Particle Size Distribution


Hydrogen peroxide, 30% (H2O2)
Sodium hexametaphosphate [(NaPO3)13.Na2O]
d make to volume (50% solution)

Sample Preparation and Procedure
For samples containing >1% organic carbon (surface soils and Bh horizons), remove organic carbon by oxidation with 30% H2O2. Weigh 100.0g soil into beakers, add approximately 200mL H20, 25mL H2O2 and place on the hot plate with low heat. It may be best to let them sit overnight to prevent foaming over. This is before putting them on the hot plate. Make subsequent additions of H2O2 as necessary to remove excess H2O2 and proceed as follows. (Note: Be sure to correct calculations based on %O.C.)

Weigh 50g (>30%clay) or 100g (<30% clay) into 400mL beakers, add 100mL sodium hexametaphosphate, let stand overnight. Do same for oxidized samples. Carefully transfer samples to mixer cup and mix for 3 minutes. Then transfer sample carefully from cup to cylinder making sure sample is completely transferred; make to 1000mL with deionized water, stopper and shake, and place into water bath. Include a blank for each run.

Important: Note time and water temperature. These elements are necessary to determine at what time and depth aliquots are to be taken. See table.

Using 25mL automatic pipette, draw off aliquot at required time and depth and carefully dispense into numbered and weighed aluminum weighing dishes. Place these samples in drying oven overnight. Remove cylinders from water bath, decant silt and clay with repeated washing until only sand remains. Wash sands into 400mL beakers, decant water, and place in drying oven overnight.

Remove the weighing dishes from oven and weigh. This is a quantitative measure of the clay fraction. Remove sands from the oven, completely transfer to a nest of sieves, and shake on Ro-tap for 3 minutes. Weigh sand separates. This is a quantitative measure of the total sand fraction. The silt fraction is obtained by difference.

Percent sand = [(sand wt.)*100] divided by (sample wt.)
Percent clay = (boat wt. + clay wt + Na3PO4 – boat wt – blank wt)(40))(100) divided by (sample wt)
Percent silt = 100% - %clay - %sand

Footnotes to Particle Size Differential Analysis Procedure
1. Samples should be weighed to 50.0g +/- 0.1g. If a sample is over 50% clay a sample weight of 25.0 +/- 0.1g may work better.
2. Aluminum weighing boats should be weighed to three decimal places.
3. Experimental laboratory error should be approximately 5%. If results fall outside of this range, the sample should probably be run again.
4. Field data can vary as much as 20% and still give meaningful information.

Soil Survey Laboratory Methods Manual. August 1992.  p.14.  U.S. Department of Agriculture.