Annals of Botany 83 : 293–302, 1999
Article No. anbo.1998.0822, available online at http:\\www.idealibrary.com on
The Effects of Soil Bulk Density on the Morphology and Anchorage Mechanics
of the Root Systems of Sunflower and Maize
A. M. G O O D M A N* and A. R. E N N O S
School of Biological Sciences, 3.614 Stopford Building, UniŠersity of Manchester, Oxford Road,
Manchester M13 9PT, UK
Received : 4 September 1998
Returned for revision : 23 October 1998
Accepted : 22 November 1998
The effects of soil bulk density and hence strength on two contrasting species of herbaceous annuals, the dicot
sunflower (Helianthus annuus L.) and the monocot maize (Zea mays L.), were investigated by comparing the
morphology and mechanics of field-grown plants in soil with a low and high bulk density. Soil with a low bulk density
had a significantly lower penetration resistance (118p4n4 kPa) than the high bulk density soil (325p12n2 kPa ; P
0n0001). Soil strength affected shoot and root systems of both species but had no significant effect on shoot height.
In both species roots were thicker closer to the stem base in strong soil compared to those in weaker soil. Sunflower
tap-roots growing in strong soil tapered more rapidly than those in weak soil. Only in maize, however, were roots
growing in weak soil stiffer than those in strong soil. Despite only small absolute differences in the penetration
resistance of the soil both species growing in strong soil had greater anchorage strength than those in weak soil. As
a consequence more plants in weak soil lodged compared with those growing in strong soil. This study shows that
plants can, to a small extent, respond to changes in soil strength, but that changes do not appear to compensate fully
for alterations in soil conditions. Furthermore it may be possible, by manipulating soil strength, to control lodging.
# 1999 Annals of Botany Company
Key words : Roots, compaction, soil strength, anchorage mechanics, bulk density, thigmomorphogenesis, lodging,
Helianthus annuus L., Zea mays L.
I N T R O D U C T I ON
It is well known that soil bulk density and strength are
important factors affecting both shoot and root growth of
plants. Areas of compact soil with a high shear strength can
be caused by agricultural machinery, and consequently
numerous studies have investigated the effects of soil bulk
density and strength on plant shoot and root growth
(Barley, 1963 ; Barley and Greacen, 1967 ; Goss, 1977 ;
Masle and Passioura, 1987 ; Atwell, 1988 ; Assaeed et al.,
1990 ; Materechera, Dexter and Alston, 1991).
Previous studies on the effects of strong soil on shoot
growth have produced conflicting results. In many studies,
both the height and weight of shoots were reduced in
strong soils when compared to those grown in weak soils
(Chaudhary and Prihar, 1974 ; Masle and Passioura, 1987 ;
Atwell, 1990 ; Lowery and Schuler, 1991). However, in other
studies shoot growth was not affected by soil strength
(Kirkegaard, So and Troedson, 1992 ; Oussible, Crookston
and Larson, 1992) and was even promoted in strong soils
compared with weak soils (Iijima et al., 1991).
The results of investigations into the influence of soil
strength on root growth are more consistent. In strong soils
there is a reduction in the elongation rate of roots (Barley,
1962, 1963 ; Goss, 1977). Barley (1962) and Goss (1977)
simulated high soil strength by applying radial pressure to
roots growing in ballotini ; the elongation rate fell sharply
* Present address : De Montfort University Lincoln, School of Agriculture, Lindsey Centre, Riseholme, Lincoln LN2 2LG.
0305-7364\99\030293j10 $30.00\0
when mechanical impedance was increased. Bengough and
Young (1993) showed that the daily elongation rate of pea
roots which were growing through a high bulk density soil
(1n4 Mg m−$) was only about 65 % of that of roots which
were growing through the weaker low bulk density soil
(0n85 Mg m−$). Not only is there a reduction in elongation
growth of roots but this is accompanied by an increase in
the diameter of roots (Barley, 1965 ; Atwell, 1988) and also
changes in the pattern of lateral root initiation (Russell,
1977 ; Tsegaye and Mullins, 1994). An increase in the
diameter of roots in response to high soil strength is well
documented in many species (Materechera et al., 1991)
including maize (Barley, 1963), cotton (Gossypium hirsutum
L.), peanuts (Arachis hypogea L.) (Taylor and Ratliff, 1969)
and barley (Hordeum Šulgare L.) (Wilson and Robards,
1977). In lupins (Lupinus angustifolius), the thickening of
roots is caused by an increase in the diameter of each cell
and is associated with a small reduction in cell length
(Atwell, 1988).
