Advances in agronomy volume 38

ADVANCES IN

AGRONOMY
VOLUME 38


CONTRIBUTORS TO THIS VOLUME
MARTINALEXANDER

ANN P. HAMBLIN

N. J. BARROW

S. SANKARAN

MARIONF. BAUMCARDNER

LEROYF. SILVA

LARRYL. BIEHL

DONALD
L. SPARKS

S. K. DE DATTA

ERICR. STONER

A. S. EL-SEBAAY

B . B . TRANCMAR

FEI HUAILIN

G . UEHARA

Guo XIANYI

0. VAN CLEEMPUT

R.

s. YosT


ADVANCES IN

AGRONOMY
Prepared in Cooperation with the
AMERICAN
SOCIETY
OF AGRONOMY

VOLUME 38
Edited by N. C. BRADY
Science and Technology
Agency for International Development
Department of State
Washington, D. C .

ADVISORY BOARD

H. J. GORZ,CHAIRMAN
E. J. KAMPRATHT. M. STARLING

J. B. POWELL J. W. BIGGAR

M. A. TABATABAI
R . A. BRIGGS,Ex OFFICIO,
ASA Headquarters

1985

ACADEMIC PRESS, INC.
Harcourt Brace Jovanovich, Publishers

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PRINT20 IN THE UNITED STATFS OF AMERICA

85868788

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CONTENTS
CONTRIBUTORS
............................................
PREFACE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ix
xi

REFLECTANCE PROPERTIES OF SOILS

Marion F. Baumgardner, LeRoy F. Silva, Larry L. Biehl, and Eric R. Stoner
I. Soil Color in Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2
5
13
28
33
39

..................
.................
IV. Reflectance Properties of Soils in Their Environment . . . . . . . . . . . . .
V. Applications of Soil Reflectance Measurements . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11. Instrumentation for Reflectance Measurements
111. Effects of Soil Constituents on Soil Reflectance

APPLICATION OF GEOSTATISTICS TO SPATIAL STUDIES OF SOIL PROPERTIES

B. B. Trangmar, R . S. Yost, and G. Uehara

...........

I. Introduction . . . . . . . . .
II. Nature of Soil Variability .
111.

IV.
V.

VI.
VII.

45
47
49
53
56
70
89
91

.........................
Traditional Methods of Describing Soil Variability . . . . . . . . . . .
Regionalized Variable Theory and Geostatistics . . . . . . . . . . . . . . . . . .
Analysis of Spatial Dependence . . . . . . . . . . . . . . . . . . . . . . . .
Interpolation by Kriging . . . . . . . .
.......................
Perspectives: Future Use of Geosta
n Soil Research . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
........

THE INFLUENCE OF SOIL STRUCTURE ON WATER MOVEMENT, CROP ROOT
GROWTH, AND WATER UPTAKE

Ann P. Hamblin
I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11. Soil Structure: Components of the Soil-Pore System . . . . . . . . . . . . .
111. Stability of the Pore System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IV. Water Flow in Agricultural Soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
V. Patterns of Root Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

VI. Water Uptake by Roots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
V

95
96
107
114
127
144


vi

CONTENTS

VII . Speculation: Are We Measuring and Averaging
at Consistent Scales? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VIII . summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

149
151
152

GASEOUS HYDROCARBONS IN SOIL

0. Van Cleemput and A . S . El-Sebaay
I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11. Formation. Transformation. and Importance of Gaseous
Hydrocarbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

111. Environmental Factors Affecting the Evolution of the Gaseous
Hydrocarbons in Soil .......................................
IV . Sampling and Analysis of the Gaseous Hydrocarbons . . . . . . . . . . . . .
V. Some Physical and Chemical Properties of the Gaseous
Hydrocarbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VI. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

159
160
167
176
178
178
178

REACTION OF ANIONS AND CATIONS WITH VARIABLE-CHARGE SOILS

N . J . Barrow
I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11. The Development of Charge on Variable-Charge Surfaces . . . . . . . . .
Ill . Adsorption on Variable-Charge Surfaces .......................
1v. Rates of Adsorption and Desorption ...........................
V . Transferring the Variable-Charge Models to Soils . . . . . . . . . . . . . . . .
VI . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

183
185
186

207
211
227
228

KINETICS OF IONIC REACTIONS IN CLAY MINERALS AND SOILS

Donald L . Sparks
I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I1. Methodologies Used in Kinetic Studies ........................
nI . Application of Chemical Kinetics to Soil Solutions . . . . . . . . . . . . . . .
IV . Rate-Determining Steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
V . Kinetics Models ...........................................
Kinetics of Ionic Exchange
VI ...............
" in Clav
. , Minerals ....................

