Advances in agronomy volume 68

Agronomy
D VA N C E S

VOLUME

I N

68


Advisory Board
Martin Alexander

Ronald Phillips

Cornell University

University of Minnesota

Kenneth J. Frey

Larry P. Wilding

Iowa State University

Texas A&M University

Prepared in cooperation with the
American Society of Agronomy Monographs Committee
Jon Bartels

Jerry M. Bigham
Jerry L Hatfield
David M. Kral
Linda S. Lee

Diane E. Stott, Chairman
David M. Miller
Matthew J. Morra
John E. Rechcigl
Dennis E. Rolston

Donald C. Reicosky
Wayne P. Robarge
Richard Shibles
Jeffrey J. Volenec


Agronomy

DVANCES IN

VO L U M E

68

Edited by

Donald L. Sparks
Department of Plant and Soil Sciences
University of Delaware

Newark, Delaware

ACADEMIC PRESS
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Contents
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

vii
ix

A LIFETIME PERSPECTIVE ON THE CHEMISTRY
OF SOIL ORGANIC MATTER
M. Schnitzer
I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
II. Soil Organic Matter (SOM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
III. Humic Substances: Analytical Characteristics . . . . . . . . . . . . . . . . . . .
IV. Chemical Structure of Humic Substances . . . . . . . . . . . . . . . . . . . . . .
V. Nitrogen-, Phosphorus-, and Sulfur-Containing
Components of SOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VI. Colloid Chemical Characteristics of Humic Acids
and Fulvic Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VII. Water Retention by Humic Substances . . . . . . . . . . . . . . . . . . . . . . . .
VIII. Reactions of Humic Substances with Metals and Minerals . . . . . . . . .
IX. Interactions of Pesticides and Herbicides with Humic Substances . . .
X. Functions and Uses of Humic Substances . . . . . . . . . . . . . . . . . . . . . .
XI. Conclusions and Outlook for the Future . . . . . . . . . . . . . . . . . . . . . . .
XII. Personal Encounters with Outstanding Scientists . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3
4
11

21
30
36
38
41
44
45
46
47
54

REPRODUCTIVE DEVELOPMENT IN GRAIN CROPS
DURING DROUGHT
Hargurdeep S. Saini and Mark E. Westgate
I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
II. Sensitivity to Drought at Various Reproductive Stages . . . . . . . . . . . .
III. Nature of Injury. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IV. Water Relations of Reproductive Tissues and Their Influence
on Yield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
V. Physiological and Metabolic Bases for Reproductive Failure
under Drought . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VI. Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
v

60
61
62
68
71

85
86


vi

CONTENTS

ADVANCES IN CHLORIDE NUTRITION OF PLANTS
Guohua Xu, Hillel Magen, Jorge Tarchitzky, and Uzi Kafkafi
I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
II. Behavior of Chloride in Soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
III. Chloride in Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IV. Chloride in Crops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
V. Chloride Management in Fertilization and Irrigation . . . . . . . . . . . . .
VI. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

98
99
103
127
134
139
140

OXISOLS
S. W. Buol and H. Eswaran
I.
II.

III.
IV.
V.
VI.
VII.
VIII.
IX.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Historical Background. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Geography of Oxisols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Definition and Kinds of Oxisols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Processes and Formation of Oxisols . . . . . . . . . . . . . . . . . . . . . . . . . . .
Soil–Landscape Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Features and Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ecosystem Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

152
153
158
163
164
167
170
180
187
187

CROP RESIDUES AND MANAGEMENT PRACTICES:
EFFECTS ON SOIL QUALITY, SOIL NITROGEN DYNAMICS,
CROP YIELD, AND NITROGEN RECOVERY
K. Kumar and K. M. Goh
I.
II.
III.
IV.
V.
VI.
VII.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Crop Residues and Their Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Decomposition of Crop Residues . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Crop Residues and Management Practices. . . . . . . . . . . . . . . . . . . . . .
Soil Nitrogen Dynamics and Crop Nitrogen Recovery. . . . . . . . . . . .
Nitrogen Benefits to Subsequent Crops . . . . . . . . . . . . . . . . . . . . . . . .
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

198
199
200
230
257
271
278
279

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

321


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

S. W. BUOL (151), Department of Soil Science, North Carolina State University,
Raleigh, North Carolina 27695
H. ESWARAN (151), Natural Resources Conservation Service, U.S. Department
of Agriculture, Washington, DC 20013
K. M. GOH (197), Soil, Plant, and Ecological Sciences Division, Lincoln University, Canterbury, New Zealand
UZI KAFKAFI (97), Department of Field Crops, Vegetables, and Genetics, The Hebrew University of Jerusalem, Rehovot 76100, Israel
K. KUMAR (197),1 Soil, Plant, and Ecological Sciences Division, Lincoln University, Canterbury, New Zealand
HILLEL MAGEN (97), Extension Service, Ministry of Agriculture, Tel Aviv
61070, Israel
HARGURDEEP S. SAINI (59), Institut de recherche en biologie vegetale, Montreal, Quebec, Canada H1X 2B2
M. SCHNITZER (1), Eastern Cereal and Oilseed Research Center, Agriculture
and Agri-Food Canada, Ottawa, Ontario, Canada K1A 0C6
JORGE TARCHITZKY (97), Dead Sea Works Ltd., Potash House, Beer-Sheva
84100, Israel
MARK E. WESTGATE (59), Department of Agronomy, Iowa State University,
Ames, Iowa 50010
GUOHUA XU (97),2 College of Resources and Environmental Sciences, Nanjing
Agicultural University, Nanjing 210095, People’s Republic of China

