Density and Porosity
SOILS 206 – Soil Ecosystem Lab
Objectives:
1.
2.
3.
4.
Define soil bulk density, particle density, total pore space, and individual pore size.
Describe the relationship between soil texture, bulk density, particle density and porosity.
Perform calculations involving bulk density, particle density, percent solids and percent pore space.
Discuss the factors influencing soil bulk density, particle density and porosity.
Introduction
Soil is composed of solid material and pores. The solid fraction consists of mineral and organic material.
The pores contain variable amounts of water and air. A soil in good condition for plant growth will
contain about 50% pore space and 50% solids. While it is sometimes helpful to think of the soil
components separately, it should be emphasized that, in a natural soil, the four main soil components
are thoroughly mixed.
The relationships and interactions between the soil solids and pores are very important and merit much
study in soil science. The following exercises are intended to introduce you to the concepts of soil
density and soil porosity.
Soil Density and Porosity
A good understanding of soil density and porosity is essential to soil scientists, engineers, and persons
involved in crop and timber production. Soil pores contain air for plant root metabolism and provide a
reservoir for water storage. Careful irrigation planning requires that one take into account the porosity
as well as the initial moisture content of the soil.
Bulk density values provide an extremely useful conversion factor for calculations involving soil mass
and soil volume. Bulk density measurements are also used to calculate the total pore volume in a soil as
well as the weight of soil to be moved during an excavation. High bulk density values may indicate the
presence of compact layers in soil that could restrict root and water penetration.
There are two important characteristics of porosity – total pore space and the size distribution of the
pores. The total pore space of a soil is that portion of the soil volume occupied by air and water. The
soil texture, structure, and organic matter content largely determine the amount or total volume of soil
pore space. A well-aggregated, fine-textured soil high in organic matter would have high pore space
while a massive, coarse-textured or compact soil would have low pore space. Soil pores are generally
grouped into two categories:
1. Micropores – pores less than 0.06 mm in diameter that are important for water retention
and,
2. Macropores – pores greater than 0.06 mm in diameter that are important to aeration and
saturated water flow.
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Density Formulas
The density of any object is its mass divided by volume it occupies. Soil density is the oven-dried weight
per unit volume expressed in g/ cm3 or Mg/ m3. We are interested in measuring two different density
values:
1. Bulk density (Db), and
2. Particle density (Dp).
Bulk Density, Db
Bulk density values represent the density of the oven dry soil as a whole; this includes solids and pore
space.
Db = Ws/ Vt
Where:
Ws = Oven dry mass of the sample (g)
Vt = Total volume of the sample, pore volume + solid volume (cm3).
Particle Density, Dp
Particle density values represent only the weight of dry soil per unit volume of the soil solids; the pore
space is not included in the volume measurement.
Dp = Ws/ Vs
Where:
Ws = Oven dry mass of the sample (g)
Vs = Volume of the solids (ONLY) (cm3).
Porosity Formula
Pore Space, PS
The pore space of a soil is that portion of the soil volume occupied by air and water.
PS = Vp/ Vt
or
PS = [1 - (Db/ Dp)]
Where:
Vp = Volume of the pores *
Vt = Total volume of the sample, pore volume + solid volume (cm3).
* Vp is difficult to measure, so it is more common to calculate PS from Db and Dp
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Soil Characteristics of Density and Porosity
Bulk density values for mineral soils commonly range from 1.0 to 1.8 Mg/m3. Highly compact subsurface
soils usually have high bulk density and low total pore space. In extremely compact soils, where the pore
space is near zero, the bulk density value may begin to approach the particle density value. Organic
soils, such as Histisols, have very low bulk densities (often less than 1).
Bulk density values are affected by soil texture and modified by structure. The large-sized particles in
sandy soils tend to pack to fairly defined volumes, which contain few micropores but many macropores.
The small aggregated soil particles in a fine-textured soil tend to be separated by many micropores.
However, in soils with well-developed structure, large macropores may exist between the peds.
In general fine-textured soils (clays and silt loams) exhibit lower bulk densities than coarse-textured soils
(sands). However, it is important to note that soil bulk density can vary significantly between soils and
within a soil profile and can be influenced by land management. Incorporation of large amounts of
organic matter will lower the bulk density while processes that compact the soil will increase bulk density.
See p. 36-37 in Gardiner and Miller for a discussion of factors affecting soil bulk density.
