the effects of wheel-induced soil compaction on anchorage strength and resistance to root lodging of...

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Soil & Tillage Research 82 (2005) 147–160

The effects of wheel-induced soil compaction on anchorage

strength and resistance to root lodging of winter barley

(Hordeum vulgare L.)

D.I. Scotta, A.R. Tamsa, P.M. Berryb, S.J. Mooneya,*

aDivision of Agriculture and Environmental Sciences, School of Biosciences,

University of Nottingham, University Park, Nottingham NG7 2RD, UKbADAS High Mowthorpe, Duggleby, Malton, North Yorkshire YO17 8BP, UK

Received 17 November 2003; received in revised form 7 June 2004; accepted 22 June 2004

Abstract

Lodging is the permanent displacement of cereal stems from the vertical. Cereal plants growing in the edge rows next to both

wheel tracks (‘tramlines’) and the gaps between experimental plots (‘inter-plot spaces’), which are traversed by farm vehicles

during planting operations and agrochemical application, are less prone to lodge than plants growing elsewhere in fields and

plots. Previous research has attributed this phenomenon to an increase in the stem strength of edge row plants, and hence their

resistance to stem lodging, resulting from reduced competition between edge row plants for resources. However, this explanation

gives no consideration to the anchorage strength of edge row plants, and hence their resistance to root lodging. Differences in soil

and plant characteristics between the edge and centre rows of plots of winter barley (Hordeum vulgare L.) were examined on

sand, silt and clay dominated soil types. Edge rows next to tramlines were investigated on the silt and clay soil types, whereas

edge rows next to inter-plot spaces were investigated on the sand soil type. Edge row plants next to both tramlines and inter-plot

spaces had 58.8% greater anchorage strength and hence resistance to root lodging than centre row plants. This was attributed to

(1) greater soil compaction in the edge rows resulting from wheel traffic in the tramlines and inter-plot spaces, which increased

the strength of the soil matrix surrounding the roots, and (2) greater plant root growth in the edge rows resulting from reduced

competition. Bulk density, root plate spread and structural rooting depth were 19, 22, and 12% greater, respectively, in the edge

rows of all soil types. The results suggest that in order to reduce lodging risk, energies should be directed towards identifying

agricultural practices that optimise soil compaction in the seedbed without causing significant limitations to root growth.

# 2004 Elsevier B.V. All rights reserved.

Keywords: Soil compaction; Root lodging; Winter barley; Bulk density; Soil strength

* Corresponding author. Tel.: +44 115 951 3199;

fax: +44 115 951 3251.

E-mail address: [email protected]

(S.J. Mooney).

0167-1987/$ – see front matter # 2004 Elsevier B.V. All rights reserved

doi:10.1016/j.still.2004.06.008

1. Introduction

The permanent displacement of cereal stems from

their upright position is known as lodging (Pinthus,

.

D.I. Scott et al. / Soil & Tillage Research 82 (2005) 147–160148

1973). Lodging can occur by stem failure (stem

lodging) (Neenan and Spencer-Smith, 1975) or

anchorage failure (root lodging) (Crook and Ennos,

1993). It has been hypothesised that root lodging may

have become the most common form in wheat due to

the advent of varieties with shorter and more rigid

stems (Crook and Ennos, 1994). This may also be the

case for other cereal species, such as barley and oats.

Lodging is a significant problem for farmers because it

causes large reductions in grain yield and quality

(Fischer and Stapper, 1987; Easson et al., 1993) and

extensive costs to the UK farming industry (Berry et

al., 1998). The spatial distribution of lodging in

commercial fields is rarely uniform, and usually

occurs in the field margins before spreading to the

centre of the field. The plants next to wheel tracks

(‘tramlines’) caused by farm vehicles during the

application of agro-chemicals in commercial fields

almost always remain standing, even when plants

within the rest of the field have lodged (Berry et al.,

1998). Similar observations have also been made

within experimental plots where the plants growing in

the edge rows next to the gaps between adjacent

experimental plots (‘inter-plot spaces’) are less prone

to lodge than others and often remain standing when

the middle of the plot lies flat (Watson and French,

1971). As a result, edge rows are usually considered

atypical and discarded from an experiment rather than

used as material for study.

The mechanism by which plants next to tramlines

in commercial fields or on the edges of experimental

plots next to inter-plot spaces have a greater lodging

resistance than other plants has never been experi-

mentally determined. The reason proposed by Watson

and French (1971) to account for the increased lodging

resistance of edge row plants is that they produce

wider and stronger stems as a result of reduced

competition for nutrients and light. However, this

explanation accounts only for an increase in stem

strength and hence resistance to stem lodging. It gives

no consideration to the anchorage strength and hence

resistance to root lodging of edge row plants, which is

influenced by the characteristics of the roots and soil.