231
233
238
251
256
258


CONTENTS

VII . Kinetics of Ionic Reactions in Heterogeneous Soil Systems . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

vii
261
264

ENHANCING NITROGEN FIXATION BY USE OF PESTICIDES: A REVIEW

Martin Alexander
I . Introduction

...........................................
....
...............................
I n . Free-Living Heterotrophs . . . . . . . . . . . .
........
IV . Blue-Green Algae in Flooded Soils ...........................
V . Resistant Isolates ..........................................
VI . Choice of Pesticides and Inocula ..............................
VII . Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VIII . Summary and Conclusions .............................
References ................................................
II. Rhizobium

267
269
273
274
276
277
278

280
281

WEEDS AND WEED MANAGEMENT IN UPLAND RICE

S . Sankaran and S . K . De Datta
I.
I1 .
III .
IV .
V.
VI .

Introduction ...............................................
Weed Flora of Upland Rice . .
.........................
Ecology of Upland Rice Weeds
.........................
Weed Competition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Land Preparation and Crop Establishment Techniques . . . . . . . . . . . .
Fertilizer Application and Weed Management . . . . . . . . . . . . . . . . . . .
VII . Soil Moisture-Herbicide Relationships in Upland Rice . . . . . . . . . . . .
VIII . Weed Control Methods in Upland Rice ........................
IX . Yield Response of Rice to Herbicides and Herbicide Combinations . .
X . Phytotoxicity of
XI . Economics of W
XI1 . Critical Research Needs .
Appendix: Common Names and Chemical Formulas of Herbicides . .
References . . . . . .

284
285
294
295
303
306
310
313
323
323
327
328
330
330

RICE-BASED CROPPING SYSTEMS AND THEIR DEVELOPMENT IN CHINA

Guo Xian Yi and Fei Huai Lin
I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

II. Environmental Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

340
340


...

Vlll

CONTENTS

111. Division of Rice Belts ......................................
IV. Reformation and Achievements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
V. Problems in Multiple-Cropping Systems ........................
VI. Approaches to Solving the Problem . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

INDEX. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

345
350
353
358
368
369


CONTRIBUTORS
Numbers in parentheses indicate the pages on which the authors' contributions begin.

MARTIN ALEXANDER (267), Department of Agronomy, Cornell University,
Ithaca, New York 14850
N. J. BARROW (183), CSIRO Division of Animal Production, Wembley, Western Australia 6014, Australia
MARION F. BAUMGARDNER ( l ) , Purdue University, West Lafayette, Indiana 47907
LARRY L. BIEHL (l), Purdue University, West Lafayette, Indiana 47907
S. K. DE DATTA (283), Department of Agronomy, International Rice Research
Institute, Manila, Philippines
A. S. EL-SEBAAY (159), Faculty of Agriculture, University of Ghent, B-9000
Ghent, Belgium

FEI HUAI LIN (339), China National Rice Research Institute, Hang Zhou, Zhe
Jiang, People's Republic of China
GUO XIAN YI (339), Crop Breeding and Cultivation Institute, Chinese Academy of Agricultural Sciences, Beijing, People's Republic of China
ANN P. HAMBLIN* (959, Western Australian Department of Agriculture, South
Perth, Western Australia 6151, Australia
S. SANKARAN (283), Department of Agronomy, International Rice Research
Institute, Manila, Philippines
LEROY F. SILVA ( I ) , Purdue University, West Lafayette, Indiana 47907
DONALD L. SPARKS (231), Department of Plant Science, College of Agricultural Sciences, University of Delaware, Newark, Delaware 1971 7
ERIC R. STONER' ( I ) , Cornell University, Department of Agronomy, Ithaca,
New York 14850
B . B . TRANGMAR (45), Soil Bureau, Department of Scientific and Industrial
Research, Christchurch, New Zealand
G. UEHARA ( 4 3 , Department of Agronomy and Soil Science, College of Tropical Agriculture and Human Resources, Universizy of Hawaii, Honolulu,
Hawaii 96822
'Present address: CSIRO, Dryland Crops and Soils Research Program, Wembley P . O . ,Western Australia 6014.
Australia.
'Present address: Cornell UniversityiTropSoils, EMBRAPAKPAC. Caixa Postal 70.0023, 73.300 Planaltina,
D.F., Brazil.