1Present address: Department of Soils, Punjab Agricultural University, Ludhiana 141 004, Punjab,
India.
2Present address: Department of Field Crops, Vegetables, and Genetics, The Hebrew University of

Jerusalem, Rehovot 76100, Israel.

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Preface
Volume 68 contains five outstanding and contemporary reviews on topics dealing
with soil chemistry, plant physiology, plant nutrition, pedology, and soil and crop
management. Chapter 1 is a classic review by one of the great pioneers in soil organic matter chemistry (SOM), Dr. Morris Schnitzer. Dr. Schnitzer clearly and
brilliantly summarizes past and present knowledge on SOM, discussing humic
substances (HS), analyses, chemical structure, N-, P-, and S-containing components of SOM, colloid chemical characteristics of HS, water retention by HS, interactions of HS with metals, minerals, and organic chemicals, and future prospects,
with a lively personal discussion of interactions with other pioneers in the field
over an almost 50-year distinguished career. Chapter 2 is a comprehensive treatise
on reproductive development in grain crops during drought by two leading experts:
Hargurdeep S. Saini and Mark E. Westgate. The authors discuss the sensitivity of
plants to drought at various reproductive stages, types of injury, water relations of
reproductive tissues and their influence on yield, and physiological and metabolic bases for reproductive failure under drought. Chapter 3, by Guohua Xu, Hillel
Magen, Jorge Tarchitzky, and Uzi Kafkafi, presents advances in the chloride nutrition of plants. Aspects of chloride in soil, plants, and crops and chloride management in fertilization and irrigation are extensively discussed.
Chapter 4 is another review in our continuing series on the 12 soil orders. S. W.
Buol and H. Eswaran, distinguished pedologists, provide a very useful review on
Oxisols. They provide a historical background on Oxisols and cogent discussions
of the geography, kinds, and processes/formation of Oxisols, soil–landscape interactions, features and processes, and ecosystem management. Chapter 5 is a
timely review on crop residues and management practices as they relate to soil
quality and nitrogen dynamics. The authors, K. Kumar and K. M. Goh, discuss aspects of crop residues, including uses, decomposition, management practices, and
soilnitrogen dynamics and nitrogen benefits to subsequent crops.
I thank the authors for their excellent reviews.
Donald L. Sparks

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A LIFETIME PERSPECTIVE ON THE
CHEMISTRY OF SOIL ORGANIC
MATTER
M. Schnitzer
Eastern Cereal and Oilseed Research Center
Agriculture and Agri-Food Canada
Ottawa, Ontario, Canada K1A OC6

I. Introduction
II. Soil Organic Matter (SOM)
A. Definitions
B. Relationship among SOM, Humus, and Humic Substances
C. Problems Associated with Extraction of SOM from Soils
D. Direct Analysis of SOM in Whole Soils by 13C Nuclear Magnetic
Resonance (NMR) and Pyrolysis–Field Ionization Mass
Spectrometry (Py-FIMS)
E. How Is SOM Affected by Long-Term Cultivation?
III. Humic Substances: Analytical Characteristics
A. Chemical Methods
B. Infrared (IR) and Fourier Transform IR Spectroscopy
C. 13C NMR Spectroscopy
D. Electron Spin Resonance Spectroscopy
E. Electron Microscopy

IV. Chemical Structure of Humic Substances
A. Oxidative Degradation
B. Reductive Degradation
C. Py-FIMS of Humic Acid (HA), Fulvic Acid (FA), and Humin
D. Curie-Point Pyrolysis Gas Chromatography/Mass
Spectrometry (GC/MS)
E. A Two-Dimensional Structure of HA
F. A Three-Dimensional Structure of HA, SOM, and Whole Soil
V. Nitrogen-, Phosphorus-, and Sulfur-Containing Components of SOM
A. Origins and Functions of Soil Nitrogen
B. Nitrogen Distribution in Soils and Humic Substances
C. Amino Acids in Soils and Humic Substances
D. Amino Sugars in Soils and Humic Substances
E. Nucleic Acid Bases in Soils and Humic Substances
F. 15N NMR Analyses of Soils and Humic Substances
G. Detection of Nitrogen Compounds in Soils and Humic Substances by
Pyrolysis GC/MS

1
Advances in Agronomy, Volume 68
Copyright © 2000 by Academic Press. All rights of reproduction in any form reserved.
0065-2113/00 $30.00


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M. SCHNITZER

VI.