Subsequent research has shown that reducing compe-

tition between plants can increase their resistance to

both stem and root lodging by influencing their shoot

and root growth respectively (Easson et al., 1993,

1995; Berry et al., 1998, 2000). In addition to reduced

competition, a further factor that may contribute to the

root lodging resistance of edge row plants is the

difference in physical soil conditions in the edge rows

compared to elsewhere in a plot. Both tramlines and

inter-plot spaces experience wheel traffic by farm

vehicles during planting and post-planting operations

(Voorhees, 1992). Consequently soil in, and close, to

wheel tracks is usually more compact than elsewhere

in a field or plot (Smith, 1987; Rowell, 1994). Based

on a soil compaction model, Smith (1987) predicted

that compaction caused by wheel traffic is accom-

modated by both vertical and horizontal compression

of soil thus resulting in an increase in bulk density both

beneath and beside a wheel track. Therefore, plants

growing in edge rows beside wheel tracks grow in

more compact soil than plants growing elsewhere in a

plot (Voorhees, 1992). This may further influence their

anchorage strength and hence resistance to root

lodging.

There are two main ways by which soil compaction

can influence the anchorage strength of plants. Firstly,

soil compaction affects soil strength, which is an

integral component of all anchorage models for

cereals. Depending on the mechanism of root lodging,

soil strength affects either the resistance of the root–

soil bond to failure by axial or shearing root

movements (Ennos, 1989, 1991b; Easson et al.,

1995), or the resistance of the soil matrix to failure

by rotation of the root–soil cone (Crook and Ennos,

1993; Ennos et al., 1993). Secondly, soil compaction

affects root growth and thus the ability of root systems

to provide anchorage to a plant. Roots are generally

unable to penetrate pores narrower than their own

diameter (Lampurlanes and Cantero-Martınez, 2003).

Consequently, the decrease in macroporosity caused

by soil compaction can cause mechanical impedance

and subsequently morphological changes to plant root

systems (Barley, 1962, 1963; Wilson et al., 1977;

Goodman and Ennos, 1999). Morphological changes

can include a reduction in length, an increase in

diameter, and alterations in the pattern of lateral root

initiation. Such changes may adversely affect the

ability of root systems to perform functions, including

anchorage (Voorhees, 1992). However, Goodman and

Ennos (1999) showed that the positive effects of

increased soil strength on the anchorage strength and

resistance to root lodging of sunflower (Helianthus

annuus L.) and maize (Zea mays L.) outweighed any

D.I. Scott et al. / Soil & Tillage Research 82 (2005) 147–160 149

negative effects resulting from morphological and

mechanical responses of the root systems.

The aim of this study was to examine the

differences in soil and plant characteristics between

edge rows and centre rows of plots in order to provide

an insight into the factors that may contribute to the

increased resistance of edge row plants to root

lodging. Edge rows next to both ‘tramlines’ and

‘inter-plot spaces’ were investigated. Examination of

these factors will help current understanding of the

lodging process and identification of the traits of soils

and plants that confer strong anchorage in field

conditions, which at present are poorly understood.

The study will focus on winter barley (Hordeum

vulgare L.), which despite its extensive growth in the

UK has received little attention in the literature to date

with respect to lodging.

2. Materials and methods

2.1. Field Sites and experimental design

Winter barley (cv. Pearl) was grown at Bunny,

South Nottingham (52.58N 1.38W) on a sand soil

(Newport Series), ADAS Rosemaund, Hereford

(52.18N 2.48W) on a silty clay loam soil (Bromyard

Series), and ADAS Boxworth, Cambridge (52.28N0.08W) on a clay soil (Hanslope Series) over the 2002/

2003 growing year. Particle size distribution deter-

mined by laser diffractometry (McCave and Syvitski,

1991) and organic matter content determined by loss

on ignition (Rowell, 1994) are shown for each soil

type in Table 1. Plants at each site were sown in six

plots (24 m � 1.5 m) separated by inter-plot spaces at

seeding densities of 100 and 400 seeds m�2. Each plot

had 12 rows of plants and the inter-row distance was

12.5 cm. Three replicate plots of each seeding rate

Table 1

Particle size distribution and organic matter content of the Ap

horizon at Bunny (sand), ADAS Rosemaund (silty clay loam)

and ADAS Boxworth (clay)

Sand Silty clay loam Clay

Sand (%) 91.9 10.0 30.4

Silt (%) 7.6 65.0 34.9

Clay (%) 0.5 25.0 34.7

Organic matter (%) 3.4 4.8 8.6

were laid out in a randomised block design. Plots at

each site were cultivated and sown in mid-October

2002. At Bunny, edge rows next to ‘inter-plot spaces’

were investigated. Inter-plot spaces were 0.5 m wide

and ran parallel with the direction of drilling. Each

inter-plot space received a total of two tractor passes

during and immediately after drilling in autumn. At

ADAS Boxworth and ADAS Rosemaund, edge rows

next to ‘tramlines’ used by farm vehicles to apply

agro-chemicals were investigated. Tramlines were

0.5 m wide and ran perpendicular to the direction of

drilling. Each tramline received a total of nine passes

by farm vehicles between October and May. The

tractor ground pressure was approximately 83 kPa at

each site. Edge row plants were growing within a

distance of 5 cm from the edges of the inter-plot

spaces and tramlines. At all sites, the centre rows of

plots running parallel with the direction of drilling

were investigated starting at a minimum distance of

1 m from the tramline.

