Donald G. McGahan, Ph.D.
The primary reference for these methods is the:
Soil Survey Staff. 2004. Soil Survey Laboratory Methods Manual. Rebecca Burt ed. USDA-NRCS Gov’t Printing Office Washington D.C.
Other references of note are listed.
The procedure for analysis of particles with
An alternative order to the methods described is to calculate the time necessary for the silt to settle and, after that interval to, decant the clay suspension from off of the sand and silt. The sand and silt can be repeatedly resuspended in a dispersant and after the sand has settled the silt decanted.Once the sand is separated from clay and silt, the sand can be dried, weighted, and the mass of the sand fractions determined.
From the fine earth fraction (< 2mm size) sample.
- Tare 250 ml (or 500 ml for sandy loams, loamy sands, and sands) centrifuge bottle and label.
- Weigh air dry soil (AD) into labeled centrifuge bottle.
- Alternate 1: To achieve a specific endpoint of clay (5 g this example) for mineralogical analysis of the clay, first estimate the clay content by the texture-by-feel method. Weigh enough soil fine earth fraction material to yield about 5 g of clay once fractionated and transfer to a 250 ml centrifuge bottle if the air dried soil is less than about 40 g, or transfer to a 500 ml centrifuge bottle if the air dried soil is greater than about 40 g. Using removable tape add a label to the bottle with the sample ID. Record the weight of the soil. This is the Air Dry Particle Size (AD-PS) weight. When replicates are preformed the suggested labeling is prefixed with the laboratory assigned sample number and AD-PS-1, AD-PS-2, etc. (365-AD-PS-1, 365-AD-PS-2, etc.)
- Alternate 2: For routine analysis. Weigh fine earth fraction soil material (10 g for clays, 20 g for loams, 40 g for sandy loams and loamy sands, and 80 g for sands) and transfer to a 250 ml (or 500 ml for sandy loams, loamy sands, and sands) centrifuge bottle labeled with sample ID. Record the weight of the soil. (e.g. 365-AD-PS-1)
Determine the AD moisture content.
- Tier a labeled weighing dish and record weight to nearest 0.01 g (the use of a balance recording to 0.001 g, or 0.0001 g, can be substituted for research grade work). (e.g. 365-tin204-AD2OD-1 where tin204 is a weighing tin numbered 204. This can also be accomplished by creating another data column for the tin number and recording its weight.)
- Add ~ 5 g of AD soil to dish, weigh to at least the nearest 0.01 g and record. (e.g. 365-AD-AD2OD-1) (again, the use of a balance recording to 0.001 g, or 0.0001 g, can be substituted for research grade work)
- Place heat at 105ºC for 15 h.
- Remove from oven and place in desiccator until ambient temperature is achieved and weigh the oven dried (OD) soil and tin to the at least the nearest 0.01 g and record. (e.g. 365-OD-AD2OD-1)
If no pretreatment are to be preformed proceed to step 5.
Removal of organics: choose Method 1 or Method 2
If the soil is a surface horizon and/or has an appreciable amount of organic matter; digesting the organic matter
Organic removal Method 1 [Read Anderson (1963) for background]
- Add 50 ml Clorox (pH 9.5), mix, and place in water bath or on hot plate for 15 min. Centrifuge and discard clear supernatant. Repeat twice (total of three treatments).
- Clorox can be adjusted to pH 9.5 by adding HCl.
- Add 25 ml water mix, centrifuge at 2400 RPM for 15 mm and decant supernatant into a clear small beaker. If the supernatant is clear, discard it, and repeat the washing with 25-mL water. If the supernatant is cloudy (suspended clay), return it to the centrifuge bottle washing any residue with a squirt bottle, and proceed to fractionation.
Organic removal Method 2 (Gee and Bauder, 1986) (Soil Survey Staff, 2004 Procedure 3A1a1 7.6 & 7.8)
- Add 50 mL of distilled water and 5 mL of 30% H2O2 to the soil sample at ambient temperature.
- Cover the soil sample with a 50 mm watch glass.
- Allow initial oxidation of organic matter to complete and then place sample on hot plate.
- If the froth from the reaction exceeds the capacity of the bleaker, transfer the sample to a larger beaker.
- Place the sample on a hot plate and heat to 90°C.
