Journalof
Soil Science,
1990,41,341-358
Mechanical impedance to root growth: a review
of
experimental techniques and root growth responses
A.
G.
BENGOUGH
&
C.
E.
MULLINS*
Cellular and Environmental Physiology Department, Scottish Crop Research Institute,
Dundee OD2 5DA and *Department
of
Plant and Soil Science, University
of
Aberdeen.
Aberdeen AB9 ?UE,
UK
SUMMARY
Mechanicalimpedancetorootgrowthisoneofthemostimportant
factorsdeterminingroot
elongation and proliferation within a soil profile. Penetrometers overestimate resistance to
root growth in soil by a factor of between two and eight and, although they remain the most
convenient method for predicting root resistance, careful interpretation
of
results and
choice of penetrometer design are essential
if

In
this paper, the effects of mechanical impedance on root morphology are reviewed and some
direct comparisons between soil resistance
to
root growth and resistance to a penetrometer are
discussed. The physical process of root growth through soil and artificial media is considered, with
emphasis on the interpretation of results from different experimental techniques. Changes which
occur in root elongation rate under both constant, and temporally and spatially varying levels of
mechanical impedance are considered together with the complicating effects of soil aeration and
water status. Finally, possible physiological mechanisms for the root responses are discussed.
Terminology
Penetrometers provide the best estimates
of
resistance to root growth in soil, short of direct measure-
ment
of
root force. Most penetrometers consist of a metal probe with a conical tip fixed onto a
34
1
342
A.
G.
Bengough
&
C.
E.
Mullins
cylindrical shaft (Fig. 1) that is generally of smaller diameter than the cone (normally 80% of the
cone diameter; Gill, 1968; Barley
&

41
ifl
Elongating
region
Meristemat i(
region
Phloem
Endodermis
Cortex
Epidermis
Xylem
‘‘j
,,’/
f
Muclgel
sheoth
‘l;
I
I
Fig.
1.
(a)
A
penetrometer,
where
F,,
A,,
oN.
and
a

1977). The increase in root diameter in mechanically impeded roots results mainly from an
increased thickness of the cortex; this is a consequence
of
both the increase in the diameter of the
outer cells, and an increase in the number of cells per unit length of root.
Mechanical impedance to root growth
343
The apical meristem and zone of cell extension
of
impeded roots is shorter (Barley, 1962; Souty,
1987), and root hairs develop closer to the tip of impeded roots (Goss
&
Russell, 1980). Lateral
initiation occurs nearer the tip and laterals occur together along the impeded axis (Goss
&
Russell,
1980; Barley, 1962). Where mechanical deflection causes roots to curve around an obstacle, the
initiation of laterals generally occurs on the convex side of the root (Goss
&
Russell, 1980). Root hair
development is greater on the opposite (concave) side and, in highly impeding media, the growing
zone of the root is much distorted. The growth of impeded lateral roots is affected by impedance
similarly to the main axis (Goss, 1977). However, if the pore size in the growing medium is such that
only the main root axes are impeded, the freely penetrating laterals attain much greater length than
in completely unimpeded root systems.
COMPARISON
OF
ROOT RESISTANCE WITH PENETROMETER
RESISTANCE
There have been relatively few studies involving the measurement of root force

between about 0.9 MPa and
1.3
MPa (Misra
et
al.,
1986b), whereas root elongation stops in soil with
a penetrometer resistance of
0.8
to
5.0
MPa (Greacen
et
al.,
1969). The results are variable because of
differences between plant species and soil types, and possibly the temperatures at which the exper-
iments were performed (Greacen, 1986). Thus, roots cease elongating in soil with a penetrometer
resistance up to six
or
more times greater than the maximum axial pressure that they can exert. The
reason for this difference must be physical differences in the way in which plant roots and metal
probes penetrate soil.
MODELLING MECHANICAL IMPEDANCE TO PLANT ROOTS AND
TO
PENETROMETERS IN SOIL
Barley
&
Greacen (1967) comprehensively reviewed the mechanics
of
soil deformation and failure
which occur around penetrometer probes, roots and underground shoots. There have since been

