Advances in agronomy volume 19

In another article Barley and Greacen, Australian authors, take up in an analytical mood one of the oldest problems of plant growth, the penetration of roots through the soil and the eme

A D V A N C E S I N

AGRONOMY

VOLUME 79

CONTRIBUTORS TO THIS VOLUME

A D V A N C E S I N

AGRONOMY

Prepared under the Auspices of the

AMERICAN SOCIETY OF AGRONOMY

COPYRIGHT @ 1967, BY ACADEMIC PRESS INC

ALL RIGHTS RESERVED

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LIBRARY OF CONGRESS CATALOG CARD NUMBER: 50-5598

PRINTED IN THE UNITED STATES OF AMERICA

CONTRIBUTORS TO VOLUME 19

Numbers in parentheses indicate the pages on which the authors’ contributions begin

BARLEY, K P ( l ) , Reader, Agronomy Department, Waite Agricultural Research Institute, The University of Adelaide, Glen Osmond, South Australb

DUDLEY, J W (45), Associate Professor, Plant Genetics, Department Agronomy, University o f Illinois, Urbana, Illinois

GREACEN, E L ( l ) , Principal Research Scientist, Diuision o f Soils, Com- monwealth Scientific and Industrial Research Orgunization, Glen Osmond, South Australia

HAGEMAN, R H (45), Professor of Plant Physiology, Department of Agronomy, University of Illinois, Urbana, Illinois

HANDRECK, K A ( 107), Experimental Oficer, Division of Plant Industry, Commonwealth Scientific and Industrial Research Organization, University of Melbourne, Parkville, Victmh, Australia

JAMES, EDWIN (87), Head, National Seed Storage Laboratory, Agricul-

tural Research Sewice, United States Department of Agriculture, Fort Collins, Colorado

JONES, L H P (107), Principal Research Scientist, Division of Plant

Industry, Commonwealth Scientific and Industrial Research Organ- ization, University o f Melbourne, Parkoille, Victoriu, Australia

LARSEN, SIGUFUJ (151), Chief Soil Scientist, Department of Soil Science, Levington Research Station, Levington, Ipwich, Suffolk, England

LENG, E R (45), Professor of Plant Breeding and Genetics, Department

of Agronomy, University of Illinois, Urbana, Illinois

MCCANTS, C B (211), Professor of Soil Science, Department of Soil

Science, School o f Agriculture and Life Sciences, North Carolina State University, Raleigh, North Carolina

QUINBY, J R (267), Head, Sorghum Breeding, Pioneer Smghum Com-

pany, Plainview, Texas

RAUPACH, M (307), Head, Soil Chemistry Section, Division of Soils,

Commonwealth Scientific and Industrial Research Organization, Glen Osmond, South Australia

WOLTZ, W G (211 1, Professor of Soil Science, Department of Soil Sci- ence, School of Agriculture and Life Sciences, North Carolina State University, Raleigh, North Carolina

V

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of the opportunities thus presented Physiological factors under genetic control are dealt with by Quinby in reviewing the maturity genes in sorghum, a crop the geographic range of which has been considerably extended in recent years

All plant breeders are properly concerned with the preservation of seed stocks and the maintenance of gene pools The unique facility erected by the U S Department of Agriculture for this purpose is described by its Director, Edwin James

More applied topics are treated in a chapter on the growth and nutrition of flu-cured tobacco by McCants and Woltz and in one on the soil and nutritional requirements of an important Australian tree crop,

Pinus radiata, by Raupach

In another article Barley and Greacen, Australian authors, take up in

an analytical mood one of the oldest problems of plant growth, the penetration of roots through the soil and the emergence of seedling shoots, as affected by the mechanical stress of the environment Recent developments in our understanding of soil forms of phosphorus and phosphorus transformation in soils are presented in a scholarly review

by Sigurd Larsen This is another old topic that is steadily reshaped because of continuing attention to the essential and dynamic role played

by this element in plant growth

The eight chapters in this volume are indicative of the diversity and vitality of researches in soil and crop science that lead to improvements

in practice and to the benefit of man

Ann Arbor, Michigan

June, 1967

A G NORMAN

vii

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CONTENTS

CONTRI~UTORS TO VOLUME 19 V

PREFACE vii

MECHANICAL RESISTANCE AS A SOIL FACTOR INFLUENCING THE GROWTH OF ROOTS AND UNDERGROUND SHOOTS BY K P BARLEY AND E L GREACEN I Introduction 1

11 Types of Deformation Produced by Plants 2

111 Forces Required to Deform Soils 5

IV Forces Exerted by Roots and Shoots 18

V Effects of Mechanical Stress on the Growth of Roots and Shoots 24

VI Growth in the Soil 30

VII Conclusion 40

References 40

A BIOCHEMICAL APPROACH TO CORN BREEDING BY R H HAGEMAN, E R LENG, AND J W DUDLEY I Introduction 45

11 Heterosis and the Gene-Enzyme Concept 46

111 Heterosis and Enzyme Activity during Germination 54

IV Genetic Control of the Initial Reaction of Nitrogen Metabolism 63

V Specific Chloroplast Activity , 72

VI Some Recent Developments in Plant Biochemistry Related to Heterosis 74 VII A Concept for the Future 80

References 83

PRESERVATION OF SEED STOCKS BY EDWIN JAMES I Introduction 87

11 Theories Regarding Seed Deterioration 88

111 Methods of Preserving Seeds 94

IV The National Seed Storage Laboratory 101

References 105

ix

X

BY L H P JONFS AND K A HANDRECK

I Introduction .

I1 111 Silica in the Plant .

IV Silica in Relation to Plant Growth .

V Silica in the Ruminant Animal .

VI The Silica Cycle .

References .

Factors Affecting the Silica Content of Plants .

I I1 I11 IV V VI VII VIII I I1 I11 IV V VI VII VIII IX X XI XI1 XI11 XIV XV SOIL PHOSPHORUS BY SICURD LARSEN Introduction .

Phosphorus in Soil Solution .

Soil Phosphorus in the Solid Phase .

Kinetics of Soil Phosphorus Reactions .

Mobility of Soil Phosphorus

Outlook .

References .

Geochemical Aspects of Soil Phosphorus .

Agronomic Considerations

GROWTH AND MINERAL NUTRITION OF TOBACCO BY C B MCCANTS AND W G WOLTZ Introduction .

Origin and Characteristics of Classes of Tobacco .

Seedling Growth .

Plant Growth and Nutrient Uptake

Nitrogen .

Phosphorus .

Potassium

Calcium .

Magnesium

Liming .

Chloride .

Boron .

Sulfur .

Manganese .

Other Minor Elements .

References .

107

108

122

129

135

144

145

151

152

154

167

182

193

196

205

206

212

213

215

216

222

233

238

243

245

248

251

254

257

258

260

261

CONTENTS xi

THE MATURITY GENES OF SORGHUM

BY J R QUINBY

1 Introduction .

I1 Cultivated Sorghum .

IV VI VII IX I11 The Four Maturity Gene Loci of Sorghum .

Effect of Environment on Time of Flowering .

V Control of Leaf Number by Time of Floral Initiation Interaction of Maturity Genes in the Milos and Hegari Interaction of Maturity Genes in the Heterozygous Condition Allelic Series at the Maturity Gene Loci .

X Influence of Time of Floral Initiation on Plant Size VIII Identification of Sorghum Varieties for Maturity .

XI Maturity Gene Loci and Heterosis .

XI1 Effect of Heterosis on Time of Flowering .

XI11 Physiology of Flowering in Sorghum .

XIV Discussion and Summary .

