Particle Size Analysis
Soils 206- Soil Ecosystem Lab
Objectives:
1. Understand the principles and assumptions of Stoke’s Law,
2. Perform particle size analysis using the hydrometer method,
3. Calculate the textural class using the results of the particle size analysis.
Soil Texture
Soil texture refers to the proportion (by weight) of sand, silt, and clay. The following chart is a
generalization of the influences of soil separates on some properties and behavior of soils.
Property
Diameter
Surface Area
Inherent Fertility
Mineralogy
Characteristic, Dry
Silt
0.05-0.002 mm
Moderate
Moderate
Often Different
Properties of Both
Clay
<0.002 mm
High
High
Often Different
Hard, brittle
Properties of Both
Plastic and Sticky
Water Holding Capacity
Drainage Rate
Sand
2 – 0.05 mm
Low
Low
Usually Parent
Soft, friable
(if granular)
Non-plastic, Nonsticky
Low
Fast
Moderate
Moderate
Porosity
Compactability
Shrink-Swell Potential
Sealing Potential
Pollutant Leaching
Low, Large Pores
Low
Very Low
Poor
High
Moderate
Moderate
Moderate
Poor
Moderate
Organic Matter Level
Decomposition of SOM
Spring Warm-Up
Erosion, Wind
Erosion, Water
Low
Rapid
Rapid
Moderate
Low
Moderate to High
Moderate
Moderate
High
High
High
Slow
(unless cracked)
High, Small Pores
High
High
Good
Low
(unless cracked)
High to Moderate
Slow
Slow
Low
Moderate
Characteristic, Wet
Determination of Soil Texture
Two methods exist for textural analysis. During a prior lab exercise you practiced using the texture by
“feel” or hand-texturing method. Today we will use a mechanical analysis known as the Hydrometer
Method. This method involves dispersing a soil sample in water and determining the sedimentation rate
of the sand, silt, and clay particles. Sedimentation rates of suspended soil particles depend primarily on
particle size. Large sand-sized particles settle faster than smaller clay-sized particles. This relationship
can by quantitatively expressed by
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Stoke’s Law:
V = d2 g (Dp – Dl)
18
Where:
V = settling velocity (m/s)
t = particle settling time (s)
L = particle settling distance (m)
d = diameter of the particle (m)
g = acceleration due to gravity (9.81 N/kg)
Dp = density of the particle (2.65x103 kg/m3)
Dl = density of the liquid (1.00 x103 kg/m3)
 = viscosity of the liquid (1.0 x 10-3 Ns/m2)
Stoke’s Law can be simplified for our use as gravity remains constant, the particle density and the
density of water remain constant and the viscosity of water remains constant. These terms can be
combined to form a constant, ‘k’. Entering the known values to the equation and leaving the variable of
particle size the equation becomes:
(9.81 N kg-1) (2.65x103 kg m-3 – 1.00 x103 kg m-3) * d2
18 (1.0x10-3 Ns m-2)
solving,
V=
V=
9x105 s-1m-1 * d2
or
V=
kd2
Substituting for velocity and solving for time (t):
V=
L/ t
so t = L/V
or
t=
L/ kd2
Units: t = seconds, L = m, d = m, k = 9x105 s-1m-1
Assumptions of Stoke’s Law:
1. Particles have the same density
2. Particles are spherical, smooth and rigid
3. Particles do not interact with each other or with the walls of the container
4. No Brownian motion
5. No turbulence
The time required for a sand particle with a diameter of 0.05 mm or 0.00005 m to fall the distance of 15
cm or 0.15 m is calculated below.
t=
L/ kd2
t=
0.15 m
= 67 seconds
9x105 s-1m-1 * (0.00005 m)2
The time required for a silt particle with a diameter of 0.002 mm or 0.000002 m to fall the distance of 15
cm or 0.15 m is calculated below.
t=
L/ kd2
t=
0.15 m
= 41 780 seconds or 11.6 hours
9x105 s-1m-1 * (0.000002 m)2
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Hydrometer method of PSA
A hydrometer relies on buoyancy to calculate the number of grams of soil material in 1 liter of suspension
at the time of the reading (it has been calibrated to do the mathematics for you). The hydrometer method
involves two basic principles; dispersion and sedimentation.
