Clay Minerals, (2010) 45, 23–34
Determination of soil aggregate
disintegration dynamics using laser
diffraction
A . B I E G A N O W S K I * , M . R Y Ż A K
AND
B.WITKOWSKA-WALCZAK
Institute of Agrophysics, Polish Academy of Sciences, Doświadczalna 4, 20-290 Lublin 27, Poland
(Received 29 April 2009; revised 28 September 2009; Editor: John Adams)
AB ST R ACT : A new practical and precise method for determining soil aggregate stability is
described. Four air-dry aggregate fractions (<0.25, 0.25 0.5, 0.5 1.0 and 1.0 2.0 mm) were added
to thoroughly stirred water in a Mastersizer 2000 laser diffractometer. The suspension obtained was
passed directly through the measuring system. The dynamics of median (equivalent diameter d50)
particle-size distribution decrease (interpolated with a logarithmic function) was assumed to be the
measure of soil aggregate stability. In order to show the applicability of the new method, the results
obtained (for selected and diverse soils) were compared with those from the wet sieving standard
method. The main conclusion is that the proposed method is convenient and can be successfully used
for the estimation of soil aggregate stability. Moreover, it has wider application because standard
sieving methods are restricted to aggregates >0.25 mm whereas, with the use of the laser diffraction
method, smaller aggregates can be measured. The energy delivered to the aggregates in the process
of aggregate disintegration is more reproducible in the method described here. The method also
provides an opportunity to verify that the soil aggregates are completely destroyed (lack of the
changes of the median value shows the end of soil aggregate disintegration).
KEYWORDS: soil aggregate stability, laser diffraction, dynamics, soil aggregate disintegration.
Resistance of structural components of soils, soil
aggregates, to the effect of external agents depends
primarily on the physical and chemical properties of
sticky substances, i.e. clay and organic matter
(Gorbunov, 1987; Dexter & Czyż, 2000; Shein &
Goncharov, 2006; Calero et al., 2008,). Various
indices have been proposed to express the
distribution of aggregate sizes; if a single
characteristic parameter is required, a method
should be adopted to assign an appropriate
weighting factor to each size range of aggregates
* E-mail: [email protected]
DOI: 10.1180/claymin.2010.045.1.23
(Hillel, 1998; Kutilek & Nielsen, 1994; Niewczas &
Witkowska-Walczak, 2003, 2005). Some indices
still in use are: the ‘mean weight diameter and its
change before and after the water acting, the
geometric mean diameter, the coefficient of
aggregation, the weighted mean diameter, the
change in mean weight diameter, the slaking loss,
etc.’ (De Boodt, 1967). Gardner (1956) reported
that the aggregates of many soils exhibit a
logarithmic-normal distribution, which can be
characterized by two parameters, namely the
‘geometric mean diameter and the log standard
deviation’. Bryan (1971) tested some aggregation
indices for English and Canadian soils and proved
that although several indices were clearly efficient
at distinguishing differences, none was adequate for
measurement or description of the complete
# 2010 The Mineralogical Society
24
A. Bieganowski et al.
disaggregation characteristics of a soil. Many
indices used today were proposed many years ago
(Rewut, 1969). They are time consuming, far from
perfect and none of them is universally preferred.
Comparison of materials described by different
indices is limited or impossible (Dexter, 1988;
Niewczas & Witkowska-Walczak, 2003; Roth &
Witkowska-Walczak, 1992; Walczak & Witkowska,
1976)
In the 1980s a new technique for measuring the
particle-size distribution was described
the laser
diffraction method. Immediately, applications of the
method in soil science were found (Cooper et al.,
1984) and since then the technique has been
improved (Buurman et al., 1997; Chappell, 1998;
Sperazza et al., 2004). The method is commonly
used in soil laboratories to determine the particlesize distribution of soils but there are limitations in
its use, the main one being the problem with
measuring the finest fractions of soil (Taubner et
al., 2009).
In work done so far the laser diffraction method
has only been used to determine the soil particlesize distribution (Arriga et al. 2006; Goossens,
2008). Because the soil particle-size distribution
characterizes the size of elementary particles in soil
aggregates, the limitation mentioned above should
be eliminated as in classical sedimentation methods
(Pini & Guidi, 1989). The aggregate disintegration
in the soil particle-size distribution measurements,
i.e. dispersion of the soil sample, is usually carried
out using a solution of sodium hexametaphosphate
or by the ultrasonic method (Levy et al., 1993;
Mayer et al., 2002; Mentler et al., 2004; Tippkötter,
1994; Zobeck, 2004).