ix


CONTRIBUTORS

X

0. VAN CLEEMPUT (159), Faculty of Agriculture, University of Ghent,

B-9000 Ghent, Belgium

R. S . YOST (43,Department of Agronomy and Soil Science, College of Tropical Agriculture and Human Resources, University of Hawaii, Honolulu,
Hawaii 96822


PREFACE
This volume continues the international focus of Advances in Agronomy. Scientists from six countries are among the authors of the nine papers included. They
remind us of the universality of agronomic problems and of the attempts of
scientists to solve them.
Four of the contributions are concerned directly or indirectly with the physical
properties of soils. One describes how the spectral properties of soils and their
vegetation are being used in satellite imagery programs. A second summarizes
our knowledge of spatial variability of soils-a topic of both practical and
scientific significance. Recent investigations on the effects of soil structure on
water movement and root growth are covered in a third. The fourth is concerned
with the quantity and nature of hydrocarbons in the soil air, a subject of considerable importance, especially in soils that are not too well aerated.
Chemical reactions in soils and clays are reviewed in two other chapters. One
focuses on the reactions taking place on variably charged surfaces, a relatively
understudied subject and one of special significance in some soils of the tropics.
The application of chemical lunetics of soil systems is covered in a second,
which reviews our understanding of the mechanisms of ionic reactions in soils
and clays, and the rates at which they occur.
The surprisingly positive role of certain pesticides on nitrogen fixation in soils
is reviewed in another contribution. Research suggests that certain soil organisms
that prey on or compete with the nitrogen-fixing bacteria are controlled by these
pesticides, thereby freeing the bacteria to fix more nitrogen.
Of the two chapters that specifically consider crop production, one focuses on
weed control, namely research on several methods of weed control in dryland
(upland) rice. This information will be especially helpful to scientists in developing countries where weeds severely restrict dryland rice production.
The second crop production chapter provides a brief review of rice-based

cropping systems in the People’s Republic of China. It gives a description of
certain extremely intensive farming systems used in China, some of which have
been modified in recent years in response to economic and agronomic factors.
Our appreciation is expressed to these crop and soil scientists for providing
their interesting reviews.

N. C . BRADY

xi


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ADVANCES IN AGRONOMY. VOL 38

R EFLECTAN CE PROP ERTl ES
OF SOILS
Marion F. Baumgardner. LeRoy F. Silva.
Larry L. Biehl. and Eric R . Stoner2y3
Purdue University. West Lafayette. Indiana
2Cornell University. Department of Agronomy. Ithaca. New York

I . Soil Color in Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A . Visible Reflectance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Nonvisible Reflectance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C. Need for Quantitative Reflectance Measurements . . . . . . . . . . . . . . . . .
I1. Instrumentation for Reflectance Measurements . . . . . . . . . . . . . . . . . . . . .
A . Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Geometrical Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

C. Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I11. Effects of Soil Constituents on Soil Reflectance . . . . . . . . . . . . . . . . . . . . .
A . Moisture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Organic Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C . Particle Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D. Ironoxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E. Mineral Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
F. Soluble Salts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G. Parent Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
H . Other Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IV. Reflectance Properties of Soils in Their Environment . . . . . . . . . . . . . . . . .
A . Atmospheric Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Physical Surface Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C. Surfacecover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D . Surface Expression of Subsoil Characteristics . . . . . . . . . . . . . . . . . . .
E. Sensor Data Dimensionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
V . Applications of Soil Reflectance Measurements . . . . . . . . . . . . . . . . . . . . .
A . SoilSurvey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Soil Degradation Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C. Soil Information Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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26
27
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30
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Present address: Cornell UniversityflropSoils. EMBRAPA/CPAC. Caixa Postal 70.0023,
73.300 Planaltina. D.F., Brazil.
1

Copyright 0 1985 by Academic Press. Inc.
All rights or reproduction in any form reserved.


2

MARION F. BAUMGARDNER ET AL.