VII.
VIII.

IX.
X.

XI.
XII.

H. Phosphorus in Soils and SOM
I. Sulfur Compounds in Soils and Humic Substances
Colloid Chemical Characteristics of Humic Acids and Fulvic Acids
A. Surface Tension, Surface Pressure, and Viscosity Measurements on HAs
and FAs
Water Retention by Humic Substances
Reactions of Humic Substances with Metals and Minerals
A. Formation of Water-Soluble Complexes
B. Mixed Ligand Complexes
C. Adsorption and Desorption
D. Dissolution of Minerals
E. Adsorption on External Mineral Surfaces
F. Adsorption in Clay Interlayers
Interactions of Pesticides and Herbicides with Humic Substances
Functions and Uses of Humic Substances
A. Functions in Soils
B. Uses and Potential Uses
Conclusions and Outlook for the Future
Personal Encounters with Outstanding Scientists
References

The author has researched the chemistry of soil organic matter for almost 50
years. In this chapter, he presents a personal account of how soil organic matter
chemistry has evolved during the second half of this century from wet to computational chemistry. The chapter begins with a definition of soil organic matter and
how it relates to humus and humic substances. Problems associated with the extraction of organic matter from soils, separation of the extract into humic substances, and purification of the resulting fractions are then discussed. New experimental approaches to the in situ analysis of organic matter in whole soils to
overcome these problems are described. Investigations on the chemistry of soil organic matter are outlined in terms of (a) an analytical and (b) a structural approach.
The analytical approach involves determinations of the characteristics of humic
substances by chemical methods, infrared, 13C nuclear magnetic resonance, electron spin resonance spectroscopy, and electron microscopy, whereas the structural
approach consists of oxidative and reductive degradations, pyrolysis–field ionization mass spectrometry, and Curie-point pyrolysis–gas chromatography/mass
spectrometry. The author recounts how the results of the analytical and structural
studies led to the formulation of a two-dimensional humic acid model structure and
how the latter was converted with the aid of computational chemistry to a threedimensional humic acid model structure and later to three-dimensional model
structures of soil organic matter and whole soils. The next topics discussed by the
author are advances in the chemistry of N-, P-, and S-containing components of
soil organic matter. Especially noteworthy is progress in the chemistry of N in soil
organic matter, which points to a prominent role of heterocyclic N. As far as colloid-chemical characteristics of humic substances are concerned, the three parameters that control the molecular characteristics (molecular weight, size, and shape)
of humic and fulvic acids are (a) the concentration of the humic substance, (b) the
pH of the system, and (c) the electrolyte concentration of the medium. In the last


A LIFETIME PERSPECTIVE

3

part of the chapter, the author discusses how humic substances interact with water,
metals, minerals, pesticides, and herbicides; lists functions and uses of humic substances; and describes personal encounters with outstanding scientists who influenced his research.
© 2000 Academic Press.

I. INTRODUCTION
I thank Dr. D. L. Sparks for not only inviting me to write this chapter but also
for suggesting the title. After almost 50 years of continuous research on the chemistry of soil organic matter (SOM), I have learned a lot about this complex material, and I am pleased to have the opportunity to communicate some of this knowledge to readers. This chapter is not a literature review but a personal account of

how SOM chemistry has evolved during the second half of this century and what
the prospects for the future are. Over the years, the success of SOM chemists in
dealing with these complex materials depended to a large extent on how well they
could adapt newly developed methods and instruments to SOM. In the late 1940s,
wet chemistry done in beakers, flasks, and test tubes was predominant. The major
instruments that were available to me at that time were pH meters, powered by batteries, and colorimeters requiring filters for changing wavelengths. In the early
1950s, recording ultraviolet (UV) spectrophotometers became available, and in the
mid-1950s, I remember convincing my director to purchase an infrared (IR) spectrophotometer. The early 1960s saw the arrival of gas chromatographs. This was
an important development because it allowed us to separate complex mixtures of
humic acid (HA) and fulvic acid (FA) oxidation products, along with organic soil
extracts containing alkanes, alkenes, fatty acids, and esters. In the mid-1960s we
purchased a mass spectrometer, which we attached to a gas chromatograph. This
allowed us to not only separate complex mixtures of organics but also to identify
the separated compounds. About the same time, we saw the arrival of an electron
spin resonance (ESR) spectrometer, which enabled us to measure concentrations
of free radicals in humic materials and to obtain information on the nature of free
radicals. ESR also helped us throw light on the symmetry and coordination of paramagnetic metals such as Fe3+, Cu2+, and Mn2+ bound to HA and FA. In the early
1980s, we purchased a liquid-state 13C nuclear magnetic resonance (NMR) spectrometer. After we had learned how to use this instrument properly, we realized
that 13C NMR was of great importance to SOM chemists. It showed, for the first
time, that aliphatic C in HAs and FAs was as important as aromatic C and that the
older theories that HAs were almost completely aromatic were no longer valid for
SOM in most agricultural soils. Finally, the mid-1980s saw the arrival of pyrolysis–field ionization mass spectrometry (Py-FIMS), which we applied to the in situ


