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After reading this article you will learn about the principle and pipette method of mechanical analysis of soil.
Principles of Mechanical Analysis:
It is based on Stokes’ law, according to which when the size of the soil particles is decreased, their time of settling below a constant of about 10 cm increases. So the time t secs of setting of particles larger than a given size say 0.05 mm which is the upper limit of the size of silt particle, is calculated from Stokes’ law.
The soil-water suspension is shaken and left undisturbed for t seconds, after which only silt clay is left at a 10 cm depth.
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A sample of a known volume of soil-water suspension (e.g. 25 ml) is withdrawn from this 10 cm depth of the soil water suspension with a pipette. In a similar way, only clay is present at a 10 cm depth after t1, second.
When another sample of known volume, i.e. 25 ml, is withdrawn from this depth. Silt + clay present in the first sample and only clay is present in the second sample. Therefore their difference gives the amount of silt present in the first sample
Pipette Method of Mechanical Analysis:
Take 10 gm. of soil in a beaker and add 20 ml of 30 per cent Hydrogen Peroxide to it; boil to destroy the soil organic matter.
Cool and add a little dilute hydro-chloric acid to destroy calcium carbonate present in the soil and to replace the divalent cations from the clay micelle by hydrogen ions as shown in the following equation:
Filter the soil and transfer it on the filter paper. Wash it with distilled water till it is free from chloride it when no white precipitate is obtained with silver nitrate solution. Than transfer the soil back to the original beaker, with distilled water and add a little dilute sodium hydroxide to de-flocculate the clay and stir it. Hydrogen ions are replaced from the clay micelle by sodium ions as shown in the following equation.
Transfer it to a tall 1000 ml cylinder and make up the volume to 1000 ml, shake and leave the cylinder undisturbed for the time t1 seconds, taken from the silt column of the mechanical analysis chart.
After the time t1 sec is over, withdraw 25 ml of soil-water suspension from the requisite depth, which is usually 10 cm, 22 cm or 28 cm depending on the temperature.
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Dry it in an oven and weigh it as silt + clay. Similarly, shake it and leave it undisturbed for time t2 taken from the clay column of the Mechanical Analysis chart. After this time is over, withdraw 25 ml of the soil water suspension oven dry it and weigh it as clay.
Calculation:
Suppose wt. of soil taken = 10 gms
Total vol. of soil water suspension = 1000 ml
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Vol. of soil water suspension withdrawn at 1st and 2nd pipetting = 25 ml.
25 ml or 1st pipetting sample contains 0.18gms of silt + clay
25 ml or 2nd pipetting sample contains 0.12 gms of clay
... 25 ml or the soil water suspension contains 0.18 – 0.12 = 0.06 gms of silt and 0.12 gms of clay
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... 1000 ml of the soil water suspension contains 0.06 x 1000/25 = 2.4 gms of silt and 0.12 x 1000/25 = 4.8 gms of clay
... 10 gms of the soil contains 2.4 gms of silt and 4.8 gms of clay
... 100 gms of the soil contains 24 gms of silt and 48 gms of clay
Pour away most of the supernatant liquid from the tall cylinder and transfer the soil particles to a beaker. Add water and stir, leave it undisturbed for time taken from the sand column of the mechanical analysis chart.
After this, slowly pour away the water. Repeat this process a number of times till it is absolutely clear. Then dry it in an oven and weigh as sand.
Suppose the weight of this sand is 2.8 gms
Then 10 gms of soil contains 28 gms. of sand.
Then the soil contains 28 per cent sand, 24 per cent silt and 48% clay.
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