Roots growing in soil experience mechanical stress to
varying extents. When roots grow through pores of
insufficient diameter the root tip deforms the soil and
hence experiences mechanical stress. Indeed, Bengough,
Mackenzie and Elangwe (1994) showed that when a
compressive force is applied to a seedling pea (Pisum
satiŠum L.) root there is a stress response. Root elongation
rate decreased by 50 % within 30 min, a smaller increase in
growth rate occurred when the force was removed. Root
growth does not return to normal as soon as the stress is
removed and there appears to be a lag-phase. In the roots of
# 1999 Annals of Botany Company
294
Goodman and Ennos—Soil Strength and Root Mechanics
pea seedlings growing out of a strong layer of soil into weak
soil, the growth rate of roots did not fully recover until
4–10 d later (Bengough and Young, 1993). Similar effects
were found by Goss and Russell (1980) who studied the
effects of releasing the confining pressure from whole root
systems of barley grown in ballotini. This suggests that
changes in root morphology may be due to increases in the
mechanical stresses each root experiences. This delay in the
recovery of elongation rates is a similar response to that
found in stems of mechanically-stimulated beans (Phaseolus
Šulgaris L.) by Jaffe (1973).
Despite the wide coverage of the effects of high soil
strength on plant growth in the literature, most studies have
concentrated on the primary growth of the root system
(Pearson, Ratliff and Taylor, 1970 ; Goss, 1977 ; Kirkegaard
et al., 1992 ; Kaspar, Logsdon and Prieksat, 1995). Only one
study has investigated the effects of soil strength on
anchorage mechanics of seedlings (Ennos, 1990), and one
the effects on the mechanics of the anchorage systems of
mature plants (Crook, 1994). Ennos (1990) suggested that
roots of leek seedlings (Allium porrum L.) grown in a weaker
soil should provide less anchorage force per unit length than
those grown in a stronger soil. In contrast, Crook (1994)
showed that coronal root development in wheat was
unaffected by soil strength ; the length, number and bending
strength were the same regardless of whether the seedbed
had been loosened or compacted.
Recent work which has investigated the effects of
mechanical stress on the roots of sunflower and maize
showed that not only are root systems able to respond
locally to mechanical stimulation but that this leads to an
overall increase in anchorage strength (Goodman and
Ennos, 1996, 1997 a, b), though different species responded
in rather different ways. Materechera et al. (1991) examined
the effects of increased soil strength on 22 monocot and
dicot species ; the dicots showed less effect of increased soil
strength on elongation rates of roots than the monocots.
In weak soil anchorage strength is reduced and stresses
might be expected to be transmitted further down the roots.
These stresses might stimulate more secondary growth of
roots further from the stem, a response which would
increase the anchorage strength and so, to some extent,
compensate for the weakness of the soil. This study examines
the effect of altering soil strength by changing its bulk
density on the morphology and mechanics of the major
anchorage roots and hence on the anchorage strength of
plants. These results may help determine the extent to which
plants compensate for changes in soil strength and how
changes in the cultivation of crops might reduce losses due
to uprooting or lodging.
the peat, gravel and sandy loam were thoroughly mixed to
produce a total weight of 600 kg of soil. This was mixed
with 1n4 kg of John Innes base and 0n3 kg of lime and left to
equilibrate for 2 d in covered dustbins. Fifteen soil samples
with a fresh weight of approx. 45 g each were randomly
collected at a depth of 20–30 cm. The moisture content was
calculated by taking the difference in sample weight before
and after oven drying for 4 d at 80 mC.
Pots were packed to a wet bulk density of 1n0 Mg m−$ for
weak, and 1n4 Mg m−$ for strong soil at a gravimetric
moisture content of 14n1p0n34 %. Sunflowers were grown
in 7n5 l (top diameter 25 cm, depth 21 cm) tapered pots, and
maize was grown in 5 l (top diameter 21 cm, depth 19 cm)
tapered pots. Soils of low strength were prepared by adding
a fraction of soil (weighed to the nearest gram) and then
tamping the pot once on a hard surface creating a single
4 cm layer of soil. This was repeated until there were six
layers in the 7n5 l pots and five layers in the 5 l pots. For the
strong soil treatment each layer was compacted to 4 cm by
releasing a 5 kg ram of diameter equal to the internal basal
diameter of the pot from a height of 30 cm onto the soil ten
times.
