METHODOLOGICAL ASPECTS OF SOIL
ORGANIC MATTER EVALUATION
Rosell
R.A., J.C. Gasparoni and J.A. Galantini
CIC (PBA), CERZOS (UNS-CONICET) Dpto. Agronomía
- UNS
8000
BAHIA BLANCA - ARGENTINA
juangalantini@gmail.com or juan.galantini@cyt.cic.gba.gob.ar
Introduction
Soil organic matter (SOM) is a complex mixture of animal and plant
residues, fresh and at all stages of decomposition, living and decaying
microbial tissue or heterotrophic biomass, and the relatively resistant humic
substances. It has a series of beneficial properties for soils, even at low
percentage levels. Organic matter (OM) is one of the most complex, dynamic and
reactive soil components. The various organic fractions have different
properties which affect the soil behavior and fertility as a whole.
Unfortunately, it is difficult to separate and quantify those fractions in
order to be able to correlate soil constituents and plant productivity.
Carbon is the main element present in SOM, comprising from 48 to 60% of
the total weight. Organic carbon determination is often used as the basis for
SOM estimation by multiplying the OC value by a conversion factor. Many
researchers use the so-called Van Bremmelen factor of 1.724 (see below). Actual
evidence indicates that the factors most appropriate for converting OC to OM
are 1.9 to 2.5 for surface and subsoil, respectively (Broadbent, 1953; Nelson
and Sommers, 1982).
Direct determination of SOM by either dry and/or wet oxidation
procedures includes several errors and, therefore, is not convenient. Several
conversion factors which produce uncertainty must be utilized. Rasmussen and
Collins (1991) have indicated that the conversion factor to transform % OC to %
SOM varies between 1.4 to 3.3. For that reason it is suggested to use the “% of
OC”, which is determined, instead of “% of SOM”, which must be estimated.
The wet oxidation procedure is based on the oxidation of OC in soil with
a dichromate-sulfuric acid mixture, either without or with external heat in
order to accelerate and complete the oxidation. It is known that these methods
do not produce complete oxidation of the OC, requiring therefore a correction
(oxidation) factor for calculating the results. In both groups of procedures
the excess dichromate (Cr2O72-) is back
titrated with ferrous ammonium sulfate by using one of the three redox
indicators (diphenylamine, o-phenanthroline, or N-phenylanthralinic acid). The
chemical reactions are :
  2
Cr2O72- |
+
|
3 Cº
|
+
|
16 H+
|
4 Cr3-
|
+
|
3 CO2
|
+
|
8 H2O
|
|
  Cr2O72- |
+
|
6 Fe2+
|
+
|
14 H+
|
2 Cr3-
|
+
|
6 Fe3+
|
+
|
7 H2O
|
Soil laboratories normally measure SOM
by the wet combustion method of Walkley and Black (Allison, 1965),
assuming that the procedure measures easily oxidizing OC. A correction
(oxidation) factor (1/0.76=1.33) is used to multiply the partially-oxidized OC
content to convert this value to total organic carbon (TOC). Another factor
(1.724) converts OC to OM. This conversion factor of 1.724 is the result of
attributing 58% to the content of OC in the OM (Tabatabai, 1996). In the
originally proposed method of Walkley and Black, the authors recovered an
average of 76% of the OC in 20 soil samples, but recovery values ranged from 60
to 86%. Other authors, reported by Shulte and Hopkins (1996), recovered 49 to
96% (average 78%) of the OC in 96 Wisconsin soils by this method.
Native, virgin soils decrease their carbon content and negatively change
their physical, chemical and biological properties upon continuos cultivation.
Prairie soils also lose SOM rapidly after they are first cultivated
(Miglierina et al., 1988). The loss is usually exponential, declining sharply
during the first 10 to 20 years. Later losses continue declining until an
equilibrium is reached after 50 to 60
years (Jenny, 1941).
Dalal and Mayer (1986) have found that after cultivation OC bound to
sand-sized particles declined rapidly; that associated with silt-sized
particles increased from 48 to 61% (Sikora et al., 1996). The OC differential
behavior when it is associated with or is present in several soil particle size
fractions is a clear indication of the importance of physically-fractionating
SOC.