4

M. SCHNITZER

analysis of SOM, i.e., the direct analysis of SOM in whole soils, without extractions, purifications, etc. Another application of mass spectrometry was Curie-point
pyrolysis–gas chromatography/mass spectrometry (GS/MS), which we used in

structural studies on HAs and which resulted in proposing a two-dimensional
structural HA model. We then converted the latter by computational chemistry into
a three-dimensional HA model structure. We similarly generated three-dimensional model structures for SOM and a whole soil with both inorganic and organic constituents. Thus, during the past 50 years, I was fortunate to have participated, along with other scientists, in the evolution of SOM chemistry from wet
chemistry to computational chemistry.
In addition to studies on the chemical structure of humic substances, I also
worked on determining colloid chemical properties of these materials, mechanisms of water retention, reactions with metals and minerals, and with pesticides
and herbicides. Thus, the overall objective of my research was to investigate the
chemical structure and reactions of humic substances. It was and still is my hope
that the results of this research will assist soil scientists, agronomists, and farmers
in the development of more efficient management and production systems so that
they can grow sufficient food for an increasing population.
At the end of the chapter, I describe personal encounters with some outstanding
scientists of the past 50 years.

II. SOIL ORGANIC MATTER (SOM)
A. DEFINITIONS
The term “soil organic matter,” as used in this chapter, refers to the sum total of
all organic carbon-containing substances in the soil. SOM consists of a mixture of
plant and animal residues in various stages of decomposition, substances synthesized microbiologically and/or chemically from the breakdown products, and the
bodies of live and dead microorganisms and their decomposing remains (Schnitzer
and Khan, 1978). Solid-state 13C NMR spectra of whole soils show the presence
of paraffinic C, OCH3-C, amino acid-C, C in carbohydrates and aliphatic structures bearing OH groups, aromatic C, phenolic C, and C in CO2H groups (Arshad
et al., 1988). From the 13C NMR spectrum, aromaticity and aliphaticity of SOM
can be calculated. Resulting data show that the C aromaticity of SOM seldom exceeds 55% and that the aliphaticity of SOM is often greater than its aromaticity
(Schnitzer and Preston, 1986). Similarly, Py-FIMS of SOM in whole soils indicates the presence of carbohydrates, phenols, lignin monomers, lignin dimers,
alkanes, fatty acids, n-alkyl mono,di, and tri esters, n-alkylbenzenes, methylnaphthalenes, methylphenanthrenes, and N compounds (Schnitzer and Schulten, 1992).
Carbohydrates, proteinaceous materials (amino acids, peptides, proteins), and


A LIFETIME PERSPECTIVE

5

lipids (alkanes, alkenes, saturated and unsaturated fatty acids, alkyl mono, di, and
tri esters) in SOM appear to be strongly retained by the aromatic SOM components and can only be separated from them with great difficulty. For example, even
after exhaustive extractions with n-hexane, followed by chloroform, Schnitzer and
Schuppli (1989) could remove only 10% of the total lipids from three agricultural soils sampled in western Canada. The separation of carbohydrates and proteinaceous materials from SOM requires prolonged hydrolyses with relatively
strong acids under reflux. Thus, the different chemical components of SOM are
closely associated to form a complex structure.

B. RELATIONSHIP AMONG SOM, HUMUS,
AND HUMIC SUBSTANCES
There is some confusion among soil chemists about the meanings of SOM, humus, and humic substances. Do these terms depict different materials? According
to Stevenson (1994), SOM is synonymous with humus. In my opinion, the term
total humic substance is also synonymous with SOM and humus as long as losses
occurring during the extraction and separation procedures are held to a minimum.
My definition of humic substances is that it is the sum of humic acid ϩ fulvic acid
ϩ humin. While essentially each of the three terms can be used, I personally, as a
SOM chemist, prefer use of the term SOM.

C. PROBLEMS ASSOCIATED WITH EXTRACTION
OF SOM FROM SOILS
The SOM content of agricultural soils usually ranges between 1 and 4% (w/w),
with most soils containing between 2 and 3% SOM. In the soil, because SOM and
inorganic soil constituents are closely associated, it is necessary to separate the two
before either can be examined in greater detail. This separation is usually achieved
by extracting the SOM with either dilute base (0.1–0.5 M NaOH solution) or by a
neutral salt solution such as aqueous 0.1 M Na4P2O7. Extraction of SOM with a
dilute base works reasonably well and was originated by Archard in 1786. Separation of the alkaline extract into HA, FA, and humin was first carried out by Sprengel (1826). The three fractions into which the alkaline SOM extract is partitioned
are (1) HA, which is that fraction of SOM that coagulates when the alkaline extract is acidified; (2) FA, which is the SOM fraction that remains in solution when

the extract is acidified, i.e., it is soluble in both acid and alkali; and (3) humin,
which is that SOM fraction that remains with the soil, i.e., it is insoluble in both
alkali and acid. Over the years, many objections have been raised against the use
of alkaline solutions, which are still the most efficient SOM extractants today.
Stevenson (1994) lists the following objections: (1) silica is dissolved from the