In addition 20 pots without plants growing in them were
placed at random in the trial ; half the pots contained weak
soil and half strong soil. These pots were split equally
between the two trials ; ten pots were placed at random in
the sunflower area and ten in the maize area.
Plant establishment
Six hundred seeds of Helianthus annuus L. (‘ Vincent ’)
were germinated in moist vermiculite in July 1996 in a
glasshouse. After 3 d 64 seedlings were transplanted singly
into 7n5 l pots, half into pots containing weak soil and half
into pots containing strong soil. Early transfer ensured that
the tap root would be unrestricted and undergo normal
growth (Ennos, Crook and Grimshaw, 1993 b). The sunflower radicle, approx. 1n5 cm long, was placed in a 2 cm
deep tapered hole (8 mm maximum diameter) made centrally
in the top layer of soil.
The same number of Zea mays L. (‘ Lg 20–80 ’) seeds were
germinated in moist Fisons F2 compost and, after 4 d, 64
seedlings were transplanted singly into 5 l pots, half into
pots containing weak soil and half into pots containing
strong soil. Maize seedlings were planted in a similar way to
sunflower seedlings, but using a 20 mm diameter hole.
Loose soil was then placed around the seedling.
All pots were placed on saucers in a glasshouse at the
University of Manchester’s experimental grounds. The
saucers were filled daily with water and a 16 h photoperiod
was maintained with sodium lights supplementing natural
daylight as required.
MATERIALS AND METHODS
Soil characteristics and core preparation
Field site and experimental design
A John Innes No. 1 growth medium was prepared using
sandy loam topsoil (0–10 cm), peat and sand ( 2 mm
diameter) in a ratio of 7 : 3 : 2 (v\v\v). The sandy loam was
sieved through a 10 mm sieve, heated for 75 min to a
maximum temperature of 100 mC, and left to cool. After 1 d
An area of bare loam soil at the University of Manchester
Firs experimental ground was prepared in April 1996 by
excavating holes 0n5 m apart and approx. 25 cm diameteri
25 cm deep. The site was sheltered with a prevailing south
westerly wind.
Goodman and Ennos—Soil Strength and Root Mechanics
After 10 d, when the seedlings were at the two leaf stage,
pots from the glasshouse were randomly placed into holes in
the trial area with the top of the pot level with the soil
surface. Both trials were arranged in a randomized twin
block design of 64 plants (excluding guard plants). Each
trial was protected by sowing two guard rows of the same
species around the perimeter.
295
first three nodes and four internodes was removed for
mechanical testing. Fresh and dry weights of the shoot
systems of each plant were also measured : the stem, leaves
and reproductive parts being weighed separately for both
species.
Root morphology
Trail management
Metaldehyde slug pellets were broadcast at 15 kg ha−"
across the whole trial to control slugs and snails. Weeds
were controlled as required by hand, and the area was
irrigated during dry spells with a rotary sprayer to prevent
water stress.
Sunflower and maize plants were randomly harvested 15
and 16 weeks, respectively, after planting [maize kernels
at the milk stage (GS 73 ; Tottman and Broad, 1987) ;
sunflower achenes fully expanded]. Half the plants were
randomly selected for morphological examination.
Measurement at harŠest
Soil strength measurements. Before testing the strength of
the soil it was brought to approx. field capacity by watering
to saturation over 3 d, and allowing it to drain under gravity
for 48 h. Two penetrometer and two shear tests were carried
out at random points within a zone at least 3 cm from the
edge of the core.
Shear strength was measured using a 33 mm (Pilcon DR
2645 ; Pilcon Engineering Ltd) shear vane. The vane was
pressed into the soil to a depth of 5 cm and was slowly
rotated. Readings of shear strength in kPa were indicated
on a dial.
The soil in each unplanted pot was also tested twice with
a penetrometer using a 6 mm diameter 60m semi-angle probe
attached to a Mecmesin portable force indicator (Mecmesin
Ltd, Broadbridge Heath, West Sussex, UK). This was
pushed into the soil to a depth of 10 cm and the maximum
force recorded in Newtons (unfortunately the force could
not be recorded as a function of depth). Penetrometer
resistance, Q, is defined in eqn (1), where F is the force
required to push the penetrometer probe through the soil,
and A is the cross-sectional area of the penetrometer cone
(Bengough and Mullins, 1990).