It has been recognized that the interaction of SOM fractions with clay
minerals is an essential factor in the behavior of the organic fractions
(Christensen, 1991). Soil aggregate fractionation is based on the principle
that the fractions of SOM are associated and/or bound to inorganic particles of
different sizes, which differ in structure and function, playing important and
differential roles in SOM turnover. Size fractionation is mainly applied to
whole soil samples. Soil samples are usually dispersed in water and then
separated by wet sieving. The procedure is considered to be very efficient
because of the solution and mechanical effects of water. In sandy soils dry
sieving is commonly used.
In general, most of the humified SOM in agricultural soils is found
associated with the silt- and clay-sized particle fractions, which are called
the fine fraction. The clay fraction can contain more than 50 % of SOM (Tiessen
and Stewart, 1983 ; Bonde et al., 1992). Others authors (Whatson and
Parsons, 1974 ; Elustondo et al., 1990) have reported that the silt-size
fraction appeared to have the highest proportion of SOM.
Recently, Galantini and Rosell (1997) studied the evolution of SOC, N,
P, and S in two granulometric fractions of an Entic Haplustoll under several
crop sequences after 10 years of cultivation. They used wet sieving and
obtained the denominated fine fraction (FF, < 100 µm particle size) and
coarse fraction (CF, 100 - 2000 µm particle size). They determined that the
level of some SOC fractions associated with the FF was maintained after
cultivation. On the other hand, mineralization of OM was more intensive in the
coarse fraction, thus showing its effects on the changes and dynamics of SOC
and plant nutrients. Knowledge of the SOC distribution, mainly the composition
of the CF, may therefore predict the OC and nutrient turnover and the present
crop-soil fertility status.
The concept behind physical fractionation of soils emphasizes the role
of soil minerals in SOM stabilization and turnover. The physical fractionation
techniques are considered chemically less destructive, and the results obtained
from physical soil fractions are considered to be related more directly to the
structure and function of SOM in situ.
The different SOM fractions and their transformation rates require a
knowledge of their composition and functions. In this paper, SOM definitions,
extraction or sampling, fractionation (physical, biological, and chemical), and
determination methodologies as well as their agronomic implications are
discussed.
Definitions
Different SOM definitions have been used by numerous authors throughout
the years. Historically, chemical concepts and procedures have primarily
influenced the methodology employed for SOM research, mainly the knowledge of
the chemical structure and properties such as elemental composition, cation
exchange capacity (CEC), total and/or partial acidity, nutrient availability
and/or sequestration, etc. in the humified fractions. In general, chemical
characterization of OM and agronomic responses have not been mutually
correlated.
A present concept of SOM components or composition can be defined as
follows (whole soil < 2 mm) :
· organic residues ( 53µm or 100-2000 µm
size), relatively fresh residues (Magdoff, 1996), decaying plant and animal
tissues or debris and their partial decomposition products, including compounds
of different biochemical origin ( amino acids, carbohydrates, fats, waxes,
organic acids, etc.) present in decaying tissues. This compartment is also defined
as the organic matter of the coarse fraction (CF) of the soil (Andriullo etal., 1990). Cambardella and Elliot (1992) have defined this fraction as
particulate organic matter (POM). (fine or coarse POM, according to the authors: Galantini et al., 2014, 2016; Duval et al., 2013, 2016, 2018)
· soil biomass (microbial size), the SOM present as live microbial tissues,
which carry out important functions related to residues decomposition, nutrient
cycling, pest controls, etc. and also promoting porosity channels (Magdoff,
1996).
· the OM compartments of the soil biomass and
the HS associated to the less than 53 µm (or 100 µm) size is also called the
fine fraction (FF) of the SOM.
