An Accurate and Inexpensive Apparatus and Method for Teaching
and Measuring Stable Aggregate Content of Soils
J. J. Patton, L. Burras,* M. E. Konen, and N. E. Molstad
ABSTRACT
Student comprehension of soil aggregation is often poor, even
following successful completion of several soil science courses.
This poor understanding is problematic because soil suitability
interpretations depend in part on the characteristics of the soil
aggregates. For example, soil aggregate stability influences
runoff, erosion, and root growth, which in turn influences assessment of soil tilth and quality. Anecdotal evidence suggests student grasp of soil aggregation improves if teaching includes
hands-on identification and measurement of soil aggregate properties. The major limitation to hands-on activities has been the
perception by some instructors that soil aggregate properties are
difficult to measure. The objective of this report is to present a
simple, inexpensive method that readily quantifies stable aggregate content along with directions for assembling its required apparatus. The apparatus consists of components that can be purchased for less than $40 and can be assembled in less than 2 h.
The method requires laboratory time and space comparable to
particle-size analysis. The method can be successfully used in undergraduate through graduate courses or for research.
S
OIL AGGREGATES are a fundamental component of soil.
Their role is summed up by Brady and Weil (1999), who
state “The formation and maintenance of a high degree of aggregation is one of the most difficult tasks of soil management,
yet it is also one of the most important, since it is a potent
means of influencing ecosystem functions.” The basis of
Brady and Weil’s (1999) assessment is the important impact
soil aggregation has on root growth, water infiltration, storage and drainage, oxygen–carbon dioxide exchange between
soil and the atmosphere, rates of wind, sheet and rill erosion,
the stability of soil organic matter, as well as a whole host of
other soil components. These interactions are detailed in
Amezketa (1999), Lal (1999), Kemper and Rosenau (1986),
Valentin and Bresson (1998), as well as in a myriad of other
publications. The presence of aggregates in soils is due to a
number of interacting chemical, physical, and biological
processes that involve texture, organic matter, pH, types and
numbers of micro- and macro fauna, and wetting and drying
(Amezketa, 1999; Jenny, 1941; Jenny, 1980).
The preceding paragraph indicates the importance of understanding soil aggregation, and especially aggregate stability, when determining soil quality and plant productivity.
In turn, this means that effective education about soil aggregates is essential to the success of a soil science program. Yet
J.J. Patton, Dep. of Plant and Soil Sciences, Oklahoma State Univ., Stillwater, OK 74078; L. Burras, Dep. of Agronomy, Iowa State Univ., Ames, IA
50011; M.E. Konen, Dep. of Geography, Northern Illinois Univ., DeKalb, IL
60115; and N.E. Molstad, SDI Consultants, Ltd, 2000 York Road Suite 130,
Oak Brook, IL 60523. Received 28 June 2000. *Corresponding author ([email protected]).
Published in J. Nat. Resour. Life Sci. Educ. 30:84–88 (2001).
http://www.JNRLSE.org
84 • J. Nat. Resour. Life Sci. Educ., Vol. 30, 2001
it is the collective experience of the authors that student understanding of soil aggregates often remains limited, even after
successful completion of several soil science courses. For example, the authors have observed upper-level students routinely confusing soil aggregation with texture and/or bulk
density. Interestingly, coverage of soil aggregation and its
synonym soil structure is typically quite extensive in soils
coursework. This is illustrated at Iowa State University, where
a 1999 survey of students found that “defining soil structure”
and “describing how organic matter affects soil aggregation”
were the 3rd and 40th (out of 177) most extensively covered
soil science performance objectives (Burras and Engebretson,
1997), respectively. That survey queried students who had
completed Agron 154 (Fundamentals of Soil Science), Agron
260 (Soils & Environmental Quality), Agron 354 (Soils &
Plant Growth), and Agron 360 (Environmental Soil Science)
using the 177 soil science performance objectives published
by the Soil Science Society of America and administered by
the Council of Soil Science Examiners in certification and licensing examinations for soil scientists (Burras and Engebretson, 1997).
In an apparent conflict, confusion within research literature
is a second reason that may explain the difficulties some instructors and students encounter when discussing soil aggregates. This is illustrated in “Soil Aggregate Stability: A Review,” which is a 69-page manuscript by Amezketa (1999) that
thoroughly discusses the terms and concepts used and misused
with respect to soil aggregation. Amezketa (1999) points out
that numerous methods are used to measure aggregate stability, which creates considerable confusion, especially because
many studies do not clearly define what method they used.
