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BBSS Application Method P003



Adapted for use Soil Physics Laboratory, NARC Islamabad by HTS Consultant and local counterparts

A downloadable Excel spreadsheet with all the calulations is available by clicking here.

Dispersion is maintained by a 50g/L sodium hexametaphospate solution. Measurement of silt plus clay and clay is by hydrometer with settling times calculated accurately from sedimentation theory using a computer. This method supercedes the Bouyoucos procedure (Bouyoucos, 1962) in which measurements are made at settling times of 40s and 2 hr. The Bouyoucos procedure cannot be used to accurately define particle size class, it is based on empirical relations between silt and clay and 40s/2hr hydrometer readings. Gee and Bauder compared different methods (1979). They estimate the error in using the Bouyoucos procedure to be 10 % clay and recommend it only for assessment of particle size class. They found that the pipette and hydrometer method can give comparable results with major differences arising largely from differences in pretreatment techniques.


Soil textural class is distinguished on a basis of the relative amounts of sand, silt and clay in the fine earth (<2mm sieved sample). Following USDA (1975) the sand silt and clay particle diameters are as follows:

sand: 0.05 - 2 mm
silt: 0.002-0.05 mm
clay: <0.002 mm

Dispersed soil particles settle largest first according to Stoke's Law, which for the hydrometer method may be written as:

X = S t -1/2

where X is the effective particle diameter and S is a sedimentation parameter.

The principle of the hydrometer method is that a dispersed soil sample is thoroughly stirred and the summation percentages representing sand, silt and clay are determined by measuring the density of the suspension after first sand and then silt have settled below the hydrometer settling depth.

The density of soil in suspension is measured using a hydrometer correcting for the density of the dispersing solution.

Sand size particles settle after between 40 s and 80 s (0.67 min and 1.33 min) depending on temperature and density of suspension. Hydrometer measurements are made at both these times and the % sand in the sample is calculated by interpolation between the two measurements. Silt size particles settle after between 2.5 and 24 hours (150 min and 1440 min). Hydrometer measurements are made and both these times and the % clay in the sample is calculated by interpolation between the two measurements. Silt is calculated by difference. For a more complete analysis of particle size distribution intermediate measurements can be made for example at 4.5 min.

For certain soils soluble salts, carbonates and organic matter may need to be removed to effect dispersion, otherwise determinations are made on a a basis of soil containing carbonates.

If separation of the sand fraction is required the suspension is washed through a nest of sieves with openings ranging from 53 um to 1mm and sand fractions are determined by weighing the material collected on each sive.


Standard hydrometer ASTM No. 152 H graduated in g/L
Electric high speed (10,000 rpm) stirrer with baffles and dispersing cup.
Measuring cylinder with 1 Litre mark 36 2 cm from bottom of the inside
600 mL beakers


1. Sodium hexametaphosphate (HMP) solution (50g/L). Dissolve 50 g of sodium hexametaphosphate in 750 mL deionized water and make up to 1.0 L. Mix well.

2. Amyl alcohol (only if frothing occurs)

Procedure for sand silt and clay determnation

1. Dispersion of soil:
Weigh 40.0 grams air dry 2 mm sieved soil (60-100 grams for sands) into a 600 mL beaker, add 250 mL distilled water and 100 mL of 50g/L HMP sodium hexametaphosphate reagent , mix and allow the sample to stand overnight.Weigh accurately about 10 grams of at least one sample from each batch for determination of oven­dry weight. Record mass. Dry overnight at 105C, cool, weigh and record mass.

Transfer the sodium hexametaphosphate treated sample to a dispersing cup and mix for 5 minutes at medium speed using the electric stirrer. Transfer the suspension to a measuring cylinder, add distilled water to the 1000 mL mark.

2. Calibration of hydrometer:
Add exactly 100 mL of the 50g/L HMP solution to a 1 litre measuring cylinder and make the volume to 1.0 L with distilled water. This is a blank solution containing 5g/L HMP . Mix thoroughly and let stand until the temperature is constant; record temperature to nearest 1 degree C. Lower the hydrometer into the solution and determine the scale reading to nearest 1 g/L at the upper edge of the meniscus surrounding the stem. Temperature and blank values must be determined each time hydrometer measurements are made.

3. Hydrometer Measurements
Allow the suspension to equilibrate to room temperature ­ record temperature on laboratory record sheet PSA.XLS. Insert the plunger and move it up and down to mix the contents thoroughly. Dislodge sediment with strong upward strokes of the plunger near the bottom and by spinning the plunger while the disc is just above the sediment. Finish stirring with 2 or 3 slow, smooth strokes. Add a drop of amyl alcohol if the surface of the suspension is covered with foam. Record the time. Lower the hydrometer carefully into the suspension and take readings to nearest 1 g/L after 40 seconds and again after 80 seconds. Remove the hydrometer carefully , rinse it and wipe it dry and record the temperature of the suspension. This procedure is followed for each sample.

After 4.5 minutes, 2.5 hours and 24 hours reinsert the hydrometer carefully one minute before the reading is due. At each settling time take hydrometer readings followed by temperature readings in each suspension Measure the blank at each time as well.


After each settling time calculate the corrected hydrometer reading (concentration of soil suspension in g/L) from

C = R ­ RL

where R is the sample reading and RL is the blank reading. The summation percentage after each settling time is

P= (C/CO) x 100

where C0 is the oven dry weight of the soil sample.