References .

. 267 268 269 271 277 278 279 282 290 296 297 298 300 301 304 SOIL AND FERTILIZER REQUIREMENTS FOR FORESTS OF Pinus radiata BY M RAUPACH I I1 111 IV V VI VII VIII Introduction .

The Importance of the Species Characteristics of Growth and Climatic Soil Factors Restricting Growth Assessment of Limiting Factors Effective Addition of Fertilizers Field Practices .

Conclusion .

References .

.

.

Tolerance .

.

.

.

.

.

.

307 308 311 314 322 343 347 349 350 AUTHOR INDEX . 355

SUBJECT INDEX . 368

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MECHANICAL RESISTANCE AS A SOIL FACTOR INFLUENCING THE GROWTH OF ROOTS

AND UNDERGROUND SHOOTS

K P Barley and E L Greacen

Waite Institute, University of Adelaide and Division of Soils, Commonwealth Scientific

and Industrial Research Organization, Adelaide, Australia

Page

11 Types of Deformation Produced by Plants 2

A Tensile Failure , 3

B Shear Failure without Compression 4

C Shear Failure with Compression , 4

111 Forces Required to Deform Soils 5

A Theory , 5

B The Assessment of Mechanical Resistance 10

C The Effect of Pore Water Pressure and Void Ratio on Mechanical Resistance 14

IV Forces Exerted by Roots and Shoots 18

A Morphological Adaptations , 18

B Magnitude 19

C Physiological Origin - 20

V Effects of Mechanical Stress on the Growth of Roots and Shoots 24

A Steady Stress 24

B Perturbation of Stress 29

VI Growth in the Soil 30

A Growth in Media of Known Mechanical Properties 30

B The Interaction of Mechanical Resistance, Water Supply, and Aeration 37

VII Conclusion 40

References 40

I Introduction , 1

I Introduction

Man has been aware of the importance of the mechanical properties

of the soil since agriculture began He cultivated the soil when it was moist because it was then easier to deform He was well aware that the emergence of his seeded crops could be hindered by a hard crust

In the late nineteenth century the work of Darwin and others stimu- lated considerable interest in the adaptation of plants to their mechanical environment In the same period fundamental discoveries were made

1

2 K P BARLEY AND E L GREACEN

about the chemistry of the nutrition of plants, and after the turn of the century interest was centered on this subject Later the center of scientific interest shifted to physical studies of the water, air and heat relations of the plant Although it was realized that the mechancial properties of the soil could sometimes be of great importance, the slow development

of soil mechanics hindered further analysis of the influence of this soil factor on plant growth The fact that soil mechanics has been the domain

of the engineer has been a further handicap in applying the subject to agronomic problems Frequently a practical empirical solution has been obtained by the engineer, which, although it solves a construction prob- lem, may do little to explain the processes involved

Recent discoveries in soil and in plant mechanics promise better understanding of the way in which mechanical properties of the soil influence plant growth In this review we intend to discuss chiefly the penetration of the soil by roots and emerging shoots We remind the reader that mechanical factors also operate in other processes of con- siderable agronomic interest; a few examples are the burial of fruiting organs by certain crop and pasture legumes, the radial enlargement of edible underground organs, and the uprooting of crops or trees

Although roots and shoots may grow mainly through existing voids

in openly structured soils, whenever these organs penetrate peds or horizons that lack wide pores they have to deform the soil The soil resists deformation, and the growing organ is stressed mechanically by the re- action of the soil to the force that the organ exerts It is well known that strongly cemented or indurated horizons exclude roots, and that strong crusts prevent emergence (Lutz, 1952); but in this review we aim to assess the importance of mechanical resistance in ordinary soils

We define mechanical resistance as the reaction of the soil to forces

exerted by the growing plant As the intercellular or “pore” space within

plant organs is normally highly permeable to both air and water, differ- ences in pore fluid pressure cannot be long sustained across a plant-soil boundary Large gradients, of course, may exist within the soil itself It follows that, except in transient states, we are concerned with the reac- tion on the plant of the solid phase of the soil

II Types of Deformation Produced by Plants

The theory of soil mechanics, and the methods used to measure the mechanical properties of soils have been developed almost exclusively for engineering applications The foundations engineer is concerned with the maximum force that a soil can withstand without undergoing a large displacement, that is, with the ultimate strength of the soil; whereas the biologist wants to know the force that will deform a soil sufficiently

MECHANICAL RESISTANCE OF SOIL 3

to allow a root or shoot to grow Differences in scale are also important: the engineer deals with stresses acting over areas of square meters and

can employ a statistical concept of stress; in plant studies we are con-

cerned with areas of the order of one square millimeter, and the plant organ is often commensurate in size with the structural or mechanical elements of the soil

A TENSILE FAILURE One manifestation of tensile failure is the rupturing of soil crusts by emerging shoots An appropriate measure of the strength of crust mate- rials being deformed in this way is the modulus of rupture (Carnes, 1934) The force required to rupture the crust depends on the dimensions

of the ruptured plates, and emergence should be related to this force rather than to the modulus itself Arndt (1965) points out that rupture

of the surface crust can be followed by jamming of the broken plates of soil (Fig l a ) , This increases the force required for emergence

FIG 1 ( a ) Examples of soil deformation by emerging seedlings The surface

seal has cracked naturally, or been ruptured by the plant, with the plates subse-

quently jamming Jamming occurs when a + dl + d' < (a' + z')"' ( b ) Shear failure

in the form of an inverted cone (From Arndt, 1965.)

Roots can also rupture soils by tensile failure Barley et al (1965) observed that radicles of peas, Pisum sutivum L., 2 mm in diameter,

were able to split cores of compact loam (Fig 2 ) In contrast, the thinner

(0.3 mm diameter) radicles of wheat, Triticurn aestivurn L., formed channels in cores of compact loam, but the bursting force was not great enough to rupture the cores

Rupturing may involve either general or local tensile failure When

4 K P BARLEY AND E L GREACEN

B SHEAR FAILURE WITHOUT COMPRESSION

Besides failing under tension, soils also fail under shearing stresses imposed by plant organs Terzaghi (1943, p.119) describes general shear failure in soils under shallow foundations In Terzaghi’s model the soil compresses little with increasing application of the load until a critical load is reached, when the soiI fails completely Failure takes place on a sliding surface described by a plane and a logarithmic spiral The load that the soil will support depends on the strength parameters, apparent cohesion, c, and the angle of internal friction, + (Terzaghi, 1943)

The kind of failure described by Terzaghi has been observed when roots first penetrate saturated clay (Cockroft, unpublished data) An example of general shear failure caused by seedling emergence has been given by Arndt (1965) (Fig l b ) ; the soil fails along the surface of an

inverted cone having its apex at the top of the seedling,

C SHEAR FAILURE WITH COMPRESSION

In unsaturated compressible soil much of the volume increase of the growing plant organ may be accommodated by compression, and the

MECHANICAL RESISTANCE OF SOIL 5

zone of shear failure in which the stresses are in “plastic equilibrium” (Terzaghi, 1943, p.23) may frequently fail to spread to a soil boundary When this is so we speak of “local shear failure.” Examples of local shear failure with compression caused by growing roots have been given by

Barley (1954, 1963) Roots were shown to have compacted coarse tex- tured media for a radial distance of several millimeters around the root The volume of the cores in which the roots were grown remained con- stant Shear, together with compression, is probably the most common way in which growing plant organs deform ordinary, unsaturated soils