The same problem was encountered in the
present study, i.e. the disintegration of soil
aggregates during measurement of soil particlesize distributions. These effects provide an opportunity for characterization of soil aggregate
disintegration.
The aim of this study was to develop a new, easy
and reliable method of characterizing soil aggregation stability. The method is based on the analysis
of the unsteady state during mechanical disintegration of air-dry soil aggregates by stirring under
controlled conditions. The principle of the proposed
method is the observation of changes in the median
of the particle-size distribution as a function of
disintegration time. To confirm the advantages of
the new method, the results were compared with
those obtained for the same soils by the standard
wet sieving method.
MATERIAL AND METHODS
Soils
The material for the study included aggregates
from the arable horizon of three types of soils
which are significantly different, i.e. they have
different granular composition and stability of
aggregates (Niewczas & Witkowska-Walczak,
2003); black earth derived from light sandy loam
(Mollic Gleysol), proper rendzina formed from
calcereous rock (Calcaric Cambisol) and chernozem
formed from loess (Haplic Phaeozem). The basic
properties of the investigated soils are shown in
Table 1. Four fractions of aggregates of air-dry soil
were investigated: 40.25, 0.25 0.5, 0.5 1 and
1 2 mm. These were obtained by sieving and the
water resistance of the aggregates was determined
by standard wet sieving methods (Savinov, 1936;
Yoder, 1936).
TABLE 1. Basic properties of the investigated soils.
Soil
Grain-size distribution*
(%) (diameter in mm)
>50
2 50
<2
Mollic Gleysol
Calcaric Cambisol
Haplic Phaeozem
53
37
13
34
35
73
13
28
14
* Analysed by areometric method
** Specific surface area (measured by water vapour)
Corg.
(wt.%)
CaCO3
(wt.%)
pHKCl
SSA**
(m2g 1)
Bulk
density
(g cm 3)
3.1
2.7
3.0
0.14
18.2
0.12
6.5
7.0
6.8
51
87
79
1.34
1.20
1.15
Soil-aggregate disintegration by laser diffraction
25
Apparatus
Measurement options
The dynamics of soil aggregate disintegration
was determined using the Mastersizer 2000 instrument from the Malvern Company (Malvern
Operators Guide, 1999), designed for standard
determination of the grain-size distribution of
particles within the size range of 0.02 mm 2 mm.
The Mastersizer 2000 makes use of laser light
scattered by the particles being measured and
calculates particle-size distributions. The measuring
setup is shown in Fig. 1.
In our investigations we used the Hydro MU
attachment to the Mastersizer 2000 which enabled
us to carry out the measurements in ~800 cm3 water
suspension using distilled water as the continuous
phase.
The software of the Mastersizer 2000 permits
several options depending on the investigated
objects
the model/theory of recalculation of
images registered on the detectors into particle-size
distributions, the homogeneity of the samples and
the shapes of particles.
Calculations related to the determination of the
aggregate-size distribution by laser diffraction using
the Mastersizer 2000 instrument were carried out on
the basis of the Mie theory (adopting a refractive
index of 1.577 and absorption index of 0.1
indices recommended for clay by the producer).
The laser light wavelength in the apparatus was 466
nm for blue light and 633 nm for red.
Measurements (averaging of 30,000 images of
laser light diffraction recorded by the detectors)
lasted 60 s (30 s for blue and 30 s for red light) and
were carried out consecutively.
The following algorithms were selected for the
measurements: (1) general purpose analysis
a
calculation procedure recommended by the apparatus producer for objects with unknown properties
or containing a large number of fractions;
(2) irregular shape ratio
although one of the
assumptions of the method is sphericity of particles,
the manufacturer has provided a module permitting
greater accuracy of results when the particles under
study are not perfect spheres.
The speed of the stirrer and the integrated pump
was determined experimentally and fixed at 2500
rotations per minute. This speed was selected to
ensure that the mixture did not suffer from
gravitation segregation of particles (slow speed) or
formation of air bubbles in the stirred mixture (fast
speed).