Calculate the corresponding particle diameter by the following procedure

1. density of the HMP solution

rl = r w(1+0.63Cs)

where Cs is concentration of HMP in g/mL

2. viscosity of the HMP solution

h = h w(1+4.25*Cs)

where hw is viscosity of water in poise

3. sedimentation parameter S

S =1000(Bh')0.5, B=30x(g(s-l)), h'=-0.164R +16.3

where s is particle density , g/cm3 and h' is hydrometer settling depth, cm

4. effective particle diameter

X (mm) = S t-1/2 /1000

where t is the settling time in minutes

An example calculation follows:

settling time = 80 s = 1.33 min, temperature is 19 C

Rl = 4.5, R = 30.5, C = 26 g/L

mass air dry soil sample = 40 g, Co = 40/(0.05+1) = 38 g

P = 26/38 x 100 = 68 %

w = 0.998, l = 1.002, w = 0.0103, l = 0.0105

B = 0.00019, h' = 11.3, = 46.89, X = 0.041 mm

Summation percentages and effective particle diameters at four settling times T1-T4

  T1 40s T2 80s T3 2.5h T4 24 h

P / %

P1 75

P2 68

P3 28

P4 22
X /mm X1 0.056 X2 0.041 X3 0.0042 X4 0.0014

Calculate %clay (summation percentage at 0.002 mm) from Pc:

Pc = P4 + m ln (0.002/X 4 )

where m is the slope of of the summation percentage curve between T3 and T4 This is plotted as P vs ln(X) and slope of curve m = (P3-P4) / ln (X3/X4)

Calculate % sand from 100 - Ps

Ps =P2 + m ln (0.05/X2 )

where m is the slope of the summation percentage curve between T1 and T2

m= (P1-P2) / ln (X1/X2)

Calculate % silt from 100 - (%sand+%clay)

Continuing with reference to the data plotted on a logarithmic x-axis (Fig 2):

m (clay) = (28-22)/-5.47+6.57=5.46 , Pc =22 +5.46 ln(0.002/0.0014) = 24 = % clay

m (sand)= 21.1 Ps = 68+21.1 ln (0.05/0.041) =72, % sand =100-72=28

Approximate clay and sand contents can be determined from the graph of summation % (Fig 1)

clay = summation % for x=0.002mm = 16

sand = 100- summation % for x= 0.05mm = 100-48 =52

Figure 1. Summation graph showing approximate particle size distribution

Figure 2. Summation graph plotted on a logarithmic scale

Table 2. Viscosity and density of water (Source:CRC Handbook of Chemistry & physics)

Temperature Viscosity Density
(C) (poise) (g mL)
18 0.01053 0.9986
19 0.01027 0.9984
20 0.01002 0.9982
21 0.009779 0.9979
22 0.009548 0.9978
23 0.009325 0.9975
24 0.009111 0.9973
25 0.008904 0.9970
26 0.008705 0.9968
27 0.008513 0.9965
28 0.008327 0.9962
29 0.008148 0.9960
30 0.007975 0.9957

Determination of sand fractions

If sand fractionation required 8 inch diameter brass sieves with openings 1000, 500, 250,106 and 53 um.

1. Secure the five test sieves together with the 1000 um sieve on top and the 53 um sieve on the bottom. Place the sieves in the sink.

2. Pour the soil suspension from the first cylinder into the nest of sieves and wash with tap water. Use the hose directed water supply to wash out particles from the measuring cylinder into the sieve. Ensure that the bottom sieve is not overflowing and that water flow is not so high that any particles are forced over the rim.

3. Move the coarse particles from one side of the sieve to the other for about two minutes.

4. Loosen the 1000 um sieve from the other sieves and tilt the the sieve to about 45o. Move all the particles down to one side of the sieve by the aid of water from the hose. Rinse the sieve and its content with distilled water from a wash bottle. Transfer particles to a weighed and labelled (vcos) beaker using a rubber policeman and distilled water.

5. Proceed with the other sand fractions as described in 3 and 4 above.

6. Dry the sand fractions in the oven at 100-105oC


5. Each sand fraction is expressed as a percentage of the fine earth, calculated as follows:

mass fraction sand (with beaker) = s

mass beaker = sb

mass of sand fraction /100g soil = (s - sb) x 100/40

very fine sand 0.05 - 0.10 mm material collected on 53 um sieve

fine sand 0.10 - 0.25 mm material collected on 106 um sieve

medium sand 0.25 - 0.50 mm material collected on 250 um sieve

coarse sand 0.50 - 1.0 mm material collected on 500 um sieve

very coarse sand 1.0 - 2.0 mm material collected on 1000 um sieve

Values are recalculated to add up to the total sand content measured by hydrometer


ASTM. 1985. Standard Test Method for particle size analysis of soils. D 422-63. American Society for Testing and Materials, Philadelphia.

Bouyoucos, G.J. 1962. Hydrometer method improved for making particle size analysis of soils. Agron J. 54: 464-465

Day, P.R. 1965. Particle fractionation and particle size analysis. pp 545­567. In Methods of Soil Analysis, C.A. Black, Editor. Agronomy No. 9, Part 1. Am. Soc. of Agron., Madison, Wisconsin, U.S.A.

Gee, G.W. and J. W. Bauder, 1986. Particle Size Analysis pp383-411 in Klute. A. (Ed) Methods of Soil Analysis, Part 1, Physical and Mineralogical Methods, Agronomy Monograph No 9 (2nd Edition). American Society of Agronomy, Madison.

Submitted on 18-4-1997 by Alan Stapleton Soil Science & Land Research Consultancy 6, Elm Grove Rd, Topsham EXETER EX3 0BW



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