In saturated clay plant organs may form channels by consolidation together with shear failure If the volume of the root is accommodated without displacing the boundaries of the clay, as water and clay are only slightly compressible, water must be either absorbed by the pene- trating root or drained through an outer boundary of the clay This process, by definition, involves consolidation ( Terzaghi, 1943, p.265) ,

but, as a hole is being formed, shear failure must also occur

The process described above differs from one-dimensional consolida-

tion as met in engineering practice In one-dimensional consolidation the

consolidating axial stress, ul, and the resulting radial stress, us, are not

in plastic equilibrium but are related by the expression u3 = K,u,, where

KO is the coefficient of earth pressure at rest For medium-textured soils with 9 = 40°, K O z 0.5, and for clays with lower values of 9, KO varies from 0.6 to 1.0 When consolidation is accompanied by shear failure the two stresses are related by the coefficient of active earth pressure, K ,

(Terzaghi, 1943, p.50); K , is as low as 0.2 for coarse-textured soils but can approach 1.0 for clays

Ill Forces Required to Deform Soils

A THEORY

1 Tensile Failure

General tensile failure of surface crusts is commonly treated in terms

of elasticity theory In the modulus of rupture test the force, F , required

to rupture a slab of length a, width b, and thickness z, for single-center point loading is given by

and for two-point loading at a / 3 and 2 a/3 by

6 K P BARLEY AND E L GREACEN

where up is the tensile strength of the soil Analyses of tensile failure for more complicated configurations are available in the theory of elasticity ( Timoshenko and Goodier, 1951 )

The tensile rupture of bulky structures can also be described theoreti- cally Applying a spherical model, the zone of plastic equilibrium around the base or point of a probe can be treated as a pressure bulb of radius R

(see Section 111, A, 3 ) The radial pressure at R, u ~ , will burst a soil clod

if the cross-sectional area of the structural element is such that tensile resistance is less than the force developed over the cross section of the pressure bulb Whether a clod will fail in tension depends then on the magnitude of uR, the tensile strength of the soil uT, and on the size of the clod If rupture occurs during radial enlargement rather than during penetration a cylindrical model should be used

Local radial cracks may develop either around individual roots or be- tween adjacent root channels (Fig 2 ) Using either a spherical or cylin-

drical model, the tangential stress U t , which reaches a maximum at R,

closely approaches the tensile strength of the soil Where the plastic zones

of adjacent roots overlap v(Tt is increased, and local rupture is likely to occur

2 Shear Failure without Compression

The conventional description of forces acting on the base of a pile

or probe (Terzaghi, 1943) shows that the bearing capacity qp of a shallow ( z = d ) foundation, of depth z and width d, failing in general shear, is given by

q p = cNc + P Z N , + pdN, (3)

where c = apparent cohesion, p = bulk density, and N,, N,, Np =

bearing capacity factors

The values of the bearing capacity factors depend only on the angle

of internal friction, +, When saturated clays are distorted with negligible drainage, the strength of the clay is not altered by an applied load since

the load is carried by the pore water (see Section 111, C, 1) Shear strength is then determined solely by c, and the soil is called a friction- less or + = 0 soil For circular shallow footings in saturated undrained clay qp z 7.5 c According to Terzaghi’s model qp increases continuously

with x This relation applies to rough probes entering saturated “un-

drained” clays, the requirement of the “undrained condition being met either because the clay is so impermeable that it fails to consolidate, or because the rate of loading or penetration is so high that there is time for only a negligible amount of consolidation

MECHANICAL RESISTANCE OF SOIL 7

With the exception of Terzaghi’s analysis for shallow foundations there are few analyses of general shear failure appropriate to biological problems The general shear failure that sometimes occurs above upward acting penetrometers and seedling shoots is described in an analysis given by Balla (1961) for the anchorage of mushroomed pylons Soh-

tions require the strength parameters c and + and the configuration of the system

3 Shem Failure with Compression

Where the soil does not behave as an ideal brittle or plastic material, but is compressed or consolidated during deformation, conventional theory is inadequate For deep piles, z > 3d, a “plasticity” theory modified from that of Terzaghi is usually employed (Meyerhof, 1951) Although Meyerhof‘s theory implicitly describes local shear failure, as shearing is depicted as occurring in a localized zone around the base of the pile, compression is not described explicitly According to Meyerhof, for homogeneous saturated clay soils failing without drainage ( 4 = 0),

qp attains a steady maximum at depth where q p = 10 c Strictly, qp cannot attain a steady maximum in such materials, because the shearing zone would have to extend to the full depth of the pile But real clays are neither truly saturated nor homogeneous, and in practice the volume of the pile may often be accommodated locally, for example by displacement

of the clay into cracks or fissures In compressible soils, following Ter- zaghi (1943, p.130) an arbitrary reduction is made in c and 4 The bear- ing capacity factors have been elaborated by Meyerhof (1961) to include the shape and roughness of the pile His theory is useful for saturated clays and for soils having 4 < 35” and failing with little compression Since the factors become highly sensitive to changes in + for values >

35”, and as a large arbitrary reduction in + must be made in compressible soils, the theory lacks general utility

An analysis of the resistance offered to probes in compressible soils has recently been made by Farrell and Greacen (1966) Following earlier work on the distribution of stress in soil around holes ( d e Jong and Geertsma, 1953 ) , tunnels ( Terzaghi, 1943), and around piles (Nishida, 196l), they postulate the existence of two main zones of com- pression around the point of a penetrating probe: a zone of shearing failure called the plastic zone, and outside this an elastic zone (see Fig

3 ) Farrell and Greacen assume that the pressure on the base of a probe

is equal to the pressure required to form a spherical cavity in the soil

This approach is not new Previously Bishop et al (1945) had used the model of an expanding cavity in a study of indentation tests in copper Ladanyi (1963) used a similar model to describe pile penetration into a

8 K P BARLEY AND E L GREACEN

saturated undrained clay, and Nishida ( 1961) calculated the pressure required to expand a cylindrical cavity in the soil

The new contribution of Farrell and Greacen is their treatment of the compressibility of the soil The analyses of Bishop et al and Ladanyi concerned incompressible material Nishida assumed that the volume change was determined by the mean principal stress, ( u1 + u2 + ~ , ) / 3 ,

where the subscripts refer to the principal stresses Vanden Berg et al

(1958) also used the mean principal stress, but Sohne (1958) used the major principal stress Farrell and Greacen largely overcome this ambi- guity by using an experimental curve for compression accompanying

PRINCIPAL STRESS U, (bar)

(a)

FIG 3 Compression curves ( a ) associated with the zones of compression I-IV ( b ) around the point of a penetrometer in compressible soil: I , e = emin, 11, failure zone, I l l , rebound zone, and lV, elastic zone

shear failure In the plastic zone there are three distinct subzones of compression (Fig 3 ) : I, where the soil is compressed to the minimum

void ratio’ emin; 11, where the soil undergoing failure behaves as a material being compressed for the first time; 111, a rebound zone where the soil behaves as an “overconsolidated” material (see Section 111, C, 2 )

After equating the change in volume of voids in the various zones with the volume of the probe, Farrell and Greacen find the radius of the plastic zone, R, and, knowing this, the pressure qp on the base of a smooth (frictionless) cylindrical probe The theoretical value of qp for a smooth