The objective of this report is to present a simple, effective
method for measuring stable aggregate content as well as directions for assembling the apparatus used in that method. The
method and apparatus are easy and inexpensive to construct
and use. The method is based on Method 4G1 in the Soil Survey Laboratory Methods Manual (Soil Survey Staff, 1996).
Apparatus construction and parts were developed by the authors at Iowa State University. The method and apparatus are
valuable because they allow students to readily obtain direct
and accurate results, which means students and instructors can
develop site-specific interpretations about the relationship
between stable soil aggregate content and other site conditions
(e.g., land use, infiltration and runoff, crusting likelihood,
erodibility, and organic matter content).
An additional need for making this method available is the
paucity of undergraduate opportunities for tactile learning
about soil aggregate stability. This general lack of hands-on
experience is thought to be due to a perception held by many
soils faculty that it is difficult to obtain accurate stable soil aggregate results in light of time and monetary constraints. Interestingly, most soils programs do offer undergraduate laboratory experience in measuring particle size and pH—both
of which are similar in set-up cost and no more accurate than
the stable aggregate procedure described herein. Furthermore,
if more soil science programs offer students a hands-on approach to learning about soil aggregation, it is thought that student understanding of soil aggregation will be improved.
MATERIALS AND METHODS
Background
An aggregate is defined as a group of primary particles that
cohere to each other more strongly than to other surrounding
soil particles (Kemper and Rosenau, 1986). Aggregate stability
is a measure of the degree to which these soil aggregates are
vulnerable to externally imposed destructive forces (Hillel,
1982). The stability of aggregates in any particular soil is affected by many factors including: the amount and type of clay
minerals present; the abundance, type, and activity of soil organisms; the amount and activity of organic matter; current
vegetation; and management practices used (Jenny, 1980;
Jastrow, 1996; Jastrow et al. 1998; Amezketa, 1999). In general, best aggregate stability occurs on soils that are well vegetated and have high clay and organic matter content (Jordahl
and Karlen, 1993). Soil erodibility increases as aggregate stability decreases (Kemper and Rosenau, 1986). Soils with stable aggregates are better able to withstand the destructive
forces of rain, and are less susceptible to runoff.
Numerous methods are currently available to measure soil
aggregate stability (e.g., see Kemper and Rosenau, 1986;
Sutherland and Ziegler, 1997; Amezketa, 1999; Soil Quality
Inst. Staff, 1999). The method discussed herein differs from
most other methods because students at all levels can do it easily. It also requires less specialized and costly equipment than
the methods of Sutherland and Ziegler (1997) or Kemper and
Rosenau (1986). By constructing the simple and inexpensive
(around $40.00) apparatus described in the following text
and by having a top- loading balance and a standard drying
oven, one will be able to measure the stability of aggregates
of many soils in two to three class periods. This method is especially well suited to fill in the down time associated with
running particle size, pH, or other methods that also require
significant periods of waiting.
Assembly of Stable Aggregate Apparatus
The stable aggregate apparatus consists of three main parts:
sieves, agitation rack, and water chamber with lid (Fig. 1). It
is assembled using the materials and equipment shown in
Table 1 according to the following instructions. The materials listed in Table 1 permit construction of 14 sieves, all of
which can simultaneously fit onto the agitation rack and into
the water chamber.
Sieve Assembly
Step 1. Using tin snips, cut the stovepipe into 14 sections
that are each 7.5 cm long. The cut edges should be straight and
level so the pipe section will rest evenly on a flat surface. The
cut edges are sharp and should either be bent over with a pair
of pliers, filed down, or covered with tape.
Step 2. Cut the metal screen into 14 sections, each having
dimensions of approximately 10 by 10 cm.
Step 3. Center a section of metal screen over one end of a
stovepipe section.
Step 4. Keeping the screen taut, bend the excess screen over
the outside of the stovepipe.
Step 5. Place a muffler clamp over the excess screen and
stovepipe. Using a flat-headed screwdriver, tighten the muffler clamp until the screen is held snugly in place (Fig. 2). It
is important the screen is flat. Waves or creases in the screen
could bias results.
Step 6. Set the assembled sieve on a flat surface. Make sure
the sieve rests evenly. If any unevenness appears, adjust the
screen or re-cut the stovepipe edge.
Step 7. Uniquely label each sieve with a permanent marker.
Agitation Rack Assembly
Step 1. Arrange the two wire cooling racks in the bottom
of the water chamber (i.e., plastic storage tub) so that both
racks lay flat. The racks will overlap.
Step 2. Noting their position, remove the racks from the
tub and secure the racks together using small rip ties or lightweight wire. The racks should be fastened as to make one
sturdy sheet (Fig. 3).
Fig. 1. Complete stable aggregate apparatus used at Iowa State University.