Measuring procedure
Air-dry aggregates (each fraction separately)
were metered out directly into a measurement
beaker with distilled water. Measurements were
made over six replications (as each replication was
in progress, a new portion of air-dry soil was
poured into the measuring system). To take into
consideration one of the main sources of uncertainty, i.e. inaccuracies during the measuring
procedure, the measurements were carried out in
two separate series (with three replications in each
series) by two investigators. For subsequent
analyses, the results of all six measurements were
averaged.
To improve the credibility of the results, the
measurement procedure employed by the
Mastersizer 2000 apparatus eliminates the socalled background particles contained in the
continuous phase.
Quantity of the sample
FIG. 1. Schematic diagram of the measurement setup
for the determination of particle-size distribution using
the laser diffraction method.
The amount of soil taken for measurement was
chosen so that the obscurance (the fraction of light
obscured by the analysed suspension) measured by
the Mastersizer 2000 at the beginning of the
measurement cycle was ~10%, which is within the
recommended range (10 20%). At lower obscurance the quality of images reproduced on the
detectors is too weak, causing a high level of
measurement uncertainty. At obscurance levels
>20%, the laser light beam may be subject to
multiple reflections from successive soil particles,
introducing errors in the results. In practice, the
26
A. Bieganowski et al.
mass of soil samples was in the range of 0.5 to a
few grams.
Procedure for reducing obscurance in the
course of measurement
In the course of measurements of soil particle
size distribution, conducted in a water medium with
the use of the Mastersizer 2000 apparatus, a
methodological problem arose. Because of the soil
aggregate disintegration during the measurement,
the obscurance increased to over the 20% figure
recommended by the manufacturer.
To eliminate this problem, a procedure was
developed for the removal of excess particles in
the course of measurement, thereby reducing the
degree of obscurance. The basic criterion that the
procedure had to meet was the proportional removal
of all aggregate fractions from the measurement
system to avoid the necessity of stopping the stirring
of the suspension circulating within the measurement
system. Stopping the stirring results in the settlement
of larger particles, with the smaller particles
remaining in the suspension; an attempt at separation
of such a suspension would cause an alteration of
the quantitative relations between the particular
aggregate size fractions. Therefore, when obscurance
exceeded 20% in the course of measurement, the
recording of laser beam diffraction on the detectors
was stopped without stopping the suspension stirring
system and the flow through the measurement cell.
While the pump stirring the sample continued to run,
the hose provided for draining the tested mixture
from the measurement cell to the flask was
disconnected (Fig. 1). This permitted a part of the
tested mixture to be drained into a separate vessel
and removed from the measurement system,
reducing the obscurance. Then the volume was
supplemented with distilled water up to 800 cm3.
This operation was repeated until the proper
obscurance level of was obtained. Following this
procedure, the drain hose was reconnected to the
measurement system and the measurement process
was resumed. If necessary, the procedure could be
repeated any number of times, ensuring that the
signal recorded in the tested suspension did not drop
below the background signal recorded prior to the
measurement. This procedure did not disturb the
measurement, as the system was dynamically stirred
all the time. This is confirmed by the results
presented in Fig. 2 which show an example of
changes in the course of measurement of the
equivalent diameter d50 (median) for the Calcaric
Cambisol fraction 0.25 0.5 mm (Fig. 2a), and
changes in obscurance for the same measurement
(Fig. 2b). It is clear that no changes occurred in the
smooth charge of the equivalent diameter d50 when
the obscurance adjustment was made.
Measure of soil aggregate disintegration
The changes of the median of the particle-size
distribution can be the measure of soil aggregate
disintegration. The median, sometimes called the
equivalent diameter, d50, defines the limit that 50%
of the population of particles have diameters >d50,
and 50% of the population have particle diameters
<d50. Analysing changes in the value of d50, one
can easily observe changes within the whole
population.
The dynamics of aggregate disintegration
expressed as changes of the median during
mechanical stirring was interpolated using the
logarithmic function:
(y = a lnx + b)
(1)
The logarithmic function was tested on the basis
of experimental results obtained during a period of
30 min (30 measurements).
The soil aggregate water stability was determined
using the most popular varieties of the standard
(classic) wet sieving methods (Savinov, 1936;
Yoder, 1936). The soil aggregate distributions
before and after water impact were expressed as
the mean weighted diameter (MWD), whereas its
differences were expressed as a change of mean
weighted diameter (CMWD) (De Boodt, 1967). The
time of measurements was 12 min for the Savinov
method and 30 min for the Yoder method.