’ I t is mathematically convenient to express the state of compaction of the soil as

void ratio, e, rather than bulk density, p e = p./p - 1, where p = absolute density

of solid phase Similarly, volumetric water content, 8 , is conveniently replaced by e ,

and air space, a, by e,

MECHANICAL RESISTANCE OF SOIL 9 probe can be checked experimentally by rotating a real probe to dissipate friction in the tangential direction When this was done Farrell and Greacen found good agreement between theoretical and measured values

of qp in a range of finely structured soils

Ordinarily, friction is mobilized both at the base (“point” friction) and along the curved cylindrical barrel (“skin” friction) of a probe Point friction is appreciable for metal probes in soil For example, it increases the value of qp for real as opposed to smooth probes by as much

as 40 percent when the angle of soil-metal friction, 8, = 23” (Farrell and Greacen, 1966) When the additional expression for point friction is incor- porated, the theory of Farrell and Greacen may be used to predict qp

for real, nonrotated probes The agreement obtained with measured val- ues for steel probes in three soils is shown in Table I (see p 15)

It seems likely that qP for root tips is less than qp for steel probes, as

an estimate of the friction angle, 6, for the interface between root tips and sand (Barley, 1962) suggests that SrOOt-SO,l < Ssteel-soil (see Section

111, A, 4) However no data are available for the immediately relevant interface between root cap and soil It is possible that the well known secretion of mucigel by cells of the root cap is a means of reducing 6

Recently Farrell and Greacen have extended their theoretical analysis

to include cylindrical enlargement Surprisingly, when 4 is large, say

40”, the pressure required for the radial enlargement of a cylindrical cavity is only one-fifth of that required for a spherical cavity The dif- ference between the two pressures decreases with decreasing values of 4

Clearly, the shape of a penetrating object may have a large influence on the resistance encountered in high 4 soils The cylindrical model is likely

to be more appropriate when the tip is acutely tapered

4 Skin Friction

In foundations-engineering the total axial pressure, q, that a pile can withstand, or, in other words, the axial pressure that has to be applied to penetrate the soil, is termed the bearing capacity and is given by

Kuz, where a, is the effective axial pressure and K is a coefficient of earth

10 K P BARLEY AND E L GREACEN

pressure Then, Qr = 2 ~ / o x K ~ Z r tan 6 dx For rough piles 6 may be set equal to 4

Little is known about the skin friction and adhesion at the interface between plant organs and the soil One value of 8, reported for a root-

“soil” interface, pertains to the root tip of maize and a moistened plate

of cemented sand (Barley, 1962) This value of 6 was obtained directly

by the following method: first, root tips with a flattened “face” were obtained by pressing roots against the plate as they grew The tip was then severed and secured to a slider with small barbs Finally, the flat face of the root tip was forced against a portion of the plate mounted on

a friction trolley The measured value of 8 was 17”

Recently Barley and Stolzy (1966) used as a crude measure of Qf the force required to pull out a penetrating root tip For peas (Pisum

sativum L.) in a moist loam Q, was one-fifth of the total resistance to

penetration Q The pulling method is used in engineering to measure Q, for piles, and it is usefuI in clays In sands the radial pressure on the pile

is relieved by the upward pull and friction is underestimated

In contrast to piles, where the whole buried length is pushed through the soil and meets with frictional resistance, in the root only the short length from the cap to the proximal limit of the zone of elongation is pushed through the soil Friction occurs behind the zone of elongation, but it is mobilized as anchorage to assist penetration, For emerging shoots the location of the zone of elongation relative to the apex differs widely

between species (Leonhardt, 1915) In many plants an appreciable part

of the shoot is pushed upward through the soil, and skin friction cannot

be safely neglected in any analysis of the resistance opposed to emergence

B THE ASSESSMENT OF MECHANICAL RESISTANCE

Estimates of the mechanical resistance opposed to growth must be based on knowledge of the type of deformation produced by the plant root or shoot The type of deformation determines not only the soil properties to be measured, but also, as we shall see, the methods to be used in measurement

1 Determinatwn of Strength Parameters

The parameters that describe the strength of a soil failing by shear with little or no compression are the classical strength parameters c and

4 The relationship between these parameters and certain derived meas- ures of strength is described diagrammaticaIIy in Fig 4 For any particu- lar normal load, un, acting on a plane of failure, c and 4 give the shear strength, sn, according to the Coulomb equation

MECHANICAL RESISTANCE OF SOIL 11 The Mohr circle for the unconfined compressive strength, uc, is shown

in Fig 4; it can be seen that uc depends on c and 4 Farrell et al (1967) have shown that, at pore water pressures as high as -0.3 bar, compact loams behave as brittle materials, for which uc = Sor (Griffith, 1924) Where the sample is in the form of a core, either natural or remolded,

FIG 4 Mohr diagram for an unsaturated soil with the failure envelope described

by c and @, u1 and u3 are the principal stresses; in a triaxial test these are the axial and the radial stresses, respectively The shear stress 7 = ( uI - u3)/2 Mohr circles for the compressive strength, uc, and the tensile strength, uT, are also shown

UT can be measured indirectly by means of the so-called Brazilian test (Kirkham et al., 1959) or uC can be measured by an unconfined loading

test Both tests are performed in a compression test machine; in the Brazilian test the lateral load required to rupture the core in tension is measured, and, in the second, the axial load required to rupture the core

in shear is measured

Rogowski (1964) has pointed out that the above methods measure bulk strength of the soil and that the bulk strength is usually limited by the inter-aggregate strength Rogowski suggests that intra-aggregate strength may be more important in controlling root penetration, because the root may often penetrate by deforming the adjacent aggregates rather than an extensive zone He proposes that aggregate density be measured, strength then being determined on cores of soil remolded and compacted

to the measured density However soil strength is known to depend on the stress history of the soil, and there is no simple relation between density and strength (Section 111, C, 2 ) Rogowski also developed a tech-

12 K P BAFLEY AND E L GREACEN

nique for measuring the crushing strength of small ( 2 to 3 mm.) aggre-

gates, by rupturing them in an unconfined compression test between two plates He postulates that roots encounter a resistance that depends on the crushing strength of the aggregates However, even if this is so, his analysis is unsatisfactory as it stands because it neglects deformations that precede and accompany failure of the aggregates

Rogowski's criticism of the measurement of bulk soil properties hardly applies when the deformation spreads over a zone that is large compared with the size of the aggregates, that is, in finely structured soil In soils where the aggregates are commensurate in width with the plant organ concerned, Rogowski's approach may be profitable

The derived measures: modulus of rupture, the Brazilian test, the compressive strength, and the crushing strength each give a single Mohr circle on the strength diagram (Fig 4 ) Because of this any one of these measures provides useful comparative data only where 4 is constant or

almost so As mentioned in Section 111, A, 2, saturated, undrained clays behave as if they were 4 = 0 materials In unsaturated soils or in fully drained clays 4 usually varies between 20" and 45" (Fountaine and Brown, 1959), not being greatly affected by changes in void ratio or pore water pressure It should be noted, however, that occasionally much lower values have been reported (Payne and Fountaine, 1952)

A satisfactory characterization of strength for failure with little or no compression is obtained by describing the failure envelope on a Mohr diagram with one of the recognized techniques The torsion shear box (Payne and Fountaine, 1952) or the direct shear box (Terzaghi and Peck, 1948) are often employed, the former being useful for small (25 cc.) samples or peds The most versatile method for soil cores is the triaxial compression test, a comprehensive account of which is given by Bishop and Henkel (1962)

Where the deformation involves local shear failure with compression, analytical estimates of mechanical resistance require the strength parame- ters c and 4 together with a measured compressibility curve The com- pressibility characteristics may be expressed as a Young's Modulus and

as the gradients of the failure and rebound curves for compression with

shear (see Section 111, A, 2) The parameters c and 4 and the compressi- bility characteristics are equally important in determining the resistance

to penetration As Farrell and Greacen (1966) have shown they can be

measured with sufficient accuracy by means of the triaxial cell,

No general relation is to be expected between void ratio, e, and the resistance that soils offer to penetration, Q When e>>e,,i, for a par- ticular soil most of the volume change occurs in the zone of compression with failure; as e approaches emin the rebound zone and the zone of

