EQUIPMENT

  1. Bouyoucos hydrometer
  2. Thermometer
  3. Humbolt mixer and dispersion cup
  4. Stop watch and clock
  5. Glass graduated cylinders capable of containing 1200 mL of solution
  6. 250 mL beakers

REAGENT

  1. 5% Calgon solution:
    1. Dissolve 50 g of Calgon in 1 liter of deionized water. (Calgon is a commercially available water softener.)

PROCEDURE

  1. Weigh 50 g of dry screened soil (use 100 g for sandy soils) into a 250 mL beaker. Add 100 mL 5% Calgon solution. Stir and let stand overnight.
  2. Transfer to a dispersion cup. Add 2 to 3 drops of oil. Place the dispersion cup on the Humbolt mixer and mix for 2 minutes.
  3. Transfer to a graduated cylinder and bring to the 1000 mL mark with deionized water. For sandy soils, dilute to 1120 mL.
  4. Stir thoroughly using the weighted disc shaped bar. Alternatively, invert the cylinder 10 times. Immediately place the hydrometer and the thermometer into the slurry. Record the hydrometer reading and the temperature 40 seconds after the last mixing stroke or inversion.
  5. Let stand for 2 hours, and again record the hydrometer reading and the temperature.
  6. Analyze a blank in the same manner using 100 mL 5% Calgon solution diluted to 980 mL. For sandy soils the blank should consist of 100 mL 5% Calgon solution diluted to 1080 mL.

COMPUTER CALCULATIONS

  1. Give the 40 second and 2 hour hydrometer and temperature readings to the data processing group, being sure to include blank values.
    1. These values will be entered into the computer and the % sand, % silt, and % clay, and texture classification will be printed out.

MANUAL CALCULATIONS

A = Sample hydrometer reading + temperature correction*

B = Blank hydrometer reading + temperature correction*

* Temperature correction:
Add 0.2 units to the hydrometer reading for each degree above 67 °F. Subtract 0.2 units to the hydrometer reading for each degree under 67 °F.

  1. 40 Second Reading
    1. %Silt + %Clay = ((A − B) ÷ wt. soil) × 100
    2. %Sand = 100 − %(Silt + Clay)
  2. 2 Hour Reading
    1. %Clay = ((A − B) ÷ wt soil) × 100
  3. Determination of Silt
    1. %Silt = 100 − %Sand − %Clay

QUALITY CONTROL

  1. Prepare one check soil and one duplicate soil with each set of samples.
    1. Check Soil – Values for sand, silt and clay must lie within the limits established for this sample.
    2. Duplicate Sample – Values for sand, silt, and clay must be within 20% of the average value.

REFERENCES

  1. Bouyoucos, G. J. 1936. Directions for Making Mechanical Analysis of Soils by the Hydrometer Method. Soil Sci. 42(3).
  2. Day, P. R. 1965. Particle Fractionation and Particle-Size Analysis. In C. A. Black (ed) Methods of Soil Analysis. Part I. Soil Sci. Soc. Amer.