Studies involving direct measurement
of
penetration resistance both to plant roots and to
Eavis Stolzy
&
Whiteley Misra
Bengough
&
(1967)
Barley(1968)
etal.
(1981)
etal.
(1986a) Mullins(1988)
Soil
Probe diameter (mm)
Probe semiangle
Penetration rate
(mm min-
I)
mm behind tip where
root diameter
measured
ratio (orobe resistance)
remoulded remoulded remoulded
sandy loam sandy loam cores and
undisturbed
clods
of
sandy

clear which figure was used.
and
a
is the cone semi-angle. On the assumption that plant roots experience very little frictional
resistance, Greacen
et
al.
(1968) have shown that this equation can account for much of the large
difference between the resistance experienced by plant roots and by metal probes: Equation
(2)
predicts that sharp penetrometers (i.e. small
a)
will experience a much higher component of
frictional resistance than blunter penetrometers. However, with a semi-angle of more than 30", soil
bodies (that move with the probe) have been observed to form around the probe tip
so
that soil-
metal friction is no longer involved and Equation
(2)
ceases to be applicable (Mulqeen
et
al.,
1977;
Bengough, 1988).
Farrell
&
Greacen (1966) and Greacen
et
al.
(1968) calculated the pressure required to expand

many agricultural soils.
Further confirmation that less stress is required for radial (cylindrical) soil deformation than for
axial (spherical) deformation was provided by Abdalla
et
al.
(1969) and Hettiaratchi
&
Ferguson
(1973). For any given (elastic) strain in a cylinder of soil ahead of the root tip, it was theoretically
predicted that less stress is required to deform the soil radially than axially (Abdalla
et
al.,
1969).
Mechanical impedance
to
root growth
345
This theory was complemented by experiments using a large modified penetrometer to demonstrate
that radial expansion behind a penetrometer (or root) tip can reduce axial resistance to soil pen-
etration. Hettiaratchi
&
Ferguson (1973) predicted theoretically that the pressure required for
cylindrical soil deformation in a frictionless cohesive medium was always less than for spherical
deformation, the difference increasing with cohesion.
Collis-George
&
Yoganathan (1985) used the spherical cavity expansion model of VesiC (1972)
to
define limiting mechanical conditions for seed germination and root growth. Although this model
may be suitable to describe germination conditions, use of spherical expansion theory will have

remain the best available method of estimating resistance to root growth in soil. It is important,
therefore, to determine what are the most important physical differences between the action of roots
and penetrometers.
Rootflexibility and spatial variation
of
soil strength
Because roots often grow through cracks and holes in the soil, or follow planes of weakness between
soil peds (Russell, 1977), penetrometers are of limited use in some structured soils. Detailed work
has
been done on the behaviour of roots growing along cracks and through pores (Whiteley
&
Dexter, 1983; Dexter, 1986; Scholefield
&
Hall, 1985), but is beyond the scope of this review.
However, in coarsely structured soil, individual soil peds may be considered as continuous even
though the soil is structured on a larger scale (Greacen
et al.,
1969) and root penetration into these
peds may be important for nutrient uptake and plant growth. The forces required to buckle root tips
growing across air gaps were measured by Whiteley
&
Dexter (198
1
).
The buckling stress decreased
as the size of the air gap increased, but attempts to predict the buckling stress from the elastic
modulus of the root tip were only partly successful. Dexter (1978) has modelled root growth through
a bed
of
aggregates by relating root growth rates to penetrometer resistance within individual

pattern
of
variation of penetrometer resistance with depth was examined using Fourier analysis, was
used by Grant
et al.
(1989, but has not yet been related to root growth.
Diameter and rate ofpenetration
Existing experimental evidence on the effects of probe
or
root diameter on penetration resistance is
based almost entirely on penetrometer measurements, and is often contradictory (Table 2).
Richards
&
Greacen (1986), in their theoretical model of cavity expansion in granular media, imply
that thin roots may deform the soil elastically, thereby encountering less resistance than thicker
roots which cause plastic deformation. However, the limited studies of several different plant species
available to date do not indicate that roots of smaller diameter are relatively less mechanically
impeded by soil or by ballotini (Gooderham, 1973; Goss, 1977). In contrast to roots, which can grow
around objects that offer high resistance to displacement, a small probe may have to displace soil
particles
of
a diameter comparable to the probe. The result is that, particularly where there is an
abundance of coarse sand
or
larger material, the effective diameter of the probe is greater than its
actual diameter
so
that smaller probes (e.g. of
1
mm rather than 2 mm diameter) can experience