MECHANICAL RESISTANCE OF SOIL 13

elastic compression become important This change of process is responsi- ble for the lack of any general relation

2 Empirical Measures of Mechanical Resistance

Although empirical measures of mechanical resistance, such as penetrometer data, contribute little to physical understanding and provide little scope for generalization, they may be useful in diagnostic work As illustrated in Fig 5 the point resistance, Qp, offered to a probe

RELATIVE DEPTH OF PENETRATION (Z/d)

A

4

FIG 5 Fractional point resistance, Qp/Qp mnx, as a function of z/d for a shallow

( z = d ) and a deep ( z > 3 d ) test in a compressible soil

increases with z to a steady maximum when x exceeds several diameters The force required to indent the soil is customarily measured by a shal-

low test or “indentation” test in which x = d It can be seen from Fig 5

that Qp is still increasing rapidly where x = d This introduces a serious

source of variability in the shallow test, as errors of +2O percent can easily be made in measuring the depth of penetration of say a 5 mm diameter probe

An alternative to penetrometer testing that has been fashionable in

foundations engineering is the vane shear test (Carlson, 1948) This

method was developed initially for saturated clays that behave in rapid tests as if 4 = 0 Evans and Sherratt (1948) have shown that for 4 < 10”

the vane shear strength can be related to c and +, but for higher values

of + the frictional component becomes overriding No adequate analysis has been made of the mechanics of the vane test in high 4 soils

14 K P BARLEY AND E L GREACEN

In a recent study emergence of shoots has been related to indentation test data using downward acting probes (Parker and Taylor, 1965) (see Section VI, A ) ; but upward acting probes would seem to be preferable in that the boundary conditions for the test are then more appropriate (Morton and Buchele, 1!360) Arndt (1965) devised an upward acting probe for use in the field, the apparatus being buried in the soil before weathering of the seed bed had taken place, As the use of Arndt’s device

in the field is extremely tedious, simpler methods should be examined Bennett et al ( 1964) measured the force required to pull up a line buried

horizontally in the soil, and showed that the pull was negatively cor- related with the emergence of cotton seedlings A simple empirical test that is mechanically more apt could be conducted by using a buried bead several millimeters in diameter and measuring the force needed to pull this from the soil with a fine wire

Although cylindrical probes provide a relative measure of resistance

to penetration, and are useful in correlative studies (see Section VI, B, 2 ) ,

probe data should not be identified with the absolute resistance encoun- tered by growing organs Discrepancies arise for many reasons; the chief reasons are as follows: ( 1) Growing organs are flexible and tend to grow

around obstructions ( 2 ) The shape of plant organs differs from that of

cylindrical probes; moreover the shape is influenced by the resistance of

the soil ( 3 ) The stress distribution around a plant organ, unlike a rigid

body, depends not only on its shape and on the soil properties, but also

on the anisotropic properties af the tissue (4) Friction and adhesion at the interface between plant and soil may differ from that between probe and soil (5) Uptake of water by roots causes local changes in the pore

water pressure and hence in the strength of the soil ( 6 ) In saturated soils

the root creates additional opportunities for drainage

The biological aspects will be further explored in Section VI, A Un- less the differences between probes and plant organs are understood we cannot hope to relate theoretical or measured values of Q to the mechan- ical resistance experienced by roots or shoots

C THE EFFECT OF PORE WATER PRESSURE AND VOID RATIO

ON MECHANICAL RESISTANCE The data in Table I provide a clear illustration of the extreme depend- ence of qp on pore water pressure, uw, and void ratio, e It is worth noting

that the strength of unsaturated soils can change considerably even when there is little change in the water content; indeed the change in strength

is most rapid when the water-filled void ratio, e,, is appreciable and the gradient de,/du, is small Note, for example, that for the Parafield loam described in Table I, at e = 0.56, qp increases from 20 to 34 bar when

MECHANICAL RESISTANCE OF SOIL 15

TABLE I

Comparison of Theoretical with Measured Values of Point Pressure ( q p )

for Steel Probes in Three Soils

Pore water pressure” Water-filled

Pore water pressure uul = -h, where h is the suction in the soil water, both uw and

h being referred to atmospheric pressure as datum It is more convenient to employ uw

in mechanical studies, as pressures above and below the datum exist simultaneously in different parts of the soil-plant system

urn is decreased from -0.3 bar to -0.7 bar, the decrease in e , being only 0.04

1 Pore Water Pressure and Effective Stress

In a saturated soil a decrease in u, has the same effect on strength as

an increase of equal magnitude in the externally applied pressure (Childs, 1955) Skempton (1960) has discussed the effect of am on the strength of saturated soils from the engineering point of view, and should

be consulted for a more detailed account

Terzaghi (1923) showed experimentally that for a saturated soil the

degree of unidirectional consolidation depended on the “effective” stress,

d, defined as d = u - urn, where u is the applied normal stress Similarly the bulk modulus, p, of a saturated soil experiencing isotropic compres- sion is given by p = dp’/dc, = d ( p - u,) /da,, where p and p’ are the applied and effective pressures and E, is the cubical dilation Generally,

if c and 9 had been defined in this review as intrinsic properties of the soil at datum pressure, effective rather than applied stresses would have had to be substituted in equations such as ( 5 ) that contain c or $ In practice it is often more convenient to work in terms of applied stresses

and use apparent values of c and 0 obtained under conditions of testing

16 K P BARLEY AND E L GREACEN

(drainage, rate of deformation) that pertain to the deformation being studied For example, if mechanical properties are to be related to root penetration, tests should be conducted with full drainage at low rates of deformation (slow drained tests)

In unsaturated soil, where the pores contain both air and water, the pore water pressure is regarded as acting over an effective area x per unit area of the soil The effective stress is then given as

When the soil is saturated x = 1 and Eq ( 6 ) may be identified with Terzaghi’s definition given above Bishop ( 1960) shows experimentally that x is a nonlinear function of the degree of saturation The function exhibits hysteresis and depends on the stress history of the soil Bishop’s relations between uw and effective stress are satisfactory where U, is held constant during deformation, or alternatively where the volume of soil being strained is so small relative to the bulk of the sample that uw is buffered by internal drainage However, where the bulk of the soil is deformed, as in most testing procedures, u, may differ markedly from the initial pressure, particularly if the test is rapid or the moisture conduc- tivity is low Croney and Coleman (1954) show that in undrained satu- rated soils uw changes considerably with the degree and rate of straining Greacen (1960) and Bishop (1960) extended this result to unsaturated soils Again, where the deformation involves compression, the influence

of uw on compressibility must be taken into account by measuring the compressibility curves at a number of initial water contents (Farrell and Greacen, 1966)

In addition to changes in uw arising from deformation of the soil, we have to remember that the transpiring plant can transmit large suctions

to the soil water The probable magnitude of gradients in uw around roots

arising from transpiration is shown, for example, by Gardner (1960) As

the elongating tip of the root is permeable (Rosene, 1937), the tip presumably takes up water together with the proximal parts of the root Indeed the local decrease in uw due to transpiration may often be more significant than the change associated with deformation

2 Void Ratio

Although it is obvious that compact soils are hard to deform, failure

to appreciate the nature of the relation between void ratio and pene- trability has hindered progress Veihmeyer and Hendrickson ( 1948) pro- posed that the inability of roots to penetrate particular soils below a certain critical void ratio could be attributed to the lack of pores of suf- ficient width I t is now recognized that the mechanical resistance of the