Q l F\A
(1)
Stem morphology
At harvest, the height and degree of taper of each stem
was measured by taking diameter measurements at the soil
level, at heights of 50 and 100 cm, and at the top of the stem
just under the flower in sunflower, or the tassel of each plant
in maize. Shoot height was taken as the distance from the
surface of the soil to the point at which the flower head in
sunflower and the tassel in maize joined the stem. Shoots
were then cut off at the base, just above the topmost
adventitious roots, and a length of 25 cm which included the
The root systems were stored in a cold room at 5 mC for
up to 14 d until all of the roots had been measured and
mechanically tested. Each was then carefully washed and all
the fine roots collected using a sieve. The soil under each pot
to a depth of 20 cm was carefully sieved to collect any roots
not contained by the pot. The method of determining the
angle of spread of the root system, adapted from Pinthus
(1967), involved measurements (to the nearest 5m) in two
planes by placing the system on a paper protractor and
reading the maximum angle of the whole root system ; the
roots were then rotated by 90m and the maximum angle
again measured. The mean of these two measurements gave
the spreading angle of the root system. For maize, the
number of roots was counted at each node and each was
classified either as entering the soil or not. The first-order
lateral roots of each species were then removed at the base
using a hacksaw and transverse basal sections were cut and
stained with phloroglucinol to reveal the extent of lignification.
The total number of structural roots (defined as firstorder lateral roots which had a basal diameter greater than
2 mm) was counted for each plant. Three first-order lateral
roots were sampled randomly from each plant for measurement of the degree of root taper : root diameters at the base,
the base plus 4 cm and the base plus 8 cm were measured
using callipers. All of the roots were placed on moist
sponges to prevent alteration of the mechanical properties
by desiccation.
The dimensions of the tap roots of sunflowers were also
recorded by measuring their diameter at 7i2 cm intervals
from the base. The length of sunflower first-order lateral
roots which showed noticeable rigidity in bending (termed
‘ rigid root length ’) was also measured from the base down
to the point at which the root no longer resisted bending.
Fresh and dry weights of roots
Root systems were divided into three different components : rigid first-order lateral roots which have a primary
anchorage role ; central anchorage element (the tap root in
the case of sunflower, and the basal internode in the case of
maize) ; and fine lateral roots. Samples were weighed before
and after being oven dried at 70 mC for 5 d. To investigate
the partitioning of biomass between the shoot and root
system, values of total root weight and total shoot weight
were used to calculate the root : shoot ratio.
Mechanical tests
Three-point bending tests were carried out both on the
stems and on all first-order lateral roots ( 2 mm base
296
Goodman and Ennos—Soil Strength and Root Mechanics
diameter) of both sunflower and maize, using a universal
testing machine (Instron, model 4301).
where dF\dY is the initial slope of the force displacement
curve. The bending modulus, E, is given by :
E l R\I
Stems
The diameter of each stem sample was measured at the
mid-point using callipers ; samples included the first three
nodes and four internodes. Stem samples were placed
between two supports which were set apart a distance of
approx. 15 times the mid-point diameter of the sample to
avoid problems with shear (Vincent, 1992). A pushing probe
of radius 20 mm was attached to the load cell and lowered
until it just touched the mid-point of the sample. The
crosshead was then lowered at a rate of 20 mm min−",
bending the sample until it eventually buckled. A computer
with an interface to the testing machine was used to produce
a graph of force Šs. displacement, permitting calculation of
the mechanical properties of the sample (Ennos, Crook and
Grimshaw, 1993 a).
Roots
The basal 60 mm of each root was cut where it joined the
tap-root in sunflower or the basal nodes in maize, stripped
of fine roots using a razor, placed between two sponges
before testing, and the diameter was measured at the midpoint. The sample was placed between two supports (set
apart a distance of approx. 15 times the diameter of the
sample) and a pushing probe of radius 2 mm was lowered
until it just touched the sample. During the test the
crosshead was lowered at a rate of 10 mm min−", bending
the sample until it failed.
Using data collected from the test an interfaced computer
calculated three mechanical properties : the bending
strength, S [eqn (2)], and the rigidity, EI [eqn (3)], of each
root ; and the bending modulus, E [eqn (4)], of the material
of which they were composed. In the analysis performed
using the Instron it was assumed that there was no taper.
The errors due to this assumption are small ; for a beam of
circular cross section, where the angle between the top edge
of the beam and the horizontal is less than 20m the errors are
less than 10 % (Gere and Timoshenko, 1991). In both
species, the roots of flexed and control plants showed a low
degree of root taper and this angle was no more than 2m.
