Bulk Density Determination by Automated
Three-Dimensional Laser Scanning
Ann M. Rossi*
Soil and Water Sciences Program
Dep. of Environmental Sciences
Univ. of California
Riverside, CA 92521-0424
Daniel R. Hirmas
Dep. of Geography
Univ. of Kansas
Lawrence, KS 66045-7613
Robert C. Graham
Paul D. Sternberg
Soil and Water Sciences Program
Dep. of Environmental Sciences
Univ. of California
Riverside, CA 92521-0424
Soil bulk density is a ratio of the mass of solids to bulk soil volume.
Values typically range from 1.2 to 1.6 g cm−3 depending on the texture, structure, degree of compaction, and shrink–swell characteristics
(Hillel, 1998; Soil Survey Division Staff, 1993; Mason et al., 1957).
Bulk density measurements are widely used in mass–volume conversions and calculating porosity and void ratios (Blake and Hartge,
1986). Bulk density values are also used as a measure of soil quality, indicating the ease of root penetration, water movement, and soil
strength (Grossman and Reinsch, 2002).
One commonly used procedure for determining bulk density is
the clod method. This method is especially useful for soils containSoil Sci. Soc. Am. J. 72:1591-1593
doi:10.2136/sssaj2008.0072N
Received 29 Feb. 2008.
*Corresponding author ([email protected]).
© Soil Science Society of America
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SSSAJ: Volume 72: Number 6 • November–December 2008
MATERIALS AND METHODS
Intact soil clods were collected from four sites representing a
range of soil textures and structures. Sample locations and descriptions are given in Table 1. Hard, nonporous, granitic rock fragments
from the Bishop Creek moraines in California were used as controls.
Triplicate samples from each site were analyzed with both the three-dimensional scanning method (nondestructive) and the paraffin-coated
clod method (destructive). Volumes and bulk density measurements
from these two methods were analyzed in an analysis of variance using
R 2.6.0 (R Development Core Team, 2007).
PEDOLOGY NOTE
This study examined the application of a relatively new automated
three-dimensional scanning technology to bulk density determination
of intact soil clods and rock fragments. The method uses an
inexpensive commercially available three-dimensional desktop scanner.
Measurements obtained by the scanning method were compared with
those of the paraffin-coated clod method determined on the same clods.
Results showed excellent agreement between volume and bulk density
measurements obtained by the two methods across a wide range of
textural classes. Use of the technology has several important advantages
over the traditional clod method. The nondestructive nature of the
scanning method makes it possible to use the same intact ped or clod for
other purposes, such as thin sections for micromorphological analysis.
In addition, high-resolution digital imaging opens up possibilities for
new physical measurements of soil morphological features.
ing rock fragments (Cunningham and Matelski, 1968; Hirmas and
Furquim, 2006) because extracting cores from these soils is problematic. Intact soil clods are coated with an impervious or semipermeable substance, such as liquid paraffin, saran, rubber, wax, or oil,
and the clod volume is measured by water displacement (Blake and
Hartge, 1986; Grossman and Reinsch, 2002; Soil Survey Staff, 1996).
Preparing paraffin-coated clods can be difficult and labor intensive
(Van Remortel and Shields, 1993). The process of removing the coating is difficult and destroys the clast, making any further analyses of
the clod impossible.
Sander and Gerke (2007) used a three-dimensional optical scanner to determine volume in measuring soil shrinkage characteristics.
This technology can also be used to measure the bulk density of soils
and geologic materials.
The objective of this study was to evaluate the application of
automated three-dimensional laser scanning technology to the determination of the bulk density of soil clods and rock fragments. A nondestructive method would have an advantage over the traditional clod
method by allowing various further analyses on the same soil clod.
Three-Dimensional Laser Scanning Method
Soil clods were oven dried at 105°C overnight and scanned using a commercially available desktop three-dimensional scanner (Fig.
1) (NextEngine Desktop 3D Scanner Model 2020i, NextEngine, Inc.,
Santa Monica, CA). This instrument is relatively inexpensive, with a
cost roughly equivalent to a desktop computer and printer setup. A
computer connected to the scanner is used to operate the scanner and
store collected data. A total of 10 scans at 0.13-mm resolution was
taken of each clod as follows. Clods were placed on the sample holder
and scanned five times. Each scan was taken at 72° intervals so that a
complete 360° scan of the sample was obtained. The sample holder is
linked to the scanner and rotates automatically (Fig. 1), making the
instrument simpler to operate than the one used by Sander and Gerke
(2007). The five scans obtained by rotation of the sample, known as a
scan family, were assembled to create a partial three-dimensional image
of the sample using ScanStudio PRO software (NextEngine, Inc., Santa
Monica, CA). Upon completion of the first scan family, the sample was
repositioned on the sample holder to include the remaining unscanned
portions of the clod. The sample was rescanned five times with a 72°
rotation between each scan and the scans were assembled into a second
scan family. Identical points on the surfaces of the two scan families
were matched and used to align the images, creating a three-dimensional virtual model of the clod. In the process, digital color images
were taken and automatically overlaid on the scanned image to create
a three-dimensional color image. Clod volumes were calculated using
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Table 1. Sample locations and characteristics.
Sample
Sampling location
Horizon Soil depth
cm
Textural class
Gravel content
%
Alfisol
North Mountain Experimental Area,
Bt
San Jacinto Mountains, CA
20–60
gravelly sandy clay loam
26
Vertisol
Entisol
Aridisol
Rock
Carlsbad, CA
Forest Falls, CA
Southern Fry Mountains, CA
Bishop Creek moraines, Bishop, CA
38–69
0–20
36–82
NA
sandy clay
gravelly loamy coarse sand
silt loam
NA
0
22
0
NA
A3
A
2Btk2
NA†
† NA, not applicable.
the ScanStudio Pro software. Each clod was placed on a balance and
the mass was recorded. Gravel was removed and weighed after volumes
were determined using the paraffin-coated clod method (see below).