MECHANICAL RESISTANCE OF SOIL 17

soil determines the chances of penetration, small pores usually but not always being associated with higher strength

For a saturated soil that is being consolidated for the first time (Fig

6, curve a b ) the relation between e and the uniaxial consolidation pres-

sure, U, is given by

where I, = a constant for the particular soil called the compression index; and e, = the void ratio at an arbitrary pressure u0

In soils that have been consolidated and then unloaded (curve b c ) ,

the void ratio depends on the maximum consolidation pressure ex- perienced, ub, and the extent of swelling following unloading The effect

of a previous cycle of compression on the compressibility of such “over- consolidated soil is evident from the reloading or “rebound” curve ( c d )

If the pressure is increased beyond ~b the soil is compressed along a continuation of the “virgin” or “normal” consolidation curve (ub )

In the classical Coulomb equation (Eq 5) the effect of void ratio on shear strength is not separated For saturated clays Hvorslev (1937)

attempted to express this in the following equation:

s = co exp (- e f / l c ) + a’, tan 40 (8)

where sf = the shear strength at failure; ef = the void ratio at failure;

~ ’ f = the effective normal stress acting on the plane of failure; and c,, +,,

are constants for the particular soil

The first term of the R.H.S of Eq ( 8 ) represents the cohesion as a

function of ef The meaning of the second term can be illustrated by

reference to Fig 6 It can be seen that any particular void ratio, em, may

be arrived at by different loading paths, and that the soil may be in equilibrium at any given value of em at distinct effective normal stresses

Similarly, shear failure at a given value of ef can occur at a number

of values of u’f It is found that sf increases linearly with u’,, and +, is the slope dsf/du‘f Scott (1963 p 383) discusses refinements of the Hvorslev equation

The effect of void ratio on the mechanical properties of unsaturated soil is not as well understood As in saturated clay, e is usually related directly to In u (Farrell and Greacen, 1966) Analogous behavior can also

be seen in a tendency for u, to decrease as overconsolidated, unsaturated soils are sheared In a soil consisting of overconsolidated aggregates the deformation is far from simple; individual aggregates can dilate positively while the soil as a whole undergoes compression and shear (Greacen,

1960)

Such observations emphasize the need to base analytical estimates

18 K P BARLEY AND E L GREACEN

FIG 6 Void ratio, e, as a function of the logarithm of consolidation pressure,

In (I, for a saturated soil

of mechanical resistance partly on empirical data, such as a measured compressibility curve, and to eschew the use of overidealized mechanical models when dealing with ordinary unsaturated soils

IV Forces Exerted by Roots and Shoots

Two classical papers prepared the way for modern work on plant mechanics: Schwendener (1874) not only described ways in which anatomical adaptations help the plant withstand mechanical stress, but

he also made the first measurements of the elasticity and strength of

plant tissues; Pfeffer ( 1893) provided the first detailed, quantitative

account of the forces exerted by roots and shoots By comparing the forces that can be exerted by plant organs with those that are needed

to deform the soil, we can see whether the mechanical properties of the soil are likeIy to influence pIant growth

A MORPHOLOGICAL ADAPTATIONS

Underground plant organs frequently show striking morphological adaptations to growth in a resistant medium One class of adaptations reduces the resistance encountered; for example, in roots the localizing

of elongation in a short zone just behind the cap makes it easier for the root to grow around obstructions and minimizes skin friction Other adaptations assist the organ to withstand the reaction of the soil; for example, the apex of rhizomes is often protected with hardened tissues

MECHANICAL RESISTANCE OF SOIL 19 Others again enable the meristem to evade stress, as does the nutant habit

of the seedling shoot in many dicotyledons

It is not proposed to go into further detail here, as clear accounts of mechanical adaptation are to be found in the literature The most com- prehensive source of information is still Haberlandt’s classical text

“Physiologische Pflanzenanatomie.” Numerous further examples of the mechanical adaptations to be found in underground shoots are given by Leonhardt ( 1915)

B MAGNITUDE

In Pfeffer’s experiments part of a root or shoot was secured within a gypsum block; a second but movable block was then cast around the exposed tip or around one side of the organ, Any force exerted on the second block by the growing organ could then be measured by balancing against a known resistance In practice Pfeffer was concerned solely with static equilibria, measuring the resistance that had to be applied to the second block to prevent it from being moved

Pfeffer found that when an organ was so confined it soon exerted a force The force increased rapidly at first and then more slowly, approach-

ing a maximum in 2 to 3 days The maximum force corresponded to a

pressure of from 5 to 10 bar distributed over the largest cross section within the growing region Although he did not make many measure- ments, Pfeffer’s results suggest that the pressure exerted by root tips is greater in the direction of the longitudinal axis than in the radial direc- tion (see Table 11) On the other hand, Pfeffer found that the axial and

TABLE I1

Pfeffer’s Data on the Maximum Pressure Developed by Confined Root Tips0

Transverse pressure (bar) Axial pressure (bar)

pith isolated from the stem of the sunflower, Helianthus annuus L., and

20 K P BARLEY AND E L GREACEN

Krabbe (1884) had reported that a radial pressure of 10 bar was needed

to prevent an increase in the girth of trees

After the publication of Pfeffer’s paper in 1893 the subject appears

to have been neglected until Williams (1956) measured the force exerted

by the arching hypocotyl of small-seeded legumes Although papers of Gill and Miller (19%) and Barley (1962) helped renew interest in the topic, these authors were mainly concerned with the efEects of stress on growth (see Section V, A, 3 ) Recently, Barley and Stolzy ( 1966) have described a method of measuring the force exerted by root tips penetrat- ing a soil The soil is supported by a force transducer that measures the reaction to the root tip Providing measurements are restricted to the time during which the hairless part of the tip is penetrating the soil, only

a small correction is needed for skin friction

From Pfeffer’s work it is clear that, for a given species and organ, the pressure developed is largely independent of the diameter attained, so that the force exerted increases with the size of the growing organ Even though roots apply a smaller pressure in the radial than in the axial direction, the force exerted in the radial direction is by far the greater, as the pressure acts over a larger area For example, roots of the broad bean,

Viciu faba L., can exert maximum radial and axial pressures of 5 and 9 bar, respectively, but the radial and axial forces that can be exerted by

a 4 cm length of root are 5 kg.wt and 0.3 kg.wt The upward acting

forces exerted by seedling shoots range from 15 g.wt for the thin hypo- cotyls of alfalfa, Medicago satiua, L., (Williams, 1956) to 401) g.wt for the thick hypocotyls of the broad bean (Pfeffer, 18913) Evidently, any environmental factor that changes the dimensions of a growing organ influences the total force that can be exerted on the surroundings The ability of roots or shoots to exert force on the soil depends not only on their physiological properties and shape, but also on the anchor- age provided by the proximal parts of the plant; that is, the force exerted cannot exceed the ability of the proximal parts to withstand the reaction Anchorage is provided by skin friction together with the resistance that has to be overcome to dislodge the seed, root hairs and root laterals Pfeffer found that forces of the order of 40 g.wt per centimeter length were required to pull the hair-covered radicles of broad bean from soils, and that several centimeters of branched root could stand a pull equal

to the maximum axial force exerted by the growing tip of the root

C PHYSIOLOGICAL ORIGIN

1 Osmotically Induced Turgor

The exertion of force by plant organs is most readily explained in terms of their osmotic behavior When pressures are measured with

