4. Soil Physical Analysis
4.4. Hydrometer Method
The hydrometer method of silt and clay measurement relies in the effect of particle size on the differential settling velocities within a water column. By this method (using Hydrometer with Bouyoucos scale in g/L) after 40 second all sand-sized particles (0.02 mm and larger) settle out of the suspension and after 4 h, particles larger than clay (0.002 mm) settle out of the suspension.
Theoretically, the particles are assumed to be spherical having a specific density of 2.65 g/cm3. If all other factors are constant, then the settling velocity is proportional to the square of the radius of the particle (Stoke’s Law).
r= "equivalent" radius of the particle (cm) ρdp = Density of particle (g/cm3), 2.65 is a
In practice, the density of water and its viscosity are both affected by temperature. Therefore, we must know and make corrections for the temperature of the liquid. Greater temperatures result in reduced viscosity due to liquid expansion and a more rapid descent of falling particles.
Relative size of soil separates
Apparatus
Soil dispersing stirrer: A high-speed electric stirrer with a cup receptacle
Hydrometer with Bouyoucos scale in g/L (ASTM 152H) Interval timer
Hot plate Drying oven
Balance, accurate to 0.5 g Desiccator
Beakers
Measuring cylinders, 1000-mL
Reagents
A. Dispersing Solution
Dissolve 40 g sodium hexametaphosphate [(NaPO3)13],
and 10 g sodium carbonate (Na2CO3) in DI water, and bring to 1-L volume. This solution deteriorates with time and should not be kept for more than 1 to 2 weeks.
B. Amyl alcohol
Procedure
1. Weigh 40 g air-dry soil (2-mm) into a 600-mL beaker.
2. Add 60-mL dispersing solution.
3. Cover the beaker with a watch glass, and leave overnight.
4. Next day, quantitatively transfer contents of the beaker to a soil-stirring cup, and fill the cup to about three- quarters with water.
5. Stir suspension at high speed for 3 minutes using the special stirrer. Shake the suspension overnight if no stirrer is available.
6. Rinse stirring paddle into a cup, and allow to stand for 1 minute.
7. Transfer suspension quantitatively into a 1-L calibrated cylinder (hydrometer jar), and bring to volume with water.
A. Determination of Blank
1. Dilute 60 mL dispersing solution to 1-L hydrometer jar with water.
2. Mix well, and insert hydrometer, and take hydrometer reading, Rb.
3. The blank reading must be re-determined for temperature changes of more than 2 °C from 20 °C.
B. Determination of Silt plus Clay
1. Mix suspension in the hydrometer jar, using a special paddle carefully, withdraw the paddle, and immediately insert the hydrometer.
2. Disperse any froth, if needed, with one drop of amyl alcohol, and take hydrometer reading 40 seconds after withdrawing the paddle. This gives reading, Rsc.
C. Determination of Clay
1. Mix suspension in the hydrometer jar with paddle; withdraw the paddle, leave the suspension undisturbed.
2. After 4 h, insert the hydrometer, and take hydrometer reading, RC. E. Determination of Sand
1. After taking readings required for clay and silt, pour suspension quantitatively through a 50-μm sieve.
2. Wash sieve until water passing the sieve is clear.
3. Transfer the sand quantitatively from sieve to a 50-mL beaker of known weight.
4. Allow the sand in the beaker to settle, and decant excess water.
5. Dry beaker with sand overnight at 105 °C.
6. Cool in a desiccator, and re-weigh beaker with sand.
Calculations
–
–
–
–
Technical Remarks
1. If possible, all hydrometer jars should be placed in a water bath at constant temperature (20 °C); in that case, temperature corrections are not needed.
2. For temperature correction, use a value of 0.4 for each degree temperature difference from 20 °C. Add or subtract this factor if the temperature is more or less than 20 °C, respectively.
3. All results of mechanical analysis should be expressed on the basis of oven-dry soil (16-24 hours drying at 105 °C).
4. In the above procedure, carbonates and organic matter are not removed from the soil.
5. The Hydrometer method, as described in this section, cannot be applied to soils that contain free gypsum (gypsiferous soils). For gypsiferous soils, see Hesse (1971).
6. Sum of silt and clay + sand should be 100 %. The magnitude of deviation from 100 is an indication for the degree in accuracy.
7. The material not passing the sieve is weighed and reported as percentage of the air-dry weight of the whole sample.
8. Dispersion is achieved by mechanical means (stirring for larger aggregates), by chemical dispersion using sodium hexametaphosphate, and by eliminating cementing agents such as organic matter (oxidized with hydrogen peroxide) and calcium carbonate (dissolved with hydrochloric acid).
9. Sodium hexametaphosphate is an effective dispersing agent for two reasons:
The sodium monovalent cation replaces polyvalent cations (predominantly calcium) usually adsorbed on soil colloids, thereby breaking one type of inter-particle linkage. The polyvalent cations are then reduced in activity by reacting with the phosphate and precipitating.
The adsorbed sodium cations are highly hydrated and raise the electro-negativity of the colloids until these particles repel each other and remain dispersed. The mixture of dispersed soil particles in water is called a soil suspension.