I. SOIL COLOR IN PERSPECTIVE

A. VISIBLEREFLECTANCE
In delineating differences between soils and in describing the characteristics of a soil profile, color is one of the most obvious and useful attributes for
documenting these differences. Soil visible reflectance, or color, is a differentiating characteristic for many classes in all modern soil classification systems
and is an essential part of the definitions for both surface and subsurface
diagnostic horizons. Although most of the differentiating characteristics
selected as diagnostic criteria in the soil classification process are verifiable by
instrumental procedures, soil color is still most commonly determined by a
human observer making a visual comparison between a given sample and the
various color chips in an array of artificially produced colors, arranged
according to hue, value, and chroma (Soil Survey Staff, 1975).
Early in this century, Munsell, a Boston artist and art teacher, developed a
color notation (Munsell, 1947). The “Munsell Book of Color” and Munsell
standard color papers were made and published by the Munsell Color
Company for many years (Nickerson, 1940).Munsell’s work was later to play
an important role in the standardization of soil color designations.
For more than 60 years soil scientists have been studying methods of
improving the measurement of soil color. In 1920 the American Association
of Soil Survey Workers (later the American Soil Survey Association, which
merged with the Soil Section of the American Society of Agronomy to form
the Soil Science Society of America) formed a committee to establish
standards for soil colors (Rice et a/., 1941). During the 1920s and 1930s soil
color research was conducted by scientists in the U.S. Department of
Agriculture (USDA) and at several state experimental stations (Hutton, 1928,
1932; Kellogg, 1937; Shaw, 1937; O’Neal, 1923).
In 1927 a laboratory devoted to the measurement of color and its

application to the grading of agricultural products was developed in the U.S.
Department of Agriculture in Washington, D. C. (Nickerson, 1946). T. D.
Rice and other soil scientists used the laboratory facilities in the study of soil
color problems in a search for better soil color measurements and descriptions. In this laboratory, under controlled lighting conditions, a set of 250 soil
samples was carefully selected by Rice and his co-workers to cover the range
of soil color. The 250 soil samples were measured in the color laboratory in
terms of the Munsell color notations.
In 1941 an important bulletin published by the USDA summarized the


REFLECTANCE PROPERTIES OF SOIL

3

results of soil color research of the preceding 20 years (Rice et al., 1941).
During this period significant progress was made in two areas: (1) the
selection of an array of soil samples which would cover the range of all
possible soil colors and (2) development of color names or descriptions to
describe the color of a soil. In an experiment in 1939 using the 250 soil
samples from the USDA collection, more than 50 U.S. soil scientists and
several from other countries, all with field experience, were asked to name
and record the colors of these samples. Although the names used generally
followed the nomenclature suggested in the Soil Survey Manual, there was
little agreement on an exact color designation for each sample. In fact, the few
observers who were requested to repeat the exercise were unable to duplicate
completely their original color designations, though there were no great
discrepancies.
The next step was to find a system of color names that could be sufficiently
standardized to be acceptable to color scientists, sufficiently useful to satisfy
the needs of soil scientists, and sufficiently commonplace to be understood by

the users of soil information. This search for standardized color names
resulted in the tentative adoption of the names used in the Inter-Society
Color Council/National Bureau of Standards (ISCC-NBS) method (Judd
and Kelly, 1939). The assignment of an ISCC-NBS color designation to each
of the samples in the USDA collection resulted in a total of only 56 color
names.
From this research 56 samples were prepared to represent the central color
of each of the designations in the soil color range. Charts, carrying 56
different color chips, were published as the “Soil Color Name Charts,” which
accompanied the “Preliminary Color Standards and Color Names for Soils”
developed by Rice and his colleagues (Rice et al., 1941). With use of and
experience with color chip matching, it naturally followed that refining and
updating of the method would occur. During the 1940s a committee of the
US. Soil Survey, chaired by E. H. Templin, replaced the earlier charts with a
much wider selection of colors. The Templin committee used 202 regular
Munsell standards instead of the 56 special colors in the early charts. They
also made adjustments in the names (Pendleton and Nickerson, 1951).
Today the standard “Munsell Soil Color Charts” are published on charts
representing 7 different hues and containing 99 different standard color chips
(Munsell Color, 1975).
Great progress was made during the period from the 1920s to the 1950s in
the standardization of methods of measuring and designating soil color.
Today the Munsell soil color notations are widely used throughout the
world. However, the fact remains that the designation of soil color as
normally made in the field or laboratory is subjective and nonquantitative.


4

MARION F. BAUMGARDNER ET AL.