Analysis of bending tests
The mechanical properties of samples were calculated
using well-known equations (Gordon, 1978). Bending
strength is given by the expression :
S l Fmax L\4
(2)
where Fmax is the maximum force a sample will withstand
before it fails and L is the distance between the supports.
The bending rigidity, R, of a uniform beam is the resistance
of that beam to curvature and is given by :
R l L$(dF\dY )\48
(3)
(4)
where R is the rigidity of the sample [eqn (3)] and I is the
second moment of area. In sunflowers, which are cylindrical
in cross-section, the second moment of area was calculated
for a solid cylinder using πr%\4 where r is the radius. For
maize, which is elliptical in cross-section, the second moment
of area was calculated for an ellipse using π ba$\4, where a
and b are the radii of the major and minor axes of the ellipse.
A high modulus indicates a stiffer material.
Lodging assessment
Lodging (the permanent displacement of the stem from
the vertical ; Pinthus, 1973) occurred in late September. The
extent and type of lodging was recorded by scoring the
severity of inclination of the stem base from the vertical.
The number of plants which had lodged (stem base inclined
20m from the vertical) growing in the high bulk density
soil was then compared with the number of lodged plants
growing in the low bulk density soil.
Statistical analysis
A Kolmogorov-Smirnov test was used to test the
normality and the similarity of the shape of the underlying
distributions before proceeding with analysis of variance.
The count data, before analysis, were normalized using a
square root transformation. Two-way analysis of variance
was used whenever possible to identify differences between
treatments and account for block effects. The soil shear
strength and penetrometer data were analysed using a
repeated measure analysis of variance.
Chi-squared tests were used to detect differences in the
incidence of lodging between plants which were growing in
a weak and strong soil. All values in the text are meansps.e.
RESULTS
EnŠironmental effects
In both sunflowers and maize there was a significant
difference in shoot height between blocks (P 0n05) ; this
was most probably due to sheltering from nearby buildings.
One of the sunflowers growing in the weak soil was badly
damaged by slugs and was excluded from the analysis.
Soil strength
There were significant differences between the soil shear
strength of the high (1n4 Mg m−$) and low (1n0 Mg m−$) bulk
density treatments (P 0n01). Soils with a low bulk density
had a significantly lower shear strength (4n7p0n21 kPa) than
those with a high bulk density (11n9p0n61 kPa). Penetrometer readings followed the same pattern ; the maximum
penetration resistance of low bulk density soil (118
p4n4 kPa) was significantly lower than the high bulk
density soil (325p12n2 kPa ; P 0n0001). There were no
297
Goodman and Ennos—Soil Strength and Root Mechanics
T     1. Morphology and mechanical properties of the mature stems (basal sample) of sunflower and maize growing in weak
and strong soils
Sunflower
Maize
Property
Weak soil
Strong soil
P
Weak soil
Strong soil
P
Shoot height (cm)
Stem diameter (mm)
Base
Basej50 cm
Basej100 cm
Top
Mechanical properties
Rigidity (Nm#)
Bending strength (Nm)
Bending modulus (MPa)
113p1n6
112p1n7
NS
110p2n7
116p2n2
NS
16n3p0n57
15n1p0n63
13n4p0n50
27p1n7
17n5p0n56
16n9p0n63
14n6p0n38
29p1n9
NS
*
NS
NS
17n9p0n31
13n4p0n43
6n8p0n37
5n1p0n10
18n1p0n29
14n0p0n50
9n0p0n81
5n0p0n15
NS
NS
*
NS
6n2p0n73
7n2p0n71
1820p96
8n5p0n85
9n5p0n81
1670p71
*
*
NS
2n6p0n24
3n5p0n33
760p37
3n1p0n16
4n7p0n28
820p76
NS
*
NS
Results were analysed using two-way ANOVA. Only maize showed a significant block effect (P 0n05). Values are means of 16 plantsps.e.m.
(except for mechanical properties where n l 15). ** P 0n01 ; * P 0n05 ; NS, not significant, P 0n05.
