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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
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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
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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
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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