2.2. Measurement of soil characteristics

Bulk density (BD), total porosity (TP), penetration

resistance (PR) and volumetric water content (VWC)

of the Ap horizon were determined for the edge row

and centre row of each replicate plot. Metal cylinders

of 7.3 cm diameter and 5.2 cm depth (218 cm3) were

used to obtain three replicate undisturbed soil

cores from the edge and centre row of each repli-

cate plot for BD and TP determination from dry

weight and volume measurements (assuming particle

density = 2.65 g cm�3) (Rowell, 1994). PR (kPa) was

measured at 3.5 cm depth increments to a maximum

depth of 14 cm at three positions along the edge row

and centre row of each replicate plot using a bush cone

penetrometer. The penetrometer cone had a 12.8 mm

diameter and 308 tip angle. Three replicate VWC

measurements were made for the edge row and centre

row of each replicate plot using a thetaprobe (Delta-T

Devices Ltd., Cambridge). Six VWC measurements

were also taken at random locations at each site using

both the thetaprobe and bulk soil samples for oven

drying for calibration purposes. In addition, undis-

turbed samples were also collected from each soil type

to permit the determination of a water release curve.

Samples were equilibrated using a combination of

sand baths and pressure membrane apparatus at matrix


potentials of 0, �5, �10, �50, �100, �500, and

�1500 kPa.

2.3. Measurement of plant characteristics

Winter barley plants were sampled between cereal

growth stage (GS) 77 and GS 87 (Tottman, 1987) at all

sites. Three plants were removed from the edge and

centre row of each replicate plot and the roots of each

plant were recovered to a depth of 10 cm. The soil

surrounding the plant roots was left intact during

transit and storage to protect the roots from damage.

Plants were stored in a cool room at 12 8C for up to 10

days prior to analysis.

2.3.1. Resistance to root lodging

Resistance to root lodging was measured in the

field for three plants at both the edge rows and centre

rows of each replicate plot using a specially designed

lodging instrument based on that described by Ennos

(1991b). The lodging instrument consisted of a

Mecmesin Smart torque cell (maximum torque

3 N m) fitted with a lodging attachment comprising

a metal rod and a handle capable of rotating both the

torque cell and lodging attachment up to 1358 from the

vertical. The lodging attachment applied a force to a

plant at a height of 105 mm from the plant base, with a

centre of rotation at 55 mm from the plant base.

Rotation of the lodging attachment by 1358 displaced

the plant to 618 from the vertical. As the plant was

rotated, the maximum torque (N m) required to

overturn the plant (‘maximum torque resistance’)

was recorded using a Mecmesin Advanced Force/

Torque Indicator (AFTI) (maximum load 250 N)

(Mecmesin Ltd., Slinfold). Each test took approxi-

mately 60 s equating to a rotational velocity of

618 min�1.

2.3.2. Shoot and root measurements

The number of fertile shoots per plant was

recorded. Plant root systems were soaked in water

so that the soil could be removed without damaging

the anchorage roots. Root plate spread (mm) and

structural rooting depth (mm) were recorded for each

plant using the rhizosheath method described by Berry

et al. (2000) for winter wheat. Individual roots

emanating from the stem base of each plant were

removed and stored in water at 12 8C for up to 10 days

prior to image analysis using WinRHIZO (Regent

Instruments Inc., Quebec). Seminal roots emanating

from the seed were not included, as these play no part

in anchorage of the mature plant (Ennos, 1991a).

Washed roots were spread out in about 10 mm depth of

water in a scanner tray. A two-D scanner was used to

acquire a digital image of the roots from which root

length was measured as a function of root diameter.

Ten root diameter classes in 0.5 mm increments were

defined ranging from <0.5 mm to >4.5 mm.

2.4. Statistical analysis

Analysis of variance (ANOVA) procedures for a

fully randomised split-split plot design were used

within Genstat 6 software (Lane and Payne, 1996) to

test for significant differences between treatments and

to calculate the standard errors of the differences

(S.E.D.s). Soil type formed the main plots, seeding

rate formed the sub-plots and row (edge or centre)

formed the sub-sub plots.

