o understand the theory behind the Particle
Size Distribution Measurements Protocol and how
the data can be used to predict percent sand,
silt, and clay
Overview
Using the measurements made in the Particle
Size Distribution Measurements Protocol, the
amount of sand, silt, and clay in grams and in
percent will be calculated. Students will also
be introduced to the theory behind the settling
experiment (Stokes Law), and instructed on
how to use the textural triangle with both the
results from their measurements and a sample
set of sand, silt, and clay measurements for
practice.
Time
One class period
Level
Intermediate and Advanced
Key Concepts
How different particle sizes in the soil are
distributed to create a specific texture
Stokes Law and particle settling
Skills
Reading a conversion table
Using mathematics to correct hydrometer
readings for volume and temperature
Calculating the amount of sand, silt and
clay in grams and in percent of the
sample
Reading information from a textural
triangle
Estimating percentages
Materials and Tools
Data from Soil Particle Size Distribution
Measurements Data Work Sheet
Copy of the textural triangle for each
student
Ruler or straight edge
Preparation
Conduct a discussion of different size particles
in soils and their distribution. See the
Introduction.
Perform the Particle Size Distribution
Measurements Protocol to obtain the
measurements required for this exercise.
Background
The amount of each size particle (sand, silt, or
clay) in the soil is called the particle size
distribution. Knowing the particle size
distribution of a soil sample helps us understand
many soil properties including how much water,
heat, and nutrients the soil will hold, how fast
water and heat will move through the soil, and
what kind of structure and consistence will form.
Sand, silt, and clay are the 3 particle sizes of
mineral material found in soils. The amount of
each of these is called the particle size distribution
and the way they feel is called the soil texture.
Sand is the largest sized particle, silt is medium
sized, and clay is the smallest. There is
disagreement in the scientific community about
the exact size ranges of sand and silt. For GLOBE,
we will measure sand and silt based on 2 different
size definitions: GLOBE 1997
Learning Activities - 31
Soil
A
ppendix
W
elcome
Intr
oduction
P
r
otocols
L
earning A
ctivities
1. The US Department of Agriculture (USDA)
which defines the size of sand as
2.0 mm - 0.05 mm, and the size of silt as
0.05 - 0.002 mm.
2. The International Soil Science Society
(ISSS) which defines the size of sand as
2.0 mm - 0.02 mm, and the size of silt as
0.02 - 0.002 mm.
Clays are the smallest particles and are defined
(by both organizations) as being smaller than
0.002 mm. Particles greater than 2 mm are called
stones or gravels and are not considered to be soil
material.
Heavy, large particles settle first, so when a soil
sample is stirred or shaken in a 500 mL cylinder,
sand particles (according to the USDA definition)
settle to the bottom of the cylinder after 2 minutes,
while the clay and silt size particles stay in
suspension. After 12 minutes, the sand, according
to the ISSS definition, has settled, leaving the clay
and silt size particles in suspension. After 24
hours, the silt size particles have settled, and only
the clay stays in suspension to be recorded by the
hydrometer.
To Determine the Amount of Sand, Silt,
and Clay in Your Soil Sample
The specific gravity hydrometer is an instrument
used to measure the density of water which has
materials suspended in it compared with pure
water. A hydrometer and temperature reading is
made at 2 minutes, 12 minutes, and 24 hours
during the Particle Size Distribution Protocol. To
determine the amount of sand, silt, and clay in
your sample, we will take each hydrometer
reading and make a temperature correction to it.
Next, we will use a conversion table (below) to
convert the corrected specific gravity of the water
to grams of suspended soil per liter (1000 mL)
which includes a correction for the density of the
dispersing agent that was added. Once we make
that conversion, we need to multiply by the
number of liters (0.5 L or 500 mL), in order to
determine the number of grams of soil in
suspension.
Obtain the data recorded on the Particle Size
Distribution Measurements Data Work Sheet, and
use the Calculation Work Sheet below to perform
the following corrections:
1. Begin with your 2 minute hydrometer
reading. From the conversion table below,
determine the value of grams of soil/liter.
At 2 minutes, this value corresponds to
the grams of silt (USDA size) plus clay in
suspension. All of the sand (USDA size)
has settled to the bottom of the cylinder.
2. Note the temperature values you obtained
2 minutes. For every degree of
temperature above 20
o
C, add 0.36 grams
to the grams of soil you obtained from the
table. Subtract 0.36 for every degree
below 20
o
C.
3. Next, multiply the value for temperature-
corrected grams of soil/L by 0.5 L to find
out how many grams of soil we have in
suspension in the 500 mL cylinders. This
answer gives you the grams of silt plus
clay in your sample.
4. Repeat procedure 1, 2, and 3 for the 12
minute and 24 hour hydrometer readings
using the temperature read at each time
period to correct for every degree above or
below 20
o
C. The 12 minute reading
corresponds to the amount of silt (ISSS
size) plus clay that is in your sample (the
ISSS sand has settled at 12 minutes). The
24 hour reading represents the amount of
clay in your sample (all of silt and sand
has settled by 24 hours).