· humic substances (< 53 µm size), “ a
series of relatively high-molecular-weight, brown to black colored substances
formed by secondary synthesis reactions” (Stevenson, 1982). According to their
molecular size and their solubility in alkali and acid aqueous solutions, well
decomposed humic substances (HS) include :
· humin,
the alkali insoluble fraction of the HS, formed possibly by the condensation of
humic and fulvic acids and residual lignin components.
· humic
acids, the dark-colored organic material insoluble in acid media but
soluble in alkali solutions and specific organic solvents
· fulvic
acid fraction, the yellow-reddish (depending on pH), water soluble, colored
material which remains in solution after precipitation of the humic acids by
acidification
The agronomic or functional aspects of SOM and its fractions have been
important in situations related to soil fertility and plant productivity. For
over a century, SOM has been separated by chemical procedures (water or alkali
- acidic extractions) to be used more in pedogenic studies than in agricultural
applications. However, Feller (1993) considers that:
·
humic substances have low turnover rate
(Anderson and Paul, 1984; Duxbury et al., 1989) and are, therefore, not
necessarily implicated in the short-term processes which occur in agronomic
situations;
·
the behavior of these chemical compartments in
relation to major soil processes such as aggregation, mineralization and
surface properties are not yet fully understood.
In the last 30 years, SOM characterization based on physical (particle
size or density) separation (Feller, 1979; Tiessen and Stewart, 1983; Elliott
and Cambardella, 1991; Christensen, 1992) has been intensified. Much evidence
has accumulated to demonstrate that fractionation of soils according to
particle size provides an useful tool for the study of SOM dynamics and
distribution. A key point in these separations is to achieve adequate soil
dispersion to prevent inclusion of microaggregates of smaller size particles in
the silt and sand size separates. Chemical dispersants used regularly in the
past may be too drastic, causing denaturalization of part of the SOM. Three
different compartments of SOM have been separated from tropical soils (Feller
et al., 1991):
· plant
debris (>20 µm size), with C/N >15;
· organo-clay (0-2 µm size), amorphous organic
material closely associated to clays, with C/N <10;
· organo-silt (2-20 µm size), with properties
intermediate between the former two and a C/N ratio ranging from 10 to 15.
FUNCTIONAL ORGANIC MATTER FRACTIONS
Normally, SOM is used as an indicator of soil quality. The most labile
compartments (light carbon or bioactive C) of SOM are thought to be
participating in supplying nutrients (N, P, S and minor elements) for plant
growth. The most labile compartments are rapidly depleted as a result of
cultivation or soil movement. The intermediate labile compartments, which have
a turnover rate of 10 to 20 years, must be periodically replenished to maintain
an adequate level of SOM, fertility and sustainability.
Particulate SOM
Actual research indicates that the POM properties are consistent with
the characteristics of the intermediate labile SOM compartments. In native
grassland soils, POM can account for up to 48 % of the total SOC and 32 % of
the total soil N (Greenland and Ford, 1964).
Numerous authors have found that POM (or POC) shows promise as a
short-term or early warning indicator of long-term changes in soil quality. The
POC has been shown to be much more responsive than SOC to changes in
agricultural management and, as such, has been proposed as an indicator of soil
quality (Gregorich and Ellert, 1993).
Others authors (Sikora et al., 1996) have indicated that the POC pool
consist of partially decomposed fractions of plant residue and has a density of
< 1.85 g cm-3 and a C/N ratio larger than 20. Light fraction (LF)
SOM, as defined originally by Greenland and Ford (1964), has a similar density
(< 2.0 g cm-3) and an approximate C/N ratio of 25.
For the propose of this paper, LF SOM is considered analogous to POM.
Soil Microbial Biomass
Live soil biomass is responsive for the mineralization of plant residue
or POM and the genesis of more stable humic substances and the production of
plant nutrients. For that reason soil biomass is essential to the dynamics of
SOM quality. Besides microbial biomass, respiration, enzymes and active OC
(carbohydrates, peptides, etc.) are factors associated with total OC and may
function as indicators of soil quality. However, Campbell et al. (1991)
suggested that an increase in biomass N does not necessarily imply an
improvement of soil quality. On the other hand, Chien et al. (1964) reported
that respiratory capacity increased with improvement of soil fertility. Sikora
et al. (1996) consider that “data suggest that microbial biomass, CO2
evolution, and specific respiration changes are much larger than SOM changes
and that these compartments may allow more rapid evaluation of treatment
effects” on soil fertility and plant production.