B. NONVISIBLE
REFLECTANCE
Remarkable improvements and changes have occurred over the past three
decades in the development of laboratory and field instrumentation for
observing and measuring physical and chemical phenomena. An area of
development as it relates to soil color is an array of new instruments which
scan a wide range of the electromagnetic spectrum and record quantitatively
the intensity of energy radiating from a specific material or scene. In soil
studies it is possible to measure soil reflectance in the laboratory or in situ and
obtain spectral curves which plot intensity of reflectance in the ultraviolet,
visible, and infrared portions of the spectrum.
Although spectrometers have been used by analysts in the laboratory for
many years, new designs of instruments have extended the use of spectroradiometers to many new applications. One of the driving forces of some of these
applications has been the kinds of sensors which have been and are being
designed for earth observation systems, primarily involving sensors on
aerospace platforms. Since atmospheric attenuation severely limits the use of
ultraviolet measurements from such platforms, this article will not discuss the
application of ultraviolet radiation to the study of soils.
On the other hand, the atmosphere is a relatively open window to the
longer visible wavelengths and to infrared reflectance (Gates, 1962, 1963).
For this reason special attention is given to visible and infrared (nonvisible)
reflectance.
With the increasing availability and continuing improvement of these
spectroradiometers during recent years, there has been an expanding interest
among soil scientists in developing techniques to obtain more precise
quantitative reflectance (visible and infrared) measurements of soils (Baumgardner and Stoner, 1982; Cipra et al., 1971b; Condit, 1970, 1972; DaCosta,
1979; Gausman et al., 1977; Karmanov, 1970; Montgomery, 1976; Obukhov
and Orlov, 1964; Stoner, 1979).

c. NEEDFOR QUANTITATIVE
REFLECTANCEMEASUREMENTS
Ever since soil science evolved into an important discipline for study and
research, color has been one of the most useful soil variables in characterizing
and describing a particular soil (Kohnke, 1968; Pendleton and Nickerson,
1951; Soil Survey Staff, 1975). The quantity and quality of soil components
and the variable conditions under which soils are observed affect soil color.
Our commonly used “measurement” of soil reflectance, usually confined to
the visible, is at best semiquantitative. Both in the field and the laboratory the
assignment of a specific soil to a specific Munsell notation or category is


REFLECTANCE PROPERTIES OF SOIL

5

subjective and is limited to the visible portion of the spectrum and by the
number of Munsell color chips.
Numerous studies in recent years have shown relatively high correlations
between soil reflectance and certain other physical and chemical properties of
soils (Baumgardner and Stoner, 1982; Da Costa, 1979; Montgomery, 1976;
Pazar, 1983; Stoner, 1979; Stoner and Baumgardner, 1981). It has also been
noted that the environmental conditions under which soils have been formed
affect soil reflectance (Montgomery, 1976; Stoner, 1979). If these relationships among soil reflectance and chemical and physical properties can be
established quantitatively and definitively for given environmental conditions, the capacity to extract useful soils information from sensor data
obtained by current and future earth observation satellite systems will be
greatly enhanced.

II. INSTRUMENTATION FOR REFLECTANCE
MEASUREMENTS

A. NOMENCLATURE
Reflective optical radiation is defined as propagating electromagnetic
energy with characteristic wavelengths between 0.4 and 3 pm. When +tical
radiation interacts with a surface, a portion of that radiation is either
absorbed in the material below the surface or is transmitted through the bulk
of the material through another surface into another medium. The remainder
of the radiation is said to be reflected from the surface. In general terms, the
ratio of the reflected radiation to the total radiation falling upon the surface is
defined as reflectance. This is contrasted to reflectivity, which is an intrinsic
material property. Reflectance is the result of a measurement concerning the
aforementioned ratio.
In order to expedite the discussion of reflectance, it is convenient to
introduce some radiometric terminology. Irradiance is the optical radiative
power falling on a unit area of surface. It has the units of watts per square
meter and is usually denoted by the symbol E . If the distribution of the power
per unit area with respect to wavelength is being described, a related term
called spectral irradiance is used. It has the units of watts/(m* - pm). The
term most frequently used to describe reflected radiation is that of radiance,
denoted by the symbol L. It has the units of watts/(m2 - sr), where sr is the
abbreviation for the unit of solid angle, the steradian. The spectral quantity
associated with radiance is called spectral radiance and carries the units of
watts/(m2 - sr - pm).


6

MARION F. BAUMGARDNER ET AL.
Normal to sample
Incident flux

I

Viewed flux

FIG. 1. Geometric parameters describing reflection from a surface: 0, zenith angle; 4,
azimuth angle; o,beam solid angle; a prime on a symbol refers to viewing (reflected)conditions.

