SOIL STRUCTURE, BULK DENSITY, PARTICLE DENSITY, AND POROSITY
Laboratory Exercise # 4
Objectives:
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
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Understand the concept of soil structure, how it differs from soil texture.
Learn how to determine and calculate bulk density
Determine particle density and understand the difference in particle density
and bulk density
Know how soil strength is affected by water content, texture, and bulk
density.
The previous lab was about soil texture, which describes the relative proportion of the
different sizes of individual mineral particles, sand, silt, and clay (remember that organic
matter is not a part of soil texture). Soil structure is the arrangement of the primary soil
particles (sand, silt, and clay) and other soil materials into discrete aggregates. Structural
units are called peds, and have distinct boundaries and well-defined planes of weakness
between the aggregates. Peds consist of primary particles bound together by cementing
agents like organic matter, clay, and hydrous oxides of iron and aluminum. Peds can take
several shapes as illustrated in figure 4.1. The terms weak, moderate, or strong are used
to describe the grade or how stable the peds are and how hard they are to break apart.
The size or class of the peds is described as fine, medium, or coarse. The shapes of peds
are platy, granular, blocky, columnar, or prismatic. If the soil has no structure it is either
massive or single grain.
Like soil texture, structure influences movement of water and air through the soil. In soils
with good structure, the pore space that occurs between peds is relatively large and
facilitates water and air movement. . Well-developed structure is very important in
clayey soils. Clayey soils with poor structure restrict water and air movement. Unlike
texture, structure can be altered by tillage or traffic. Tilling soils that are too wet, or
compacting soils with heavy equipment can break down the natural structural units.
Figure 4.1 Types of soil structure
Granular: Resembles
cookie crumbs and is
usually less than 0.5 cm
in diameter. Commonly
found in surface
horizons where roots
have been growing.
Blocky: Irregular
blocks that are usually
1.5 - 5.0 cm in
diameter.
Prismatic: Vertical
columns of soil that
might be a number of
cm long. Usually found
in lower horizons.
Columnar: Vertical
columns of soil that
have a salt "cap" at the
top. Found in soils of
arid climates.
Platy: Thin, flat plates
of soil that lie
horizontally. Usually
found in compacted
soil.
Single Grained: Soil is
broken into individual
particles that do not
stick together. Always
accompanies a loose
consistence. Commonly
found in sandy soils.
Massive: Soil has no
visible structure, is hard
to break apart and
appears in very large
clods.
BULK DENSITY, PARTICLE DENSITY, AND POROSITY,
Soil bulk density is the mass per unit bulk volume of soil that has been dried to a constant
weight at 105 degrees C. Particle density is the mass per unit volume of soil particles.
Both of these terms are usually expressed in grams per cubic centimeter (g/cm3). If both
bulk density and particle density are known, the total porosity can be calculated using
these values.
The objective of this exercise is to determine bulk density, particle density, and total
porosity of the following: (1) a loamy soil (2) sand (3) clay.
Lab Activity:
BULK DENSITY DETERMINATION
1.
2.
3.
4.
5.
6.
7.
8.
Make duplicate determinations.
Add soil to about the 20 ml mark of a 100 ml graduated cylinder.
Compact the soil by tapping the cylinder base on the palm of your hand.
Add about 20 ml more of the soil and compact as above.
Repeat step four until 80 to 100 ml of the material is in the cylinder.
Record the soil level as volume in cc (1 ml = 1 cc).
Weigh the soil and record the weight.
Calculate bulk density.
9. Calculate the porosity assuming the particle density to be 2.65 g/cm3
● Using three soils in the lab, we determined the following information. You now need
to fill in the empty blanks to determine bulk density and porosity, as well as guessing
what soil type we used.
Soil #1
Soil #2
Soil #3
Weight of cylinder (g)
128.1
128.4
128.2
Weight of cylinder + soil (g)
242.5
275.6
265.3
99.5
99.7
99.1
Weight of soil (g)
Volume of soil (mL or cm3)
Bulk Density of soil (g/cm3)
Porosity
PARTICLE DENSITY DETERMINATION
Graduated Cylinder Method
1. Use the materials provided. Make duplicate determinations.
2. Add 70 ml of water to a 100 ml graduated cylinder.
3. Using a mortar and pestle, grind about 30 grams of soil to destroy aggregates.
Grinding is not required for sand.
4. Weigh the sample and record the weight.
5. Using a funnel, pour the weighed sample into the cylinder containing the
measured amount of water.
