experi-

mental procedure, after preleaching, and conditions

are illustrated in Table 2. The leachate samples were

analysed for the concentrations of chloride and

bromide.


Table 2

Experimental leaching procedure and column data (u = volumetric water content, p = pore volume)

Fig. 2. Changes in porosity due to different loads.

4. Results and discussions

4.1. Pore volume

The total pore volume, as calculated from the

measurements of the bulk density, was slightly higher

in the conservatively tilled site, compared to the

conventionally tilled site, see Table 1. This was not

expected, as Horn et al. (2004) found a higher pore

volume, and lower bulk density, for the conventionally

tilled soil. The difference is probably due to the time of

sampling and the influence of roots on the bulk

density. Tillage increases the porosity of the soil but

with time the soil settles again, and becomes more

compact, with a decrease in porosity. The conserva-

tively tilled site also had a larger proportion of large

pores at the time of sampling, as can be seen from the

water retention curve (not shown).

The changes of the total porosity, or pore volume,

due to different loads as obtained from the precom-

pression test are shown in Fig. 2 for the two different

treatments. As expected the change in porosity, and

thus compression, increases with increasing load in

both treatments. The decrease is, despite the higher

initial pore volume, smaller for the conservatively

tilled site than the conventionally tilled site, suggest-

ing higher internal strength of the former site. In both

cases about 10% of the decrease in porosity during

compression is due to elastic deformation, as can be

seen from the increase in porosity following stress

release.


Fig. 3. Changes in air permeability due to different loads.

Fig. 4. Stress strain curves as a function of time of loading.

Fig. 5. Settlement curve showing the calculation of the precompres-

sion value (PCV).

4.2. Air permeability

The change in the air permeability index due to the

stress application is shown in Fig. 3 for the two different

treatments. Whereas the air permeability for the

conventional treatment had already decreased substan-

tially at low loads, large changes at the conservatively

tilled site occur only at loadings of 100 kPa and higher.

This suggests that the change from conventional to

conservative tillage improves the soil structure and

pore functioning with respect to air permeability.

This in turn improves the aeration and gas exchange

in the soil, and thus crop growth conditions.

4.3. Saturated hydraulic conductivity

The saturated hydraulic conductivity was higher in

the conservatively tilled site compared to the

conventional-tilled site (see Table 1). This suggests

that, not only the porosity is higher, but also the

connectivity of the pores, or at least the macropores.

The movement of water, nutrients and gas through

soils are affected by both the porosity and connectivity

of the pores. Similarly, Ehlers et al. (1994) found

increased macroporosity and pore continuity as well

as increased aggregate stability under conservation

tillage due to increased biological activity and soil

swelling and shrinking.

4.4. Soil strength and precompression

The soil strength can be quantified by the

precompression value, which can be calculated from

the settlement curve during load application. Fig. 4

shows the load application and the settlement curve for

one of the samples from the conventionally tilled soil.

The stress strain curve was used to calculate the

change in void ratio with increasing load application,

which is shown in Fig. 5 for one of the measurements.

The inflection point of the curve is then used to

calculate the PCV.

Fig. 6 gives the measured PCV values for both

treatments and the mean value from the four

replicates. The slightly higher mean PCV of the


Fig. 6. Precompression value for the two different treatments.

conservation-tilled topsoil suggests natural structure

formation (Horn, 2004). Over more than 10 years the

site has been under conservative tillage. Several other

studies have also proved that aggregation increases the

stability of a soil against deformation by mechanical

loads, and thus the precompression value (Lebert et al.,

1989; Baumgartl and Horn, 1991; Kuhner, 1997).

4.5. Leaching experiments

4.5.1. Water flow

The measured outflow rates, or Darcy flux

densities, qw, achieved with the applied pressure of

�10 kPa are given in Table 3 and are quite similar for

three of the columns, A2, B1 and B2. The flux

densities in these three columns ranged from 3 to

14 mm h�1, and were always higher during the second

leaching run with chloride than the first. This suggests

that with time more pores become connected and

participate in the transport of water and chemicals.

Another explanation could be changes in the pore size

Table 3

Model parameters obtained from the CDE

Column #

Conservative Conventional

A1 A2 B1

Tracer Br Cl Br Cl Br C

q (mm h�1) 66 44 7 14 3 1

l (mm) 156 46 95 44 103 19

um (%) 47 49 59 48 67 6

distribution during the duration of the leaching

experiment. Column A1, which came from the

conservation-tilled plot had a much higher Darcy flux

density of 44 and 66 mm h�1 than the conventional

tillage system. This suggests a larger fraction of

connected pores with a diameter �30 mm. Under

conventional tillage such connected macropores are

likely to be disturbed. For all leaching events water

flux densities were well below the saturated hydraulic

conductivity (see Table 1).