T     2. Fresh and dry weights of shoots of mature sunflower and maize growing in weak and strong soils
Sunflower
Fresh weight (g)
Stem
Leaves
Reproductive
Basal node
Total
Dry weight (g)
Stem
Leaves
Reproductive
Basal node
Total
Maize
Weak soil
Strong soil
P
Weak soil
Strong soil
P
148p13
72p6n9
140p11
192p16
105p12
205p15
*
*
**
360p28
502p41
**
110p4n6
71p2n5
42p4n6
4n5p0n3
228p9n6
126p3n6
69p2n1
55p3n9
5n5p0n25
256p6n0
*
NS
NS
*
NS
29p2n2
16p1n5
12p0n88
35p2n3
22p1n8
18p1n4
NS
*
**
57p4n4
75p5n7
*
16n3p0n77
16n1p0n50
6n4p0n50
1n2p0n08
40p1n6
18n8p0n55
17n3p0n37
8n2p0n51
1n2p0n04
46p1n0
*
NS
*
NS
**
Results were analysed using two-way ANOVA and there was a distinct block effect in maize (P
** P 0n01 ; * P 0n05 ; NS, not significant, P 0n05.
significant differences in the shear strength or penetrometer
readings between sunflower and maize trials (P 0n05).
Shoots
Morphology. Soil strength had a small effect on the shoots
of both species. There was no significant difference in the
height of sunflower or maize grown in strong soils compared
with those grown in weak soils (Table 1). Neither did plants
show any significant difference in basal diameter between
treatments, although sunflowers grown in strong soil were
thicker at 50 cm, and maize at 100 cm, than those grown in
weak soil.
However, there were differences in fresh and dry weights
of shoots between treatments. Sunflowers grown in strong
soil had a greater stem, leaf, reproductive and total shoot
fresh weight than those growing in weak soil. There was
no significant difference in the dry weight of sunflower stems
between treatments. Maize in strong soil only showed
significant increases in the fresh weight of the stem and basal
node, and increased dry weight of the stem and reproductive
parts (Table 2).
0n05). Values are means of 16 plantsps.e.m.
Mechanics. Both species showed significant differences
between treatments in the mechanical properties of stems.
Sunflowers grown in strong soil had more rigid and stronger
stems than those grown in weak soil. However, maize plants
grown in strong soils had stronger, but not more rigid stems
than those in weak soil (Table 1).
Roots
Morphology. The root systems of sunflower and maize
showed a greater response to soil strength. There was no
significant difference in the number or weight of first-order
laterals of sunflower or maize growing in weak compared to
strong soil (Tables 3 and 4). Neither was there a visible
difference between treatments in the degree of lignification
of roots of either species. However, there were other
differences in root morphology. In sunflower, the root
system of plants grown in strong soil had a greater angle of
spread than those grown in weak soil (Table 3). The basal
diameter of first-order lateral roots growing in strong soil
was also significantly thicker than that of plants grown in
weak soil (Fig. 1). Sunflower tap-roots growing in strong
298
Goodman and Ennos—Soil Strength and Root Mechanics
T     3. Root morphology of mature sunflower and maize growing in weak and strong soils
Morphological characteristics
Sunflower
Root angle of spread (degrees)
Root lateral number (basal diameter 2 mm)
Length of rigid root (cm)
Maize
Root angle of spread (degrees)
Root lateral number (basal diameter 2 mm)
First internode length (cm)
Weak soil
Strong soil
P
108p3n2
35p1n5
65p2n1
121p1n8
35p1n4
67p1n7
**
NS
NS
82p3n7
18n1p0n88
2n2p0n09
82p1n9
19n6p0n94
2n5p0n10
NS
NS
*
Results were analysed using two-way ANOVA. Values are means of 16 plantsps.e.m. (except for sunflower root angle where n l 15). ** P
0n01 ; * P 0n05 ; NS, not significant, P 0n05.
T     4. Fresh and dry weights of the first-order lateral roots of mature sunflower and maize growing in weak and strong
soils
Sunflower
Fresh weight (g)
First-order
laterals
Tap root
Fine roots
Total
Dry weight (g)
First-order
laterals
Tap root
Fine roots
Total
Maize
Weak soil
Strong soil
P
Weak soil
Strong soil
P
21p1n9
27p2n5
NS
20p1n1
22p1n1
NS
20p2n0
22p2n2
63p4n5
20p1n7
23p3n1
70p5n4
NS
NS
NS
68p2n9
88p2n4
63p2n2
85p2n5
NS
NS
3n4p0n33
4n3p0n42
NS
3n0p0n14
3n2p0n14
NS
4n9p0n51
2n3p0n18
10n6p0n86
4n8p0n47
2n4p0n27
11n5p0n88
NS
NS
NS
5n9p0n28
8n9p0n24
5n7p0n22
8n9p0n28
NS
NS
Results were analysed using two-way ANOVA. Only maize showed a significant block effect (P
** P 0n01 ; * P 0n05 ; NS, not significant, P 0n05.
0n05). Values are means of 16 plantsps.e.m.