Figure 1 illustrates the basic geometric relationships between incoming
radiation and outgoing radiation using the previously described terminology.
The reflecting properties of a surface are most precisely described using a
parameter called the bidirectional rejectance distribution function (BRDF).
The defining equation for the BRDF is

The angles are shown in Fig. 1. The BRDF is the ratio of a radiance to an
irradiance; therefore, it has the units of sr-'. If the numerator and denominator of the expression are spectral quantities, then a spectral BRDF has been
defined and is usually denoted by the symbol A. A careful examination of Fig.
1 reveals that the BRDF is the ratio of two differential solid angles. This is a
mathematical abstraction that is closely realized by many physical situations
in which the incident and reflective solid angles are small enough to
approximate the differential case. The physically measured BRDF is therefore an average fr value over the parameter intervals. The incident and
reflected solid angles, however, need to be small to obtain a good estimate of
the true BRDF.
The measurement of the BRDF is, however, a particularly difficult
problem. It would be necessary to place a sensor at the surface to measure the
incoming radiation and then take that sensor, or another sensor, and place it
in the viewing position necessary to measure the reflected radiation. Although this represents a possible approach, an experimentally more convenient method uses a reflectance standard in the measurement procedure. The


REFLECTANCE PROPERTIES OF SOIL

Reference

7

Targef

FIG.2. Illustration of procedure for measuring the reflectance factor. The response of the
sensor to a perfectly diffuse, ideal reference is recorded, and then the response of the sensor to a
target of interest is recorded under the same illumination conditions.

geometry of this method is illustrated in Fig. 2. In this method a single sensor
located in the viewing position is used to view the reflected radiation from a
perfectly diffuse ideal reflector as well as that from the scene of interest. If the
scene and the perfectly diffuse ideal reflecting surface are viewed and
illuminated under identical conditions, the ratio of the two measurements is
referred to as the reflectance factor of the scene (Nicodemus et al., 1977).
If the geometry of the situation resembles that of Fig. 1, then the quantity
we are measuring is referred to as the bidirectional reflectance factor (BRF). If
differential solid angles are not assumed and real conical solid angles are the
case, then the quantity being measured is called the biconical reflectance
factor. If the conical solid angles for both incident and reflected radiation
include all directions, i.e., the hemisphere, then the measurement is referred to
as the bihemispherical rejectance factor. Also, the measurement may be
referred to as the directional-hemispherical reflectance factor, hemisphericalconical reflectance factor, etc., depending on the instrumentation setup. In
many real measurement situations the magnitudes of the solid angles
involved are small enough so as to approximate the differential situation. For
this reason, the results of the reflectance factor measurement are referred to as
the bidirectional reflectance factor, whereas in reality they are actually the
results of a biconical reflectance factor measurement.
The principal advantage of the reflectance factor method of measurement

is that the sensor can be kept in its viewing position and it is only necessary to


8

MARION F. BAUMGARDNER ET AL.

measure the radiation from the reflectance standard and that from the scene
of interest within a short time period and then take the ratio to obtain the
desired reflectance factor measurement. The method is amenable to both
laboratory and field measurement situations.
An important part of the measurement procedure just discussed is the
reflectance standard. A perfectly diffuse reflecting surface is one that reflects
equally in all directions. An ideal reflector is a surface in which all of the
energy falling on the surface is reflected. Again, this is an abstraction, but
physical surfaces have been prepared which approximate this ideal situation.
Examples of some of the reflecting materials and coatings used for diffuse
surfaces include magnesium oxide, barium sulfate (Grum and Luckey, 1968;
Billmeyer et al., 1971; Shai and Schutt, 1971; Young et al., 1980), homopolymers and copolymers with fluorine substitution (Schutt et al., 1981),
and canvas panels (Robinson and Biehl, 1979). Barium sulfate surfaces have
received the widest acceptance.
Specially prepared barium sulfate surfaces approximate a perfect diffuser at
angles within 45" of a normal to the surface. The reflectance of properly
prepared barium sulfate surfaces is over 0.9 for most wavelengths in the
reflective spectrum. Departures from perfectly diffuse or ideally reflecting
properties of the reflectance standard can be documented and accounted for
during the analysis of the measurement data (Robinson and Biehl, 1979).
Primary reflectance factor calibration measurements are usually made with
tablets that are fabricated out of pressed barium sulfate powder. The results
of these measurements are referred to standard data that have been made

available by the manufacturers of this special barium sulfate reflectance
powder. The resulting measurements are then used to calibrate painted
barium sulfate reflectance standard panels. The pressed barium sulfate tablets
can often be directly used in the laboratory, but for field situations 1.25 x
1.25 m painted panels are usually prepared. The panels are covered when not
in use and are otherwise handled very carefully to avoid contamination from
dust.
B. GEOMETRICAL
CONSIDERATIONS
Figure 3 is a schematic diagram of the angles involved in a physically
realizable measurement situation. In this case, the source and the sensor have
real apertures which produce conical solid angles rather than differential
angles as indicated in Fig. 1. In addition, the sensor has a field of view
determined by its internal optics and indicated on the diagram by the angle p.
The amount of reflected power gathered by the sensor is proportional to the
square of the field of view, the sensor aperture area, the irradiance, the