RESULTS AND DISCUSSION
Results
The dynamics of the process of soil aggregate
disintegration for four fractions of the investigated
soil are shown in Fig. 3. The larger aggregate has
lower water stability. This lack of stability appears
in the first second of the measurement. Because the
time of measurement was 1 min (30 s for the
measurement at each of two wavelengths), the first
measurement in each series is associated with the
largest error. This does not influence the final result
because the trend is repeatable.
Soil-aggregate disintegration by laser diffraction
27
FIG. 2. Changes in the equivalent diameter (median), d50, (a) and obscurance (b) at the moment of removal from
the suspension excess particles causing them to exceed of the upper limit of obscurance.
Values of the parameters of the interpolated
logarithmic functions and the statistical study are
given in Table 2. It can be concluded from the
analysis of the data contained in Fig. 3 and Table 2
that the greatest rate of aggregate disintegration of
the investigated soils was for Calcaric Cambisol
whereas the smallest was for Haplic Phaeozem; this
could be expected from the physical and chemical
properties of the soils. It was also expected that the
largest dynamic changes would occur for the
coarser fraction.
Because the mean values in Table 2 are
obtained from six replications they can be used
to determine statistical significance. For instance,
the a parameter from the logarithmic equation for
the two fractions 0.5 1.0 ( 28.620) and 1.0 2.0
( 24.317) mm of Mollic Gleysol are not statistically different (for a = 0.05), while for other
fractions of the same soil they differ statistically.
Because, as mentioned earlier, the aim of this
paper was not to characterize the soil but to
describe a new method for determination of the
dynamics of soil aggregate disintegration, the
discussion will concentrate on aspects of this
method.
Interpolated function
Initially, the problem of selection of the
mathematical function should be discussed. From
the shape of the relationship and from experience
one can expect that the best function would be a
logarithmic or power function. The equation of the
power function is:
(y = cx d)
(2)
The power function was rejected in our approach.
The reasons were its limits: when x approaches 0
28
A. Bieganowski et al.
FIG. 3 (above and facing page). Changes in the equivalent diameter (median), d50, over time for the aggregate
fractions under study. The graphs are averages of six replications.
(x?0) then y approaches infinity (y??). This is
equivalent to the assumption that the soil aggregates
have infinite sizes; when x approaches ? (x??)
then y approaches 0 (y?0). This is equivalent to
the assumption that the aggregates completely
dissolve in water. Both assumptions are evidently
false.
The measure of good fitting of the interpolated
logarithmic function can be the coefficient of
determination, R2 (or r, the correlation coefficient).
From data presented in Table 2, it can be concluded
that R2 occurs in the range 0.704 to 0.994
(corresponding to r in the range 0.839 0997).
Statistical tests showed the significance for all
correlations in Table 2.
Somewhat surprisingly, the largest values of R2
are for Calcaric Cambisol (for all granulometric
fractions). The methodical problems of measuring
the grain-size distribution of this soil type are
reported in the literature (Kalicka et al., 2008).
However, it is not surprising that the largest
values of R2 were obtained for the finest fraction of
all soils. The explanation is that the number of
aggregates in this fraction is the smallest and
because the aggregates are not large, their
disintegration is repeatable.
The information on the dynamics of disintegration of aggregates is included in the a parameter of
the logarithmic equation. The smaller the value of
a, the larger the dynamics of disintegration. The b
parameter indicates the shift of the graph along the
Y axis and is a measure of the aggregate size at the
start of the disintegration process (just after the
addition of the soil to the measuring system).
Soil-aggregate disintegration by laser diffraction
Reproducibility
Because the six replications were carried out in
two series by two operators, the standard deviation
and coefficient of variation reflect the reproducibility of measurement rather than repeatability.
When analysing data from Table 2 it can be
stated that the average coefficients of variation are
equal: 0.096 for a; 0.047 for b (both parameters
from equation 1). The repeatability of results
obtained by one operator in a short time is
desirable, but we decided to include this source of
uncertainty because this situation can occur when
comparing results from different laboratories.