Bengough (1988)
field soil
(various textures)
remoulded
sandy loam
undisturbed
undisturbed clods
and remoulded cores
of
sandy loam
remoulded
(various textures)
undisturbed cores
of
sandy loam
10,20,30,40
3.8,5.1
1.00,1.25,1.50
1.75.2.00
1.00,1.25,1.50
1.75,2.00
0.5,l
.O
sphere smallest probe
conical no difference
-
smallest probe
(30")
conical no difference
conical no difference

et al.
(1972)
and Gooderham (1973) revealed decreases of less than 20% in penetrometer resistance for decreases
in penetration rate
of
between one and three orders of magnitude down to penetration rates of
Mechanical impedance
to root
growth
347
1
mm h-’
or
slower. Although Waldron
&
Constantin (1970) found a large effect of penetration rate,
an intermittently rotated penetrometer was used, which would have resulted in larger decreases in
soil frictional resistance at slower rates of penetration. In very wet soil, penetrometer resistance is
more clearly linked to penetration rate because of its interaction with pore water pressure (Cockroft
et al.,
1969). This effect will be greater in less permeable soils (especially remoulded soil) containing a
higher propertion of silt and clay, than in sands. Penetrometer resistance doubled in
a
sandy loam
soil remoulded at approximately field capacity for a 100-fold increase in penetration rate, whereas a
250-fold increase in penetration rate resulted in only a 25% increase in penetrometer resistance in
air-dry sand (Bengough, 1988). Similarly, Cockroft
et al.
(1969) found a doubling of penetration
resistance for

(Greacen &Oh, 1972; Voorhees
et al.,
1975).
Interactions between roots, water extraction
by
roots, root swelIing and root nutation
Because roots seldom grow through soil in complete isolation, it is important to consider inter-
actions between neighbouring
roots.
Greacen
et al.
(1969) measured resistance
to
penetration of a
narrowly-tapered probe, surrounded by six identical probes. Penetration resistance for the central
probe of a group was considerably lower than when the probe was used on its own. Tensile cracking
occurred between the probes, and similar cracks were also observed in a separate experiment
between neighbouring pea radicles growing into a loam. The drying action of roots is very important
in the formation of such cracks, which must facilitate the subsequent growth
of
lateral roots in a soil
of high resistance (Gerard
et al.,
1972). Although this cracking is generally likely to be advan-
tageous, there are soils of high tensile strength (e.g. Mullins
et al.,
1987, 1990) which may not crack
readily under the drying action of roots. In such soils, the increase in penetration resistance caused
by the soil drying may further impede root growth.
It has been suggested by Abdalla

1964).
348
A.
G.
Bengough
&
C.
E.
Mullins
EFFECTS OF MECHANICAL IMPEDANCE
ON
ROOT GROWTH
Experimental techniques
Existing experimental techniques can be divided into several different catagories.
Soil.
Experiments
in
soil are more realistic, but it is difficult to ensure that mechanical impedance
is the only soil factor limiting root growth. If a soil is compacted to increase resistance to root
growth, the resulting decrease in porosity may result in poor aeration. Similarly, increasing soil-
water tension to increase soil strength may result in water stress. However, the greatest difficulty is in
determining the penetration resistance experienced by roots in soil. This is ideally determined by
direct measurement (e.g. Stolzy
&
Barley, 1968; Eavis
&
Payne, 1969; Bengough
&
Mullins, 1988),
but practical difficulties have led most researchers to use penetrometer resistance measurements to