- Add 5 ml increments of 30% H2O2 at 30 min intervals until the oxidation has completed or until 30 ml of 30% H2O2 have been added.
- Heat the sample for an additional 45 min to decompose excess H2O2. If the reaction is violent, add small increments of ethyl alcohol to the sample or remove the sample from the hot plate to slow the reaction or transfer sample to a 1000-mL beaker.
- Record any unusual sample reactions.
Optional Carbonate removal is not necessary if absolutely no effervesce is detected.
To determine if carbonates are present. Place a sufficient amount of soil matrix material into a spot-plate depression, add 1 to 2 drops of 1 M HCl, observe the initial reaction, and wait about 2 minutes before assessing the final extent of effervescence and assigning the appropriate effervescence class. Table 1 can be used to assign the effervescence class.
|Noneffervescent||No bubbles detected|
|Very slightly effervescent||Few bubbles seen|
|Slightly effervescent||Bubbles readily seen|
|Strongly effervescent||Bubbles form low foam|
|Violently effervescent||Thick foam forms quickly|
Removal of carbonates (Gee and Bauder, 1986) (Soil Survey Staff, 2004 Procedure 3A1a2).
Carbonate Removal: Carbonates are destroyed with a 1 N NaOAc solution buffered to pH 5. The NaOAc solution is added to sample until carbonate bubbles no longer evolve. The NaOAc solution is then washed from the sample. After destruction of carbonates.
- Add 50 ml pH 5 1M NaOAc. (to prepare 1 N sodium acetate (NaOAc) solution, buffered to pH 5: Dissolve 680 g of NaOAc in 4 L of RO water. Add ≈ 250 mL of acetic acid. Make to 5-L volume with RO water.)
- Screw on lid, shake to mix, remove lid, and place in 80°C water bath or on an 80ºC hot plate for 30 mm. Check for fizzing as an indication of solution of carbonates. Wash down inside of bottle with a small amount of water with a jet from a squeeze bottle.
- Centrifuge at 2000 RPM for 10 minutes, decant and discard the clear supernatant.
- If the soil fizzed, repeat the treatment with NaOAc (heating, centrifuge, decant) until carbonates are removed. If the soil did not fizz, the single NaOAc treatment serves to Na-saturate the sample.
- Add 15 ml water, disperse, centrifuge and discard clear supernatant.
- Add 25-mL of sodium hexametaphosphate solution to each sample. Add distilled water to bring volume to about 175-mL (2.5 cm head space in centrifuge bottle).
- Recipe for sodium hexametaphosphate solution. Dissolve 35.7 g of (NaPO3)6 and 7.94 g of Na2CO3 in 1 L of RO water.
- Add 25-mL of sodium hexametaphosphate solution to a centrifuge bottle and bring to volume with DI water. This is the blank. An aliquot of the blank must be sampled in the same manor as the clay and dispersant. The weight of the dispersant in the blank is deducted from the clay and dispersant sample weights.
- Place the sample in a horizontal shaker set at 120 oscillations min-1 and shake for 15 h (overnight).
It is best to separate the sand before taking a clay suspension alliquote when the clay fraction is to be “collected for further analysis.” Otherwise, the sand may be wet sieved, oven dried and weighed, from the silt and clay after the clay aliquot is removed. When collection of the clay and silt is not required, simply move steps 6 through 8 and 10 through 12 after step 17 and omit the silt separation from the clay portion below.
- Remove the sample from the shaker and pour through a Tyler no. 270 [300-mesh] sieve mounted on a ring stand.
- Place a funnel below the sieve and a 1-L cylinder below the funnel. Collect the silt and clay in the 1-L cylinder. Avoid using jets of water in washing the sample. Wash and rub all particles from the centrifuge bottle into the sieve. Continue to wash until the suspension volume in the cylinder is ~ 800-mL. Sand and some of the coarse silt remain on the sieve. Rinse all
- Fill the cylinder to 1 L and cover with a 65-mm watch glass.
- Prepare a distilled water and dispersant blank to measure temperature fluctuations and contributions of the dispersant salts. Allow the cylinder to stand overnight to equilibrate the suspension with the room temperature.
- Weigh and mark a sand evaporation dish (a 400 ml beaker works well) and record the weight.