In discussing the reproducibility of results
expressed by the coefficient of variation it should
be stated that for the majority of cases the smallest
values of these coefficients were obtained for the
finest class and the largest values for the coarsest
class. This would be expected because the number
29
of coarser aggregates is larger. Moreover, the
aggregates consist of soil particles of different
shape and size. During disintegration these soil
particles are free and their heterogeneity increases
the statistical dispersion, reflected in an increase of
the coefficient of variation.
Mechanism of disintegration
Two mechanisms are responsible for the disaggregation of air-dry aggregates during the measurement on contact with the intensively stirred water:
wetting and mechanical energy of stirring. It is not
possible to separate the effects of each of these, but
in this procedure separation is not necessary. The
most important factor is the repeatability of
disintegration conditions. This repeatability can be
ensured by using a similar volume of dispersion
water and the same speed of the stirrer and the
pump in the measuring system. Because the
Equation
parameter
16.583
134.583
0.968
6.585
62.265
0.980
Calcaric Cambisol
a
b
R2
Haplic Phaeozem
a
b
R2
* SD, standard deviation
** CV, coefficient of variation
6.598
77.625
0.979
0.360
1.824
0.009
1.269
1.634
0.007
0.419
1.589
0.004
0.055
0.029
0.009
0.077
0.012
0.007
0.063
0.020
0.004
— Fraction <0.25 mm —
Mean value
SD*
CV**
of parameter
a
b
R2
Mollic Gleysol
Soil
17.433
94.487
0.778
51.050
277.567
0.983
10.540
233.267
0.704
1.183
3.016
0.021
1.766
5.909
0.003
1.889
16.469
0.124
0.068
0.032
0.027
0.035
0.021
0.004
0.179
0.071
0.176
– Fraction 0.25 0.5 mm –
Mean value
SD
CV
of parameter
15.600
87.422
0.727
79.500
339.283
0.994
28.620
204.120
0.790
0.657
1.882
0.035
5.373
17.770
0.001
5.983
19.263
0.097
0.042
0.022
0.048
0.068
0.052
0.001
0.209
0.094
0.122
– Fraction 0.5 1.0 mm –
Mean value
SD
CV
of parameter
TABLE 2. Values of interpolated parameters of the logarithmic function.
14.200
79.065
0.761
101.650
387.533
0.986
24.317
138.717
0.801
1.630
4.858
0.047
9.255
40.478
0.006
6.050
13.060
0.097
0.115
0.061
0.062
0.091
0.104
0.006
0.249
0.094
0.122
– Fraction 1.0 2.0 mm –
Mean value
SD
CV
of parameter
30
A. Bieganowski et al.
Soil-aggregate disintegration by laser diffraction
reproducibility of experimental conditions is
realized in the Mastersizer 2000, the information
on these parameters should be included in the
experiment description.
Dynamics of disintegration
It is obvious that the dynamics of disintegration
is most rapid immediately after pouring air-dry soil
aggregates into the measurement system.
Considering Fig. 3, one can observe that a point
is reached where the changes are slight and the
values of the equivalent diameter stabilize at a
specific level. Therefore, the frequency of measurements and the time of individual measurement
should be considered.
The Mastersizer 2000 software permits the time
of individual measurement to be set from 0.5 to
65.5 s for each laser (blue and red). This implies
that the real time of individual measurement is
twice as long. Taking into account the monitoring
of changes at the start of the disintegration process
one can expect the measuring time to be as short as
possible, but the meaning of ‘shortest time’ needs to
be discussed.
When measuring time is too short, the problem of
the representativeness of the measurement arises.
When there are only few larger particles (aggregates or disaggregated soil particles), statistically
they will enter the measuring cell less frequently
than smaller particles and the measuring system
will fail to identify them.
An additional argument for not shortening the
measuring time is the necessity for using the
obscurance reducing procedure (described in the
Material and methods section above). This procedure was often used in measuring large aggregates
of Calcaric Cambisol and Mollic Gleysol. When the
measuring time is very short compared with the
time interval necessary for reducing obscurance, it
strongly influences the dynamics of aggregate
disintegration and, in consequence, the determination of the coefficient of the interpolated function.
We decided, on the basis of experience, to set the
individual measurement interval to 60 s (2630 s).
In our opinion, this measurement parameter is not
critical because rapid changes of the median in the
first few seconds do not influence the parameters of
the interpolated equations significantly. Of course,
for other applications, monitoring for shorter periods
can be used. However, it seems better to interpolate
the changes in this short time by a linear function.