J. Nat. Resour. Life Sci. Educ., Vol. 30, 2001 • 85
Table 1. Materials and equipment used in the construction of soil aggregate stability wetting chamber.
Item
No. needed
Plastic storage tub with lid
Wire cooling racks
Galvanized stove pipe
Adjustable metal clamp (e.g., muffler clamp)
Wire mesh screen, 0.5-mm openings
S hooks
Lightweight chain
Wire or plastic rip ties
Tape
1
2
1
16
1
4
2
8
1
Approximate size per item†
Total approximate cost and location
40 cm (W) by 40 cm (L) by 25 cm (H)
40 cm (W) by 26 cm (L) with openings of approximately 1 cm
120 cm long by 7.5 cm diam
7.5 cm diam
2000 cm2
2 cm (L)
30 cm (L)
15 cm (L), then cut to necessary length
roll
$ 5, available at hardware stores
$ 3, available at hardware stores
$ 5, available at hardware stores
$12, available at automotive stores
$10, specialty hardware stores‡
$ 1, available at hardware stores
$ 1, available at hardware stores
$ 1, available at hardware stores
$ 1, available at hardware stores
Equipment: tin snips to cut wire mesh screen and stove pipe, screw driver to tighten adjustable metal clamps around mesh screen and stove pipe, scissors to cut plastic rip ties, pliers
to bend top edge of sieves, permanent marker to number stove pipe sections.
† Abbreviations: W = width, L = length, H = height.
‡ We used aluminum wire mesh screen such as available from McMaster-Carr Supply Company, Atlanta, GA (http://www.mcmaster.com), item no. 9227T413.
Step 3. Fasten the chain sections to opposite sides of the
cooling racks using the S hooks. The chains should be fastened
in such a manner as to allow enough slack in the chain to allow
for hands to grab on and use the chains as handles for the rack
(i.e., lift chains).
Collecting and preparing soil samples for stable aggregate
analysis is straightforward, requiring a modicum of generally
readily available field and laboratory equipment and reagents
(Table 2). The method described herein is a synthesis, with
minor modifications, of the sample collection, laboratory
preparation, and aggregate stability methods provided in the
Soil Survey Laboratory Methods Manual (Soil Survey Staff,
1996).
Step 1. For each soil of interest, collect (or have a student
collect) between 200 and 1000 g using a shovel, spade, or garden trowel. The sample should be stored in a labeled bag or
bucket.
Step 2. Allow samples to air-dry for at least 1 or 2 d. If a
soil sample is stored in a bag, the simplest approach is to prop
the bag open in an upright position. Quicker drying will occur
if the soil is transferred to a drying container of some kind, e.g.,
the authors commonly transfer each soil sample to a disposable aluminum bread pan and then use a fan to accelerate drying. The aluminum bread pans can be cleaned and reused indefinitely.
Step 3. Crush each soil sample gently by hand onto a 2-mm
sieve (which is nested on a 1-mm sieve); shake the nest of
sieves so all of the soil passes through the 2-mm sieve, being
careful not to unnecessarily damage peds. It is valuable to remind students that one part of the definition of soil material
is “material that passes through a 2-mm sieve.”
Step 4. Keep the soil material retained on the 1-mm sieve.
This is the material used to determine stable aggregate content. Discard the <1-mm diameter soil (i.e., the material that
Fig. 2. Example of a complete sieve, which consists of a piece of galvanized stove pipe (7.5 cm diam. by 7.5 cm length) that has wire mesh
screen (0.5-mm openings) held in place with a muffler clamp.
Fig. 3. Agitation rack made by joining two wire cooling racks with a series of plastic rip ties and lightweight chains on two sides, which are
held in place using small S hooks.
Final Assembly
Step 1. Place the agitation rack into the water chamber by
lowering it in place with the lift chains.
Step 2. Place the sieves (screen side down) on the agitation rack screen.
Step 3. Fill the plastic box with water until the water level
is approximately 20 millimeters above the bottom of the sieve
screen.
Step 4. Mark the water level on the outside of the plastic
box with a permanent marker. This ensures a constant water
level is used each time data is collected.
The apparatus is now fully assembled and ready to be used
in measuring stable soil aggregate contents.
Sample Collection, Preparation, and Analysis
86 • J. Nat. Resour. Life Sci. Educ., Vol. 30, 2001
Table 2. Materials needed in collecting, preparing, and analyzing stable
aggregate content in soils.