- Wash the sand into the evaporation dish and dry the sand at 105°C overnight.
- Weigh the oven dried sand and beaker and record the weight.
To fractionate the sand
- Transfer the dried sand to a nest of sieves that has a top-to-bottom order of 1.0, 0.5, 0.25, 0.1, and 0.047 mm. A pan catches any silt and the silt is added to the silt determined [very coarse sand (vcs) = 2 to 1 mm; coarse sand (cs) = 1 to 0.5 mm, medium sand (ms) = 0.5 to 0.25 mm; fine sand (fs) = 0.25 to 0.10 mm; very fine sand (fvs) = 0.1 to 0.05 mm]
- Shake the sand for 3 min on a shaker that has 1.3-cm vertical and lateral movements and oscillates at 500 strokes min-1. Record the weight of each separate sand fraction to at least the nearest mg.
- Place sand fractions in labeled storage.
- Record temperature of blank.
- Determine the sampling time for a 5 cm or 10 cm sampling depth based on the temperature.
- Place the cylinder on a stable, vibrationless table and stir with a hand stirrer in an up-and-down motion for 30 s. Timing is started upon completion of the stirring. Record the time that stirring is stopped.
- At the time specified by the settling table (Table 1 Pipett settling times) according to the temperature of settling solution (and average of start and measuring temperature may be used if temperature has fluctuated) slowly lower the closed pipet to a 5 or 10-cm depth in the suspension and withdraw an aliquot (at a rate of about12 seconds per 25ml) at the calculated time (Table 1 Pipett settling times). Regulate the vacuum such that the pipet fills in ~12 s.
- Dry the aliquots at 105°C overnight and cool in a desiccator. Record the weight of the residue to the nearest 0.1 mg. Blank weight is subtracted from residue weight to determine net residue weight. This is 1/40 of the clay in the cylinder. Silt is calculated by difference.
If fine clay analyses are requested, use the remaining suspension and follow procedure 3A1b (Soil Survey Staff, 2004) (Silt separation from clay). Save the sediments for optical mineralogy.
Silt separation from clay
- Stir the silt and clay suspension with mechanical stirrer for 6 min or for 8 min, if the suspension has stood for >24 h.
- Decant the suspension and transfer the sediment to a 250mL to 400-mL beaker. Fill the beaker to a 5-cm (or any known) height. Stir the sediment and allow settling time for 2 um particles to settle from top to bottom of column. See the attached pipett sampling times chart to adjust for temperature. Decant the supernatant into 1L beaker (label this beaker as clay and sample #) (for safety sake decant supernatant of clay suspension to an empty clean beaker then transfer to the 1L beaker of Clay). Refill the 250-mL to 400-mL beaker to 5-cm height. Stir again, allow to settle again, and then decant again into 1L beaker (If 1L beaker becomes full label another). Repeat the filling and the stirring; allow to settle; and then decanting until top half of suspension is clear. Transfer the sediment, which is dominantly 20 to 50 um, to a labeled drying dish. Wash with ethanol, air-dry, and save in the drying dish for optical mineralogy.
- To the 1L decanted clay suspensions add 1-2 tablespoons of NaCl stir and allow flocculation overnight.
- Siphon off the clear supernatant from the 1-L beakers containing flocculated clay suspensions. Wash flocculated clay into storage containers for mineralogical analysis.
Anderson, J.U. 1963. Clays and Clay Minerals 10:380-388.
Gee, G.W. and Bauder, J.W. 1986. Particle size analysis. pp. 383-411 In: A. Klute (ed), Methods of soil analysis: Part 1 Physical and mineralogical methods. ASA, Madison, WI.
Kilmer, V.J., and L.T. Alexander. 1949. Methods of making mechanical analyses of soils. Soil Sci. 68:15-24.
Soil Survey Staff. 2004. Soil survey laboratory methods manual – soil survey investigations report No. 42 Ver. 4.0. Available URL: https://nrcspad.sc.egov.usda.gov/DistributionCenter/pdf.aspx?productID=886 [Accessed 10 March 2016]
|Pipette Sampling Times (hr:min)|
|TEMP||10 cm Sampling Data||5 cm Sampling Depth|
|°C||50 µ||2 µ||1 µ||50 µ||2 µ||1 µ|