3. Results

3.1. Soil characteristics

Bulk density (BD) in the upper 5 cm of the Ap

horizon was significantly greater (P < 0.001) in the

edge rows of plots than in the centre rows (Table 2).

The difference in mean BD between rows for all soil

types was 0.23 g cm�3 indicating that the mean BD in

the edge rows of plots was 18.5% greater than in the

centre rows as a result of wheel traffic in the inter-plot

spaces. The largest difference in BD between rows

was observed in the silty clay loam (0.34 g cm�3) and

the smallest in the sand (0.14 g cm�3) indicating BD

increases in the edge rows of 26.9 and 10.8%

respectively. Similarly, there were significant differ-

ences between the edge and centre of rows for TP,

ranging between 0.13 in the silty clay loam and 0.06 in

the sandy soil. Field measured VWC was significantly

higher (P < 0.01) in the edge rows than in the centre

rows of plots in both the sand and clay, however the

opposite was observed in the silty clay loam (Table 2).

The mean difference in VWC between rows for all soil

types was small (0.01 cm3 cm�3) with the largest

difference in the clay (0.03 cm3 cm�3) and the


Table 2

Bulk density (BD), total porosity (TP) and volumetric water content (VWC) of the upper 5 cm for the centre and edge rows of plots in different

soil types

Row BD (g cm�3) TP VWC (cm3 cm�3)

Centre Edge Centre Edge Centre Edge

Sand 1.31 1.45 0.51 0.45 0.12 0.13

Silty clay loam 1.27 1.61 0.52 0.39 0.20 0.19

Clay 1.12 1.31 0.58 0.51 0.38 0.41

P-values

Soil type (2 d.f.) P < 0.001 P < 0.001 P < 0.001

Row (1 d.f.) P < 0.001 P < 0.001 P < 0.01

Soil type � Row (2 d.f.) P < 0.01 P < 0.01 P < 0.001

S.E.D.s

Soil type 0.017 (4 d.f.) 0.007 (4 d.f.) 0.003 (4 d.f.)

Row 0.021 (12 d.f.) 0.008 (12 d.f.) 0.003 (12 d.f.)

Soil type � Row 0.031 (16 d.f.) 0.01 (16 d.f.) 0.005 (11 d.f.)

smallest in the sand (0.01 cm3 cm�3). During sam-

pling, the clay soil type was near saturation and the

sand was close to field capacity. In contrast, the VWC

of the silty clay loam was at less than field capacity.

Penetration resistance (PR) measurements recorded in

the Ap horizon (0–14 cm depth) were significantly

greater (P < 0.001) in the edge rows of plots than in

the centre rows (Fig. 1). This was observed in each soil

type throughout the depth profile, with the exception

of the upper 3.5 cm depth and at 14 cm depth in the

sand soil type where PR was slightly greater in the

centre rows (Fig. 1a). The difference in mean PR

between rows for the whole depth profile of all soil

types was large (10.3 kPa) with the greatest difference

in the silty clay loam (22.3 kPa) and the smallest in the

sand (0.69 kPa). PR increased significantly with depth

in each row and soil type (Fig. 1). The differences in

PR between rows were greatest at depths of 7 cm in the

silty clay loam (Fig. 1b) and clay (Fig. 1c) soil types,

and at 10 cm in the sand soil type (Fig. 1a). The

smallest differences in PR between rows occurred at

depths of 3.5 cm in the sand (Fig. 1a) and silty clay

loam (Fig. 1b) and at 14 cm in the clay (Fig. 1c). There

were no significant differences in soil characteristics

between plots sown at different seeding rates.

The gross differences in the porous environments

of the three soils can be seen from their water release

characteristics (Fig. 2). At saturation, the clay soil

clearly had the greatest water retention

(0.54 cm3 cm�3), whereas the sand and silty clay

loam behaved similarly (ca. 0.45 cm3 cm�3). These

differences were still evident at field capacity

(�5 kPa) which illustrated significantly more water

was retained in the clay soil in comparison to the silty

clay loam and sand soil. As expected, the loss of water

between field capacity and permanent wilting point

(�1500 kPa) was considerably greater in the sand soil

(water loss = 0.24 cm3 cm�3), in contrast to the clay

and silty clay loam soils, which had water losses of

0.12 and 0.13 cm3 cm�3 respectively.

3.2. Plant characteristics

The proportion of seeds drilled that established

plants was always greater at the low seed rate. The

percentage establishment for the low and high seed

rates respectively were 96 and 80% at Bunny, 78 and

65% at ADAS Rosemaund and 52 and 36% at ADAS

Boxworth. Previous seed rate experiments have also

observed lower establishment for high seed rates

(Whaley et al., 2000).

