Particle Size Determination by Hydrometer Method
Gobena Huluka and Robert Miller
Application and Principle
Soil is a mixture of minerals, organic matter, water and air. The solid particles, minerals
and organic matter, constitute about 50% of the soil by volume. Soil texture is more or less a
permanent physical property soil. Soil texture affects water and air movement, nutrient holding
capacity, pore sizes and rooting growth. It is one of the master physical properties of a soil. Soil
particles dominant in clay, silt and sand will be sticky, floury and gritty, respectively at field
capacity.
Soil particles are made of materials with varying diameters, from sub-meter to
nanometers. Soil particle physical activity is inversely proportional to its diameter. Thus,
particles with 2.00 mm or less are used for soil particle size, also known as soil texture
determination. The United States Department of Agriculture (US Soil Salinity lab Staff, 1954)
groups soils into sand (2 mm–0.05 mm), silt (0.05 mm –0.002 mm) and clay (< 0.002 mm).
The hydrometer analysis is described by the Stokes’ law that is based on the relationship
among the velocity of free falling spherical soil particles in water. The viscosity of water is affected
by temperature, and hence correction is necessary when measurements are not made at standard
temperature of 20 OC. There are two important steps in determining soil texture analysis which
make soil particles free and independent particles by chemical treatment and mechanical
dispersion. High speed stirring and addition of sodium metaphosphate or calgon solution will
accomplish the task. Density of the suspension is measured using the bouyoucos hydrometer at
different time??? (Gee and Bauder, 986).
The removal of organic matter, carbonates, soluble salts and some oxides may be
necessary depending on the on the concentration of each component and the accuracy expected.
These soil constituents are removed during pretreatment of the soil samples and
detailed procedures how to remove each is given by Sheldrick and Wang (1993), and Gee and
Bauder (1986).
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
Reagents
1. Deionized water, ASTM Type I grade
2. Sodium metaphosphate (NaPO3)x.Na2O
3. Sodium carbonate (Na2CO3)
4. Amyl alcohol
5. Dispersing agent: Dissolve 7.93 g of Na2CO3 and 35.7 g of (NaPO3)x.Na2O in 1 L
volumetric flask.
Determine the moisture content of the soil by taking the subsample of the soil if air-dry samples
used.
Procedure
1. Weigh 50 ± 0.05 g of fine or 100 g of coarse textured air-dried soil pulverized to pass
10 mesh sieve (< 2.0 mm) soil into a blender cup.
2. Add deionized water approximately within 10 cm of the rim.
3. Add 50 mL of dispensing agent and let it soak for 20 minutes.
4. Place the blender cup on electrical mixer and stir for 10 minutes.
5. Transfer the soil suspension to 1.0 L measuring cylinder. Use additional water to transfer
all soil if necessary.
6. Fill to the 1.0 L mark with deionized water.
7. Use a plunger with an up and down stroke at least five times to mix sediments from the
bottom of the cylinder.
8. Immediately place a stopper on the top of the cylinder and mix it by turning upside down
at least five times.
9. Place it on a counter table and immediately start a timer.
10. Slowly lower a dry hydrometer into the suspension, and add three drops of amyl alcohol
if the suspension is covered with foam.
11. Take a reading exactly after 40 seconds (First reading).
12. Measure the temperature of the suspension.
13. Remove the hydrometer and place it in deionized water and dry with towel before
reuse.
14. Repeat steps 6-13.
15. The average of the two readings is the final value.
16. Include a blank (without a soil) following the same steps as for the sample.
17. Take two readings after 8 hours (Second readings) without mixing and no timing
necessary.
18. Measure the temperature of the suspension.
Calculations
1. Sand + silt (%) = ((First reading-blank)/weight) x 100 (A) Clay (%) = ((Second reading –
blank)/weight) x 100 (B) Silt (%) = A-B
2. Sand (%) = 100-A-B
3. 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.
Comments
The soaking time can be increased to overnight with 16 h shaking time for better results.
But Clay reading time could be reduced to two hours for less accurate results depending on the
objective of the analysis.
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. 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.
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.