Soil Texture
Once the percentage of sand, silt, and clay is measured, the soil may be assigned a textural class using the USDA textural triangle. Within the textural triangle are various soil textures which depend on the relative proportions of the soil fractions. Twelve soil textural classes are recognized and their compositions are designed on a textural triangle. The class name can be determined by plotting these values on a textural triangle using the following technique:
1. Clay; extend line horizontal from the percent clay (parallel with base of triangle, sand).
2. Silt; extend line downward from percent silt at 60° (parallel with right side of triangle, clay).
3. Sand; extend line upward from percent sand at 120° (parallel with left site of triangle, silt).
For example, if a soil is 50 % sand, 35 % silt, and 25 % clay, the texture is sandy clay loam (see Figure.
5).
Figure 5. USDA soil textural triangle; indicated example, sandy clay loam: 50 % sand, 35 % silt, and 25 % clay
4.4.1. Pipette Method
The pipette is a laboratory instrument used to transport a measured volume of suspension.
Therefore, the pipette method is based on the difference in sedimentation velocity between large and small soil particles. The sedimentation of the particles is the result of two opposite forces, i.e., gravity and friction as a result of movement in a liquid medium. In the pipetting method, a sample is pipette from a suspension in a graduated measuring cylinder at various times and depths. Times and depths are determined with Stokes’ law. The pipetted suspension is evaporated down and dehydrated, and the mass percentage of the pipetted fraction is determined by weighing.
Apparatus
Cylinders (1000-mL) provided with rubber stoppers Soil dispersing stirrer: a high-speed electric stirrer with a cup receptacle
Water bath
Pipette apparatus consists of:
- Frame with a runner with pipette holder fixed on top of it.
- Pipette balloon.
- Glass tank (113 L capacity).
- Heating element with thermostat and stirring device.
Reagents
A. Amyl Alcohol
B. Deionized or distilled water C. Dispersing Solution
Dissolve 40 g sodium hexametaphosphate [(NaPO3)13], and 10 g sodium carbonate (Na2CO3) in DI water, and bring to 1-L volume with DI water. This solution deteriorates with time and should not be kept for more than 1 to 2 weeks.
Procedure
A. Pre-treatment
1. Each sample is given a laboratory number.
2. The sample is spread uniformly over a plastic or paper sheet, let them air dry.
3. Remove any straw, roots, dead leaves and large stones.
4. Prepare the subsamples by ‘quartering’. The mixed soil material is coned in the center of the mixing sheet with care to make it symmetrical with respect to fine and coarse soil material.
5. Calculate the oven dry weight of the soil sub-sample (soil moisture content).
B. Sand fraction determination
1. The pretreated soil suspension is passed through the appropriate sized sieve (usually 200 µm sieve). Therefore, the soil should be crushed by pestle and porcelain mortar and pass through 200 µm sieve.
2. Repeat until 20% of the soil is retained on the top of the 200 µm sieve.
3. The sand fraction is washed, and quantitatively transfer the sediment suspension through a 270 mesh (53 um) sieve and wash with DI water using a wash bottle. Transfer the sand to a tare beaker, dry at 105 oC and weigh. The dried sand may be placed in nested sieves to determine individual sand fraction size analysis.
C. Silt and clay determination
1. Weigh 40 g air-dry soil (2-mm) into a 600-mL beaker.
2. Add 60-mL dispersing solution.
3. Cover the beaker with a watch-glass, and leave overnight.
4. Next day, quantitatively transfer contents of the beaker to a soil stirring cup, and fill the cup to about three quarters with water using a soil stirring device, stir suspension at high speed for 2 minutes.
5. Transfer suspension quantitatively into a 1-L calibrated cylinder (hydrometer jar), and bring to volume with water.
6. Place the volumetric cylinders in the large temperature-controlled water bath (should be placed on a vibration-free table). The temperature of the water of the controlled water bath must be kept as constant as possible with the aid of the heating element with thermostat and stirring element.
7. Place the stirring paddle in the volumetric cylinder. Shake up-down and mix the suspension intensively for 20 times. Remove stirring paddle and wait until the swirling motion of the practices has just given way to steady settling under gravity.
8. Start the timer or stop-watch. Leave this to settle for 4 hours.
Note
The suspension must be complete, since/ otherwise particles could inter-aggregate. One indication of incomplete suspension is the forming of stratification of the suspension in the measuring cylinder.
9. About 30 seconds before the appropriate sampling time, lower the pipette (previously fixed above the centre of the cylinder) slowly until the tip just touches the surface of the suspension. Note the vertical scale reading and then lower the pipette to the required depth.
Calibrated and fill the pipette by gentle and steady suction (capacity of the pipette around 20 mL (see below the using of the calibrated pipette).
10. Remove the pipette from the suspension. Transfer the aliquot to an evaporating dish and wash the pipette twice with water, adding the washing to the dish.