7
6
Diameter (mm)
5
4
3
2
1
0
4
Distance from base (cm)
8
F. 1. Diameter of first order lateral roots of sunflower (=, >) and maize (, ) grown in weak (=, ) and strong soil (>, ). Mean diameter
was taken at 4 cm intervals from the base of the root, and the results were analysed using two-way ANOVA (n l 16), showing significant
differences (P 0n05) at the base for both species and 4 cm from the base in maize only. There was no significant block effect (P 0n05). Vertical
bars indicateps.e.m.
299
Goodman and Ennos—Soil Strength and Root Mechanics
30
Diameter (mm)
25
20
15
10
5
0
2
4
6
8
Distance from base (cm)
10
12
14
F. 2. Diameter of the tap roots of sunflower plants grown in weak (=) and strong soil (>). Mean diameter was taken at 2 cm intervals from
the base of the stem, and results were analysed using two-way ANOVA (n l 16) showing significant differences (P 0n05) at the base, 12 and
14 cm from the base. There was no significant block effect (P 0n05). Vertical bars indicateps.e.m.
0·45
0·40
Mean root to shoot ratios
0·35
0·30
0·25
0·20
0·15
0·10
0·05
0·00
Dry weight
Fresh weight
Sunflower
Fresh weight
Dry weight
Maize
F. 3. The effects of soil strength on the root : shoot ratio of sunflower and maize grown in weak () and strong soil (). Only sunflowers showed
a significant difference between the fresh and dry weight root : shoot ratios of plants grown in weak soil compared to those grown in strong soil
(P 0n01). The ratios were analysed using two-way ANOVA (n l 16). Vertical bars indicateps.e.m.
soil tapered more rapidly than those in weak soils (Fig. 2).
In contrast, there was no significant difference in the angle
of spread of the root system of maize between treatments
(Table 3). However, the first 4 cm of the first-order lateral
roots were thicker in strong than in weak soil (Fig. 1).
There were also differences in the root : shoot ratio between
treatments. In sunflowers, plants grown in strong soil had
significantly lower root : shoot ratios than those grown in
weak soil (Fig. 3).
Mechanics. There was only a small effect of soil strength
on the mechanical properties of first-order lateral roots. In
sunflower, there were no significant differences between
roots growing in strong compared to weak soil. In maize,
roots growing in weak soil were stiffer than those in strong
soil (Table 5).
Lodging and anchorage failure
In late September both sunflower and maize plants
lodged ; failure occurred in the roots with no buckling of
300
Goodman and Ennos—Soil Strength and Root Mechanics
T     5. Mechanical properties of the roots of sunflower and maize growing in weak and strong soils
Sunflower
Maize
Mechanical property
Weak soil
Strong soil
P
Weak soil
Strong soil
P
Rigidity (Nm#i10−$)
Bending strength (Nmi10−#)
Bending modulus (MPa)
1n5p0n25
1n8p0n22
221p31
1n8p0n37
2n1p0n28
206p23
NS
NS
NS
5n0p0n53
4n9p0n43
958p74
5n0p0n38
4n7p0n29
709p45
NS
NS
**
Results were analysed using two-way ANOVA. Neither species showed a significant block effect (P 0n05). Values are means of 16
plantsps.e.m. ** P 0n01 ; * P 0n05 ; NS, not significant, P 0n05.
stems. In sunflower, there was a significant difference
between the number of plants lodged in the different soil
density treatments (χ# l 6n2, d.f. l 1, P 0n05) ; from a total
of 64 sunflowers, 11 plants from the weak soil treatment
lodged compared with two in the strong soil treatment. In
maize there was also a similar significant effect (χ# l 24n1,
d.f. l 1, P 0n01) ; 22 plants from the weak soil treatment
lodged compared with two in the strong soil treatment.
D I S C U S S I ON
This study shows that despite differences in soil bulk
density, and hence strength, there were only small effects on
the morphology and mechanical properties of roots of
sunflower and maize. The effects of soil strength on the
thickness of roots in this study are, however, consistent with
those found in the literature (Barley, 1965 ; Atwell, 1988 ;
Materechera et al., 1991) ; both species showed an increase
in the diameter of first-order lateral roots in the stronger
soil.
