SOIL TEXTURE, STRUCTURE, AND BULK DENSITY

Knowledge of soil texture and density is essential in field sampling. Texture can be described using either soil science or engineering descriptions. Density is described as the mass per unit of dry soil, Mg/m³ or g/cc, and is universally the same.

2.4.1. Texture

The two different methods of describing soil texture depend on whether the soil is to be used for agriculture or engineering. In agriculture it is used for growing plants, and in engineering as a medium to support structures. Soil scientists are most interested in the relative amounts of sand, silt, and clay, as well as the biological characteristics of soil.

Engineers are interested in the characteristics of larger stone and gravel, sand, and fine particles. Each of these components is called a soil separate and is defined as having a particular size. (See Table 2.1.) The soil in a field to be sampled may be described by either of these approaches, so it is important to be familiar with both.

2.4.1.1. Texture—Soil Scientist Definition

Soil scientists name soil textures using the percentage of sand, silt, and clay present. Sand is between 2.00 and 0.02 mm in diameter, silt between 0.02 and 0.0002 mm in diameter, and clay less than 0.002 mm in diameter. Using the percentages of sand, silt, and clay and a textural triangle (see Figure 2.5), a

TABLE 2.1 Size Characteristics of Soil Separates Using Soil Science and Engineering Definitions

Soil component

Subdivision Engineering designation

Soil science designation^a Cobbles/gravel 75/4.75

Coarse 4.75–2.00

Sand Medium 2.00–0.425 2.00–0.02

Small 0.425–0.075

Silt >0.075 0.02–0.002

Clay >0.002

aThis is the international definition—the U.S. Department of Agriculture definition is somewhat different.

Note: (All units are in mm.) Source: Data from Refs. 15 and 18.

FIGURE 2.5 Textural triangle used by soil scientists to assign textural names to soils of different textures.

textural name of a soil can be determined. A soil containing 10% sand, 15% clay, and 75% silt would have a silt loam texture. The term loam is used for textures for which the sand, silt, and clay fractions contribute equally to the characteristics of the soil. The percentage of clay is lower than silt or sand in loam because it has much higher activity and so it takes less to have an equal effect. Table 2.1 gives the relationship between the soil scientist’s and the engineer’s definition of soil particle size. If appropriate, a soil’s textural name may be prefixed by an indication of larger components. Thus, there might be a gravely silt loam. This would indicate that the soil has a large percentage of gravel-sized material in it. For the soil scientist the particles larger than 2.00 mm in diameter are not as important as the sand, silt, and clay.

In addition to the above names there are other common, nonscientific, less specific terms in common usage, such as light, heavy, clayey, and sandy. Heavy soils are high in

clay and light soils high in sand. In regions of the world with a predominance of sandy soils, however, a soil with 1–2% clay might be called clayey. In other areas a soil with 20–30% clay might be called clayey. A similar sort of thing happens with sand. In areas with clayey soils a significant sand content might be called sandy. These common local usages need to be checked by referring to laboratory determinations of texture [17].

2.4.1.2. Texture—Engineering Definition

Engineers classify soil textures using a different classification scheme. They approach the texture of the loose material on the Earth’s surface from a broader perspective than does the soil scientist, being concerned with a much larger range of sizes of separates. From an engineering perspective the ability of a material to carry a load is most important. Also from a load-carrying perspective, the uniformity of the material in terms of size is important.

Engineers size soil components by noting the size of the sieve retaining the soil component. The size can then be reported as the sieve number, which is sometimes confusing because as the sieve number increases the size of the holes in the sieve decreases. Table 2.2 gives a number of important sieve numbers and corresponding hole sizes. Material not passing a #4 sieve is cobbles, which are larger than about 75 mm in diameter. On the other hand, gravel is smaller, but is larger than 4.75 mm. Sand is designated as coarse (smaller than 4.75, but larger than 2.00 mm), medium (smaller than 2.00 but larger than 0.425 mm), and fine (between 0.425 and

0.075 mm in diameter). Material smaller than 0.075 mm is silt or clay. (See Table 2.2.) Engineers use a combination of capital letters to indicate the texture of the material one is working with. The letters and what they signify are given in Table 2.3. Combinations of letters are used in pairs to indicate a particular type of material. For instance, GW would be well-graded gravels with no fines (silt or clay). On the other hand, CL would be clay with low or slight plasticity. Another possibility would be OH, which is organic matter with high plasticity. In this way a wide variety of materials with varying size components and plastic or organic matter contents can be described.