01
2
cm
cm
Fig.
2.
(a)
A
triaxial
cell
(after Barley, 1963), and
(b)
a pressurized diaphragm apparatus (after Barley, 1962).
Fig. 2(a) is reproduced from
K.P.
Barley, Influence of
soil
strength on growth
of
roots,
Soil
Science
1963,96(3),
175-180
(0
Williams
&
Wilkins, 1963).
Resistance to root growth in ballotini has been taken as either equal to (Russell
&

applied externally to the ballotini cell. In contrast, when the tube is not allowed to deflate during
pressurizing of the cell, the pressure required to expand the tube beyond its initial volume is between
5
and 10 times higher than the pressure applied externally to the ballotini cell (Richards
&
Greacen,
1986; Bengough
&
Mullins, 1990). This is attributable to the frictional resistance to deformation of
the ballotini. Since roots penetrating the ballotini must exert pressure to expand a new cavity in
previously undisturbed ballotini, the latter experiment gives more accurate representation
of
the
resistance experienced by growing roots. Although the pressure required to inflate a tube in the
ballotini will depend on both the tube diameter and on the frictional properties of the tube walls, it
seems reasonable to conclude that the resistance to root growth in the cells considerably exceeded
the external confining pressure.
Richards
&
Greacen (1986) also used a finite-element model
to
predict the effect
of
tube inflation
pressure on tube diameter in ballotini and in sand. Tube diameter increased slowly until the internal
pressure reached a certain critical value, when the diameter increased much more rapidly (Fig.
3).
Predicted inflation pressures were much higher than the external confining pressure, and the
difference was greater
for

I
I
I
/
/
/
I
1
I
1
I
I
0
0.05
0.10
0.15
0.20
0.25
Inflation
pressure
(MPa)
Fig.
3.
Outside diameter
of
tube
vs
inflation pressure in ballotini at several constant external cell pressures
(indicated at the top
of

times the diaphragm pressure.
Gill
&
Miller (1956) observed that if roots were well-covered with ballotini, elongation ceased at
lower diaphragm pressures. The authors suggested that ‘arching’ of the ballotini caused small
displacements of single glass beads to require larger displacements of the diaphragm. Thus,
resistance to root elongation was considerably greater than the diaphragm pressure.
Pressurized airlwater.
Chaudhary
&
Aggarwal(l984) proposed growing seedlings in moist sand
inside a pressure vessel as an experimental technique to measure the effect of mechanical resistance
on root elongation. However, although application of air pressure would cause a corresponding
350
A.
G.
Bengough
&
C.
E.
Mullins
increase in the absolute value of root cell turgor pressure, the decrease in osmotic potential or cell
wall tension required for root cell extension would
be
independent of the externally applied pressure
(assuming the cell permeability to water remained unchanged). Thus, the experiment does not truly
represent the situation of roots elongating against an external mechanical resistance. The reason for
the large decrease in root growth rates observed by the authors could be the 10-fold increase in
dissolved gas concentration in the water inside the pressure chamber, and ultimately in the plant,
resulting from the 10-fold increase in air pressure that they applied (Henry's law).

?
5;
60
t
m
W
3
0
-
c
0

5
20
c
0
W
-
Penetrometer resistance (MPa)
Fig.
4.
Root elongation
vs
penetrometer resistance obtained by Voorhees et
al.,
1975
(A
=
sandy loam,
B

root penetration resistances of between 0.39 and 0.48 MPa (based on the initial root tip cross-
sectional area). Stolzy
&
Barley (1968) found that the elongation rates
of
two pea radicles were
reduced to
44%
of
their unimpeded rate by a root penetration resistance of 0.46 MPa.
Mechanical impedance
to root growth
35
1
100
-
+
c
E
+
0
80
P
+
ul ul
P
60
c
0
._