Presently, the more used biomass indicators are : Biomass C (kg ha-1) ;
Biomass C / total OC ratio (%) ; CO2-C evolution (kg ha-1
d-1) ; potentially mineralizable N (kg ha-1 y-1) ;
and CO2-C / Biomass C (d-1). Enzyme production and evolution have
been studied, but results are not yet conclusive.
The biomass indicators must have a common feature : they require a
good, fresh sample, with no excess of humidity and temperature, and incubation
processes under controlled conditions when required.
Physical separation and
characterization of SOM
Many authors (Turchenek and Oades, 1979 ; Anderson et al.,
1981 ; Tiessen and Stewart, 1983 ; Christensen, 1985 ; Balesdent
et al., 1988 ; Christensen, 1992 ; Cambardella and Elliot, 1993) have
employed soil physical fractionation according to particle-size or density to
study the behavior of SOM in these fractions.
The procedures have been useful to determine differences in the
structural and dynamics properties of SOM. Ultrasonic disruption is commonly
used to obtain a good level of dispersion of soil structure. Different
ultrasonic levels may generate changes in the distribution of the SOM in
particle size fractions. “For example, increasing the intensity of sonication
can result in the distribution of more organic material into finer soil
fractions compared with a lower intensity of sonication energy” (Elliot and
Cambardella, 1991).
As indicated previously, Andriulo et al. (1990) have separed two main
particle-size fractions : the fine (FF) and the coarse (CF) fractions,
wich have important agronomic significance.
Mineral particles in size fractions alterate estimations of OC turnover
time within the fractions. The more complex densitometric fractionation allows
one to physically separate OC found within an specific size class from the more
dense mineral components.
Cambardella and Elliot (1993) developed a dry and wet sieving procedure
to isolate and characterize SOM fractions that were originally occluded in the
soil aggregates and evaluated their effect on the turnover of the OM in several
cultivation systems.
Nutrient contents were determined for discrete size/density OM fractions
isolated from within the macroaggregate structure. Eighteen percent of the
total SOC and 25 % of the total N in a no-till soil was associated with
fine-silt size isolated from inside macroaggregates (enriched labile fraction,
ELF) and particles having a density of 2.07 - 2.21 g cm-3. Sodium
matatungstate was used to obtain different solution/suspension densities (1.85,
2.07 and 2.22 g cm-3). The amount of OC and N sequestered in the ELF
decreased as the intensity of tillage increased, when comparing no-till and
conventional tillage.
The procedure proposed by Cambardella and Elliot (1993) has improved the
quantitative determination of SOM fractions, which in turn increases
understanding of SOM dynamics in cultivated grassland systems.
Aggregate disruption was done by suspending 10 g dried aggregate soil in
60 mL of water at room temperature and immersing an ultrasonic probe to a depth
of 3 - 5 mm into the soil suspension. The disrupted soil suspension was passed
through a series of sieves in order to obtain four size fractions : 1)
> 250 µm ; 2) 53 - 250 µm ; 3) 20 - 53 µm ; and 4) < 20
µm, which were dried over-night at 50º C.
The specific rate of mineralization (µg net mineral N / µg total N in
the fraction) from microaggregate-derived ELF was not different for the various
tillage treatments (bare fallow, stubble mulch, and no-till) but was greater
than for intact macroaggregates.
Sampling
The objective of soil sampling is to obtain reliable information through
samples collected from a large soil body called, for statistical purposes, a
“population”. The sample may or may not be representative of the population,
depending on how the sample is selected and collected.
Sampling is aimed at obtaining the true value of certain properties such
as organic matter, nitrogen (N), phosphorous (P) and other elements or property
values and/or contents. Also the purpose of sampling is to estimate these
parameters with an accuracy that will meet our needs at the lowest possible
cost.