TABLE 2.2 Sieve Numbers and Hole Sizes

Sieve number Hole size (mm)^a

#4^b 4.75

#10^c 2.00

#40 0.425

#200 0.075

aActually, the average diameter of the particles of gravel, sand, or silt passing through it.

bThere are larger hole sieves that do not have the # designation.

cThis is the upper limit for sand in the soil science textural designations.

As can be seen in Table 2.3, plasticity is an important characteristic of soils from an engineering perspective. A plasticity index (PI) can be calculated for any soil or similar material. It is the difference between the plastic limit (PL) and the liquid limit (LL).

When water is added to an air dry soil, individual soil peds are moistened first. At a certain water content the soil becomes plastic (PL), which means that it can be molded into a shape and will retain it. If more water is added it becomes liquid and runs; this is the LL. The difference between these two limits is the PI, which is simply calculated using the formula below.

Soils with a high PI typically contain large amounts of clay. This gives an indication of the water movement and the retention of contaminants spilled on soil.

There are several other terms or physical characteristics of soils and soillike media that engineers use. One is the Attenburgh limits, which refer to the PL and LL of a particular medium. There is also COLE, which is the coefficient of linear extensibility. As the name implies, this is a measure of the amount of swelling and shrinking a material undergoes between wetting and drying. It indicates the amount and type of clay in a soil. In working with engineers one is likely to hear these terms used frequently.

Engineers will also talk of the A-line. This is a graph that relates the PI to the LL. Soils with characteristics above the A-line and with a LL greater than 50 are inorganic clays of high plasticity. In the same region but below the A-line are fine sands or silts and elastic

PI=LL−PL

TABLE 2.3 Engineering Classification of Soils

Symbol Meaning

GW Gravel (G) larger than #4 sieve—well (W) sorted GP Gravel poorly (P) sorted

GM Gravel containing silt (M) GC Gravel containing clay (C)

SW Sand (S) smaller than #4 but larger than #200 sieve—well sorted SP Sand poorly sorted

SM Sand containing silt SC Sand containing clay

ML Inorganic silts with low plasticity (L) CL Inorganic clay with low plasticity OL Organic matter with low plasticity MH Inorganic silts of high plasticity (H) CH Inorganic clay of high plasticity OH Organic matter of high plasticity Pt High organic matter soils Source: Taken from Ref. 18.

silts. Below the A-line and less than a 50 LL are inorganic silts and very fine sands with low plasticity. In the same region but above the A-line are inorganic clays of low to medium plasticity. (See Figure 2.6.)

When developing a sampling plan the soils in the area may be described by either soil science or engineering terminology. It is important to be able to understand both types of descriptions and use these to determine how sampling should be done. In most cases both descriptions will be valuable. In situations in which gravel is prevalent the engineering

FIGURE 2.6 The A-line used by engineers to describe fine soil fractions. Data taken from Ref. 14.

descriptions will be very useful in deciding on the type of sampler to be used [18].

2.4.2. Structure in Soil

Figure 2.4 shows the components in an idealized, well-developed soil profile. All of the loose material—including water-saturated layers—above rock is called the regolith. The material above the saturated zone is called the vadose zone. The material in which active soil development is taking place is called the solum. The characteristics of these areas is determined by both texture and structure.

The texture of soil is extremely important because it controls important characteristics related to the movement of water and the sorption and retention of contaminants. Texture is not, however, the only factor in movement in the soil. Soil particles, sand, silt, clay, and organic matter do not act independently of each other, but are cemented together to form secondary particles called peds, which are the soil structure. The cementing agents are organic matter, clay, microbial gums, and various cations.

