Advances in agronomy volume 19

ADVANCES IN

AGRONOMY
VOLUME 79


CONTRIBUTORS TO THIS VOLUME
K. P. BARLEY
J. W. DUDLEY
E. L. GFEACEN
R. H. HAGEMAN
K. A. HANDBCK
EDWINJAMES
L. H. P. JONES
SIGURDLARSEN
E. R. LENG
C. B. MCCANTS
J. R. QUINBY
M. RAUPACH
w. G. WOLTZ


ADVANCES IN

AGRONOMY
Prepared under the Auspices of the
AMERICANSOCIETY
OF AGRONOMY

VOLUME 19

Edited by A. G. NORMAN
The University of Michigan, Ann Arbor, Michigan

ADVISORY BOARD
R. R. DAVIS
F. A. HASKINS

J. A. JACKOBS
J. P. MARTIN

W. A. RANEY

1967

ACADEMIC PRESS

New York and London


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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, Commonwealth 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, Agricultural 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 Organization, 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 Company, 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 Science, School of Agriculture and Life Sciences, North Carolina State
University, Raleigh, North Carolina
V


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PREFACE
The purpose of this serial publication is to provide authoritative reviews of progress in crop science, soil science, and agronomic practice.
If there is a single theme, it is the soil-plant relationship. Most of the
articles in this volume exemplify the theme. One indeed goes further and
in an interesting way brings in consideration of the role of the animal in
the cycling sequence.
Great strides have been made in the improvement of crop plants
through genetic recombination. The acre yield of many crops has been

substantially increased by developing varieties better adapted to the
environment, but there are still potential gains to be made, both in yield
and quality, if the limiting biochemical processes can be identified. In
their chapter on this topic, Hageman and colleagues discuss the nature
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.
A. G. NORMAN
Ann Arbor, Michigan

June,1967
vii


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CONTENTS
CONTRI~UTORS
TO VOLUME19
PREFACE

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vii

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MECHANICAL RESISTANCE AS A SOIL FACTOR INFLUENCING THE
GROWTH OF ROOTS AND UNDERGROUND SHOOTS
BY K. P. BARLEYAND E. L. GREACEN

I.
11.
111.
IV.
V.
VI.
VII.

.

Introduction .
. . . . . . . . . .
Types of Deformation Produced by Plants
. . . . .
Forces Required to Deform Soils
. . . . . . .
Forces Exerted by Roots and Shoots .
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Effects of Mechanical Stress on the Growth of Roots and Shoots .
Growth in the Soil
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Conclusion .
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References .

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18
24
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40
40

I. Introduction
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Heterosis and the Gene-Enzyme Concept
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Heterosis and Enzyme Activity during Germination
. . . .
Genetic Control of the Initial Reaction of Nitrogen Metabolism .
.
Specific Chloroplast Activity
. . . . , . . . .
Some Recent Developments in Plant Biochemistry Related to Heterosis
A Concept for the Future . .

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References .
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45
46
54
63
72
74
80
83

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A BIOCHEMICAL APPROACH TO CORN BREEDING

BY R. H. HAGEMAN,E. R. LENG, AND J. W. DUDLEY

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11.
111.
IV.
V.
VI.
VII.

PRESERVATION

OF SEED STOCKS

BY EDWINJAMES
I.
11.
111.
IV.

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Introduction
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. . .
Theories Regarding Seed Deterioration
Methods of Preserving Seeds .
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The National Seed Storage Laboratory
References .
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94

CONTENTS

X

SILICA IN SOILS. PLANTS. AND ANIMALS
BY L. H . P. JONFS
I.
I1.
111.
IV
V.
VI .

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AND

K . A. HANDRECK

Introduction .
. . . . . .
Factors Affecting the Silica Content of Plants
Silica in the Plant
. . . . .
Silica in Relation to Plant Growth .
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Silica in the Ruminant Animal .
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The Silica Cycle . . . . . .
References .

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258
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SOIL PHOSPHORUS
BY SICURDLARSEN
I.
I1
I11
IV
V
VI
VII
VIII .

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Introduction .
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Geochemical Aspects of Soil Phosphorus .

Phosphorus in Soil Solution
. . .
Soil Phosphorus in the Solid Phase .
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Kinetics of Soil Phosphorus Reactions .
Mobility of Soil Phosphorus
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Agronomic Considerations .
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Outlook .
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References .
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GROWTH AND MINERAL NUTRITION OF TOBACCO
BY C. B . MCCANTSAND W . G . WOLTZ
I.
I1.
I11.
IV .
V.
VI .
VII .
VIII .

IX .
X.
XI .
XI1.
XI11.
XIV.
XV .

. . . . . . .
Introduction .
Origin and Characteristics of Classes of Tobacco
Seedling Growth .
. . . . . .
Plant Growth and Nutrient Uptake .
. .
Nitrogen
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Phosphorus .
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Potassium
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Calcium
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Magnesium .
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Liming .
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Chloride
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Boron .

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Sulfur .
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Manganese .
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Other Minor Elements .
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References .
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xi

CONTENTS

THE MATURITY GENES OF SORGHUM

.

BY J R . QUINBY

1.
I1.
I11.
IV .
V
VI
VII .
VIII
IX .

X.
XI

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XI1.

XI11.
XIV.