Field equipment
Laboratory equipment and reagents
Shovel, spade or garden trowel
Nest of sieves
a. 2-mm openings (square-holed)
b. 1-mm openings (square-holed)
c. collection pan (optional)
Plastic bags or buckets
Stable aggregate apparatus
Balance (500 g capacity, ± g sensitivity)
Oven set at 105°C
Timer or clock that denotes minutes and seconds
30 L of distilled water
30 L of dispersing solution, which is made by
dissolving 50 g of Calgon detergent† (i.e.,
sodium hexametaphosphate) in 30 L distilled
water
Small pipette or syringe
Squirt bottle
† Calgon detergent can be purchased either with or without sodium hexametaphosphate
as an active ingredient. Calgon boxes identified with an R contain 8.7% phosphorus
as sodium hexametaphosphate, whereas boxes identified with a Z contain no phosphorus. If one has difficulty obtaining the Calgon containing phosphorus, he or she
mix up the dispersing solution simply by replacing the 50 g Calgon detergent with
35.7 g reagent-grade sodium hexametaphosphate and 7.9 g sodium carbonate (Soil
Survey Staff, 1996).
fell onto the collection pan that comprises the bottom of the
nest of sieves) as well as any nonsoil (e.g., roots and pebbles)
that remained on the 2-mm sieve.
Step 5. Set up the aggregate stability apparatus by placing
the sieves on the agitation rack in the water chamber. Slowly
add distilled water until the mark on the water chamber is
reached. This should be 20 mm above the base of the sieves.
If air bubbles are present in the sieves (e.g., under the screens),
remove this with a pipette or syringe.
Step 6. Weigh 3.00 ± 0.05 g of soil collected on 1-mm sieve
in Step 4, record the exact weight as Wor, which stands for the
original weight of the sample. Carefully pour a soil sample into
its appropriate sieve such that the aggregates are evenly distributed across the sieve’s screen. Aggregates should not
touch. Leave the soil on the sieve in the water overnight. (If
a class only meets once per week, it is recommended the students leave their weighed samples in small envelopes with the
instructor and that she or he place the samples on the sieves
in the water chamber the day before the next class period.)
Step 7. Agitate the samples using the lift chains to raise and
lower the agitation rack 20 times in 40 s. On the upward
stroke, drain the sieve but do not raise the rack so high that
air is allowed to enter the sieves from below. The downward
strokes should end just above the bottom of the water chamber.
Step 8. Remove the sieves from the water and dry in an
oven for 2 h or until dry. The oven temperature should be
105°C.
Step 9. Remove the sieves from the oven. Let the sieves
cool until they can be comfortably handled. Weigh each sieve
and the soil remaining in it. Record weight as Wc. This number is the combined weight of the sieve, aggregates, and sand
particles with diameters between 0.5 and 1.0 mm. Return
each sieve to its place on the agitation rack.
Step 10. Pour the distilled water out of the water chamber.
Refill the water chamber with dispersing solution (see Table
2). Lower the agitation rack (with sieves) into the dispersing
solution. Agitate periodically until aggregates are soft (e.g.,
30 min is normally adequate). Remove the sieves from the dispersing solution and use the tap or a squirt bottle to rinse with
distilled water until only the sand (>0.5 mm) remains in the
sieve. Place sieve in oven and dry for 1 h at 105°C.
Step 11. Remove sample from oven. Weigh the sieve containing the sample. Record weight as Ws. This value is the
weight of the sieve and sand having diameters between 0.5 and
1.0 mm. This step is necessary because discrete sand grains
between 1.00 and 0.5 mm can be present, which will artificially inflate the stable aggregate content if their weight is not
corrected for.
Step 12. Discard the sand and reweigh the sieve. Record
weight as We, which is the weight of the empty sieve.
Calculation
Part 1. Eliminate the empty weight of the sieve from all
values:
Wc – We = Was
[1]
where Was is the weight of aggregates + sand.
This calculation uses the values from Steps 9 and 12.
Ws – We = Wsd
[2]
where Wsd stands for weight of sand. This calculation uses the
values from Steps 11 and 12.
Part 2. Calculate the percentage of stable aggregates:
Stable aggregate content (%)
= (Was - Wsd / Wor - Wsd) × 100%
[3]
Wor is from Step 6. It should be very near to 3.00 g. The other
values come from Eq. [1] and [2]. Table 3 shows an example
calculation.
The ease of sample collection, preparation, and analysis
means this method is well suited for a variety of teaching purposes. For example, an instructor can easily determine and
compare stable aggregate content of soils from sites having
various land uses, textures, vegetation, organic matter content,
or pH. Other uses completed by the authors include evaluating the relationship between stable aggregate content and
landscape position and analyzing stable aggregate content
with depth and horizonation within a single soil profile.
