Soil Texture

(Particle Size Analysis or Mechanical Analysis)

Introduction

Texture, or size distribution of mineral particles (or its associated pore volume), is one of the most important measures of a soil because finely divided soil particles have much greater surface area per unit mass or volume than do coarse particles. Thus, a small amount of fine clay and silt will be much more important in chemical reactions, release of nutrient elements, retention of soil moisture, etc., than a large volume of coarse gravel or sand.

 

Soil (mineral) particles are broadly segregated into three size classes (1) sand - individual particles visible with the naked eye, (2) silt - visible with a light-microscope, and (3) clay - some may not be visible with a light-microscope, especially the colloidal size (i.e., < 1 micrometer or 0.001 millimeter). These sand, silt and clay groups are commonly referred to as the soil separates; soil texture is defined as the relative proportions of each class. However, the specific diameter limits for each class may be different, depending upon the organization making the definition (see enclosed figure, Gee & Bauder 1986).

 

It is important to recognize two distinct but overlapping uses of the term "clay":

1.         The clay particle size class includes those particles smaller than 2 micrometers (in the USDA classification).

2.         The clay minerals include particles with mineralogy corresponding to one of the layer aluminosilicate groups (kaolinite, monmorillonite, etc.), and particles of various amorphous or semicrystalline hydrated oxides of iron and aluminum.

Most clay mineral particles are small enough to also fall in the clay particle size class; on the other hand, non-clay minerals (e.g., feldspar or quartz) often dissolve away before reaching such small sizes. The physical and chemical activity of clay in soil is related to both of these features: small particle size is related to high specific area (surface area per unit mass or volume of particles), while mineralogy results in high surface charge properties.

 

Soil content of fine particle sizes is so important in determining the fertility and water-supplying capacity as well as tillage characteristics of soils that it is used as one of the primary descriptive characteristics for classifying soil horizons and soil profiles. The U.S. Department of Agriculture soil textural classes are shown on the soil texture triangle (see the enclosed figures).

 

The major features of particle-size analysis are the destruction or dispersion of soil aggregates into discrete units by mechanical or chemical means, and then the separation of the soil particles by sieving or sedimentation methods (Gee & Bauder 1986). Chemical dispersion is accomplished by first removing cementing substances, such as organic matter and iron oxides, and then replacing calcium and magnesium ions (which tend to bind soil particles together into aggregates) with sodium ions (which surround each soil particle with a film of hydrated ions). The calcium and magnesium ions are removed from solution by complexing with oxalate or hexametaphosphate (Calgon) anions (Baver et al., 1972; Gee & Bauder 1986; Sheldrick & Wang 1993).

Methods

Commonly used methods to determine soil texture include:

Hand texture method

Qualitative, but with experience many people can easily discern the different textural classes. Kimmins (1987, p. 224-225) cites some useful criteria for field texturing (see enclosed Tables).

 

Separation by sieving

Mostly used for sand fractionation only (or between 0.05 and 2 mm diameter particles) using American Society for Testing & Materials (ASTM) sieve numbers between 270 or 300 and 10 (openings/inch), respectively). One limitation is that the probability of a particle passing through a sieve in a given time of shaking depends on the nature of the particle, the number of particles of that size, and the properties of the sieve (e.g., particle shape and sieve-opening shape affect probability of passage) (Gee & Bauder 1986). Sieving may be accomplished using either a wet (washing type) or dry method.

 

Separation by sedimentation

This type of analysis depends fundamentally upon Stokes' Law. One form of this equation is:

 

v = g(rs - rl)c2/(18h)

 

where v = particle velocity of fall,

g = acceleration due to gravity,

rs = particle density,

rl = liquid density,

c = particle diameter, and

h = fluid viscosity.

Basic assumptions used in applying Stokes' Law to sedimenting soil suspensions are:

 

1) Terminal velocity is attained as soon as settling begins,

2) Settling and resistance are entirely due to the viscosity of the fluid (hydrometer or pipet and the sedimentation-cylinder wall may also influence the settling rate),

3) Particles are smooth and spherical (clay particles especially may be platy),

4) There is no interaction between individual particles in the solution,

5) Ordinarily rs (particle density) is considered to be 2.65 or 2.60 Mg m-3 (equivalent to g cm-3), however it may vary between 2.0 to 3.2 Mg m-3, and

6) Temperature of the water should be constant throughout sedimentation.