6

M. SCHNITZER

mineral matrix, which contaminates the SOM extract; (2) protoplasmic and structural components of fresh organic tissues are dissolved, and these mix with the
SOM extract; (3) autooxidation of some organic components occurs in contact
with air when the extracts are allowed to stand for extended periods of time; and
(4) other chemical changes can occur in the alkaline solutions, including condensation between amino acids and CuO groups of reducing sugars or quinones to
form Maillard reaction products. Some of these changes can be minimized by doing the extractions in the presence of an inert gas such as N2, but not all possible
changes can be excluded.
Another serious difficulty with the extraction of SOM from soils and the partitioning of the extract into HA, FA, and humin is that these are laborious and timeconsuming procedures that are not suitable for the analysis of large numbers of soil
samples. A new approach, not involving the use of wet chemical methods, is required to overcome these problems.

D. DIRECT ANALYSIS OF SOM IN WHOLE SOILS BY 13C
NUCLEAR MAGNETIC RESONANCE (NMR) AND PYROLYSIS –
FIELD IONIZATION MASS SPECTROMETRY (PY-FIMS)
In recent years we have witnessed the development of two analytical methods,
based on “high technology,” that appear to be suitable for the direct analysis of
SOM in situ, i.e., in whole soils. These methods are solid-state 13C NMR and PyFIMS. The solid-state 13C NMR analysis of whole soils has been described by Wilson (1987) and Arshad et al. (1988). This type of analysis requires that the soil contain at least 3% C and that the concentration of paramagnetic ions, e.g., Fe3+, in
the soil not be too high because paramagnetic ions interfere with the recording of
acceptable 13C spectra. According to Arshad et al. (1988), the C:Fe (w/w) ratio is
an important indicator for obtaining satisfactory solid-state 13C NMR spectra of
whole soils and particle-size fractions separated from them. If the C:Fe ratio is

ϾϾ1, the quality of the spectrum will be good; if the ratio is Ͼ1, a reasonable
spectrum will be obtained, but if the ratio is Ͻ1, the spectrum will be poor. The
quality of the spectrum can be improved by reducing the Fe3+ to Fe2+ by dithionite and then removing it. Another option is to separate the soil into particle-size
fractions and running 13C NMR spectra on the fractions (Wilson, 1987). Arshad et
al. (1988) report that SOM-rich soil and particle size fractions can be prepared by
flotation and that these yield well-defined 13C NMR spectra. Another approach that
can be used is to separate the soil on a magnetic separator into magnetic and nonmagnetic fractions, but this method requires specialized equipment and is too timeconsuming. Figure 1 shows solid-state 13C NMR spectra of particle-size fractions
separated from Culp and Rycroft soils from northwestern Alberta, enriched in
SOM by flotation (Arshad et al., 1988). Flotation increased the C content of the


A LIFETIME PERSPECTIVE

7

Figure 1 Solid-state 13C NMR spectra of (a) OM-enriched fine sand fraction isolated from Culp
soil, (b) OM-enriched sand fraction isolated from Rycroft soil, and (c) OM-enriched silt and clay fraction separated from Rycroft soil. From Arshad et al. (1988), with permission of the publisher.

sand fraction separated from the Culp soil from 2.05 to 20.15%, the C content of
a similar fraction separated from the Rycroft soil increased from 3.16 to 15.62%,
whereas the C content of the silt and clay fraction separated from the same soil increased from 3.08 to 8.59%. The three resulting spectra are well defined and are
characteristic of SOM or humic materials (Schnitzer and Preston, 1986). The major signals are at 30 ppm (paraffinic C), 73 and 102 ppm (carbohydrate C), 130
ppm (aromatic C) 150 ppm (phenolic C), and 173 ppm (C in CO2H groups). The
aromatic C content is lower than that of many soil HAs. It is hoped that with improvements in 13C NMR equipment and technology, it will be possible to analyze
soils that contain Ͻ3%C, which would include many agricultural soils.