Introduction .
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Cultivated Sorghum .
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The Four Maturity Gene Loci of Sorghum .
. . .
Effect of Environment on Time of Flowering .
. . .
Control of Leaf Number by Time of Floral Initiation .
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Interaction of Maturity Genes in the Milos and Hegari .
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Interaction of Maturity Genes in the Heterozygous Condition
Identification of Sorghum Varieties for Maturity .
. .
Allelic Series at the Maturity Gene Loci .
. . . .

Influence of Time of Floral Initiation on Plant Size .
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Maturity Gene Loci and Heterosis .
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Effect of Heterosis on Time of Flowering
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Physiology of Flowering in Sorghum
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Discussion and Summary .
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References .
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267
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269
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277
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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 .

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Introduction .
. . . . . . .
The Importance of the Species .
. . .
Characteristics of Growth and Climatic Tolerance
. . .
Soil Factors Restricting Growth .
Assessment of Limiting Factors .
. . .
Effective Addition of Fertilizers
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Field Practices
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Conclusion .
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References .
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307
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AUTHORINDEX.

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355

SUBJECTINDEX
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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

I. Introduction .
. . . . . . . . . .
11. Types of Deformation Produced by Plants .
. . . .
A. Tensile Failure .
. . . . . . . ,
B. Shear Failure without Compression .
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. . .
C. Shear Failure with Compression .
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. . . ,
111. Forces Required to Deform Soils
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A. Theory .
. . , . . . . . .
B. The Assessment of Mechanical Resistance .
. . .
C. The Effect of Pore Water Pressure and Void Ratio on
Mechanical Resistance .
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IV. Forces Exerted by Roots and Shoots
. . . . .
A. Morphological Adaptations .
. . . . . ,

B. Magnitude
. . . . . . . . . .
C. Physiological Origin .
- . . . . . .
V. Effects of Mechanical Stress on the Growth of Roots and Shoots
A. Steady Stress .
. . . . .
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B. Perturbation of Stress .
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VI. Growth in the Soil
. . . . . . . . .
A. Growth in Media of Known Mechanical Properties .
.
B. The Interaction of Mechanical Resistance, Water Supply,
. . . . . . . . .
and Aeration .
VII. Conclusion
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References
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20

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 stimulated 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 problem, 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 considerable 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 reaction 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, differences 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 reaction 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 concerned 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 materials 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(a' + z')"'. ( b ) Shear failure
quently jamming. Jamming occurs when a + dl d'
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

-

0

5 cm.

FIG.2 . Tensile failures produced in a core of compact loam by pea radicles.
(From Barley et al., 1985.)

a failure is general, by definition, it spreads to a soil boundary; in local
failure the tension cracks do not extend to the boundary but are accommodated by compression of the soil.
B. SHEARFAILURE
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,
(Terzaghi,
apparent cohesion, c, and the angle of internal friction,
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. SHEARFAILURE
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 textured media for a radial distance of several millimeters around the root.
The volume of the cores in which the roots were grown remained constant. 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 penetrating 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 consolidation 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 Oz 0.5, and for clays with lower values of 9, K O 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 theoretically. 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 between adjacent root channels (Fig. 2 ) . Using either a spherical or cylindrical 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
qp = 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 frictionless or = 0 soil. For circular shallow footings in saturated undrained
clay qpz 7.5 c. According to Terzaghi’s model qp increases continuously

with x. This relation applies to rough probes entering saturated “undrained” 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.

+,

+


7

MECHANICAL RESISTANCE OF SOIL

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. Sohtions 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 qp = 10 c. Strictly, qpcannot
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 Terzaghi (1943, p.130) an arbitrary reduction is made in c and 4. The bearing 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 (de Jong
and Geertsma, 1953) , tunnels ( Terzaghi, 1943), and around piles
(Nishida, 196l), they postulate the existence of two main zones of compression 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
u2
~,)/3,
change was determined by the mean principal stress, ( u1
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 ambiguity 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 qpin 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 incorporated, the theory of Farrell and Greacen may be used to predict qp
for real, nonrotated probes. The agreement obtained with measured values 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
< Ssteel-soil (see Section
and sand (Barley, 1962) suggests that SrOOt-SO,l
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 difference 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

(4)
where qp = point pressure; qf = axial pressure needed to overcome skin
friction on the curved cylindrical wall of the pile.
Usually adhesion and skin friction are lumped together and estimated
empirically. For rough piles in “undrained clay, skin friction per unit
curved wall area may be s e t equal to c, and the bearing load due to skin
friction Qf = %JOzcrdz, where r is the radius of the pile. For drained
conditions Eide et al. (1961) represent the radial load on the shaft as
Kuz, where a, is the effective axial pressure and K is a coefficient of earth
P=

QP

+,Qf

10

K. P. BARLEY AND E. L. GREACEN

pressure. Then, Qr = 2 ~ / o x K tan
~ Z r6 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. THEASSESSMENT
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 measures of strength is described diagrammaticaIIy in Fig. 4.For any particular normal load, un, acting on a plane of failure, c and 4 give the shear
strength, sn, according to the Coulomb equation
sn = c

+

U~

tan

ip

(5)


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.

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 techUT

12

K. P. BAFLEY AND

E. L GREACEN

nique for measuring the crushing strength of small ( 2 to 3 mm.) aggregates, 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 parameters c and 4 together with a measured compressibility curve. The compressibility 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 compressibility 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 particular soil most of the volume change occurs in the zone of compression
with failure; as e approaches eminthe rebound zone and the zone of