MECHANICAL RESISTANCE OF SOIL 21

respect to the ambient solution as datum, for a semipermeable tissue at osmotic equilibrium

where x = osmotic pressure of the cell contents, and T = hydrostatic pressure within the cell Strictly, an equilibrium expression for an imper- fectly permeable osmometer should be given here, but the nature of cell permeability does not affect the present argument We disregard varia- tion in and T within the turgid cell, arising from the presence of differentially permeable cytoplasmic membranes Treating forces directed toward the center of the cell as positive, at the cell wall,

where W = pressure exerted by the wall (“wall” pressure); B = pressure exerted by other cells (“tissue” pressure); and P = pressure applied externally by the plant

Thermodynamically, osmotic and swelling pressures are identical

(Hermans, 1949); so, if we assume that meristematic cells offer little

internal resistance to water transfer, then the vacuolar liquid and pro- toplasm should be in or near osmotic equilibrium Further, providing supply of water is not limiting, osmotic equilibrium with the ambient solution is thought to be attained, or nearly so, throughout the zone of cell enlargement ( Ordin et al., 19.56)

If plant forces are osmotic in origin, they may be mobilized either

by an increase in x or by relieving W and B The pressure exerted by the plant attains a theoretical maximum, P,,,,, = x ~ , when W = B = 0 Pfeffer believed that both processes were operative Measuring ro with the plasmolytic method of de Vries (1884) and with the “minimum

length” method often ascribed to Ursprung (1923) in modern texts, Pfeffer (1893) concluded that in broad bean T o rose gradually after the root tip or seedling shoot had been confined, Secondly, Pfeffer showed that elastic strain disappeared from the cells of confined root tips He found that root tips confined for 48 hours or more failed to shrink when plasmolyzed This was not due merely to maturation of the apical tissue, as the tips at once began to elongate when transferred to iced water

Unfortunately, as Pfeffer used potassium nitrate as the osmoticum, his r0 values are excessively high ( > 15 bar) It is now known that this

salt penetrates excessively into root cells Using sucrose at 2”C., Barley

(1962) did not find any increase in x in compressed growing radicles of the tick bean (Viciu faba L., var MINOR) Neither Pfeffer nor Barley detected any increase in T in compressed radicles of corn Whether or not there is a buildup in x in some species, the relief of wall and tissue

22 K P BARLEY AND E L GREACEN

pressure appears to offer a ready means of mobilizing osmotic turgor to perform external work Although the plant material is not directly com-

parable, it is interesting to note that the value of P,,, found by Pfeffer for the root tips of corn agrees with the r,, value obtained by Barley: P,.,,

= To = 11 bar

2 Nonosmotic Contributions to Turgor

Even if we can account for the magnitude of the pressure measured

by Pfeffer without the need to invoke other than osmotic processes, this

in itself does not show that osmosis is the only process involved How- ever, no other process has conclusively been shown to raise the hydro- static pressure within plant cells Bennet-Clark ( 1959), having reviewed the evidence in favor of “active” uptake of water by plant cells, suggested that the strongest evidence was provided by data showing the osmotic pressure of expressed sap to be generally less than the plasmolytically

determined value A more straightforward explanation of this dis-

crepancy, however, is provided by the tendency for osmoregulation to occur during exposure to an osmoticum, either by solute transfer or by hydrolysis of cell polymers

In commenting on the water relations of Nitella, Dainty (1963) notes that although small differences in electrical potential across charged pores might theoretically lead to substantial turgor differences across the membranes concerned, such differences could not in fact be realized

in Nitella as outward flow can occur through numerous uncharged pores Similar reasons may rule out electroosmotic or other “active” contribu- tions to turgor in higher plants, but at present too little is known about the properties of cell membranes for us to decide

3 Other Forces of Metabolic Origin

So far we have considered only those forces that depend on cell turgor We also need to ask whether forces might not arise from the propensity of growing tissues to accumulate, synthesize, or transform materials other than water A sol + gel transformation, for example, is associated with cell division; before furrowing begins protoplasmic sols are converted to gels Furrowing and cleavage are then brought about

by the contraction of the gels, and energy used in building up the structure of the gel can be expended as work as the gel contracts and reverts to a sol (Landau et al., 1955) Forces that might be associated with the surface extension of the cell wall or cell membranes also need

to be considered, whether or not they are adsorptive in origin as Bell

(1961) suggests

Although such phenomena provide interesting examples of ways in

MECHANICAL RESISTANCE OF SOIL 23

which metabolic energy may be expended as work, it has to be remem- bered that the rigidity of meristematic tissue is almost wholly dependent

on cell turgor When the tissue is turgid, the cell walls cannot themselves

be load bearing, as they are stretched, not compressed, and wall pressure

is directed centripetally Only when turgor is fully mobilized against an external resistance, and when wall tension is removed, can the tendency for surface extension of the wall lead to the exertion of a force By measuring the force exerted by root tips of broad bean growing at incipi- ent plasmolysis, Pfeffer (1893) concluded that wall growth gave rise to forces about one-tenth as large as those produced by turgor His experi- ment has not yet been repeated One might expect that compression of thin, flexible cell walls would lead to buckling and bending of the wall, and changes of this kind have been described by Hottes (1929)

Where cell walls have been strengthened, continued growth of the wall may well give rise to forces independent of those produced by tur- gor Even so, the ability of thin-walled cells within an organ to withstand compression may continue to set a limit to the pressures developed during growth In this connection it is worth noting that the pressures exerted

by enlarging trunks of trees, in which many of the cells have strong walls, are comparable with those produced by delicate root tips (see Section

IV, A )

4 Energy Expended on External W o rk

We have considered contributions to plant forces that may be made

by osmotic and “active” uptake of water, by cell division, and by wall growth The forces observed arise most obviously from osmotically induced turgor Whatever contributions may or may not be made by other processes, it is important to consider also the energy required for external work in relation to the total energy available to the plant

To give an example, a root of 1 mm diameter, elongating at 1 mm hr.-l against a resistance of 10 bar, performs external work at the rate

of 0.2 erg sec.-l; whereas energy is released during respiration by the root tip at rates of the order of lo2 erg sec.-l Work may also be per- formed in stretching the cell wall, but again this is small ( Frey-Wyssling, 1952) It is clear that the energy expended on mechanical work during growth is trivial compared with the output of respiratory energy, Because

of this, it is sometimes inferred that mechanical resistance is not likely

to be important However, little is known about the efficiency with which the plant “engine” performs mechanical work Moreover, even if suffi- cient energy is available, growth may be altogether prevented by a sufficient resistance, as there is a definite upper limit to the force that a plant organ can exert on its surroundings

24 K P BARLEY AND E L GREACEN

V Effects of Mechanical Stress on the Growth of Roots and Shoots

In Section I11 we saw that large pressures are often required to create channels in soils For example, in loams of modest strength the pressure needed to lengthen a channel is of the order of 10 bar Clearly, root tips or emerging shoots experience large stresses as they penetrate finely structured layers or peds of soil

Although the study of stress-stain relations in a particular organ may help us to interpret a growth response, we are much less concerned here with the strains produced in a given organ when a stress is first applied, than we are with the way in which growth proceeds after a stress has been applied

It may sometimes be overlooked that in studying underground shoots

we are dealing with dark-grown or etiolated organs, and that conclusions reached with shoots growing in the light may not apply Particular care needs to be taken in extrapolating from experiments with specialized shoots such as tendrils, that show marked growth responses both to contact stimuli and to tension ( Brush, 1912)

as the external normal stress acting in the direction of the longitudinal axis of a plant organ, and a,, ay as the external normal stresses acting in the direction of the remaining Cartesian axes When = uy we replace them by ur, the radial stress Although we deal only with applied stresses we note that these are superposed on whatever stresses arise within the plant organ