8

M. SCHNITZER

Although 13C NMR spectroscopy provides information on the different types
of C in SOM, a method that yields data on SOM at the molecular level is Py-FIMS.
This method is more sensitive than 13C NMR and can also be used for the direct
analysis of SOM in soils.
The Py-FIMS spectrum of the whole Armadale soil (Schnitzer and Schulten,
1992), shown in Fig. 2, is dominated by carbohydrates (m/z 60, 72, 82, 84, 96, 98,
110, 112, 114, 126, 132, 144, and 162), phenols (m/z 94, 108, 110, 122, 124, 126,
138, and 154), monolignins (m/z 164, 166, 178, 180, 182, 194, 196, 208, 210, and
212), dilignins (m/z 246, 260, 270, 272, 274, 284, 286, 296, 298, 300, 310, 312,
314, 316, 326, 328, 330, 340, 342, and 356), and suberin derived esters (m/z 446,
474, 502, and 530). The signals at m/z 170 and 184 arise from tri- and tetramethylnaphthalene, respectively, whereas m/z 178, 192, 206, 220, and 234 are
due to phenanthrene, and methyl-, dimethyl-, trimethyl-, and tetramethylphenanthrene, respectively. Also, n-C10 to n-C18 alkyl diesters are present. Normal alkylbenzenes range from m/z 442 (C6H5иC26H53) to m/z 470 (C6H5иC28H57). The occurrence of N-containing compounds is indicated by m/z 59 (acetamide), 67
(pyrrole), 79 (pyridine), 81 (methylpyrrole), 93 (methylpyridine), 103 (benzonitrile), 117 (indole), 131 (methylindole), and 167 (not identified).
The two methods just described allow SOM chemists to obtain significant information on the chemical composition of SOM in whole soils, i.e., in situ. They
also make it possible to study the chemistry of SOM without extracting it from the
soil, without partitioning it into HA, FA, and humin, and without having to lower
the ash content of each of these fractions. It is noteworthy that while both 13C NMR
and Py-FIMS provide similar chemical information on SOM, the problem that
faces SOM chemists at this time is to decide whether the analysis of SOM in whole
soils by advanced methods is the path to follow or whether they want to continue
using the “classical” approach that involves the extraction and separation of HA,
FA, and humin. As shown in Section III,C, 13C NMR spectra of HA and FA demon-

Figure 2 Py-FI mass spectra of the whole Armadale soil. From Schnitzer and Schulten (1992),
with permission of the publisher.


A LIFETIME PERSPECTIVE

9

strate that the main structural features, as well as the aromaticity and aliphaticity
of the two humic fractions, are quite similar. Also, the 13C NMR spectrum of
humin, after deashing, is similar to that of HA (Preston et al., 1989). These findings do not support conclusions of earlier workers (e.g. Sprengel, 1826) that HA,
FA, and humin are different substances that are separated by the “classical” extraction procedure.
The terms HA, FA, and humin do not stand for distinct chemical substances.
Both 13C NMR and Py-FIMS show that the three are closely related materials and
that the separation scheme proposed by earlier workers has no chemical validity.
The obvious solution to the problem is the direct SOM analysis of the whole soil
or soil fractions by 13C NMR or Py-FIMS. Data so obtained tell us about the chemical composition of SOM in terms of aliphatics, proteins, carbohydrates, aromatics, phenols, heterocyclic N compounds, etc. These are chemical classes of compounds, and analytical data are readily understood by all chemists. It is essential
that we start to express ourselves in the language of chemistry and no longer use
terms that have no chemical meaning. I propose that SOM chemists use the term
SOM for all C-containing compounds in the soil, high molecular weight SOM for
HA, low molecular weight SOM for FA, and insoluble SOM for humin. Similarly, water chemists could use the term natural organic matter (NOM) for SOM, high
molecular weight NOM for HA, low molecular weight NOM for FA, and insoluble NOM for humin.
While it is true that enormous literature on the chemical and physical properties
of humic substances (HA, FA, humin), consisting of thousands of scientific papers,
has accumulated over the past 200 years, it is not necessary to abandon or disregard this huge literature. Older data can be reinterpreted and will help us to better
understand the information generated by the new analytical approaches.

E. HOW IS SOM AFFECTED BY LONG-TERM CULTIVATION?
Little is known about how SOM is affected by long-term cultivation. Schulten et
al. (1995) employed Py-FIMS of whole soils, a method described in the previous
section, to throw light on this problem. Soil samples originated from a Typic Haploboroll under a long-term crop rotation established in 1910 at Lethbridge, Alberta.
One soil sample, collected in 1910 from the Ahorizon after breaking the native grassland, had been stored. Another soil sample was collected in June 1990 from the A
horizon under a wheat-fallow, nonfertilized, rotation. This sample had been under
cultivation for 80 years. The native sample (collected in 1910) contained 3.03%
SOM but the cultivated sample (collected in 1990) contained only 2.23% SOM. Percentages of sand, silt, and clay and the exchange capacities of the two samples were,
however, almost identical. The aggregate stability of the native sample was 65%,

whereas that of the cultivated sample was only 42%. There were significant reduc-