 

The pipet method is often used as the standard to which other methods are compared. It depends upon the fact that sedimentation eliminates from the depth, h, in a time, t, all particles having settling velocities greater than h/t, while retaining at that depth the original concentration of particles having settling velocities less than h/t. The taking of a small volume element by a pipet at a depth h at time t furnishes a sample from which all particles coarser than c (particle diameter as determined by Stokes' equation) have been eliminated, and in which all particles finer than that size are present in the same amount as initially. The volume element at depth h has, in effect, been "screened" by sedimentation, so that the ratio of the weight, w, of particles present in that volume at time t, divided by the weight of particles present in it initially, w0, is equal to P/100, where P is the percentage of particles, by weight, smaller than c. Now, the ratio, w/w0, can also be written as the concentration ratio, c/c0, giving c/c0 = P/100. This equation connects the concentration, c, of the pipet sample, in grams per liter, to the parameter P of the particle-size distribution, c0 being the weight of solids in the entire sample divided by the volume of the suspension. (Sheldrick & Wang 1993; Gee & Bauder 1986).

 

The Bouyoucos hydrometer method is somewhat less accurate than the pipet method, but is easier to perform. The theory of the hydrometer method is similar to that of the pipet method except for the manner of determining the concentration of solids in suspension. Letting r represent the suspension density, rl the density of liquid, and rs the particle density, all in grams per liter, we have the equation, r = rl + (c/1000)(1 - rl/rs). Although the buoyant force on a hydrometer is determined directly by the suspension density, r, hydrometer scales can be calibrated in terms of c for particular values of rl and rs. The large size of hydrometer bulb necessary to give adequate sensitivity reduces the depth discrimination of the instrument, but this limitation can be overcome by a simple correction (Day 1965).

 

Depending on the degree of accuracy of separation required, and the particle sizes of interest, the hydrometer method is well adapted for fast determinations of general categories of sizes present and is used in our analyses.

 

References:

Baver, LD, WH Gardner, and WR Gardner. 1972. Soil Physics. 4th ed. John Wiley & Sons, Inc. New York.

Day, PR. 1965. Particle fractionation and particle-size analysis. p. 545-567. In CA Black et al (ed.) Methods of soil analysis, Part I. Agronomy 9:545-567.

Gee, GW and JW Bauder. 1986. Particle-size analysis. p. 383-411. In A Klute (ed.) Methods of Soil Analysis, Part 1. Physical and Mineralogical Methods. Agronomy Monograph No. 9 (2ed). American Society of Agronomy/Soil Science Society of America, Madison, WI.

Kimmins, JP. 1987. Forest Ecology. Macmillan Publishing Co, New York.

Sheldrick BH and C Wang. 1993. Particle Size Distribution. p. 499-511. In MR Carter (ed.) Soil Sampling and Methods of Analysis. Canadian Society of Soil Science. Lewis Publishers. Ann Arbor.


 

Procedure: Soil Texture by the Bouyoucos Hydrometer Method

 

See notes at end for optional pre-treatments to remove organic matter and iron oxide coatings.

Purpose

To measure soil texture by the hydrometer method.

Materials

1.         Sieved soil (50 g dry wt equivalent if fine-textured, 100 g if sandy).

2.         Electric mixer and cup.

3.         Sedimentation cylinder (1000 mL).

4.         Bouyoucos hydrometer.

5.         Thermometer (-20 - 110C).

Reagents

1.         Sodium hexametaphosphate, 1N.

Procedure

 

NOTE: If soil is not oven dried, take a subsample for water content determination.

 

1.         Place 50 - 100 g of soil (dry weight equivalent) into a soil dispersing cup. Record the weight to at least 0.1g.

2.         Fill cup to within two inches of the top with tap water. If local tap water is hard, use distilled water. Water should be at room temperature, not directly out of tap.