REFLECTANCE PROPERTIES OF SOIL

9

n

source

'sensor field of view

'source

azimuth angle

FIG. 3. Schematic diagram of illumination and viewing angles involved in a physically
realizable measurement situation.

irradiance angles, the sensor view angles, and, of course, the bidirectional
reflectance distribution of the target. The angular relationships of the source,
target, and sensor can significantly affect the measured reflectance of the
target. For example, the reflectance measured by a sensor vertically viewing
the soil in a plowed field with deep furrows will be higher when the solar
azimuth angle is parallel with the furrow (no shadows) than when the solar
azimuth is perpendicular to the furrows (shadows present). If the target is
smooth, then the azimuthal orientation of the sensor (or source) is irrelevant.
Usually laboratory samples of a soil are prepared in such a way as to be free
of azimuthal variation. Most field observations are usually made at a sensor
zenith angle of 0" and the data are taken at a variety of solar illumination
angles. Laboratory observations are, again, usually made at a sensor zenith
angle of 0" and an illumination zenith angle of 10 to 30".
As previously stated, the signal power received by the sensor is proportional to the square of the field of view of the sensor. However, this is not the sole
factor in the choice of the particular field of view that is going to be used in a
measurement situation. As shown in Fig, 4, a narrow field of view may not
properly integrate the geometric features of the scene into the signal received
by the sensor. A "wide enough" field of view is necessary in order to represent
properly the geometric features of a target to the sensor. Examples of such
structure are the plant rows in a row crop and the roughness of a plowed field.
However, if the field of view is "too wide," then it is difficult to characterize
properly the sensor zenith angle. In field situations, an appropriate compromise is a field of view of approximately 15". In laboratory situations where
the surface roughness of the target can be controlled, a narrow field of view
such as 1" may be properly used.

10

MARION F. BAUMGARDNER ET AL.

FIG.4. Illustration of a field of view (FOV) which is large enough to represent properly the
geometric features of the target and a FOV which is too small to integrate the geometricfeatures.

In some cases, the surface roughness of the target may be so extreme as to
make it impossible to characterize properly the reflectance properties with
one measurement. In this case, several observations of the target are made at
a variety of illumination angles or viewing angles to characterize the target
completely. In this way, shadowing effects which might otherwise obscure the
reflectance properties of the target can be taken into consideration. This
method is available only in a low-altitude field instrumentation situation.
Extreme surface roughness may seriously complicate high-altitude observations from aircraft or satellites.
Another factor that enters into the instrumentation geometric problem is
that of the ratio of direct to diffuse illumination. On a very clear, lowhumidity day, most of the incoming illumination (in a field situation) is
directly from the sun. On hazy days, a significant amount of incident
illumination may be indirect due to atmospheric scattering. This is generally
referred to as the diffuse component of the incident illumination. Moreover,
the diffuse component is a function of the wavelength, being proportionally
more at shorter wavelengths. If the diffuse component of incident illumination is a significant part of the total illumination, then it is difficult to
characterize properly the zenith angle of the incident illumination. In a
laboratory situation, the instrumentation system is arranged so as to negate
the effects of diffuse component illumination. In a field situation, it is often
possible to reduce diffuse effects by restricting data acquisition times to those
in which atmospheric conditions do not give rise to extreme diffuse effects. If
it is necessary to take data under hazy atmospheric conditions, then the
diffuse component may be determined by shadowing the target (and standard) from direct illumination and making a reflectance factor measurement.

REFLECTANCE PROPERTIES OF SOIL

11

This diffuse component is then subtracted from the total illumination in
order to obtain the reflectance factor associated with the direct illumination
component.