Barley (1963): external confining pressure
D.
Eavis (1967): measured root penetration resistance
E.
Gill
&
Miller (1956): diaphragm pressure
F.
Goss (1977): external confining pressure
G.
Greacen
&
Oh
(1972): normal stress on penetrometer cone.
The results of these direct experiments represent the maximum effect
of
mechanical impedance
to be expected on root elongation rate for any particular root resistance. Reduced oxygen or
nutrient supply to the roots can further limit the observed 'root elongation rate, and similarly
anchorage of the tip by root hairs can result in the resistance being underestimated.
All
the experimental relationships in Figs
4
and
5
show similar patterns in which elongation rate
decreases with increasing mechanical resistance.
If
the results of direct experimental measurements
of root resistance and elongation rate (Stolzy

roots
growing into
a
zone
of
greater mechanical impe-
dance has been observed to decrease with time (Barley, 1962, 1963). However, Barley notes that
352
A.
G.
Bengough
&
C.
E.
Mullins
Table
3. Techniques used in studies of the effect of mechanical impedance on root elongation
rate (Figs
4
and
5)
Plant Root elongation
Reference species Root growth medium plotted against
Abdalla
e?
a/.
(1969)
Barley
(1962)
Barley

peas
oats
maize
barley
cotton and
peanuts
peas
200
pm ballotini
in cell subject to a
confining pressure
porous plate under
diaphragm covered by
a nylon cloth
1C70
pm ballotini
in cell subject to
a confining pressure
remoulded
2
mm sieved
sandy loam soil at
different bulk
densities and matric
potentials
tilled and no-tilled
silt loam field soil
50
pm ballotini between
porous plate and rubber

measured directly
penetrometer
resistance
(1
1 mm
diameter,
30"
semi-
angle probe)
pressure on
diaphragm
external confining
pressure
penetrometer
resistance after
subtracting
friction
(5"
semi-
angle)
penetrometer
resistance
(3.2
mm
diameter,
30"
semi-
angle probe)
penetrometer
resistance

at
the greater resistances more often found in soil.
Temporal variation.
Mechanical pressure applied to the apical 15 mm of previously unimpeded
roots can halt root elongation which resumes only after a lag time (13 to 50 h), which itself increases
with increasing stress (Barley, 1962). Localized moderate pressure (0.1 MPa) applied to the root
apex results in a bigger reduction in elongation rate than when fully enlarged tissue is compressed
(Barley, 1965). Higher pressures applied to the older tissue caused browning,
loss
of turgor and
stopped elongation, although no tissue damage resulted if the root cells were allowed
to
enlarge and
mature under the mechanical pressure (Barley, 1965). When the external confining pressure is
reduced to zero, mechanically impeded roots growing in pressurized cells of ballotini only gradually
return to the unimpeded elongation rate (Goss
&
Russell, 1980). The delay, of 2 to 7 d, was longer for
the more severely impeded roots. Thus, roots which develop under mechanical impedance probably
undergo physiological changes to adapt to the stress.
Interaction
of
mechanical impedance with matric potential and aeration
Roots grow more slowly in poorly aerated soil (Blackwell
&
Wells, 1983; Greenwood, 1969) and in
soil of low (i.e. more negative) matric potential (Eavis
&
Payne, 1969; Eavis, 1972; Yappa
et

root to the tip, and for ethylene transport away from the tip.
It
is more difficult to ascertain whether low matric potentials interact with mechanical
impedance, because soil strength and root penetration resistance increase as the matric potential
decreases. Taylor
&
Ratliff (1969) measured the root elongation rates of cotton and peanuts in
remoulded soil at several different bulk densities and matric potentials in the range -17 to
-700 kPa for cotton, and
-
19 to
-
1250 kPa for peanuts. Root elongation rate clearly depended
on the penetrometer resistance of the soil and not on the matric potential
per se.
Similar results
were obtained by Greacen
&
Oh (1972) for peas at matric potentials below -0.5 MPa, and by
Taylor
&
Gardner (1963) for cotton root penetration at matric potentials between
-
20 kPa and
-
67 kPa. These results imply that there is no interaction between matric potential and mechanical
impedance.
In contrast to these findings, Mirreh
&
Ketcheson (1972) found that, in soil with penetrometer

