Particle Size Determination by Hydrometer Method
Gobena Huluka and Robert Miller
Application and Principle
Soil texture is a basic property of that soil affects soil physical properties and management.
Under normal conditions, it is considered a permanent property of a soil. Soil texture affects soil water
and nutrient holding capacities, water and air movements, pore sizes and plant root growth. Because of
these important roles, soil texture is consider a master soil variable.
Soil is a mixture of minerals, organic matter, water and air of which minerals and organic
matter, constitute about 50% of the soil by volume. Soil solids are made up particles of different
sizes. Soil texture is defined by relative proportions of sand, silt, and clay. The United States
Department of Agriculture (US Soil Salinity lab Staff, 1954) defines sand, silt, and clay as particles
with sizes from 2 to 0.05 mm, 0.05 to 0.002 mm, and less than 0.002 mm, respectively.
Soil particle size is determined by hydrometer analysis using Stokes’ law which predicts the
velocity of free falling spherical soil particles in water based on particle size. The larger the particle size,
the faster the settling velocity. The viscosity of water is affected by temperature, and thus a
measurement correction is necessary for temperatures deviating from a standard temperature of 20 OC.
The determination of soil texture analysis is accomplished first by the addition of a chemical
dispersant sodium metaphosphate, with subsequent mechanical agitation. Density of the soil
suspension is measured using the Bouyoucos hydrometer at specific times as a function of the
particle size being measured (Gee and Bauder, 1986). As an alternative, the sand percentage can be
determined removal of the sand fraction using a 270 mesh (53.3 µM) screen and subsequent
gravimetric measurement.
The chemical dispersant may not completely disperse soil particles in the presence of soil
constituents such as organic matter, carbonate minerals, soluble salts, and some oxides. These soil
constituents coagulate particles via various forces that are stronger than the force of dispersion
provide by the dispersant. Thus, the removal of organic matter, carbonates, soluble salts, and oxides
may be necessary depending on the concentration of the components and the accuracy required.
These soil constituents can be removed by pretreating the soil. Pretreatment options are presented in
this method. A more detailed presentation of pretreatment options are provided by Sheldrick and
Wang (1993), and Gee and Bauder (1986) and in ASTM D422 - 63(2007) Standard Test Method
for Particle-Size Analysis of Soils.
Equipment and Apparatus
1. A.S.T.M. (1985), ASTM No. 1. 152H-Type with Bouyoucos scale in g/L
2. Electrical stirrer (10,000 rpm motor)
3. Stainless steel blender cup
4. Sedimentation cylinder marked at 1-L at top with 40 cm space above the mark
5. Plunger and rubber stopper for 1-L cylinder
6. Analytical balance: 100 g capacity, resolution ± 0.01 g
7. A timer
8. Thermometer
9. Centrigfuge capable of 1500 rpm with 250mL centrifuge tubes.
Reagents
1. Deionized water , ASTM Type I grade
2. Sodium metaphosphate Na 2 O
3. Sodium carbonate (Na2CO3)
4. Sodium hydrosulfite (Na2S2O3)
5. Amyl alcohol
6. Hydrogen peroxide (H2O2 ), 30% Solution
7. Citrate bicarbonate buffer. Prepare a 0.3M sodium citrate (88.4 g/L) and add 125 mL of
1 M sodium bicarbonate (84 g/L) to each liter of citrate solution.
8. Dispersing agent: Dissolve 7.93 g of Na2CO3 and 35.7 g of (NaPO3)6.Na2O in 1 L volumetric
flask.
Determine the moisture content of the soil by taking the subsample of the soil if air-dry sample is
used.
Procedure
1. Prepare soil with air drying and pulverizing to pass a 10 mesh sieve (< 2 mm). Weigh 50 ±
0.05 g of fine-textured soil or 100 ±0.05 g of coarse-textured soil into a 300 mL beaker.
2. Removal of carbonates: Add 50 mL DI water and sufficient 1 M HCl to reduce the soil pH
to between 3.0 and 4.0. Stir and allow to equilibrate 10 minutes until there is no
effervescence.
3. Removal of organic matter: Add 10 ml of hydrogen peroxide (H 2O2, 30 or 50%)
to beaker. When frothing subsides heat to 90 oC and add H 2O2 until frothing
subsides. Rinse down walls of beaker and continue heating until no excess H2O2
is consumed.
4. Removal of iron oxides: Add 150 mL of citrate-bicarbonate buffer to sample in
beaker. Add 3 g of sodium hydrosulfite (Na 2S2O3) gradually as samples may
froth. Place in water bath at 80 oC for 20 minutes. Place the blender cup on
electrical mixer and stir for 10 minutes.