C. INSTRUMENTATION
Instruments that have been used to measure the reflectance of soil can
generally be divided into two broad classes-spectroradiometers and multiband radiometers. The discussion that follows is a brief summary of the
instrumentation. More detailed information can be found in Robinson and
DeWitt (1983) and Zissis (1979).
Multiband radiometers contain several optical filters to define the spectral
bandpasses. These spectral bandpasses are selected to sample discrete portions of the optical spectrum, e.g., the Landsat multispectral scanner (MSS)
or the thematic mapper (TM) bands. Multiband radiometers can be of two
types-nonchopping (dc) or chopping (ac). The detectors in dc multiband
radiometers are directly coupled to the output amplifiers. The most reliable
dc multiband radiometers are limited to the spectral range of the silicon
detector because detectors such as lead sulfide, which are sensitive beyond
1 pm, are not sufficiently stable.
Many dc multiband radiometers being used to measure the reflectance of
soils in situ contain the Landsat MSS spectral bands or the first four TM
spectral bands. Some companies have built multiband radiometers that make
it relatively easy for researchers to interchange different sets of optical filters.
ac multiband radiometers contain a chopper in front of the detectors to
allow the field of view of the detector to be alternately filled with the internal
reference source and the target. The signal measuring the radiance from the

target is now the ac signal. The dc signal, which is due to the instability of the
detector, is removed. ac multiband radiometers generally use lead sulfide
detectors to measure the radiant power in spectral bands from 1.0 to 3.0 pm,
such as the last three reflective TM bands.
Spectroradiometers.are distinguished from multiband radiometers because
they measure flux in much narrower, continuous spectral bands. Spectroradiometers can also be chopping or nonchopping. Similar to multiband radiometers, the most reliable nonchopping spectroradiometers are limited to the
silicon detector spectral range.
In spectroradiometers, optical dispersion devices replace the simple optical
filters. The dispersion device may be a segmented filter wheel, a circular
variable filter (CVF), a prism, or grating. The differences in dispersion devices
relate to wavelength resolution and spectral scan speed. Circular variable
filter spectroradiometers that operate from 0.4to 2.4 pm have been widely


12

MARION F. BAUMGARDNER ET AL.

used for laboratory and in situ reflectance measurements. The wavelength
resolution of these CVF instruments is generally 1-2 % of the wavelength.
Spectroradiometers and multiband radiometers may also have an integrating sphere and an artificial light source attached to allow for the measurement of the directional-hemispherical reflectance factor. Many of the
reflectance data cited in the literature are directional-hemispherical reflectance factor measurements.
The systems described above have their own set of advantages and
disadvantages. Multiband radiometers tend to be less costly to buy and
operate. They are lighter and therefore easier to mount on simple pickup
truck booms. Generally, one can obtain a set of multiband radiometer
measurements much faster than a set of spectroradiometer measurements
-fractions of a second compared to 10-120 sec. However, the spectral range
and resolution of multiband radiometers are limited. For example, one can
use reflectance measurements from a multiband radiometer with Landsat

MSS bands to help interpret Landsat MSS data; however, the measurement
could not be easily used to interpret TM data.
Spectroradiometers, on the other hand, lend themselves well to obtaining
sets of reflectance data which can be used to interpret many different sets of
broadband satellite data, e.g., Landsat MSS and TM, the National Oceanic
and Atmospheric Administration’s advanced very-high-resolution radiometer (NOAA AVHRR), and the Nimbus 7 coastal zone color scanner
(CZCS). Spectroradiometers, however, tend to be costly to operate, require
significant time to obtain a set of spectral measurements, and are cumbersome to move in the field. Recent advances in multilinear arrays, however,
may negate some of these disadvantages in the near future.
Instrumentation systems that have been used to measure the reflectance of
soil can be divided into laboratory and in situ or field systems. The laboratory
systems can be further divided into bidirectional reflectance factor systems
and directional-hemispherical reflectance factor systems. Reflectance measurements were initially done in the laboratory using directional-hemispherical spectroradiometer systems. I n situ measurements were made of soil
reflectance as field-worthy spectroradiometer systems were developed during
the late 1960s and early 1970s. With the launch of the Landsat MSS scanners
and the attention given to the Landsat MSS bands, multiband radiometers
became a primary source of field measurements during the late 1970s and
early 1980s. Laboratory measurements of reflectance also continued during
this time. Instrument systems were developed to measure the bidirectional
reflectance factor of soils in the laboratory (DeWitt and Robinson, 1976).
Many instrumentation systems are now available to make reflectance
measurements. However, to utilize and compare measurements from different
systems, researchers need to follow well-defined calibration procedures and