31
Problem of initial preparation of the soil
sample
The prior preparation procedures for the soil
samples could lead to errors in the results obtained.
For instance, using moist aggregates would cause
inaccuracies because the water would cause
disaggregation. The extent of this prior disintegration would be difficult to estimate as it depends on
the soil type, amount of water used, possible
mixing, etc. The use of an ultrasonic probe built
into the apparatus can accelerate the speed of
aggregate disintegration and in the experimental
conditions make it impossible to monitor the
changes of the median. These problems led us to
use dry soil.
Comparison of dynamics of soil aggregate
disintegration with the standard wet-sieving
method
Comparisons of water-resistance results obtained
by the laser diffraction method and the standard wet
sieving method are shown in Table 3.
Our first conclusion drawn from Table 3 is that it
is not possible to use the standard wet-sieving
method for ~0.25 mm fractions. However, in the
laser diffraction method it is possible.
It can be seen from the data in Table 3 that there
is a characteristic relationship for all fractions of all
soils. The values of the measured parameters
decrease in the following order: from the largest
value for MWD12, through MWD30, median12 to
the smallest value of median30. It is obvious that
MWD30 (30 min of soil aggregate disintegration) is
smaller than MWD12 (12 min of soil aggregate
disintegration). A similar situation occurs in laser
measurements when the median is smaller after 30
min than after 12 min. However, the relatively large
difference between values of MWD (weighted
average) and the median should be stressed. Both
quantities express different statistical parameters but
the general trend shows that the aggregatedestruction energy during laser diffraction measurements is markedly higher than in the sieve method.
The main reason for the lack of repeatability in
the sieve method is the fact that the manner and
speed of mixing is different in the different
methods. There are numerous pieces of equipment
for the measurement of the aggregate water stability
of soils but they differ in construction and in the
relationship between mass of soil samples and
32
A. Bieganowski et al.
TABLE 3. Diameters of aggregates before and after water impact.
Diameter before
water impact
(mm)
MWD*
MWD12
Mollic Gleysol
<0.25
0.25 0.5
0.5 1.0
1.0 2.0
0.125
0.375
0.750
1.500
0.270
0.530
0.900
0.257
0.496
0.734
0.105
0.220
0.600
0.118
0.254
0.766
0.061
0.207
0.124
0.076
0.056
0.198
0.116
0.062
Calcaric Cambisol
<0.25
0.25 0.5
0.5 1.0
1.0 2.0
0.125
0.375
0.750
1.500
0.360
0.660
0.870
0.225
0.317
0.340
0.015
0.090
0.630
0.150
0.430
1.160
0.095
0.157
0.143
0.133
0.075
0.098
0.068
0.051
Haplic Phaeozem
<0.25
0.25 0.5
0.5 1.0
1.0 2.0
0.125
0.375
0.750
1.500
0.280
0.380
0.820
0.244
0.277
0.700
0.095
0.370
0.680
0.131
0.473
0.800
0.045
0.048
0.045
0.041
0.041
0.041
0.039
0.036
Soil
Aggregate
fraction
(mm)
———— Diameter after water impact (mm) ————
MWD30 CMWD*12 CMWD30 Median12
Median30
* MWD, mean weight diameter
** CMWD, change of mean weight diameter
volume of water. Also, relations between sieves and
water are different. The sieves can be immovable
(water flows); these can be moved up and down or
deviate from the perpendicular together with the
container by up to 60º (De Boodt, 1967; Walczak &
Witkowska, 1976)
Soil aggregate disintegration determinations
using the laser diffraction method are repeatable
because in modern devices the speed of stirrer can
be programmed. This parameter should be defined
in the description of the method.
Most of the methods of measurement of soil
aggregate stability (including the sieve method) do
not provide an opportunity to verify the accuracy of
the measurement. A strict compliance with the
operating procedure decreases the measurement
uncertainty. In the laser diffraction method the
operator is able to verify if the measurement is
acceptable and complete. This verification can be
achieved by observing changes of the median
during the measurement. The stabilization of the
median indicates that the process of soil aggregate
disintegration is completed. Therefore, for some
soils, it is possible to extend the measurement to
materials which are impossible with the sieve
method.
