2.2: Soil Composition

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Figure 2.1. Composition of a soil volume.

$Bulk soil showing typical fractions of air (20%), water(25%), organic matter (5%) and minerals (50%) and water films on solids.$

As Figure 2.1 illustrates, soil is composed of three major components: soil particles, air, and water. The fractions of water and air are contained in the voids between soil particles. The ratio of the volume of pores (voids) to the total (bulk) volume of a soil is the porosity (φ). One way to determine porosity is to measure the volume of a soil that is composed of soil particles and the fraction made up of the pores. The porosity may also be determined using the soil bulk density (ρ_b) Bulk density is the density of the undisturbed (bulk) soil sample described by:

$ρ_b=\left(\dfrac{M_s}{V_b}\right)$ (2.1)

where: ρ_b = soil bulk density (g/cm³),
M_s = mass of dry soil (g), and
V_b = volume of bulk soil sample (cm³).
Given the bulk density of a soil, the porosity (in percent) can be calculated as:

$φ=\left(1-\dfrac{ρ_b}{ρ_p}\right)100\%$ (2.2)

where: φ = soil porosity
ρ_p = soil particle density (a common value for mineral soils is 2.65 g/cm³).

Example $\PageIndex{1}$

An undisturbed soil sample with a volume of 80 cm3 is taken from an irrigated field. The mass of the soil
sample after drying is 100 grams. What is the soil bulk density? What is the porosity?
Given: M_s = 100 g
V_b = 80 cm³
Find: ρ_b and φ

Solution

$ρ_b=\left(\dfrac{M_s}{V_b}\right)=\left(\dfrac{100\text{g}}{80 \text{cm}^3}\right)=1.25\text{g/cm}^3$

$φ=\left(1-\dfrac{ρ_b}{ρ_p}\right)100\%=\left(1-\dfrac{1.25}{2.65}\right)100\%=53\%$

The soil sample in Example 2.1 consists of 47% soil particles and 53% pore space (air and/or water).

The mineral fraction of the soil volume is composed of sand, silt, and clay separates. With the USDA classification system, the equivalent diameter size limits are: Clay < 0.002 mm, silt 0.002 to 0.05 mm, and sands 0.05 to 2.0 mm. The relative proportions of the various soil separates are used to define the soil texture using the USDA soil textural triangle shown in Figure 2.2. These textural classes are referred to frequently in this book.

Figure 2.2. USDA soil textural triangle.

The percent sand (x-axis), silt, and clay(y-axis) associated with each of the 12 soil texture classes can read directly off this chart. Sum of 100%.

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