Sampling errors may be larger than those obtained from different
laboratories for the same determination. Therefore, the precision of field
sampling is a major limiting factor in a soil testing program.
Information from the sample is of interest only insofar as it yields
information about the population, but the information may or may not be
representative, depending on how the sample was selected.
Unless the soil sample can be assured of a true field representation,
then differences in laboratory methods, fertilizer strategy and computer
adaptations can only be of academic interest.
Important aspects to be taken into account in sampling are:
1. accuracy and precision;
2. sample areas that are representative of the
field;
3. effect of field size on accuracy;
4. when, how deep, and how often to sample ;
5. in-site variability of soil properties ;
6. soil bulk density at the time the sample is
taken ;
The failure to adjust for bulk density when a sample is taken is perhaps
the greatest errors in determining soil C content in the field.
In order to obtain a truly representative sample it should be considered
the uniformity of the area, topography,
texture, cropping patterns, etc., taking into account horizontal, vertical, and
temporal variations. Soil map units derived from changes in topography,
underlying geology, and dominant vegetation type can be used for horizontal
subdivisions. Soil horizons are excellent subdivisions of vertical change.
The deposition of dung and urine on grazed fields increase the variation
associated with soil sampling (Friesen y Blair, 1984). It also can exist a
biological activity (large animals and/or earthworms) which will disrupt the
soil properties (Subler and Kirsch, 1998).
In general, the best sampling design is one that provides the maximum
precision (smaller error) at a given cost or that provides a specified
precision (acceptable error) at lowest cost.
Effort has sometime been directed toward the use of worker judgment in
searching the most “typical” site or a representative view of the population.
However, this results is samples with biased errors. If a small sample is to be
taken, a “judgment” sample may have a lower error than a random one. As sample
size increases, however, the error with a random sampling become smaller
whereas that with a “judgment” sampling becomes larger.
Stratified composite random sampling combines composite sampling with
sampling by sub-areas having each of them similar topographic characteristics.
Rigney (1956) concluded that errors in the zigzag and stratified random
sampling methods were consistently greater than those of the random
distribution. Because sampling by the zigzag method is relatively easier, it
has been used by many soil testing programs.
Simple
random sample
Simple random sampling is defined as a sample obtained in such a way
that each possible combination of n units to be taken has an equal chance of
being selected.
When more than one sampling units are included, estimation of mean (y)
and variance (s) are
given by:
Sampling and analytical work are expensive. But cost can be reduced by
taking only as many samples as are needed for a given level of precision. We
can calculated the size of the sample needed (n) by (Crépin and Johnson, 1993):
where:
t, is a number taken from a “t” table for a chosen level of probability
and the degree of freedom (n-1, arbitrarily fixed the first time and modified
by reiteration);
s2, is the variance of SOM levels known from other
studies or estimated by s2=(R/4)2, where R is the
estimated range likely to be found in sampling;
D, is the variability in mean estimation of SOM we are willing to accept.
Sample
management
Prevention
of contamination
Great care must be taken to prevent sample contamination during sampling
collection (dirty containers, sampling tools contaminated with previous
samples, etc.) and drying processes (dusty places).
Mixing
A three - four L sample should be thoroughly mixed for subsample
homogenization and reduce the volume down to 0.5 - 1 L of representative
sample.
Drying
and sieving
Wet samples should be dried at temperatures not greater than 35-50°C.
Higher temperatures can alter nutrient solubility from organic and mineral soil
fractions.
Soil aggregates are crumbled in a porcelain or agate mortar and passed
through a 2 mm sieve. Homogenization of fine soil for further analysis is made
with a mechanical grinding mill.
Dust from sample processing is a nuisance and a important contaminant. A
mask should be worn to avoid breathing the dust.
Splitting
and storing
The field-collected material must be thoroughly mixed and a sub-sample
for analysis and storage should be taken. Clearly identified samples should be
kept in a cool, ventilated, and dry storeroom.