4.5.2. Bromide and simulation results using the

CDE

The normalised outflow concentration data of

bromide are shown in Fig. 7a and b for the

conservation tillage treatment, and Fig. 7c and d the

conventionally tilled treatment. The breakthrough

curves (BTC’s) of Br for the two replicates from the

conservatively tilled treatment are quite different, with

more preferential flow in column A1, the column with

the faster flow rate, than column A2. Also shown are

the fitted curves using the CDE (Eq. (2)) and least

square optimisation. The drop in the Br concentration

at about 1.5p is due to the interruption of leaching for

12 h, and suggests physical non-equilibrium. The drop

is highest in the column with the higher flow rate

(column A), which is probably due to the limited time

available for molecular diffusion between faster and

slower moving water. In the other three columns the

drop was smaller and of similar magnitude. For the

fitting procedure only the data collected before the

flow interruption were used. The values found for the

dispersivity (l) and the mobile water fraction are given

in Table 3. The values of l are quite large, but within

the order of values found for undisturbed cultivated

field soils (Vanderborght et al., 1997). Differences in lfor the two replicates per treatment were quite large,

Conservative

compacted

Conventional

compacted

B2 A3 A4 B3 B4

l Br Cl Cl Cl Cl Cl

2 5 12 5 17 7 4

1 360 181 72 160 25 41

7 63 63 63 86 91 81


Fig. 7. Normalised effluent concentration of bromide (C/C0) as a function of pore volumes ( p) for the conservatively tilled site and columns (a)

A1, (b) A2, and the conventionally tilled site and columns (c) B1, (d) B2. Also shown are the predictions using the CDE (black line) and the MIM

(grey lines).

suggesting high spatial variability. Due to the large

variation within the treatment, no difference is obvious

for the two different treatments. The values for the

mobile water content range from 47 to 67% and are

slightly higher in the conventional-tilled site com-

pared to the conservatively tilled site.

4.5.3. Choride and simulation results using the

CDE

The normalised outflow concentration from chlor-

ide is shown in Fig. 8, again with the simulations using

the CDE and least square optimisation. In this second

Fig. 8. Normalised effluent concentration (C/C0) for chloride as a function o

and A2 (*) and (b) conventionally tilled site, columns B1 (&) and B2 (*).

B2 grey lines, using the CDE.

leaching event the BTC’s of the replicates are quite

similar. This, and the fact that the values of the

dispersivities obtained for the chloride BTC are for all

columns different to those obtained from the BTC of

bromide (Table 3), suggest that during the break and

over the duration of the experiment the flow path

geometry changed (Roth and Hammel, 1996). The

mobile water fraction was again slightly lower in the

conservatively tilled soil compared to the convention-

ally tilled soil.

The dispersivity values found from the two

leaching events, with first bromide followed by

f pore volumes ( p) for (a) conservatively tilled site, columns A1 (&)

Also shown are the predictions for A1 and B1 black lines and A2 and


Table 4

Model parameters obtained from the modified MIM

Conservative Conventional

Column # A1 A2 B1 B2

Tracer Br Br Br Br

qw (mm h�1) 66 7 3 5

b (mm) 20 20 20 20

a (s) 0.05 0.03 0.03 0.02

um (%) 21 32 23 16

uim (%) 29 32 33 31

ux (%) 50 36 44 53

chloride, show no relation to the flux density, as has

been suggested by Brusseau (1993). He found that the

dispersivity in aggregated soils is constant at low

velocities, and increases at higher velocities due to the

effect of intraparticle diffusion. The mobile water

fraction remained quite similar for the two leaching

events, with a slightly lower average um in the

conservatively tilled soil of 51% compared to 65% in

the conventionally tilled site. This means that a larger

fraction of water was accessible to the percolating

bromide solution in the conventionally tilled site. This

could be due to a higher aggregation in the conser-

vation tillage plot, with the formation of immobile

water pockets within aggregates. The relatively low

fraction of mobile water in both treatments could

either be due to the effect of the plant roots or soil

compaction. Preferential flow due to soil compaction

from a sugar beet harvester was also found in a field

study by Kulli et al. (2003). They suggested that the

fine pore structure in the topsoil were locked up and

no longer available for water infiltration. The water

was thus ponding on the surface and preferentially

entering open wormholes, bypassing the main root

zone.

4.5.4. Bromide and simulation results using

the MIM

In the mobile–immobile model approach the soil’s

water content is generally divided into two fractions, a

mobile, and an immobile one. With this approach,

however, it was not possible to simulate the measured

Br data successfully, with respect to the timing, and

shape of the BTC and the drop in concentration during

the 12 h break. Thus, we tried an approach using three

different fractions: a ‘mobile’ (um), an ‘quasi

immobile’ (uim) and a ‘totally immobile’ (ux) fraction.