In the stronger soil, roots were thicker closer to the base of
the stem probably because in stronger soil they would sway
less in the wind and would therefore experience higher peak
stresses. However, stresses would be transmitted faster from
the roots to the ground in the strong soil (Ennos and Fitter,
1992), so roots would not be stressed so far away from the
base of the stem. Therefore it is not surprising that the taper
of the tap-roots of sunflower was greater in strong soils.
However, despite the increase in the thickness of roots, there
was no change in root weight, possibly because the shorter
root axes are often thicker (Atwell, 1993).
Surprisingly, we found few differences in the mechanical
properties of first-order lateral roots between treatments.
Earlier work has shown that large differences can occur in
the mechanical properties of roots in response to mechanical
stimulation (Goodman and Ennos, 1996, 1997 a, b) ; the
roots of mechanically-stimulated plants were thicker,
stronger and composed of a stiffer material than those of
untreated plants. In this study there were few effects of
mechanical impedance on the mechanical properties of
roots. We might have expected differences in root strength
and stiffness closer to the stem base between plants grown in
strong soil compared to those grown in weak soil because of
the increase in stress at that point. The effects of soil
strength on the mechanical properties of roots may have
been greater if the difference in soil strength between
treatments was larger. Soil strength measurements in this
study were small compared to the strength required to stop
root growth in the field. It is usually considered that soil
strength is a problem for field grown crops if soil
penetrometer resistance exceeds 2 MPa (Materechera et al.,
1991).
Soil strength also had noticeable effects on the anchorage
mechanics of sunflower and maize. Despite comparatively
small absolute differences in soil penetration resistance and
root morphology there were large differences in the
susceptibility of sunflower and maize to lodging. Sunflowers
grown in strong soil were more stable (i.e. lodged less) than
those grown in weak soil. A similar effect was seen in maize
with plants growing in weak soil being more likely to lodge
than those growing in strong soil. Significant differences
between lodging indicate that any compensatory changes in
root growth in weak soil are inadequate to restore the
same anchorage as in strong soil.
This study suggests that it may be possible to reduce
lodging in crops grown in very loose soil by compacting soil,
without a significant reduction in shoot height. Oussible
et al. (1992) showed that in compacted soils, even at
penetration resistances of 1n5-4 MPa, compaction had no
consistent effect on shoot height and no effect on shoot dry
weight. Crook (1994) showed that the resistance to
overturning was lowest in loosely cultivated seedbeds and
greatest in winter wheat plants grown in the most compact
seedbeds. In contrast, other studies have shown that shoot
fresh and dry weight is lower in plants grown in compacted
soil with a similar penetration resistance (1n4-4 MPa)
(Atwell, 1990 ; Cook et al., 1996). Atwell (1990) found that
the root : shoot ratio in winter wheat was smaller when roots
were growing in stronger soils than in weak soils because
soil compaction consistently inhibited the elongation of
seminal root axes. A possible explanation for the contradictory evidence existing in the literature may be that in
some studies plants could have been affected by periods of
water stress. In dry environments growth rate might be
improved in stronger soils because of increased water
availability in the root zone (Masle and Passioura, 1987).
This could explain why, in this study, shoot growth was
unchanged or increased in the stronger soils. Alternatively,
nutrient deficiency may develop as a consequence of
restricted root growth and, in the long-term, limit shoot
growth in mature plants (Masle and Passioura, 1987).
There were differences in the way in which sunflower and
maize responded to soil strength. Sunflowers showed
Goodman and Ennos—Soil Strength and Root Mechanics
increases in the angle of spread of the root system and the
thickness of roots, but maize only showed differences in the
thickness of lateral roots. These results are consistent with
those of Materechera et al. (1991) who also showed a
contrast in the way in which monocot and dicot species
respond to changes in soil strength. Their results showed
that roots of plant species differed considerably in their
ability to thicken under stress. Dicot species were also better
at penetrating compacted soil layers than monocots ;
generally dicots had more roots penetrating to a greater
depth in both the compact and deep tilled soils (Materechera
et al., 1993).
This study has shown that plants are, to some extent, able
to adapt their roots in response to soil strength, but that
changes in root growth do not appear to fully compensate
for alterations in soil conditions. Furthermore it has been
shown that there are differences in the way in which
sunflower and maize respond to high soil strength and that
even small increases in soil strength can reduce the likelihood
of anchorage failure.
A C K N O W L E D G E M E N TS
We thank Sue Challinor, Thurston Heaton and David
Newton for technical assistance. The work was funded by
the Biotechnology and Biological Sciences Research Council
of the United Kingdom.
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