Tillage operations may have both favorable and unfavorable effects on bulk density and porosity. The
short-term effects of tillage are generally beneficial as large clods are broken up and organic matter is
returned to the soil. However, intensive cultivation over long periods of time can be detrimental to soil
structure. Repeated mixing of the soil hastens the oxidation and loss of organic matter. Numerous
passes over a soil with heavy tillage equipment also may break down stable aggregates, increase
compaction, and decrease porosity. Furthermore, excessive cultivation significantly increases the
potential for soil erosion.
Measurements
Bulk density is most commonly determined by one of two methods:
1. The core method or
2. The clod method
Similarly, particle density may be determined by one of two methods:
1. The displacement technique or
2. The saturation method.
Once the bulk and particle density values are known, it is a straightforward calculation to determine pore
space.
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Example 1
A soil core with a height of 6 cm and a diameter of 5 cm weighs 215 g when collected and 188 g when
oven-dried. When the core was ground and pored into 100 ml of water, the final volume of the soil-water
mixture was 171 ml. Calculate the soil bulk density, particle density, and percent pore space.
In this case, use the core method for bulk density and displacement method for particle density. Pore
space can be calculated using the bulk and particle density values. Remember that 1 ml of water equals
1 cm3 of water and that water has a density of 1 Mg/m3.
Bulk density (Db)
Ws
= oven-dried soil weight
Vt
= r2h
Dp
= Ws/Vt
= 188 g
= (3.14)(2.5cm)2(6cm)
= 188g/ 117.8 cm3
= 117.8 cm3
= 1.6 g/cm3
Particle density (Dp)
Vs
= volume of water displaced by ground soil
Dp
= Ws/Vs =
188 g/ 71 cm3
= 71 ml or 71 cm3
= 2.65 g/cm3
Pore space (PS)
PS
= 1- (Db/ Dp)*100%
= 40%
= 1- (1.6/ 2.65) * 100
Example 2
A soil clod has an oven-dried weight of 48 g, a saturated weight of 59 g, and weighs 29 g when
suspended in water.
Bulk density (Db)
Vt
= volume of water displaced by coated clod
26 cm3
Db
= Ws/ Vt
= 48 g/ 29 cm3
= 1.66 g/ cm3
Particle density (Dp)
Ws
= oven-dried soil weight
Vp
= Vw at saturation
Vs
= Vt - Vp
Dp
= Ws/ Vs
= 48 g
= 59 g – 48 g
= 29 cm3 - 11 cm3
= 48 g/ 18 cm3
= 11 g or 11 cm3
= 18 cm3
= 2.66 g/ cm3
=11 cm3/ 29 cm3
= 0.38 cm3/ cm3
= 1 – (1.66/ 2.66) * 100%
= 0.38 cm3/ cm3
or 38%
Pore space (PS)
PS
= Vp/ Vt
or 38%
= 26 g =
or
PS
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= 1 – (Db/ Dp) * 100%
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Example 3 Weight to Volume Conversion
Bulk density is useful to convert soil weight to volume (and vice versa).
What is the volume (in cubic meters) of 300 kg of soil with a bulk density = 1.27 g/cm 3?
300 kg
1
1000 g
1 kg
1 cm3
1.27 g
1 m3_ = 0.0236 m3
(100 cm)3
In today’s lab, you will be using the core method to calculate the bulk density of your assigned soils at
three different depths.
Procedure:
1. Measure the height and diameter of the soil core to calculate the volume
Volume = (area)(depth)
Where: area of a circle = πr2 and the depth is the length of the core
2. Obtain soil moisture cans with oven-dried samples taken from cores.
3. Determine the sample weight and record it in the data table below
4. Calculate the bulk density of each core.
Data sheet:
Inside Diameter _______ _ Radius_________
(Radius)2 __
Soil___________________
Sample
Depth
(cm)
Tin Weight
(g)
_
Length
____ _ __
Volume
____ _ __
Oven-dry
Total
(g)
Dry Soil Wt.
Oven-dry
Total
(g)
Dry Soil Wt.
(g)
Volume of
Core
(cm3)
Bulk Density
Volume of
Core
(cm3)
Bulk Density
(g/cm3)
Soil____________________
Sample
Depth
(cm)
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Tin Weight
(g)
(g)
(g/cm3)
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