Bulk densities (ρb) were calculated as
ρb =
W c −W g
(
V c − W g 2.65
)
[1]
where Wc is the mass of the clod, Wg is the weight of the gravel, Vc is
the volume, and 2.65 g cm−3 is the assumed average particle density
of the gravel. Volumes and bulk densities of the rock samples were
determined using the same method.
W c −W r −W g
(
V c − W g 2.65
)
where Wr is the mass of soil lost from the clod during handling. Rock
fragment volumes were obtained by displacement of water without
paraffin coats because the samples were nonporous.
RESULTS AND DISCUSSION
After soil clods were scanned, volumes were reanalyzed using the
paraffin-coated clod method (Grossman and Reinsch, 2002; Soil Survey
Staff, 1996; Blake and Hartge, 1986). This method has been found to
yield bulk density values indistinguishable from the similar saran-coated clod method (Shipp and Matelski, 1965). Oven-dried clods were
secured in a hairnet and weighed. The mass of the hairnet was obtained
and recorded separately. Soil material that separated from the clod during handling was collected in a tared dish and weighed. Each sample
was repeatedly dipped in liquid paraffin until the entire clod was sealed.
Clods were suspended until the paraffin solidified and masses were recorded. A 1000-mL beaker filled with approximately 600 mL of water
was weighed on a top-loading balance and the mass was recorded. The
clod was suspended and completely submerged in the beaker, keeping
Volume measurements by the three-dimensional scanner and the
clod method in Fig. 2 showed excellent agreement (r2 = 0.999, P
< 0.001). Bulk densities calculated from the volume measurements
showed close agreement across the range of textures used in this study
(Fig. 3). Both methods accurately measured the densities of the control (rock) samples at 2.60 g cm−3 (Fig. 3).
Scanning time varies depending on the number of scans needed
and the resolution settings used. With the resolution settings used in
this study (0.13 mm), the entire scanning and image processing procedure took approximately 40 min per sample. Since sample holder
rotation is automatic, the user only needs to position the sample once
for each scan family. Under the resolution settings used in this study,
each scan family took 15 min to scan.
The paraffin-coated clod method is more labor intensive than
the scanning method, although multiple samples can be processed simultaneously. The clod method becomes more complicated in coarse-
Fig. 1. The three-dimensional scanner and sample holder.
Fig. 2. Three-dimensional (3D) scanned volumes as a function of paraffincoated volumes; 1:1 line shown for reference.
Paraffin-Coated Clod Method
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ρb =
the clod away from the sides
and bottom. The beaker was
reweighed and the mass recorded. The change in mass
is equivalent to the volume
of water displaced by the
clod. Gravel was separated
from the paraffin-coated
clod, weighed, and the mass
recorded following Hirmas
and Furquim (2006). Bulk
density was calculated as
SSSAJ: Volume 72: Number 6 • November–December 2008
Fig. 3. Bulk density measurement comparisons using volume
determinations from the three-dimensional (3D) scanning method
and the paraffin-coated clod method; ALF = Alfisol, VERT = Vertisol,
ENT = Entisol, ARID = Aridisol. Error bars show one standard
deviation. Note: Rock samples were nonporous and did not require
paraffin coating to determine volume by displacement.
textured samples because the paraffin coat must be removed from the
clod to sieve the sample. Gravel and soil particles are cemented by
the paraffin and must be separated by hand or by melting the paraffin (Hirmas and Furquim, 2006). Paraffin remaining on the gravel
or gravel-sized cemented soil aggregates will contribute to the mass
of gravel, resulting in underestimates of bulk density (Hirmas and
Furquim, 2006). The three-dimensional scanning method eliminates
the use of paraffin or other coatings, so gravel may be easily removed
from the clod by sieving. In addition, inconsistencies in the paraffin
coat can introduce errors into bulk density calculations. Incomplete
coats allow water to enter soil clods, resulting in inaccurate volume
measurements. Variations in paraffin temperature and soil texture can
influence the depth of paraffin penetration into soil pores, changing
the coating thicknesses (Soil Survey Staff, 1996) and affecting volume
measurements. Since the scanning method is nondestructive, clods
may be used for other analyses, such as for preparation of thin sections
for micromorphology studies or porosity studies using computed tomography scanning. The destructive nature of the traditional clod
method makes further analyses on the same clod impossible.
Detailed three-dimensional images of peds can also be used to
display soil structural units as in Fig. 4. This opens up the possibility
for quantitative determination of ped properties that relate to structure type, size, and grade. Ped surface roughness can be assessed with
the detailed images produced by this technology, allowing correlation
with other physical properties such as linings and slickensides.
In summary, the automated three-dimensional scanner provides
a relatively inexpensive solution to the limitations of the traditional
clod method. While the time required to analyze each clod is typically
greater than with the paraffin-coated clod method, the three-dimensional scanner’s real advantage is that it preserves the clod for continued analyses. High-resolution three-dimensional imaging opens up
new possibilities for quantitative soil morphological analyses.
SSSAJ: Volume 72: Number 6 • November–December 2008
Fig. 4. Three-dimensional images of scanned clods: (A) angular blocky
ped, (B) subangular blocky ped, (C) columnar ped, (D) prismatic ped.
Scale bars equal 1 cm.
ACKNOWLEDGMENTS
This research was funded in part by the University of California Kearney
Foundation of Soil Science. We thank NextEngine of Santa Monica,
CA, for ScanStudio Pro software and for providing technical support.
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