5. Removal of soluble salts: Place soil in centrifuge tube and add 100 mL water
and centrifuge for 10 min at 1500 rpm until supernatant is clear. If the
centrifuge is not clear, repeat the process until it is clear.
6. Place 50 ± 0.05 g of air-dried soil pulverized to passing a 10 mesh sieve (< 2.0 mm) sample
of medium to fine textured soil (i.e. loam, silt loam, clay loam) or 100 g of coarse textured
soil (i.e. sand, loamy sand, sandy loam) into a blender cup. If soil was pretreated, transfer
all contents from the pretreatment beaker into the blender cup with rinsing.
7. Add deionized water approximately within 10 cm of the rim.
8. Add 50 mL of dispensing agent and let it soak for 20 minutes.
9. Place the blender cup on electrical mixer and stir for 5 minutes.
10. Transfer the soil suspension to 1.0 L measuring cylinder. Use additional water to transfer all soil
if necessary.
11. Fill to the 1.0L mark with deionized water.
12. Use a plunger with up and down stroke at least five times to mix sediments from the bottom
of the cylinder.
13. Immediately place a stopper on the top of the cylinder and mix it by turning upside down at
least five times.
14. Place it on a counter table and immediately start a timer, and remove the stopper. If
the suspension is covered with foam add three drops of amyl alcohol
15. Slowly lower a dry hydrometer into the suspension
16. Take a reading of the hydrometer level at the solution surface at exactly 40 seconds.
This is the first reading (R1st).
17. Measure the temperature of the suspension
18. Remove the hydrometer and place it in deionized water and dry with towel before reuse.
19. Repeat steps 7-13 for the same soils to get duplicate readings.
20. The average of the two readings is the final value for the first reading.
21. Take two readings after 8 hours (second reading) without mixing (R2nd).
22. Measure the temperature of the suspension.
23. Include a blank without soil following the same steps as for the sample. Hydrometer
level readings from the blank are designated as RC1, Rc2, respectively.
Calculations
1. (Clay + silt) (%) = ((First reading - blank1)/weight) x 100
2. Clay (%) = ((Second reading – blank2)/weight) x 100
3. Silt (%) = (Clay + silt)% - Clay(%)
Sand (%) = 100 –((R1st – RC1) / w)*100
Clay (%) = (R2nd – RC2) / w)*100
Silt (%) = 100 – Sand (%) – Clay (%)
Where R1st is the first hydrometer reading at 40 s, R2nd is the second hydrometer reading at 8 h,
RC1, RC2 are the hydrometer reading of the control cylinder without soil, and w is the weight of
soil in the cylinder.
4. For temperature correction, add or subtract 0.36 g/L for each degree above or below 20 OC,
respectively.
Analytical Performance
Range and Sensitivity
1. The method has a detection limit of 2.0% sand, silt and clay (dry basis) and is generally
reproducible to within ± 8%.
Interferences
1. Organic matter, carbonates, soluble salts and iron oxides are shown to interfere with
measurement when they are at significant concentration. If organic matter is greater than
3.5% or carbonates is greater than 2%, soil should be pretreated to remove these
constituents for more accurate analysis.
Comment
The theoretical time for the 2nd reading is 7.72 h, but durations as low as 2 h (Bouyoucos,
1962), and 6 h (Ashworth, et al., 2001) have been suggested depending on the required
accuracy, dominant soil texture, and type and amount of interferences.
References
American Society for Testing and Materials. 1985. Standard test method for particle-size analysis of
soils D 422-63 (1972). Annual Book of ASTM Standards 04.08:117-127. American Society for Testing
Materials, Philadelphia.
American Society for Testing and Materials. 2008. Standard test method for particle-size
analysis of soils D422 - 63(2007). Annual Book of ASTM Standards 04.08:117-127. American
Society for Testing Materials, Philadelphia.
Ashworth, J., D. Keyes, R. Kirk and R. Lessard. 2001. Standard procedure in the
Hydrometer method fir particle size analysis. Commun. Soil Sci. Plant Anal. 32:633-642.
Bouyoucos, G.J. 1962. Hydrometer method improved for making particle size analyses of soils.
Agron. J. 54:464-465.
Gee, G.W. and J.W. Bauder. 1986. Particle-size Analysis. Pages 383-411 in Methods of Soil Analysis
Part 1. A. Klute (edi.), Soil Science Society of America Book Series 5, Madison, Wisconsin, USA.
Sheldrick, B.H. and C. Wang. 1993. Particle size distribution. Pages 499-511 in Soil Sampling and
Methods of Analysis, M.R. Carter (edi.), Canadian Society of Soil Science, Ottawa, Ontario, Canada.
U.S. Soil Salinity Lab. Staff. 1954. Methods for soil characterization. p. 83-147. Diagnosis and
improvement of saline and alkali soils. Agr. Handbook 60, USDA, Washington, D.C.