response
of
roots
to mechanical impedance
The growth rate of root cells is controlled by the balance of the internal and external pressures
together with the rheological properties of the cell wall. Lockhart’s (1965) equation for cell growth
was modified by Greacen (1986) to allow for root penetration resistance
gr
(Pa) giving,
dlldt
=
ZKrlm(n
-
W
-
0,)/(2K
+
r’m)
(3)
where dl/dt is the rate of increase in cell length (m
s-’),
K(m Pa-ls-’) is the water permeability
of
the
cell membrane per unit surface area, m (m-’Pa-Is-’) is the cell wall extensibility,
n
(Pa) is the
osmotic pressure,
r
(m) is the cell radius and W(Pa) is the yield stress of the cell wall. Greacen (1986)

of a simple build-up of solutes because
of
the slower root growth rate. Such an excess of solute
supply over demand has also been suggested as the source of osmotic adjustment in plants growing
in dry or saline soils (Munns, 1988).
However, a model of root response to mechanical impedance based entirely on osmotic adjust-
ment does not explain the many morphological changes that occur in mechanically impeded roots.
Barley (1976) and Feldman (1984) suggested that ethylene may be involved in root response to
mechanical impedance. Kays
et
al.
(1974) noted that the rate of ethylene evolution increased by up
to six times when bean roots were impeded by a barrier. Exogenous applications
of
ethylene can
cause root radial thickening, a reduction in the root elongation rate, and a profusion
of
root hairs
within
1
to 2 mm of the swollen root apices (Kays
et al.,
1974; Smith
&
Robertson, 197
I);
all of these
have been observed in mechanically impeded roots.
Cell shape can be determined by the orientation of the cellulose microfibrils as they are laid down
in the cell wall (Osborne, 1976). Initially these are aligned transversely to the root axis, but as cellular

et
al.,
1988). Moss et
al.
measured a two- to 2.5-fold increase in the rate
of ethylene evolution by mechanically impeded maize roots, and found that supplying ethylene in
the air-flow to unimpeded roots simulated closely the effects of impedence. However, two inhibitors
of ethylene production,
aminoethoxyvinylglycine,
and action, 2,5-norbornadiene, which overcame
the effects of ethylene in unimpeded roots, did not modify the growth
of
mechanically impeded
roots, although the increased rate of ethylene evolution was entirely suppressed.
Moss
et
al.
(1988) suggest that the increased rate
of
ethylene evolution by impeded roots may not
be a result of a direct effect of mechanical impedance, but of the physical wounding of the radially
expanding root by the ballotini (such self-inflicted damage has been observed during time-lapse
studies of bean root growth in ballotini; personal observation, AGB). However, the wounding
hypothesis does not entirely explain why an increase in the rate
of
ethylene evolution occurred
within
1
h ofwhen bean roots grown inside perforated tubes were mechanically impeded by a plastic
barrier (Kays

has caused the apparent disagreement with results from penetrometer studies in soil. Pressurized
ballotini cells still provide a valuable technique for studying the effects
of
mechanical impedence on
root morphology and nutrient uptake (Goss, 1977; Lindberg
&
Pettersson, 1985), and are poten-
tially
of
considerable use for investigating the largely unresearched ability
of
different species and
varieties to grow against mechanical impedance (Goss, 1974). The changes in root growth rate
which occur as a root enters or leaves a zone
of
high mechanical impedance are of considerable
relevance to real soils, but have received surprisingly little attention.
The mechanism
of
root response to mechanical impedance may involve osmotic adjustment,
although insufficient evidence has been presented to test such a model. The role of ethylene must be
re-assessed in the light of some recent experiments, and its type
of
involvement in the response
of
roots to mechanical impedance must be more clearly established.
ACKNOWLEDGEMENTS
We thank Drs D. Linehan, B. Marshall,
I.
Young, D. Robinson and other colleagues at SCRI for

13,95-110.
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96,175-180.
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Australian Journal
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20,173-182.
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&
GREACEN, E.L. 1967. Mechanical
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&
FARRELL,
D.A. 1965.
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WELLS, E.A. 1983. Limiting oxy-
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&
AGGARWAL, G.C. 1984. A
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&
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