3.2.1. Resistance to root lodging

During rotation of plants from 08 to 618 from the

vertical, movement centred around the base of the

plant with the root system moving through the soil and

there was only limited bending of the stems. On

removal of the force applied by the lodging instru-

ment, plants recovered somewhat but still leaned at an

angle of ca. 20–308 from the vertical indicating that

anchorage failure had occurred. Plants growing in the

edge rows of plots had a significantly higher


Fig. 1. Penetration resistance (kPa) measured at 3.5 cm depth increments in the centre rows and edge rows of plots in the sand (a), silty clay loam

(b), and clay (c) soil types. Error bars indicate �1 standard error of the mean.

(P < 0.001) mean maximum torque resistance

(2.329 N m) than plants growing in the centre rows

(1.370 N m). The maximum torque resistance for

plants was not significantly different across soil types

and seeding rates. There was a significant positive

relationship between BD (and TP) and maximum

torque resistance of plants (P < 0.001), with a linear

regression accounting for 45, 72, and 74% of variation

between the parameters in the sand, silty clay loam

and clay soil types respectively (Fig. 3). Furthermore,

there was a significant positive relationship between

PR at 7 cm depth and maximum torque resistance of

plants (P < 0.001), with a linear regression account-

ing for 72 and 65% of variation between the

parameters in the silty clay loam and clay soil types

respectively (Fig. 4). However, there was no positive

relationship between these parameters in the sand soil

type.

3.2.2. Shoot and root measurements

Plants growing in the edge rows of plots had a

significantly greater number of shoots (P < 0.001)


Fig. 2. Water release curve of the three soil types examined.

than those growing in the centre rows (Table 3). Also,

plants established at 100 seeds m�2 had a significantly

greater number of shoots (P < 0.01) than those

established at 400 seeds m�2 (Table 3). The difference

in shoot numbers per plant between rows was

significantly different between seeding rates

(P < 0.05), with the largest difference between the

edge and centre rows occurring at 100 seeds m�2

(Table 3). Shoot numbers per plant were also

significantly different between soil types (P < 0.05),

with the greatest number of shoots recorded for plants

growing in the silty clay loam soil type, and the

smallest number of shoots recorded for plants in the

sand soil type (Table 3). Each plant had a large number

of adventitious roots emanating from the base of the

stems and extending outwards before bending down-

wards to form an inverted cone. Each adventitious root

had a thick basal region with a dense covering of root

hairs attached to a rhizosheath of soil. The distal

portions of the adventitious roots were well branched

with large numbers of fibrous roots. Plants growing in

the edge rows of plots had a significantly greater root

plate spread (P < 0.001) and structural rooting depth

(P < 0.001) than plants growing in the centre rows

(Table 4). Both root plate spread and structural rooting

depth were also significantly greater for plants

established at 100 seeds m�2 compared to those

established at 400 seeds m�2 (P < 0.01), and for

plants growing in the clay soil type, compared to those

growing in the sand and silty clay loam soil types

(P < 0.01) (Table 4). The differences in root plate

spread and structural rooting depth between rows were

consistent across the soil types and seeding rates.

Total root length per plant increased with an

increase in shoot numbers per plant in each soil type

and seeding rate (Table 3). Plants growing in the edge

rows of plots had a significantly greater (P < 0.001)

total root length than plants growing in the centre rows

(Table 3). Total root length was also significantly

greater for plants established at 100 seeds m�2

compared to those established at 400 seeds m�2

(P < 0.001), and for plants growing in the clay soil

type, compared to those growing in the sand and silty

clay loam soil types (P < 0.05) (Table 3). The

differences in total root length per plant between

rows were consistent across seeding rates and soil

types. The diameter of plant roots ranged between

<0.5 mm to >4.5 mm, however only very short root

lengths were recorded in diameter classes >2 mm and

so these were combined to form a single diameter

class. Edge row plants had a significantly greater root

length in each diameter class (P < 0.001) than centre

row plants. An exception to this was observed for the

length of roots with a diameter >2 mm in the clay soil

type (Fig. 5c). The differences in root length in each

diameter class between rows were consistent across

seeding rates, however significantly different between

soil types (P < 0.001) (Fig. 3). The differences in root

length in each diameter class between rows were

larger in the sand (Fig. 5a) and silty clay loam (Fig. 5b)


Fig. 3. Relationships between bulk density (g cm�3) and maximum torque of plants (N m) in the (a) sand (y = �2.69 + 3.45x; R2 = 0.45), (b)

silty clay loam (y = �3.16 + 3.45x; R2 = 0.72), and (c) clay (y = �2.49 + 3.45x; R2 = 0.74) soil types.

soil types than in the clay soil type (Fig. 5c). The

greatest differences in root length between rows in

each soil type were observed for roots in the diameter

classes ranging 0–1.5 mm, whereas much smaller

differences were observed for roots with a diameter

>1.5 mm (Fig. 5). Plants growing in the edge rows had

a significantly smaller mean root diameter than centre

row plants (P < 0.001) (Table 3), reflecting their

greater proportion of thin roots (Fig. 3). Mean root

diameter was significantly different between soil types

(P < 0.001), with the largest values recorded in the

silty clay loam and the smallest in the sand (Table 3).