11. Dry the evaporating dishes with the suspension (evaporated down).
12. Determine the mass percentage of the pipette fraction by weighing.
13. Decant the cylinder sediments and suspension into analytical sieves to calculate the sand fractions.
14. Repeat the sampling a second time after remixing the suspension with the stirring paddle.
15. Evaporate to dryness on a water bath, dry the residue in an oven at 105 oC for 16-18 h, cool in desiccators and weigh to the nearest 0.2 mg.
Technical Remarks
Using of the calibrated pipette
1. Make sure that the two-way cock (K1) and the cock (K3) of the pipette are closed, and the two-way cock (K2) of the pipette (P) is opened toward the pipette before you place the pipette (see sketch of pipette) over the measuring
cylinder with the suspension, 30 second before pipetting (see Figure.6).
2. Insert the pipette into the suspension at the required depth (lower the pipette slowly to avoid disturbance).
3. Connect exhaust point (A) to the pipette balloon.
4. Open the two-way cock (K1) and steadily draw the pipette full until the liquid level is above two-way cock (K2).
5. Close the two-way cocks (K1) and (K2).
6. Move the runner to the right (outside the glass tank).
7. Open two-way cock (K2) above drain (D) to drain off any excess suspension.
8. Rinse overflow ball (F) with water from storage ball (W) by opening cock (K3).
9. Open the two-way cock (K2) above pipette (P).
10. Open two-way cock (K1) above drain (E) and empty the pipette in an evaporating dish.
11. Rinse the pipette (P) with water from the storage ball (W). Collect also this scouring water in the same evaporating dish.
12. Open two-way cock (K1) above pipette point (S) and remove the pipette from the suspension.
13. Collect the suspension of the pipette in evaporating dish.
Determination of the in-feed depth of the pipette
1. Determine the temperature of the blank and read the in-feed depth of the pipette and the sedimentation time of the desired fraction from the table below.
2. Mix the suspension intensively for at least 1 minute and start the chronometer. Pipette (after the prescribed sedimentation time) the particles ≤ 2 μm to determine the lutum content (lower the pipette slowly to avoid disturbance).
3. To determine the particle-size distribution, first pipette the fraction smaller than or equal to 35 μm, and subsequently the other desired fractions in descending order according to particle size.
Figure 6. Calibrated pipette
Temperature of
In-feed depth of the pipette in
cm at Temperature of
In-feed depth of the pipette in cm at
the suspension
Upper limit particle fraction in µm
the suspension
Upper limit particle fraction in µm
in oC in oC
35 2 35 2
After a sedimentation time of After a sedimentation time of
90 second 4 hours 90 second 4 hours
15.0 8.4 4.4 20.5 9.7 5.1
15.5 8.6 4.5 21.0 9.8 5.1
16.0 8.7 4.5 21.5 9.9 5.2
16.5 8.8 4.6 22.0 10.1 5.3
17.0 8.9 4.6 22.5 10.2 5.3
17.5 9.0 4.7 23.0 10.3 5.4
18.0 9.1 4.8 23.5 10.4 5.4
18.5 9.2 4.8 24.0 10.6 5.5
19.0 9.3 4.9 24.5 10.7 5.6
19.5 9.5 5.0 25.0 10.8 5.6
20.0 9.6 5.0
Calculation the depth of pipette sampling
The depth of pipette sampling is based on the velocity (Stokes’ Law). The re-arranged equation is:
Where:
T = Time of pipetting h = Depth of pipette V = Velocity of fall (cm/sec) Ρdp= Density of particle (g/cm3)
ρdl = Density of liquid (g/cm3), for water this is ~1.0
g= Acceleration of gravity (cm/sec), value at the sea level is 981
η= Viscosity of liquid (g/cm/sec) X2= Particles diameter
Assumptions used in applying Stokes’ Law to soil sedimentation measurements are as follows:
1. Terminal velocity is attained as soon as settling begins.
2. Settling and resistance are entirely due to the viscosity of the fluid.
3. Particles are smooth and spherical.
4. There is no interaction between individual particles in the solution.
5. It is recommended to use horizontal reciprocating to disperse the samples. The use of electric stirrer at high revolutions per minute (rpm) may result in significant grinding of sample primary minerals.
6. For soils having clay particle densities <2.65 g/cm3, settling time increases, and for soils >2.65 g/cm3, settling time decreases.
7. An error of 0.001 ± g in the dry weight of the pipette sample results in an error of ± 0.32 % for clay size fraction.
8. Since soil particles are not smooth and spherical, the radius of the particle is considered an equivalent rather than an actual radius. In this method, particle density is assumed to be 2.65 (g/cm3) and density of liquid is assumed to be 1 (g/cm3).
9. Water density and viscosity at different temperatures.
Temperature
(oC) 10.0 12.5 15.0 17.5 20.0 22.5 25.0 27.5 30.0
Density (g/cm3) 0.99973 0.99947 0.99913 0.99871 0.99823 0.99768 0.99707 0.99640 0.99567 Viscosity
(g/cm/sec) 0.01301 0.01219 0.01138 0.01072 0.01006 0.00950 0.00895 0.00847 0.00800