10

M. SCHNITZER

tions in enzyme activities after 80 years of cultivation. The activity of dehydrogenase dropped by 60%, that of acid phosphatase by 77%, and that of urease by 82%.
While qualitatively the Py-FIMS spectra were similar, (total ion intensities) (TII),
which are related to SOM concentrations, were dramatically different for the two
samples. The TII value per milligram of soil for the native sample was 31.25 ϫ 104
counts compared to only 3.96 ϫ 104 counts for the cultivated sample. As illustrated
in Fig. 3, TII values of the summed signals characteristic of the major SOM components that are carbohydrates (carboh), phenols and lignin monomers (phenols),
alkylaromatics (alkylar), N-containing compounds (N-comp), peptides, lipids, and
lignin dimers (lignin) show quantitatively different compositions.
Despite similar SOM contents, the TII for the SOM in cultivated soil is only
one-sixth of that in the native soil. Signals in Py-FIMS spectra indicating the presence of carbohydrates, phenols and lignin monomers, alkylaromatics, and N-compounds in SOM of the cultivated sample are only between one-fifth and one-seventh of the intensities generated by the same compound classes in the SOM of the
native sample. TII values of the signals for peptides, lipids, and dimeric lignins in
the cultivated sample constitute even smaller proportions of similar components
of the SOM in the native sample.

Figure 3 Total ion intensity (TII) values of summed signals for major SOM components that are
carbohydrates (carboh), phenols and lignin monomers (phenols), alkylaromatics (alkylar), N-containing compounds (N-comp), peptides, lipids, (alkanes, alkenes, fatty acids, esters), and lignin dimers
(lignin). (a) Native sample, (b) cultivated sample. From Schulten et al. (1995), with permission of the
publisher.


A LIFETIME PERSPECTIVE

11

A possible explanation for data obtained is that the components identified by
Py-FIMS in the cultivated sample originate from more thermally stable and higher molecular weight SOM than those present in the native sample. Thus, cultivation causes increased polymerization and cross-linking of the major SOM components, leading to the formation of larger molecules with higher molecular
weights, stability, and complexity (Schulten et al., 1995). Increases in molecular
size and chemical complexity of the SOM in the cultivated sample may explain
the observed decreases in enzyme activities involving the C, N, and P cycles.
Thus, anthropogenic disturbances through cultivation may induce significant
changes in the quality, chemical composition, and molecular size of SOM. While
these changes may help preserve and maintain SOM in agricultural soils, little is
known on how they affect soil biology.

III. HUMIC SUBSTANCES: ANALYTICAL
CHARACTERISTICS
A. CHEMICAL METHODS
Humic substances contain per unit weight relatively high concentrations of oxygen-containing functional groups (CO2H, OH, CuO). It is through these groups
that these materials are capable of attacking and degrading soil minerals by complexing and dissolving metals, transporting these throughout the soil, and making
them available to plant roots and microbes. Metal–humic complexes with widely
differing chemical and biological stabilities and characteristics are formed. Interactions between humic substances and metal ions have been described as ion
exchange, surface adsorption, chelation, peptization, and coagulation reactions
(Schnitzer, 1978). FA at any pH and HA at pH Ͼ 6.5 can form stable water-soluble metal complexes in competition with hydrolysis reactions. HA is water insoluble at pH Ͻ 6.5 but exhibits sorption properties that lead to the concentration of
metals, especially trace metals, and organics on its large surface.
The elemental composition and functional group content of a typical HA (extracted from the A horizon of a Haploboroll) and a FA (extracted from the Bh horizon of a Spodosol) are shown in Table I.
A more detailed analysis of data shows that (1) the HA contains approximately
10% more C, but 36% less O than the FA; (2) there are quantitatively smaller differences between the two materials in H, N, and S contents; (3) the total acidity
and CO2H content of the FA are significantly higher than those of the HA; (4) both
materials contain per unit weight significant concentrations of phenolic OH, total
CuO, and OCH3 groups, but the FA is richer in alcoholic OH groups than the HA;
(5) about 74% of the total O in the HA is accounted for in functional groups, but

12

M. SCHNITZER
Table I
Analytical Characteristics of a Haploboroll
HA and a Spodosol FA
HA
Element (g kgϪ1)
564
55
41
11
329
Functional groups (cmol kgϪ1)
Total acidity
660
COOH
450
Phenolic OH
210
Alcoholic OH
280
Quinonoid CuO
250
Ketonic CuO
190
OCH3
30
E4 /E6
4.3

C
H
N
S
O

FA

509
33
7
3
448
1240
910
330
360
60
250
10
7.1

all of the O in the FA is similarly distributed. The E4 /E6 ratio of the FA is almost
twice as high as that of the HA, which means that the FA has a lower particle or
molecular weight than the HA (Chen et al., 1976).