The effects of mechanical stress on the processes of cell division, cell enlargement, and differentiation have rarely been separated in experi- ments, so that it is more expedient to classify the available data according

to the nature of the applied stress We begin by considering the influ- ence on growth of a simple axial tension or pressure

In what follows we define

The influence of tension on stem growth has been studied intensively

by physiologists for two distinct reasons First, following claims by Pfeffer’s school at Leipzig, considerable interest was taken at the turn

of the nineteenth century in the question of whether applied tension led

MECHANICAL RESISTANCE OF SOIL 25

to the regulatory development of woody tissues in stems Unfortunately from our present point of view the work concerned was conducted entirely with stems grown in the light Although good evidence was obtained showing that the tensile strength of certain stems increased when grown under tension (see, for example, Bordner, 1909) results were often contradictory, The literature on the topic has been reviewed

by Schwarz (1930) Secondly, following proposals of Heyn (1931) that the rate of cell elongation was limited by the plasticity of the wall material, considerable attention was given to the behavior of cellulose fibers and samples of cell wall material under tension For example, it has been shown that, above a certain yield stress, strips of Nitella cell wall creep at a rate that is roughly proportional to the applied stress (Probine and Preston, 1962) Obviously, these studies need to be supple-

mented by experiments with living shoots, but, as any applied stress

disturbs the turgor relations and tissue stress initially present in a shoot, results are difficult to interpret Recently, Lockhart et al (1964) avoided this problem by working with sections of pea hypocotyl incubated in a slightly hypotonic solution, and found that the living sections underwent irreversible extension in response to tensions greater than 50 g.wt ( uz z

-2 bar) Such studies are of considerable interest in relation to growth processes, but they are of less interest in relation to emergence as the emerging shoot is subject to axial compression rather than tension Before proceeding to examine the effects of compression, it is worth noting that roots are subject to simple tension in many plants, as part of the root proximal to the zone of elongation tends to shorten, sometimes

to a considerable degree For example, de Vries (1879) measured exten- sions (d/E) in the primary roots of red clover, TrifoEium prutense L., as

large as -0.25 over a period of several weeks This process helps to

anchor the plant to the ground, and young seedlings can sometimes be drawn further into the soil

The influence of a steady push, in the opposite sense to growth, on the elongation of etiolated shoots has been described by Sedgley and Barley (1963), who found that this slowed elongation In their experi- ment, a load of 35 g.wt ( uz = 0.5 bar) was applied to the top of the plumular hook of etiolated epicotyls of tick bean The reduction in elon- gation rate that followed was due to a change in shape, epicotyls grown under axial compression being wider than controls The rate of volumetric enlargement was unchanged As the epicotyl of tick bean lacks an inter- calary meristem, the growth response observed in this particular experi- ment cannot have been due to any change in cell division

In general it is known that, where internal controls are not over- riding, as in poorly differentiated dividing tissues, the direction of cell

26 K P BARLEY AND E L CREACEN

division can be influenced by an applied stress For example, Kny (1896)

showed that in the periderm of cut slices of tuber of the potato, SoZunum

tuberosum L., the plane of cell division became oriented normal to an applied tension, and parallel to an applied pressure Clearly, such changes could influence the form of growth, at least in simple tissues

2 Plane Stress ( a1 = 0, uX, u, # 0) The state of plane stress is found in nature when roots or rhizomes tend to enlarge radially against the resistance offered by a strong soil Underground organs may also be compressed radially by the swelling action of wetting clay In an isotropic soil the stresses uZ = uy can be replaced by the radial stress, ur

Roots sometimes grow through compact layers of soil when the soil is moist, but if the soil subsequently dries its increasing strength may prevent an increase in girth, Tabenhaus et al (1931) described field

situations where lengths of the taproot of cotton had been constricted in

this way; and Taylor et al (1963) showed that gross constrictions could lead to a reduced yield of tops In earlier work Newcombe (1894)

studied the influence of radial confinement on the development of stems

of a number of species, and noted that halving the diameter of a short length of stem reduced transpiration at high but not at moderate rates of transpiration

A recent report suggests that translocation toward the tip of the root can be reduced by radial compression of proximal tissues Barley (1965) found that, when a pressure of 1 bar was applied to a proximal length

of corn radicle, the apical part gained weight less rapidly, even though

it received a plentiful ambient supply of water and oxygen The experi- ment also showed that, while the application of pressures > 3 bar damaged cells, radicles that had developed under pressure showed no

signs of cell damage Perhaps compression has little effect on transloca-

tion within tissues that have developed under stress In general the response of a growing tissue to stress is likely to be much influenced by the rate at which the stress builds up, as this determines the degree to which the stress may be accommodated by changes in the pattern of growth

3 Triaxial Stress ( 0 1 , (I=, U~ # 0)

Root tips or underground shoots experience stress in each of the three principal directions when penetrating the soil In an isotropic soil ox =

q,, = ur, but, due to skin friction and shape factors (see Section 111, A, 3 ) ,

u1 > U P

Using closely related techniques Gill and Miller (1956) and Barley

MECHANICAL RESISTANCE OF SOIL 27 (1962, 1965) measured the effect of triaxial compression on root growth Small corn seedlings were grown between a rigid plate and a flexible diaphragm, gas pressure being applied to the diaphragm to compress the seedlings In the experiment of Gill and Miller the seedlings grew in

a thin bed of 50 p beads between an impermeable plate and the dia- phragm In Barley’s experiments beads were omitted and the seedlings were grown between a porous plate and a thin diaphragm The stresses operating on the root were difficult to ascertain, particularly when beads were present Without the beads the radial pressure, u ~ , acting on the root only slightly exceeded the gas pressure on the thin diaphragm, but

uZ exceeded ự at the proximal end of the zone of elongation because of the force needed to overcome skin friction Whether or not beads were present, the rate of root elongation decreased continuously as gas pressure on the diaphragm increased, until elongation almost ceased at gas pressures of 4 to 5 bar The first increment of gas pressure reduced

elongation more than later increments, but the large initial effect vanished when elongation was plotted against estimated values of V Z (Barley, 1962); elongation then decreased steadily almost ceasing at U Z = 7 bar When the applied stress is not isotropic, the apparent growth response may be largely due to a change in shapẹ Data on cell shape obtained in one of the above experiments (Barley, 1965) at one pressure, ự = 1 bar, show that the decrease in length of the cortical cells (-68 percent), compared with the control, accounted for most of the observed reduction

in radicle elongation (-80 percent) Setting aside the change in shape,

a genuine reduction in growth rate may well have been caused by the influence of compression on the internal aeration of the tissues (Section

VI, B, 1 ) Gessner (1961) points out that, as gas-filled intercellular spaces are always present in the tissues or higher plants, their compressi- bility is high

From the physiological point of view data on the effects of isotropic

compression would be particularly informativẹ As meristematic cells do

not fall below a certain size, the rate of cell division declines rapidly after

an organ has been completeIy confined (Hallbauer, 1909) But this does not tell us what will happen when an organ enlarges against a steady ambient pressurẹ

The ideal method of compressing an organ isotropically is to elevate

the pressure of an ambient fluid However, a clear distinction should be

made between experiments with permeating and nonpermeating fluids

In particular, if a permeable plant organ is compressed by raising the pressure of an ambient aqueous solution, the original turgor, T, defined

as the pressure difference between the intracellular liquid and the ambient solution, is restored when osmotic equilibrium is regained This