Discussion of Example Data Collected
Using this Method
The authors have used this apparatus and method to determine stable aggregate content in >250 soils. Results indicate it generates highly reliable and reproducible data. This is
illustrated by Table 4, which shows the values obtained by the
authors as well as three undergraduate students for a single
sample repeatedly run during a 6-mo period. Tables 5 through
7 are included to provide example values of stable aggregate
content for a variety of soil conditions. These values are consistent with those reported in research articles that used the
range of approaches and methods discussed in the introduction.
J. Nat. Resour. Life Sci. Educ., Vol. 30, 2001 • 87
Table 3. Example data and calculations of stable aggregate content in a
soil sample from Iowa.
Soil series: Nicollet
Table 5. Stable aggregate content of the surface A horizon in switchgrass
and cornfields from south-central Iowa (values given are mean ± SD
followed by the number of observations).
Stable aggregate content
Classification: fine-loamy, mixed, superactive, mesic Aquic Hapludoll
Sample collected from Ap horizon (0–20 cm)
Landscape position: backslope
Vegetation: corn field
Date collected: 30 May 2000
Landscape position
Switchgrass
Cornfield
t test results
assuming
equal variances
P(T £ t)
%
Summit and shoulder 53.7 ± 16.5, n = 11
Backslope
69.9 ± 14.3, n = 14
Footslope
62.5 ± 17.3, n = 11
Sample pH: 6.4
Sample texture: loam (45% sand, 33% silt, 22% clay)†
34.0 ± 15.8, n = 11
0.001
Stable aggregate content initial data
Wo
3.03 g
Wc
95.61 g
Ws
94.74 g
We
93.89 g
Table 6. Stable aggregate content by depth in soils from various corn,
small grain alfalfa rotations in Minnesota, Ohio, and Illinois (values
given are mean ± SD followed by the number of observations).
Calculations
Wc - We = Was Y 95.61 g - 93.89 g = Was = 1.72 g [1]
Ws - We = Wsd Y 94.74 g - 93.89 g = Wsd = 0.85 g [2]
Depth from surface
Stable aggregate content (%) = [(Was – Wsd )/ (Wor – Wsd)] × 100% Y
[(1.72 g - 0.85 g)/(3.03 g - 0.85 g)] × 100% = 39.9%
[3]
† This data shows 0.85 g sand per 3.03 g soil, which converts to 28% sand. That sand
content is considerably different than the 45% sand in the whole sample, which is not
surprising because this method only examines the 1.00- to 0.50-mm component.
Table 4. Reproducibility of stable aggregate content in a single soil sample† during a 6-mo period.
Replication no.
Stable aggregate content
n
g kg-1
1
2
3
4
5
6
7
8
9
10
11
12
13
190
163
193
177
210
143
157
157
200
153
157
170
157
Range
Mean
SD
Mode
Median
CV, %
143–210
171
20.8
157
163
12.2
Stable aggregate content
cm
%
0–10
10–30
30–50
33.4 ± 13.8, n = 34
20.8 ± 11.5, n = 45
14.4 ± 9.7, n = 23
Table 7. Stable aggregate content of surface horizons from fields in grass
and crops (values given are mean ± SD followed by the number of observations).
Field type
Stable aggregate content
%
19.0
16.3
19.3
17.7
21.0
14.3
15.7
15.7
20.0
15.3
15.7
17.0
15.7
14.3–21.0
17.1
2.08
15.7
16.3
12.2
† Soil sample is from the Ap horizon of a Nicollet polypedon that has been row
cropped for at least the past 20 yr. The complete taxonomic classification of the Nicollet series is fine-loamy, mixed, superactive, mesic Aquic Hapludoll. The textural class
and texture of the sample are loam and 43% sand, 36% silt, and 21% clay, respectively. The sample contains 1.7% organic C.
SUMMARY AND CONCLUSION
Using this apparatus and method in a class to determine
stable aggregate contents offers a number of benefits. The
method is simple, effective, and accurate. The apparatus is inexpensive and easy to construct. Thus, students and instructors can readily use this method to learn about soil aggregation as well as the quantitative relationship between stable aggregate content and soil parameters such as land use, crusting
likelihood, infiltration, erodibility, or organic matter content.
In an advanced class, an instructor could use this simple experiment to facilitate the discussion on the role of organo-clay
chemistry in the formation and stability of soil structure. In addition or alternatively, an instructor could explain energy
transfer physics of raindrops striking aggregates and how aggregate properties influence soil erodibility.
88 • J. Nat. Resour. Life Sci. Educ., Vol. 30, 2001
%
Grassland
Cropped field
73.3 ± 7.7, n = 180
33.4 ± 13.8, n = 34
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