In this approach it is assumed that water moves

through the mobile water fraction, and diffuses into

the immobile fraction. The totally immobile fraction is

not involved in solute transport. We have successfully

used this approach for simulating chloride movement

through a silt loam (Vogeler et al., 1998). The values

for the three different water fractions, and the

remaining parameters in Eqs. (4) and (5), a and bwere found by trial and error, and are given in Table 4.

The simulations are also shown in Fig. 7. The values

for the totally immobile water content range from 36

to 53% and are much higher than the values found in

the Manawatu soil (Vogeler et al., 1998), where the

totally immobile fraction was thought to be due to

anion exclusion from the double layer. Here it was

more likely that the pore network was not well

connected under the unsaturated conditions, resulting

in the isolation of stagnant regions within the column.

The mass transfer coefficient, a, is similar to values

reported in the literature (Vogeler et al., 1998; Tillman

et al., 1991).

This mobile–immobile approach with three different

water fractions described reasonably well the BTC of

bromide ions with a drop in concentration, when

leaching was interrupted. The simulation is however not

better than the one obtained by the more simple CDE,

with only two fitting parameters, compared to five in

this mobile–immobile approach, namely the threewater

fractions, a and b. It therefore seems more applicable to

use the much simpler classical CDE, in which the

effects of matrix diffusion are lumped into the

hydrodynamic dispersion coefficient (Valocchi, 1985).

4.5.5. Chloride transport through compacted

soil columns

The measured and predicted BTC’s of chloride

obtained from the columns, which were compacted by

a load of 70 kPa prior to leaching, are shown in Fig. 9.

The model parameter values found for the CDE are

given in Table 3. For the conservatively tilled site, the

transport of chloride through the compacted soil

columns was similar to the one observed in the non-

compacted soil columns. In the conventional-tilled

soil, however, the load of 70 kPa changed the transport

behaviour. The flow became more uniform, with a

decrease in dispersivity and an increase in the mobile

water content, and thus less preferential flow

behaviour. This suggests a higher stability of the

conservation tillage plot compared to the convention-


Fig. 9. Normalised effluent concentration (C/C0) as a function of pore volumes ( p) for compacted columns (70 kPa) taken at the (a)

conservatively tilled site: columns A3 (&) and A4 (*) and (b) conventionally tilled site, columns B3 (&) and A4 (*). Also shown are the

predictions for A3 and B3 black lines and A4 and B4 grey lines, using the CDE.

ally tilled site, with respect to pore functioning and

solute transport.

However, it has to be pointed out that the effect of

soil compaction on preferential flow also depends on

the soil type. Whereas Mooney and Nipattasuk (2003)

found that soil compaction reduces the likelihood of

preferential flow in a clay loam, compaction increased

the likelihood of preferential flow in the sandy loam

used in their study.

5. Conclusions

The current study has shown that conservation

tillage over a period of 10 years increased the strength

of the topsoil compared to the conventional tillage

practice. The precompression value of the conven-

tional-tilled site was 21 kPa, compared to 28 kPa of

the conservatively tillage site. The increased strength

is probably due to the formation of aggregates within

the topsoil of the conservatively tillage system. The

water flow and transport of non-reactive tracers,

chloride and bromide, was variable for the replicate

columns within the two different tillage practices. This

variability masks any possible differences in solute

transport behaviour due to the formation of aggregates

within the conservatively tilled topsoil.

The CDE could be used to model the movement of

chloride and bromide through the two different

managed sites reasonably well, apart from the drop

in concentration when leaching was interrupted for

12 h. This drop was probably due to molecular

diffusion from faster to slower moving water. The

dispersivities obtained from the CDE were quite large,

and variable within the replicates, again suggesting

high spatial variability. Solute transport in both

treatments was preferential, with a slightly lower

average um in the conservatively tilled soil of 51%

compared to 65% in the conventionally tilled site. This

could be due to a higher aggregation in the

conservatively tilled soil, with the formation of

immobile water pockets within aggregates. The

modified version of the MIM model, with three water

fractions, did not improve the simulation of the

experimental data much, despite the increased number

of parameters that were included.

Compacting the soil columns prior to leaching with

a load of 70 kPa did not affect solute transport in the

conservation tillage site. In the conventional-tilled

soil, however, the flow became less preferential. This

suggests a higher stability of the conservatively tilled

soil compared to the conventionally tilled site, with

respect to pore functioning and solute transport.

These results suggest that further research is

needed to better understand and to quantify the affect

of different management practices on changes in soil

structure and pore functioning with respect to both

water and solute transport.

Acknowledgements

We thank Orshi Fazeskas, Christian Albrechts

University, Kiel, for her technical help. Funding was

provided by an International Science and Technology

Linkage Fund (ISAT), a Trimble Agriculture Research

Fellowship, and by FRST (Foundation for Research

Science and Technology) Contract C06X0306.


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