A further observation from Table 3 is that the
largest median after 12 and 30 min in all cases is
for finer fractions (0.25 0.50 mm) and not for
coarser ones (0.5 1.0 and 1.0 2.0 mm) as could be
expected (as shown in MWD12 and MWD30). This
can be caused by the large rate of disintegration of
coarser aggregates at the time of addition of soil to
the measuring system. When the speed of the pump
and stirrer is 2500 r.p.m., and when the measurement interval is 1 min in the laser diffraction
method, it is impossible to observe the starting
dynamics of aggregate disintegration (in the initial
seconds when the rate is greatest). This can be
confirmed by examining Fig. 3. The median of the
coarser fraction in the first minute is much greater
than the median for the finest fraction.
Simultaneously, a sudden decrease of the median
for coarser fractions can be observed and after a
few minutes they are smaller than for the
0.25 0.50 mm fraction.
Comparing the stability of soil aggregates
expressed by parameters CMWD12 and CMWD30
(Table 3) with the dynamics of aggregates disintegration expressed by parameter a from the
logarithmic equation 1 (Table 2) shows that values
of CMWD for both methods are directly correlated
Soil-aggregate disintegration by laser diffraction
with the size of the fraction. A different situation
arises in the case of the a parameter. The
monotonicity can be observed only for Calcaric
Cambisol (the increase of fraction size and decrease
of a parameter (from 16.583 to 101.650) which
means that the dynamics of disintegration
increases). For Mollic Gleysol the greatest rate of
disintegration occurs for the 0.5 1.0 mm fraction
and, for Haplic Phaeozem, for the 0.25 0.5 mm
fraction. For the coarser fractions of both soils the
values of the a
parameter are similar. The
differences obtained for the sieving and laser
diffraction methods do not mean there is an error
in either of them. The sieve method permits
estimation of the difference before and after the
disintegration process and the a parameter describes
the dynamics of change but not the difference.
Laser diffractometry was used for the assessment
of micro-aggregation by Beuselinck et al. (1999).
They obtained good correlation with the sieve
method and their results can be a good supplement
to the data presented in this paper but they cannot
be compared directly. They used the laser
diffraction method for the measurement of the
grain-size distribution of previously dispersed soil.
Because of this, their work was rather a comparison
of the results obtained by the sieve and laser
diffraction methods than an investigation of the
dynamics of soil aggregate disintegration.
CONCLUSIONS
(1) The laser diffraction method for grain-size
distribution can be used successfully for the
determination of the dynamics of soil aggregate
disintegration. This can be achieved by analysing
the median changes during the mechanical disintegration of aggregates in water.
(2) Use of the laser diffraction method provides
an opportunity to measure the water resistance of
all fractions of aggregates when standard sieve
methods preclude the measurement of the finer
fractions.
(3) Soil aggregate disintegration is more repeatable in the laser diffraction method than in
standard sieve method. This gives less uncertainty
of measurement.
(4) With the laser diffraction method it is
possible to verify if the soil aggregates were
completely destroyed. This is achieved by observing changes of the median of grain-size distribution. Lack of the changes of the median values
33
indicated the conclusion of soil aggregate disintegration. This is largely impossible in previously
described methods (including sieve methods).
(5) Changes of the median of grain-size
distribution can be interpolated satisfactorily by a
logarithmic function.
REFERENCES
Arriaga F.J., Lowery B. & Mays M.D. (2006) A fast
method for determining soil particle size distribution
using a laser instrument. Soil Science, 171, 663 674.
Beuselinck L., Govers G. & Poesen J. (1999)
Assessment of micro-aggregation using laser diffractometry. Earth Surface Processes and
Landforms, 24, 41 49.
Bryan R.B. (1971) The efficiency of aggregation indices
in the comparison of some England and Canadian
soils. Journal of Soil Science, 22, 166 178.
Buurman P., Pape Th. & Muggler C.C. (1997) Laser
grain-size determination in soil genetic studies. 1.
Practical problems. Soil Science, 162, 211 218.
Calero N., Barron V. & Torrent J. (2008) Water
dispersible clay in calcareous soil of southwestern
Spain. Catena, 74, 22 30.
Chappell A. (1998) Dispersing sandy soil for the
measurement of particle size distribution using
optical laser diffraction. Catena, 31, 271 281.
Cooper L.R., Haverland R.L., Hendricks D.M. & Knisel
W.G. (1984) Microtrac particle-size analyzer: an
alternative particle-size determination method for
sediments and soils. Soil Science, 138, 138 146.