6. Shake or stir to release any entrapped air bubbles.
7. Read and record the new water level.
8. The change in water level is the volume of water displaced by the soil, and
therefore the volume of the soil particles.
● Using the same soils as above, we determined the following information in the lab.
Please use the given information to fill in the blanks to determine particle density.
Soil #1
Soil #2
Soil #3
Weight of soil (g)
30.1
30.2
30.9
Volume of water before adding soil (mL or cm3)
Volume of water after soil has been added (mL or
cm3)
71.2
70.6
71
82.8
82
82.6
Volume of water displaced (mL or cm3)
Particle Density (g/cm3)
SOIL POROSITY AND PERMEABILITY
Soil porosity is the percentage of a soil that is pore space or voids. The average soil has a
porosity of about 50%, and the pores are filled with air or water depending on the
moisture content. Sands have larger pores, but less total pore space than clays.
Soil permeability is the ease with which air, water, or plant roots penetrate or pass
through soil. Soils with large pores that are connected are more permeable. Rainwater
soaks in readily and moves down through the soil profile. Clayey soils can have greater
total porosity than sand and still be less permeable than sand since the pores are small.
SOIL STRENGTH
Soil strength is an important physical property affecting plant growth and seedling
emergence. Soil strength is defined as the amount of force required to move or rearrange
soil particles. It is affected by three main factors; moisture content, soil texture, and bulk
density.
Water content is the most important factor determining soil strength. The lower the soil
water content, or the drier the soil, the greater the soil strength. Soils that are saturated, or
waterlogged have weak soil strengths.
Soil texture also affects soil strength. Soil strength of aggregated soils increases as clay
content increases. Poorly aggregated or single grain soils (sands, loamy sands, sandy
loams) usually have the weakest soil strengths unless they are cemented or compacted.
Individual particles of single grain (sandy) soils are easy to rearrange, but these soils are
susceptible to compaction, sometimes resulting in the formation of hard pans. This often
occurs in the Coastal Plain region, where tillage pans form in sandy soils.
Bulk density is the third factor affecting soil strength. As bulk density of a given soil
increases soil strength also increases. Bulk density is the oven dry weight of soil divided
by the volume or space occupied by the soil. Remember that soil is composed of solids
and pores, and the greater the bulk density the greater the amount of solids, and the
smaller the amount of pore space. For a particular soil type, as bulk density increases, soil
strength increases.
Two examples of management problems caused by increasing soil strength are soil crusts
and tillage pans. A soil crust is a thin soil layer that forms at the soil surface following
heavy rains. The raindrops compact the soil surface and develop a layer having a high
bulk density that hardens upon drying. Soil crusts may prevent seedlings from emerging.
Tillage or hard pans are high bulk density (>1.7 g/cm3) layers that occur within the Ap
and E horizons. Tillage pans are formed by compaction produced by tractor wheels and
tillage equipment. When dry, tillage pans have high soil strengths and may prevent roots
from growing into lower soil horizons.
Compaction caused by traffic of equipment, vehicles, or even foot traffic often increases
soil strength to levels that restrict root penetration and plant growth.
MEASURING SOIL STRENGTH
The force required to push a rod into the soil is a measure of soil strength. Penetrometers
are devices used to measure the resistance of a soil to penetration to estimate the effect of
compaction on growth, and to detect layers of different soil strength.
PREPARATION FOR NEXT WEEKS LAB
Place soil used in next week’s lab in the oven.
Study Questions
1. What is soil structure? How is it different from soil texture?
2. What are the 5 basic types of soil structure?
3. Soil without structure may be single grain or massive. Which soil separate would
most likely be dominant if the soil were single grain?
4. What does class and grade of structure refer to?
5. How does good soil structure improve infiltration of water into the soil and down
through the soil profile?
6. Is the oxygen content likely to be higher in pores within peds or pores between
peds? Why?
7. What kinds of activity would destroy soil structure?
8. If the oven dry weight of a soil is 350 g and the volume is 250 cm 3. What is the
bulk density?
9. What is the main difference between bulk density and particle density?
10. If bulk density is increased by compaction, does percent pore space increase or
decrease?
11. Which soil would have a greater bulk density, one that is sandy or one that is
clayey?
12. Why does air and water move more easily through sandy soil than finer textured
soil?
13. What 3 factors have the most effect on soil strength?
14. What effect does a subsurface soil layer with high soil strength have on plant
growth?
15. What instrument is used to measure soil strength?