5. To find out how many grams of sand
(according to the USDA) you have in your
sample, subtract the amount of silt plus
clay you calculated in step 3 above by the
original amount of soil you used in the
GLOBE Particle Size Distribution Protocol
(25 grams). The percent sand is equal to
the grams of sand in the sample divided
by 25 grams (the original amount of soil),
and multiplied by 100 to get percent.
6. To calculate how many grams and the
percent of sand (according to the ISSS),
repeat step 5 for the grams of silt plus clay
you obtained at 12 minutes.
Partic
le Size Distribution
Measur
ements GLOBE 1997
Learning Activities - 32
Soil
7. The grams of clay in your sample is the
amount of clay determined above from the
corrected reading at 24 hours. Dividing
the grams of clay by the original weight of
the sample used (25 grams) will give the
percent of clay in the sample.
8. The amount of silt can be calculated by
adding the grams of clay (step 7) and sand
(step 5 for USDA or step 6 for ISSS)
together, and subtracting that amount
from the weight of soil added to the
cylinder (25 grams). The percent silt is
determined by dividing the grams of silt
by 25 grams, or by subtracting the sum of
the percent sand plus percent clay from
100 percent.
9. Repeat these calculations for the samples
from each horizon in your soil profile. Use
the Calculation Work Sheet to help your
work. You can compare your results with
the final results that will be returned to
you after you submit the raw data from
your Particle Size Distribution
Measurements Data Work Sheet to the
GLOBE Student Data Server.
10. You can use the Textural Triangle
procedure to determine the texture name
of your sample that corresponds with the
particle size distribution. GLOBE 1997
Learning Activities - 33
Soil
Table SOIL-L-1: Conversion Table (specific Gravity to Grams of Soil/L)
Specific
Grams Soil/L
Specific
Grams Soil/L
Specific
Grams Soil/L
Gravity
Gravity
Gravity
1.0024
0.0
1.0136
18.0
1.0247
36.0
1.0027
0.5
1.0139
18.5
1.0250
36.5
1.0030
1.0
1.0142
19.0
1.0253
37.0
1.0033
1.5
1.0145
19.5
1.0257
37.5
1.0036
2.0
1.0148
20.0
1.0260
38.0
1.0040
2.5
1.0151
20.5
1.0263
38.5
1.0043
3.0
1.0154
21.0
1.0266
39.0
1.0046
3.5
1.0157
21.5
1.0269
39.5
1.0049
4.0
1.0160
22.0
1.0272
40.0
1.0052
4.5
1.0164
22.5
1.0275
40.5
1.0055
5.0
1.0167
23.0
1.0278
41.0
1.0058
5.5
1.0170
23.5
1.0281
41.5
1.0061
6.0
1.0173
24.0
1.0284
42.0
1.0064
6.5
1.0176
24.5
1.0288
42.5
1.0067
7.0
1.0179
25.0
1.0291
43.0
1.0071
7.5
1.0182
25.5
1.0294
43.5
1.0074
8.0
1.0185
26.0
1.0297
44.0
1.0077
8.5
1.0188
26.5
1.0300
44.5
1.0080
9.0
1.0191
27.0
1.0303
45.0
1.0083
9.5
1.0195
27.5
1.0306
45.5
1.0086
10.0
1.0198
28.0
1.0309
46.0
1.0089
10.5
1.0201
28.5
1.0312
46.5
1.0092
11.0
1.0204
29.0
1.0315
47.0
1.0095
11.5
1.0207
29.5
1.0319
47.5
1.0098
12.0
1.0210
30.0
1.0322
48.0
1.0102
12.5
1.0213
30.5
1.0325
48.5
1.0105
13.0
1.0216
31.0
1.0328
49.0
1.0108
13.5
1.0219
31.5
1.0331
49.5
1.0111
14.0
1.0222
32.0
1.0334
50.0
1.0114
14.5
1.0226
32.5
1.0337
50.5
1.0117
15.0
1.0229
33.0
1.0340
51.0
1.0120
15.5
1.0232
33.5
1.0343
51.5
1.0123
16.0
1.0235
34.0
1.0346
52.0
1.0126
16.5
1.0238
34.5
1.0350
52.5
1.0129
17.0
1.0241
35.0
1.0353
53.0
1.0133
17.5
1.0244
35.5
1.0356
53.5
1.0359
54.0
1.0362
54.5
1.0365
55.0 GLOBE 1997
Learning Activities - 34
Soil
Calculation Work Sheet
A. 2 minute hydrometer reading_________
B. temperature at 2 minutes ___________
o
C
C. grams/L of soil (USDA silt + clay) from table___________g/L
D. temperature correction[(0.36 x (B - 20
o
C)] _______g
E. corrected silt (USDA) and clay in suspension (C+D)______g
F.
grams of soil (USDA silt + clay) in 500 mL (E x 0.5)_________g
G. grams of sand (USDA) (25 g - F)_______g
H. percent sand (USDA definition) [(G/25) x 100] __________%
I.
12 minute hydrometer reading_________
J.
temperature at 12 minutes ___________
o
C
K. grams