ESTIMATION OF SOM
Soil OM has been defined as “the organic fraction of the soil exclusive
of undecayed plant and animal residues”
(SSSA, 1997) and has been used synonymously with “humus”. However, for
laboratory analysis, the SOM generally includes only those organic materials
that accompany soil particles through a 2-mm sieve (Nelson and Sommers, 1982).
It is difficult to quantitatively estimate the amount of SOM. Procedures
used in the past involve determination of change of weight of the soil sample
resulting from destruction or descomposition of organic compounds by hydrogen
peroxide (H2O2) treatment or by ignition at high
temperature. Both techniques are subject to error.
Numerous methods are available for the determination of OC and the
estimation of OM, all of then having inherent associated problems. For that
reason, the method used should be most applicable to the soils being analyzed
and the required accuracy of the results. All current methods are included in
one of the following principles (See Table 1 for a classification and comments
on the different methodologies) :
I.
Wet
oxidation of OC
a.
using acid
dichromate solution
1.
followed
by back trititation of the remaining dichromate or photometric determination of
Cr3+;
2.
followed
by collection and determination of evolved CO2;
b.
by H2O2
followed by measurement of weight change;
II.
Dry
oxidation of OC in a furnace
a.
followed
by measurement of weight change (weight-loss-on-ignition, WLOI)
b.
followed
by collection and determination of evolved CO2;
Note :
for high temperature combustion techniques carbonate-C will also be included in
the total C recovered and, therefore, must be subtracted to obtain organic C.
I.a.1.
Titrimetric dichromate redox methods
The potassium dichromate procedures are widely used in soil OC
determination because of their rapidity and simplicity compared with others wet
or dry combustion methods. Advantage of acid oxidation is the removal of
carbonate C.
Dichromate redox methods potentially suffer from a number of
interferences and low recoveries. In the Walkley and Black (1934) method, OC is
oxidized solely by the action of dichromate, heated by the exothermic mixing of
aqueous dichromate and concentrate sulfuric acid (»120°C). As indicated in the Introduction, a
factor must be used due to the incomplete oxidation reached with this
temperature (Walkley & Black, 1934). This factor is not valid for different
soils (Gillman et al., 1986), different horizons of a soil profile or organic
fractions (Galantini et al., 1994).
Some authors proposed supplementary heating on a hot plate or block
digestor (Mebuis procedure) to improve OC oxidation (Mebius, 1960;
Schollemberger, 1945; Heans, 1984). In some cases two blanks are recommended
due to the thermal instability of dichromate acid.
Several errors can affect the quantification of OC by this procedure:
·
a source of positive errors is the presence of
Cl- (producing CrO2Cl2) and Fe2+ or
Mn2+ (producing Cr3+ and Fe3+ or Mn3+)
·
the higher oxides of Mn (largely MnO2)
compete with dichromate for oxidizable substances when heated in an acid medium
resulting in a negative error (Tabatabai, 1996).
·
the acid dichromate digestion solution
decompose at temperature above 150°C, limiting the heating procedure (Charles
and Simmons, 1986).
Despite the difficulties and inaccuracies, the dichromate redox method
is widely used because it requires a minimum of equipment, can be adaptable to
handle large number of samples, is suitable for comparative work on similar
soils and it is not costly.
I.a.2.
Wet oxidation of OC with collection and determination of evolved CO2
The collection and determination of evolved CO2 can eliminate
many of the interferences of the titrimetric dichromate redox procedure and can
be employed at a higher digestion temperature (Allison, 1960). The evolved CO2
may be trapped in solid absorbents (followed by measurement of weight changes)
or in a alkali solution (followed by back titration). An adaptation described
by Snyder and Trafymov (1984) allows the processing of large number of samples
at a time.
I.b.
Wet oxidation with hydrogen peroxide
The H2O2 method proposed by Robinson (1927)
produces the incomplete oxidation of OM varying the extent of oxidation among
soils (Gallardo et al., 1987). Additional errors are introduced both in
filtering and drying the oxidized residue and the filtrate at 110 °C. The
method is, therefore, unsatisfactory as a means of determining total SOM.
II.a.