De Boodt M., editor (1967) West-European Methods for
Soil Structure Determinations. University of Ghent
Press, Ghent, Belgium.
Dexter A.R. (1988) Advances of characterization of soil
structure. Soil and Tillage Research, 11, 199 238.
Dexter A.R. & Czyż E.A. (2000) Effects of soil
management on the dispersibility of clay in a sandy
soil. International Agrophysics, 14, 269 272.
Gardner W.R. (1956) Representation of soil aggregatesize distribution by a logarithmic-normal distribution. Soil Science Society of America, Proceedings,
2, 151 153.
Goossens D. (2008) Techniques to measure grain-size
distributions of loamy sediments: a comparative
study of ten instruments for wet analysis.
Sedimentology, 55, 65 96.
Gorbunov N.I. (1987) Soil Clays, Properties and
Methods of Investigation. PWRiL Press, Warsaw,
Poland (in Polish).
Hillel D. (1998) Environmental Soil Physics. Academic
Press, London-New York-Tokyo.
Kalicka M., Witkowska-Walczak B., De˛bicki R. &
Sławiñski C. (2008) Impact of land use on physical
properties of rendzinas. International Agrophysics,
34
A. Bieganowski et al.
22, 333 338.
Kutilek M. & Nielsen D. (1994) Soil Hydrology. Catena
Press, Cremlingen-Destedt, Germany.
Levy G.J., Agassi M., Smith H.J.C. & Stern R.(1993)
Microaggregate stability of kaolinitic and illitic soils
determined by ultrasonic energy. Soil Science
Society of America, Journal, 57, 803 808.
Malvern Operators Guide(1999) MAN 0247, Issue 2.0,
Malvern Instruments, Malvern, UK.
Mayer H., Mentler A., Papakyriacou M., Rampazzo N.,
Merxer Y. & Blum W.E.H.(2002) Influence of
vibration amplitude on ultrasonic dispersion of soils.
International Agrophysics, 16, 53 60.
Mentler A., Mayer H., Straub P. & Blum W.E.H. (2004)
Characterization of soil aggregate stability by
ultrasonic dispersion. International Agrophysics,
18, 39 45.
Niewczas J. & Witkowska-Walczak B. (2003) Index of
soil aggregates stability as a linear function value of
transition matrix elements. Soil and Tillage
Research, 70, 121 130.
Niewczas J. & Witkowska-Walczak B. (2005) The soil
stability index and its extreme values. Soil and
Tillage Research, 80, 69 78.
Pini R. & Guidi G. (1989) Determination of soil
microaggregates with laser scattering. Soil Science
and Plant Analysis, 20, 47 59.
Rewut I.B. (1969) Methods of Soil Structure
Investigations. Kolos Press, Leningrad, Russia.
Roth C. & Witkowska-Walczak B. (1992) Comparison
of three methods for measuring the water stability of
soil aggregates from temperate and tropical zones.
Polish Journal of Soil Science, 25, 11 16.
Savinov N.O. (1936) Soil Physics. Sielchozgiz Press,
Moscow, Russia (in Russian).
Shein E.V. & Goncharov V.M. (2006) Agrophysics.
Feniks Press, Rostov, Russia (in Russian).
Sperazza M., Moore J.N. & Hendrix M.S. (2004) Highresolution particle size analysis of naturally occurring very fine-grained sediment through laser
diffractometry. Journal of Sedimentary Research,
74, 736 743.
Taubner H., Roth B. & Tippkotter R. (2009)
Determination of soil texture: comparison of the
sedimentation method and the laser-diffraction
analysis. Journal of Plant Nutrition and Soil
Sciences, 172, 161 171.
Tippkötter R. (1994) The effect of ultrasound on the
stability of mezoaggregates (60 2000 mm).
Zeitschrift fûr Pflanzenernährung Bodenkunde, 157,
99 104.
Walczak R. & Witkowska B. (1976) Methods of
investigations and description of soil aggregation.
Problemy Agrofizyki, 19, 5 52 (in Polish).
Yoder R. (1936) A direct method of aggregate analysis
of soil and a study of the physical nature of erosion
losses. Journal of the American Society of
Agronomy, 28, 337 35.
Zobeck T.M. (2004) Rapid soil particle size analyses
using laser diffraction. Applied Engineering in
Agriculture, 20, 633 639.