The differences in mean root diameter between rows

were also significantly larger in the silty clay loam

compared to the sand and clay soil types (P < 0.001)

(Table 3).

4. Discussion

The measurements of maximum torque resistance

indicated that plants growing in the edge rows of plots

had greater anchorage strength and hence resistance to

root lodging than plants growing in the centre rows.


Fig. 4. Relationships between penetration resistance (kPa) measured at 7 cm depth and maximum torque of plants (N m) in the (a) silty clay

loam (y = 0.17 + 0.04x; R2 = 0.72), and (b) clay (y = 0.59 + 0.04x; R2 = 0.65) soil types.

Table 3

Shoot numbers per plant, total root length and mean root diameter for plants with different treatments of soil type, seed rate and row

Treatment Shoots per plant Total root length (cm) Mean root diameter

(mm)

Centre Edge Centre Edge Centre Edge

Sand 4.3 7.5 366 637 0.61 0.59

Silty clay loam 7.2 13.6 299 565 1.30 1.02

Clay 7.1 9.8 758 861 0.77 0.74

400 Seeds m�2 4.5 7.3 306 437 0.88 0.80

100 Seeds m�2 7.9 13.3 644 938 0.90 0.77

P-values

Soil type (2 d.f.) P < 0.05 P < 0.05 P < 0.001

Seed rate (1 d.f.) P < 0.01 P < 0.001 NS

Row (1 d.f.) P < 0.001 P < 0.001 P < 0.001

Soil type � row (2 d.f.) NS NS P < 0.001

Seed rate � row (1 d.f.) P < 0.05 NS NS

Soil type � seed rate � row (2 d.f.) NS NS NS

S.E.D.s

Soil type (4 d.f.) 0.77 66.4 0.048

Seed rate (6 d.f.) 1.00 149.5 0.031

Row (12 d.f.) 0.60 39.0 0.021

Soil type � row 1.06 (11 d.f.) 81.8 (9 d.f.) 0.054 (6 d.f.)

Seed rate � row 1.16 (10 d.f.) 154.5 (7 d.f.) 0.037 (11 d.f.)

Soil type � seed rate � row 1.78 (18 d.f.) 206.1 (9 d.f.) 0.071 (14 d.f.)


Table 4

Root plate spread and structural rooting depth for plants with

different treatments of soil type, seed rate and row

Treatment Root plate

spread (mm)

Structural rooting

depth (mm)

Sand 37.6 37.4

Silty clay loam 40.9 44.9

Clay 52.6 53.2

400 Seeds m�2 39.4 40.9

100 Seeds m�2 48.0 49.5

Centre Row 39.5 42.6

Edge Row 48.0 47.8

P-values

Soil type (2 d.f.) P < 0.01 P < 0.01

Seed rate (1 d.f.) P < 0.01 P < 0.01

Row (1 d.f.) P < 0.001 P < 0.01

Soil type � seed

rate � row (2 d.f.)

NS NS

S.E.D.s

Soil type (4 d.f.) 2.37 2.56

Seed rate (6 d.f.) 2.20 1.74

Row (12 d.f.) 1.87 1.55

Soil type � seed rate � row 4.83 (21 d.f.) 4.27 (18 d.f.)

An examination of the differences in soil and plant

characteristics between rows suggests that there were

two main differences contributing to this increased

anchorage strength. These include (1) greater BD and

PR, and reduced TP in the edge rows resulting from

compaction, and (2) greater plant root growth in the

edge rows resulting from reduced competition. The

extent to which increased BD and PR contributed to the

enhanced anchorage strength of edge row plants may

have been greater in the edge rows next to tramlines than

those next to inter-plot spaces. This is indicated by the

smaller differences in BD, TP and PR between rows at

Bunny, where edge rows next to inter-plot spaces were

investigated, than at ADAS Boxworth and ADAS

Rosemaund, where edge rows next to tramlines were

investigated, despite the difference in anchorage

strength between rows remaining the same across all

sites. Importantly, the results indicate that the greater

lodging resistance of edge row plants is caused by

greater anchorage in addition to greater stem strength as

suggested by Watson and French (1971).

The greater BD and PR in the edge rows is

attributed to compaction caused by wheel traffic in the

tramlines and inter-plot spaces during planting

operations and agro-chemical application, which

causes horizontal compression of soil either side of

the wheel track (Smith, 1987; Rowell, 1994). The

greater differences in BD and PR between rows where

edge rows next to tramlines were investigated, than

between rows where edge rows next to inter-plot

spaces were investigated, can be attributed to the seven

additional vehicle passes received by the tramlines

compared to the inter-plot spaces.