Different sizes and shapes of peds are found in different soil horizons. The different kinds of peds and their typical location in a soil profile are given in Figure 2.4. Note that platy structure or peds can be found in any horizon, but are commonly found between the

A and underlying B horizons. Good water-stable peds result in a soil having increased percolation and aeration rates, making it easer to work with and easier to sample and remediate [19].

The increased rates of percolation and aeration are a result of increased porosity in the soil. Pores occur both within peds and within the lines of weakness between peds. Some pores are large and drain readily, while other smaller pores retain water, which is used by plants. Still smaller pores remain filled with water even under the driest soil conditions.

More information about pores and their effect on water movement is given in Chapter 7.

2.4.3. Bulk Density

Both soil texture and structure are related to a soil’s bulk density. Because bulk soil is composed of both solid and void space or pores it has a variable density, which is specifically called its bulk density. Bulk density is the dry mass of oven-dried soil divided by its volume. Typically the density is obtained by inserting a ring of known volume into the soil using an instrument that does not cause compaction of the sample being taken. The ring is removed and the soil in it leveled. The ring and the soil are placed in a oven at 105° for 24 hr.

The ring plus soil is weighed again, and the weight of the ring is subtracted from the total to give the soil weight. (Equations 2A and 2B in Figure 2.7 are used for these calculations.) At this point one need only divide the mass by the volume of the ring to obtain the bulk density. (Equation 2C is used for this calculation.) Bulk density is usually obtained as grams per cubic centimeter. Soil bulk density is most often reported as Mg/m³, however.

Typically soil bulk densities range from 1 to ~1.7 Mg/m³. Sandy soils generally have higher and silty and clayey soils lower bulk densities. This is variable, however, depending on the structure and compaction of the sand, silt, and clay. Subsoils have higher bulk densities than surface soils, partially because of lower organic matter content and pressure from overlying soil. Examples of common bulk densities, their associated void space, and associated soil types and conditions are given in Table 2.4.

The void volume in soil is determined using the average particle density of individual soil particles. On the basis of a great many measurements, soil scientists take the particle density of soil to be 2.65 Mg/m³. The amount of solids in a soil sample is calculated by dividing this into the bulk density. Subtracting this from 1 gives the amount of void space. Multiplying these by 100 gives the percentage of each. Equations for these calculations are given in Figure 2.7, Equations 2D and 2E.

Knowing the bulk density of soil in a field we can make many useful and important calculations. Calculations of the kilograms or tons of soil that must be removed or remediated can be determined, as can the volume of soil. Such calculations will also allow mass balance calculations, which allow accounting for all the contaminant present.

In field sampling

FIGURE 2.7 Equations for calculating a soil’s percentage of solids and void space. In Equation 2A r is the radius of the sampling ring and h is the height.

situations one may find layers of soil with different bulk densities. The density of the layers must be known if one wishes to obtain comparable samples (on a weight basis). To obtain the same amount of soil solid and the contaminant it contains, it is essential to take different volumes of soils, so that the mass of solid obtained is equivalent. This would mean that the volume of sample would need to be different for the different layers. This type of calculation is illustrated below.

TABLE 2.4 Common Soil Bulk Densities

Bulk density (Mg/m³)

Void volume (%)

Soil type Condition

1 or less 62 Indicates organic soil or soil with high organic matter contact

Light in weight subject to wind and water erosion 1.25 52 Common average bulk density for

agricultural soils

Good structure

1–1.6 62–39 Normal range for soils Good to poor ped structure 1.5 and above 43 Usually clayey soils Compacted limited water

movement through soil 1.7 and above 35 Usually clayey Compacted

Source: Ref. 17.

Calculating Volume of Soil Needed for Equal Mass of Samples

A soil with a bulk density of 1.2 Mg/m³ is to be compared to a soil layer with a bulk density of 1.5 Mg/m³. The sampler being used has a cross-sectional area of 5 cm². The sampler itself is 30 cm long. How many

The bulk density of soil horizons and underlying layers is important in determining the volume of soil needed during the sampling process. In addition, knowing the bulk density of the horizons can indicate the likely path of water and contamination through the soil (see Figure 2.8) [19].