B. INFRARED (IR) AND FOURIER TRANSFORM
IR SPECTROSCOPY
IR and FTIR spectra of humic substances show bands at 3400 cmϪ1 (H-bonded

OH), 2900 cmϪ1 (aliphatic C–H stretch), 1725 cmϪ1 (CuO of CO2H, CuO stretch
of ketonic CuO), 1630 cmϪ1 (COOϪ, CuO of carbonyl and quinone), 1450 cmϪ1
(aliphatic C–H), 1400 cmϪ1 (COOϪ), 1200 cmϪ1 (C–O stretch or OH deformation
of CO2H), and 1050 cmϪ1 (Si–O of silicates). The bands are usually broad because
of the extensive overlapping of individual adsorbances. IR and FTIR spectra reflect
the preponderance of oxygen-containing functional groups, i.e., CO2H, OH, and
CuO in humic materials. While IR and FTIR spectra provide worthwhile information on the functional groups and their participation in metal–humic interactions,
they tell us little about the chemical structure of humic materials.
Celi et al. (1997b) applied FTIR to the analysis of CO2H groups in a number of
HAs. Concentrations of CO2H groups in HAs were determined directly from FTIR
spectra by totaling adsorbances at 1720–1710 cmϪ1 (CO2H) and 1620–1600
cmϪ1 (COOϪ). Good correlations were found between total carboxyl groups determined by FTIR, a wet chemical method, and by 13C NMR. Thus, depending on


A LIFETIME PERSPECTIVE

13

the equipment and facilities available, soil chemists have a choice of methods that
can be used for measuring CO2H groups in HAs.

C.

13C

NMR SPECTROSCOPY

Until about the mid-1980s, when the use of liquid- and solid-state 13C NMR became more widespread, most soil chemists thought that the chemical structure of
humic substances was predominantly aromatic. 13C NMR demonstrated that
aliphatic structures in humic substances were often as important, and sometimes

even more important than aromatic structures (Schnitzer and Preston, 1986; Wilson, 1987; Norwood, 1988; Schnitzer, 1991). This was a very significant development in our understanding of the chemistry of humic substances. Aromaticities
of HAs extracted from soils of widely differing pedological origins range from 30
to 60% (Schnitzer, 1991). A substantial portion of aliphatic carbons in HAs consists of paraffinic carbons. Of considerable interest are the prominent resonances
in both liquid- and solid-state 13C NMR spectra of humic substances near 130 and
132 ppm, which can be assigned to C in aromatic rings that are not substituted by
strong electron donors such as O and N but by C. Alkaylaromatics are typical structures that produce such resonances (Breitmaier and Voelter, 1978).
Of special interest in the context of this discussion is a comparison of solid-state
13C NMR spectra of a HA (extracted from the Ah horizon of a Haploboroll) and a
FA (extracted from the Bh horizon of a Spodosol). The HA spectrum in Fig. 4 shows
several distinct peaks in the aliphatic (0–105 ppm), aromatic (106–150 ppm), phe-

Figure 4 Solid-state 13C NMR spectra of HA (extracted from the Ah horizon of a Haploboroll)
and FA (extracted from the Bh horizon of a Spodosol).


14

M. SCHNITZER

nolic (155–160 ppm), and carboxyl (170–180 ppm) regions. The signals at 17, 21,
25, 27, and 31 ppm are likely due to alkyl C. The resonance at 17 ppm is characteristic of terminal CH3 groups and that at 31 ppm of (CH2)n in straight paraffinic
chains. The resonance at 40 ppm could also include contributions from both alkyl
and amino acid C. The broad signal at 53 ppm and the sharper one at 59 ppm may
be due to C in OCH3. Amino acid C may also contribute in this region (Breitmaier
and Voelter, 1978). Carbohydrates in HA would be expected to produce signals in
the 60 to 65, 70 to 80, and 90 to 104 ppm regions, although other types of aliphatic
C bonded to O could also do so. The aromatic region contains a relatively sharp maximum near 130 ppm due to alkyl aromatics. The peak at 155 ppm indicates the presence of O- and N-substituted aromatic C (phenolic OH and/or NH2 bonded to an
aromatic C). The broad signal near 180 ppm is due to C in CO2H groups, although
amides and esters could also contribute to this resonance.
The 13C NMR spectrum of the FA (Fig. 4) consists of a number of aliphatic resonances in the 20- to 50-ppm region, followed by signals from C in OCH3 groups,

amino acids, and carbohydrates between 50 and 85 ppm. Broad signals between
130 and 133 ppm indicate the presence of C in alkyl aromatics. The strong signal
between 170 and 180 ppm shows the presence of C in CO2H groups. In general,
fewer sharp signals are observed in the 13C NMR spectrum of the FA than in that
of the HA, possibly because of more H bonding in the FA.
13C NMR data for HA and FA are summarized in Table II in terms of the distribution of C in the different spectral regions. An examination of data in Table II
shows a similar C distribution in the two humic fractions. HA is slightly more aro-

Table II
Distribution of C (%) in a Haploboroll HA and a Spodosol FA as Determined
by 13C NMR
% of C
Chemical shift range (ppm)

HA

FA

0–40
41–60
61–105
106–150
151–170
171–190
Aliphatic C (0–105 ppm)
Aromatic C (106–150 ppm)
Phenolic C (151–170 ppm)
Aromaticitya

24.0

12.5
13.5
35.0
4.5
10.5
50.0
35.0
4.5
44.1

15.6
12.8
19.3
30.3
3.7
18.3
47.7
30.3
3.7
41.6

((Aromatic C ϩ phenolic C) /(Aromatic C ϩ phenolic C ϩ aliphatic C)) x 100.

a