Dispersion: Individual soil particles must be separated from each other and remain separated during the
determination of particle size distribution. Since aggregates of solid particles are usually held together by
some kind of binding agent, it is first necessary to remove these substances, or at least render them
ineffective. Once the particles are separated they are said to be dispersed.
Dispersion is achieved by chemical and mechanical means. Mechanical stirring is effective in dispersing
larger aggregates, but is ineffective on small aggregated clay groups. Chemical dispersion is required in
this case. Sodium hexametaphosphate is an effective chemical dispersing agent for two reasons:
1. The sodium monovalent cation replaces polyvalent cations (predominately Ca2+) usually adsorbed on
soils thereby breaking one type of interparticle linkage. The polyvalent cations are then reduced in
activity by reacting with the phosphorus and precipitating.
2. The adsorbed sodium cations are highly hydrated and raise the electronegativity of colloids until these
particles repel each other and remain dispersed.
The mixture of dispersed soil particles in water is called a suspension.
Sedimentation: Large particles will settle out of suspension more rapidly than small particles because
small particles present more specific area and therefore will experience greater frictional resistance. In
practice the amount of material still in suspension at any one time is measured with a hydrometer, which
indicates the density of the suspension at the hydrometer’s center of buoyancy.
Calculations
Percent Silt and Clay
= Corrected Silt and Clay Average Reading (g/l) * 100
Original Sample Weight
Percent Sand
= 100% - (Silt & Clay Percent)
Percent Clay
= Corrected Clay Reading (g/l) * 100
Original Sample Weight
Percent Silt
= 100% - (Sand Percent + Clay Percent)
Use the percent sand, silt and clay to determine the textural class using the textural triangle.
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Procedure
1. Weigh 50g fine-textured soil (100 g of coarse-textured soil) into a labeled wide-mouth Erlenmeyer
flask. The label should indicate the soil name and the precise weight of the soil. Add 100 ml of SHMP
(50 g/l), and 50 ml of distilled water. Stir.
2. Quantitatively transfer the contents of the Erlenmeyer flask into a dispersing cup with distilled water.
Do not fill the dispersing cup more that ½ full but fill to ½ full, if necessary before mixing. Place on the
mixing machines and stir until soil aggregates are broken down. This will usually require 3-4 minutes
with coarse textured soils and 5-6 minutes for soils high in clay.
3. Quantitatively transfer stirred mixture to a settling cylinder with distilled water. Fill the settling cylinder
to 1000 ml mark with distilled water.
4. Gently mix the suspension using full strokes from the bottom to the top of the cylinders with an
agitation plunger. Stir the suspension until the soil material is equally distributed throughout the
cylinder, this usually requires 3-4 minutes. Start timing as soon as the plunger is removed from the
cylinder. Gradually and carefully insert the hydrometer into the suspension in the settling cylinder.
At exactly the calculated time for the silt and clay reading, record the hydrometer reading. Repeat
Step 4 three times and record each reading on the data sheet. (NOTE: The hydrometer is calibrated
to read in grams of soil material remaining in suspension.)
3. Re-stir the suspension. Take a hydrometer reading at the calculated time for the clay fraction,
approximately 11 hours later.
4. Temperature Correction: For each degree above 18 C, add 0.25 to the hydrometer reading. For
each degree below 18 C, subtract 0.25 from the hydrometer reading. Note the temperature and the
correction for the 40 second readings and for the 11:36 hour reading on your data sheet.
5. Density Correction: Gradually and carefully place the hydrometer you are using in the settling
cylinder that contains only the distilled water and the SHMP solution. Note the correction on your
data sheet.
6. Following completion of this exercise, the liquid portion can be discarded in sink drains, but transfer
settled materials to the soil bucket located in the sink.
Don’t forget to leave your labeled soil tin here so it can be oven dried for lab next week!
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