Compaction decreases the volume, size and

continuity of pores in the soil (Hillel, 1998). The

water release data (Fig. 2) supports the BD and TP

results with respect to the differences described

between the soil types. The main difference between

the three soil types was observed at high tensions

(micropores) where the sandy soil retained signifi-

cantly less water than the clay and silty clay loam

soils. This suggests that in addition to number of

tractor passes, the porous architecture of a given soil

texture has an important role in determining the nature

of the compaction mechanism since the soil types with

the greatest differences were also those with the

highest percentage of micropores (clay from ADAS

Boxworth and silty clay loam from ADAS Rose-

maund).

Importantly, changes in porosity resulting from

compaction increase the degree of contact between

plant roots and soil (Lampurlanes and Cantero-

Martınez, 2003). A reduction in pore size and

continuity increases the probability that plant roots

will encounter and penetrate soil aggregates thus

creating new root channels in which they will have

complete contact with the surrounding soil matrix

(Kooistra et al., 1992). An increase in root–soil contact

in the edge rows caused by soil compaction is likely to

increase the strength of the root–soil bond, which is

considered to be an important point of failure during

root lodging (Ennos, 1989, 1990; Easson et al., 1995).

Whilst the degree of root–soil contact was not

measured in this study, the positive relationships

between BD and maximum torque of plants in each

soil type (Fig. 3) suggest that this may be the case.

The differences in PR between edge and centre

rows indicate that wheel traffic influenced the soil

conditions in the edge rows to a depth of at least

14 cm. These differences were greatest at depths of

either 7 or 10.5 cm in each soil type (Fig. 1), which

correspond closely with the structural rooting depth of

plants growing in the edge rows (Table 4). This


Fig. 5. Root length as a function of diameter for plants growing in the centre rows and edge rows of experimental field trial plots in the sand (a),

silty clay loam (b), and clay (c) soil types. Error bars indicate �1 standard error of the mean.

indicates that the soil matrix beneath the root plate of

plants growing in the edge rows had higher shear

strength than in the centre rows (Hillel, 1998). The

mechanism of root lodging in winter wheat (Crook

and Ennos, 1993) and maize (Ennos et al., 1993)

involves the rotation of a cone of rigid coronal roots at

its windward edge below the soil surface, the soil

inside the cone moving as a block and compressing the

soil beneath. If the mechanism of root lodging in

winter barley is similar to that of winter wheat and

maize, the stronger soil beneath the root plate of edge

row plants will be more resistant to plastic deforma-

tion under compression by the root–soil cone, hence

increasing anchorage strength. Evidence of this is

provided by the positive relationships between PR at

7 cm depth and maximum torque resistance of plants

in the silty clay loam and clay soil types (Fig. 4). The

absence of a positive relationship between the two

parameters in the sand soil type is probably due to the

small difference in PR between the edge and centre

rows at this site (Fig. 1). This reflects the smaller

number of vehicle passes received by the inter-plot

spaces (two vehicle passes) compared to the tramlines

(9 vehicle passes).


The greater number of shoots recorded for edge row

plants (Table 3) has previously been attributed to their

ability to obtain more nitrogen from the soil as a result

of reduced competition (Watson and French, 1971).

However, enhanced shoot growth in the edge rows is

unlikely to contribute directly to the increased lodging

resistance of edge row plants. This is because greater

shoot numbers per plant increase the leverage force

that the aerial parts of the plant exert on the plant base

to cause lodging (Berry et al., 2000). Instead,

increased shoot growth may indirectly increase the

lodging resistance of edge row plants by increasing the

number of roots and hence anchorage strength

(Pinthus, 1973). This is indicated by the close

relationship between shoot numbers per plant and

total root length observed in this study (Table 3).

Importantly, the greater root length in each diameter

class in the edge rows (Fig. 5) indicated that the extent

of soil compaction in the root zone was not sufficient

to limit the growth of winter barley roots. Conse-

quently, edge row plants were able to establish

extensive, adventitious root systems despite growing

in more compact soil. This is supported by research

into minimal and no tillage systems, which has shown

that plant roots can continue to extend into moderately

compacted soil that has received little or no

cultivation. Finney and Knight (1973) demonstrated

that direct drilling into uncultivated, and thus more

compact soil, had no effect on the number, elongation

and diameter of adventitious roots of winter wheat

compared to plants grown in loose ploughed soil.

Similarly, Crook (1994) showed that compaction of

the seedbed did not affect the length, number and

bending strength of the adventitious roots of winter

wheat. According to Lampurlanes and Cantero-

Martınez (2003), the unrestricted extension of plant

roots in moderately compact soil reflects their ability

to grow in the inter-aggregate spaces, provided that the

soil is reasonably well structured or has preserved

biochannels such as in non-tilled soils.