3.         Add 5 ml of 1N sodium hexametaphosphate.

4.         Allow to slake (soak) for 15 minutes (high-clay soils only).

5.         Attach cup to mixer; mix 5 minutes for sandy soils, 15 minutes for fine-textured soils.

6.         Transfer suspension to sedimentation cylinder; use tap water from squirt bottle to get all of sample from mixing cup.

7.         Fill cylinder to 1000-mL mark with tap water.

8.         CAREFULLY mix suspension with plunger. After removing plunger, begin timing. Carefully place hydrometer into suspension; note reading at 40 seconds. This 40-second reading should be repeated several times to improve accuracy. Because the suspension is opaque, read the hydrometer at the top of the meniscus rather than at the bottom.

9.         After final 40-second reading, remove hydrometer, carefully lower a thermometer into the suspension and record the temperature (C). Mixing raises temperature by 3-5C, so it is important to record the temperature for both hydrometer readings (40 sec and 2 hr).

10.     Mix suspension again and begin timing for the two-hour reading. Be sure that the cylinder is back from the edge of the counter and in a location where it wont be disturbed.

11.     Make up a blank cylinder with water and sodium hexametaphosphate. Record the blank hydrometer reading. If the reading is above 0 (zero) on the hydrometer scale (in other words, if the zero mark is below the surface), record the blank correction as a negative number. Read at the top of the meniscus as before.

12.     Take a hydrometer reading at 2 hours, followed by a temperature reading.

Calculations

1. Temperature correction factor, T (may be different for each reading):

 

T = (Observed temperature - 20C) * 0.3

 

2. Corrected 40-second reading:

 

40-sec(c) = 40-sec - Blank + T

 

3. Corrected 2-hour reading:

 

2-hr(c) = 2-hr - Blank + T

 

 

4.

 

 

5.

 

 

6. % Silt (0.05 - 0.002 mm) = 100% - (% sand + % clay)

 

Determine your sample's textural class from the textural triangle.

 

 

 

Optional pre-treatment for soil high in organic matter

Materials

1.         400-ml beaker .

Reagents

1.         Hydrogen peroxide, 30%.

2.         Hydrochloric acid, 1N.

Procedure

1.         Transfer sufficient soil to yield 100 g dry wt, to a 400-ml glass beaker. Add distilled water to give a 1:1 or 1:2 soil-water ratio.

2.         If necessary, make the suspension acid to litmus paper with a few drops of 1N HCl. Oxidation with H2O2 requires an acid medium.

3.         Add hydrogen peroxide carefully, a few milliliters at a time, allowing effervescence to subside before adding more, until no more frothing occurs.

4.         Place beaker on a hot plate and heat suspension to 65 to 70C. When reaction slows, add peroxide slowly as above. Evaporate excess liquid between additions of H2O2 to maintain a 1:1 or 1:2 soil-water ratio.

5.         Transfer soil to mixing cup, using squirt bottle to get it all out of beaker.

 

 

Optional pre-treatment for soil high in free iron oxides:

 

See Kunze 1965, pp.574-576.

 

 

References

Anderson, J.M., and J.S. Ingram, eds. 1993. Tropical Soil Biology and Fertility: A Handbook of Methods. 2nd ed. CAB International, Wallingford, UK. P. 93-94.

Day, P.R. 1965. Particle fractionation and particle-size analysis. Chap. 43 in Methods of Soil Analysis, Part 1. C.A. Black, ed. American Society of Agronomy, Madison. Pp. 545-567.

Gee, GW and JW Bauder. 1986. Particle-size analysis. p. 383-411. In A Klute (ed.) Methods of Soil Analysis, Part 1. Physical and Mineralogical Methods. Agronomy Monograph No. 9 (2ed). American Society of Agronomy/Soil Science Society of America, Madison, WI.

Kunze, G.W. 1965. Pretreatment for mineralogical analysis. Chap. 44 in Methods of Soil Analysis, Part 1. C.A. Black, ed. American Society of Agronomy, Madison. Pp. 568-577.

Wilde, S.A., R.B. Corey, J.G. Iyer, and G.K. Voigt. 1979. Soil and Plant Analysis for Tree Culture. Oxford and IBH Publishing Co., New Delhi. Pp. 12-13.