Dry combustion followed by measurement of weight changes
The dry combustion is followed by the
measurement of changes in weight-loss-on-ignition (WLOI). This is a alternative method (non Cr contaminant) for
the dichromic acid digestion, which is widely used in soil testing
laboratories. The method assume that SOC is oxidized completely within a narrow
temperature range at which losses (water of hydration for example) from
minerals is negligible. Unfortunately, this is not the case, so temperature
selection is somewhat arbitrary but, at the same time, critical to minimize
errors (Schulte and Hopkins, 1996). Temperature higher than 500°C can result in
errors from loss of CO2 from carbonates, structural water from clay
minerals, oxidation of Fe2+, and decomposition of hydrated salts
(Schulte and Hopkins, 1996). Temperatures below 500°C should eliminate many of
these errors but may result in incomplete SOM oxidation (Gallardo et al.,
1987).
Hygroscopic water can be removed by heating 24
h at 105°C, but more temperature is required to remove water from gypsum or MgSO4.7H2O.
Some authors have reported a complete SOM
oxidation at temperatures between 430º C and 500º C (Davis, 1974 ;
Giovanni et al., 1975 ), with no loss of carbonates. Others studies showed
that part of the soil humic compounds resist 600°C (Gallardo et al., 1987).
Differential thermal analysis showed an
endothermic peak around 250°C (due to gibbsite interference), a first
exothermic peak around 350°C (oxidation of humic acid lateral chains) and a
second peak around 450°C (oxidation of
humic acid central nucleous).
Dupuis (1971) found that the presence of sodium
salts thermically stabilized the SOM, while aluminum slows down (humic-aluminum
complexes) or accelerates (excess of aluminum) combustion.
Gallardo et al. (1987) concluded that part of
the SOM may be thermostable in presence of certain inorganic compounds such as
the transitional elements.
A review of papers comparing WLOI method is
presented by Schulte and Hopkins (1996). Significant regression equations were
found in all cases and r2 higher than 0.90 in most of them.
Differences in slopes for regressions may be attributed to differences in
heating time and temperature, and/or differences in the nature of the clay and
SOM fractions.
WLOI may be reasonably accurate and economical
for estimating SOM if precautions such as hygroscopicity and hydrated salt
content are taken into account.
II.b. Dry
oxidation of OC with collection and determination of evolved CO2
The use of a stream of O2 in the
tube furnace at temperature over 900°C provides for near complete combustion of
OC. The addition of MnO2 and CuO as catalysts convert any evolved CO
to CO2 (Tiessen et al., 1981).
The most convenient combustion methods use
automated instruments, which heat the sample by induction and determine the
evolved CO2 by infrared absorption. Many of the commercially
available units are equipped for simultaneous C, H, N, and/or S analysis.
In soils with significant levels of carbonates,
however, organic carbon can be overestimated if carbonates are not removed
before analysis or account for by analysis.
Soil thickness and bulk density as
errors in an analytical result
Chemical analysis indicate the concentration of elements in soils, but
thickness and bulk density of the soil layer in the field must be considered to
estimate quantities of element per unit of area. For management-induced changes
in SOM sequestration to be well assessed, the masses of soil being compared
must be equivalent (Ellert and Bettany, 1995).
Sampling activities require quality assurance principles such as the
development and use of standardized sampling procedures, training and
documentation of sampling personal, and creation of traceable and defensible
information through the use of labels and chains of custody (Kesta and Bartz,
1996)
Comments
SOC determination and SOM estimation can be obtained by using wet and
dry combustion procedures. The wet combustion titrimetric method is rapid,
simply and rather accurate. The WLOI oxidation determination is also rapid and
accurate provided the procedure errors are evaluated and applied .
The main sources of errors in the SOM estimation are :
· erratic field sampling
· the use
of correction factors : oxidation of OC (1/0.76 = 1.33) and conversion of
OC to OM (1.724) coefficients
· inherent titration and volumetric analytical
procedure
· presence of inorganic C (carbonates)
· the need
to express results on a volumetric basis by using the soil bulk density taken
at sampling
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