

© BBSS Application Method
P003 PARTICLE SIZE ANALYSIS BY HYDROMETER
METHOD OF GEE & BAUDER Adapted for use Soil Physics Laboratory,
NARC Islamabad by HTS Consultant and local counterparts 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. Principle 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:
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.
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: 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: 3. Hydrometer Measurements 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. Calculations After each settling time calculate the corrected hydrometer reading (concentration of soil suspension in g/L) from C = R R_{L} where R is the sample reading and R_{L} is the blank reading. The summation percentage after each settling time is P= (C/C_{O}) x 100 where C_{0} is the oven dry weight of the soil
sample. Calculate the corresponding particle diameter by the following procedure 1. density of the HMP solution r_{l }= 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 h_{w }is
viscosity of water in poise 3. sedimentation parameter S S =1000(Bh')0.5, B=30x(g(sl)), 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 t1/2 /1000 where t is the settling time in minutes An example calculation follows: settling time = 80 s = 1.33 min, temperature is 19 C R_{l }= 4.5, R = 30.5, C = 26 g/L mass air dry soil sample = 40 g, C_{o} = 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 T1T4
Calculate %clay (summation percentage at 0.002 mm) from Pc: Pc = P_{4 }+_{ }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 = (P3P4) / ln (X3/X4) Calculate % sand from 100  P_{s} P_{s} =P2 + m ln (0.05/X_{2 }) where m is the slope of the summation percentage curve between T1 and T2 m= (P1P2) / ln (X1/X2) Calculate % silt from 100  (%sand+%clay) Continuing with reference to the data plotted on a logarithmic xaxis (Fig 2): m (clay) = (2822)/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
=10072=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 = 10048 =52
Figure 1. Summation graph showing approximate particle size
distribution Figure 2. Summation graph plotted on a logarithmic
scale
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 100105oC
Calculation 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 ReferencesASTM. 1985. Standard Test Method for particle size analysis of soils. D 42263. American Society for Testing and Materials, Philadelphia. Bouyoucos, G.J. 1962. Hydrometer method improved for making particle size analysis of soils. Agron J. 54: 464465 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
pp383411 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.

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