Particle Size Analysis for Soil Texture Determination (Hydrometer Method)

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In use from 2008-07-01

Abstract

This method quantitatively determines the physical proportions of three sizes of primary soil particles as determined by their settling rates in an aqueous solution using a hydrometer. Proportions are represented by stated class sizes: sand ranging from 2000 – 50 µm; silt ranging from 50 – 2.0 µm and clay < 2.0 µm and those stated by the USDA Soil Survey and Canadian Soil Survey Committee. Settling rates of primary particles are based on the principle of sedimentation as described by Stokes’Law and measured using a hydrometer. The use of the ASTM 152H-Type hydrometer is based on a standard temperature of 20 oC and a particle density of 2.65 g cm-3 and units are expressed as grams of soil per liter. For specific samples the method may require the pretreatment removal of soluble salts, organic matter, carbonate sand iron oxides with subsequent dispersion using sodium hexametaphosphate .Corrections for temperature and for solution viscosity is made by taking a hydrometer reading of a blank solution. Generally this method is of lower precision than the pipette or sedimentation methods and is used to determine soil texture. The method has a detection limit of 2.0% sand, silt and clay (dry basis) and is generally reproducible to within ± 8%.

Protocol

Equipment

  1. Analytical balance: 100 g capacity, resolution ± 0.01 g.
  2. Standard hydrometer, ASTM No. 1. 152H-Type with Bouyoucos scale in g L-1.
  3. Reciprocating horizontal mechanical shaker, capable of 180 oscillations per minute.
  4. Sedimentation cylinder with 1.0 L mark 36± 2 cm from the bottom.
  5. Shaker bottle 200 mL and cap (polypropylene or glass).

Reagents

  1. Deionized water, ASTM Type I grade.
  2. Amyl alcohol.
  3. Sodium Hexametaphosphate (HMP), 5% dispersing solution. Dissolve 50.0 g Na-hexametaphosphate in 1.0 L.

Procedure

  1. Weigh 40.0 ± 0.05 g of air-dried soil pulverized to pass 10 mesh sieve (<2.0mm) into 200mL container. Determine oven dry soil moisture on a 2nd sample of soil.
  2. Add 100 mL of HMP solution, cap and place on reciprocating horizontal shaker for sixteen (16) hours.
  3. Quantitatively transfer the suspension to the sedimentation cylinder and add deionized water to bring to 1.0 L final volume.
  4. Allow the suspension to equilibrate to room temperature for two (2) hours.
  5. Insert plunger and thoroughly mix contents, dislodging sediment from the bottom of the cylinder. Finish stirring with two or three smooth stokes. As an alternative mixing procedure stopper the cylinder and use end over end shaking for one (1) minute. Add 2 mL of amyl alcohol to the surface to suspensions covered in foam. Repeat the process and determine hydrometer reading on a blank solution and to the nearest ±0.5 g L-1 as “RC1”.
  6. Lower the hydrometer carefully into the suspension after thirty(30)second sand take a reading after forty (40) second sand record to the nearest ±0.5 g L-1 as “R sand”.
  7. Remove the hydrometer carefully, rinse and wipe dry.
  8. After six (6) hours record temperature of the suspension to the nearest ±1oC. Using the temperature correction values in Table 1 determine the settling time for the 2.0 µm size fraction. Based on time after initiation of settling, reinsert the hydrometer carefully and take a reading and record as “R clay” to the nearest ±0.5 g L-1. Repeat the process determining hydrometer reading on a blank solution and record as “RC2” to the nearest ±0.5 g L-1.

Table 1: The influence of suspension temperature on the hydrometer determination of soil clay (2 µm) based on a particle density of 2.65 g cm-3 and a solution density of 0.5 g L-1.




Temperature oC Settling time for clay (hours and minutes)
188:09
197:57
207:45
217:35
227:24
237:13
247:03
256:53
266:44
276:35
286:27

Calculations

Report results to the nearest 0.1% content:


  1. Sand % =( (oven dry soil mass) – ( Rsand – RC1 ))/ (oven dry soil mass) x 100

  2. Clay % = ( Rclay – RC2 ) / (oven dry soil mass) x 100

  3. Silt % = 100 – (Sand % + Clay %)

References

American Society for Testing and Materials. 1985 Standard test method for particle-size analysis of soils D 422-63 (1972). 1985
Annual Book of ASTM Standards 04.08:117-127. American Society for Testing Materials, Philadelphia.
Day, P.R. 1965. Particle fractionation and particle-size analysisp.545-567. In C.A. Black et al (ed.) Methods of soil analysis, Part 1.Agronomy 9:545-567.
Gee, G. W., and J. W. Bauder. 1979. Particle size analysis by hydrometer: a simplified method for routine textural analysis and a sensitivity test of measured parameters. Soil Sci Soc. Am. J. 43:1004-1007.
Gee, G. W., and J. W. Bauder. 1986. Particle-size Analysis. P. 383 – 411. In A.L. Page (ed.). Methods of soil analysis, Part1, Physical and mineralogical methods. Second Edition, Agronomy Monograph 9, American Society of Agronomy, Madison, WI.
U.S. Salinity Lab. Staff. 1954. Methods for soil characterization.p.83-147.Diagnosis and improvement of saline and alkali soils. Agr. Handbook 60, USDA, Washington, D.C.

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