The use of the rhizosheath as an indicator of root

plate spread and structural rooting depth, as described

by Berry et al. (2000) for winter wheat, also proved

suitable for winter barley. The rhizosheath for winter

barley roots was more difficult to identify than

suggested by Berry et al. (2000) for winter wheat due

to the greater lateral branching of the adventitious

roots of winter barley with fibrous roots. However,

most roots of individual plants had a similar length of

rhizosheath, thus reducing the uncertainty in the

measurements. If the anchorage model developed by

Crook and Ennos (1993) for winter wheat is

appropriate for winter barley, the greater root plate

spread of edge row plants suggests that they have a

larger resisting moment to overturning than centre row

plants. Furthermore, the greater structural rooting

depth of edge row plants suggests that they require

more rainfall to reduce the strength of the soil

surrounding their anchorage roots than centre row

plants (Baker et al., 1998). The significantly greater

values recorded for root plate spread and structural

rooting depths in the clay soil type were probably

caused by the low proportion of plants established at

ADAS Boxworth. Edge row plants also had 98.6, 74.8

and 16.7% greater lengths of roots with a diameter of

0.5 mm or more than centre row plants in the sand,

silty clay loam, and clay soil types respectively. It has

been shown that an increase in the diameter of winter

wheat roots increases both their bending strength

(Crook and Ennos, 1993) and tensile strength (Easson

et al., 1995). Whilst these mechanical properties were

not measured in this study for winter barley roots, it is

expected that edge row plants have a higher proportion

of stronger roots than centre row plants reflecting their

greater proportion of thicker roots. This has been

shown to be an important characteristic of root

systems increasing anchorage strength for a variety of

plant species (Ennos, 1989, 1991b; Ennos et al., 1993;

Crook and Ennos, 1993).

Reduced seeding rate and subsequent reduced

competition between plants increased both shoot and

root growth of winter barley as reported by other

authors for winter wheat (Easson et al., 1993, 1995;

Berry et al., 2000). Surprisingly, reducing the seeding

rate had no significant effect on torque measurements

and hence anchorage strength. This may reflect an

increase in the variability of torque measurements

between seeding rates due to the inclusion of

measurements taken for plants growing in the edge

rows of plots.

5. Conclusion

Plants growing next to tramlines used by farm

vehicles during the application of agro-chemicals in


commercial fields almost always remain upright, even

when the rest of the field has lodged. Similar patterns

have also been observed within experimental plots

where plants growing in the edge rows next to inter-

plot spaces used by farm vehicles during planting

operations are less prone to lodge than plants within

the middle of the plot. Previous research has attributed

the increased lodging resistance of edge row plants

solely to an increase in stem strength resulting from

reduced competition between edge row plants for

nutrients and light. This study provides evidence that

the increased lodging resistance of winter barley

(Hordeum vulgare L.) growing in the edge rows of

plots, 5 cm from the edges of both tramlines and inter-

plot spaces, is also likely to reflect an increase in

anchorage strength caused by two contributing factors.

These factors include (1) greater soil compaction in

the edge rows resulting from wheel traffic in the

tramlines and inter-plot spaces, and (2) greater plant

root growth in the edge rows resulting from reduced

competition. Importantly, the extent of soil compac-

tion recorded in the edge rows next to both tramlines

and inter-plot spaces was moderate and insufficient to

cause significant limitations to root growth and hence

anchorage strength. Instead, soil compaction is

thought to have increased the anchorage strength of

edge row plants by increasing the strength of the soil

matrix surrounding the anchorage roots. The extent to

which soil compaction contributed to the enhanced

anchorage strength of edge row plants may have been

greater in the edge rows next to tramlines than in the

edge rows next to inter-plot spaces. This was attributed

to the greater number of vehicle passes received by the

tramlines (nine vehicle passes) compared to the inter-

plot spaces (two vehicle passes). This study indicates

that in order to reduce lodging risk, energies should be

directed towards identifying more suitable agricultural

practices that can optimise soil compaction in the

seedbed without causing significant limitations to root

growth. Such practices may include minimum or no

tillage methods, as opposed to traditional tillage

methods that involve loosening of the seedbed to

minimise soil compaction. Also, rolling the soil after

drilling or any time up to the beginning of stem

extension has been shown to increase soil shear

strength (Berry et al., 2002). Furthermore, this study

provides experimental data to complement current

research into modelling of the lodging process (Baker

et al., 1998; Berry et al., 1998), which is being used to

identify the relative importance of different soil and

plant characteristics in affecting the risk of lodging in

field conditions.

Acknowledgments

The use of the field sites at ADAS Boxworth and

Rosemaund is gratefully acknowledged. The technical

assistance of Melanie King from the University of

Nottingham is also duly acknowledged.

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