We use the hydrometer method as described by Gavlack, Horneck and Miller (2005). The basic procedure is copied below; please review the reference for potential pretreatment steps and procedural comments. In general, this method is of lower precision than the pipette or sedimentation methods, has a detection limit of 2.0% sand, silt and clay (dry basis), and is generally reproducible to within ± 8% (Gavlack et al., 2005).

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 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 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 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 30 seconds and take a reading after 40 seconds and record to the nearest ±0.5 g L^-1 as “R sand”.
7. Remove the hydrometer carefully, rinse and wipe dry.
8. After 6 hours record temperature of the suspension to the nearest ±1 ^oC. 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.

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 %)

Reference and Citation of Protocol

Gavlak, R., D. Horneck, and R. Miller. 2005. Plant, soil and water reference methods for the Western Region. Western Regional Extension Publication (WREP) 125, WERA-103 Technical Committee, http://www.naptprogram.org/files/napt/western-states-method-manual-2005.pdf.