SOIL PHYSICAL PROPERTIES
Clip Art: Soil Quality Institute
A. You need to know the definitions and
characteristics of the following:
- Soil Texture
- Soil Structure
- Bulk Density
- Pore Space
- Soil Color
- Soil Temperature
B. Solve Problems and Properly Interpret Tables Relating to These Six Soil
Properties
Misconceptions
- Soil texture and soil structure are synonymous.
- Clay textured soils have less pore space than sandy textured soils.
Soil Texture
soil separates: Mineral particles ranging between specified size
limits. The names and size
limits of major categories of separates recognized in the USA
are:
| soil separate fraction name | size range |
| very coarse sand | 2.0 to 1.0 mm; |
| coarse sand | 1.0 to 0.5 mm; |
| medium sand | 0.5 to 0.25 mm; |
| fine sand | 0.25 to 0.10 mm; |
| very fine sand | 0.10 to 0.05 mm; |
| silt | 0.05 to 0.002 mm; |
| clay | <0.002 mm. |
Image: University of Nebraska, How soil holds water, http://ianrpubs.unl.edu/fieldcrops/g964.htm accessed 01/21/2005.
Given that 1000 microns (u) = 1 mm, answer the "?" below:
sand ____? u to ____ ? u
silt ____ ? u to ____ ? u
clay ____ ? microns
Given the definition of a soil separate, we can now define soil texture.
soil texture - The relative proportions of
in a soil as described by
the classes of soil texture shown in Fig. 1. A continuum exists between sandy and clayey textured soils.
Figure: Courtesy of the Soil Science Society of America
It is rare that you find a soil texture composed entirely of a specific soil separate.
Nearly
all soils are mixtures of the three soil separates. Soils that have
varying proportions of
the three soil separates are called
soils.
Equal proportions of separates
doesn't mean doesn't mean equal influence on
soil properties. Only a relatively small
proportion of clay is necessary to engender
clayey properties to a soil. Small amounts
of sand and silt have a lesser
influence on how soil behaves.
In class exercise: Using the Soil Texture Triangle to Determine Soil Texture:
parallel to the triangle's base. Next, find the sand % along the triangle's base by reading
the lines parallel to the triangle side labeled as "% silt." At this point you have two options to
derive the % silt: summing the clay and sand percentages and subtracting from 100% OR
find the % silt directly from the triangle by finding the % silt along the right side of the
triangle and reading the lines parallel to the "% clay" side of the triangle. Whatever method you use, record
this value. The name of the compartment in which these lines intersect indicates the textural class of the
soil sample. The % of soil separates always sums to 100% in designating a soil texture.
Take out a sheet of paper, put your name on it, number it 1-4, and answer the following:
| soil | % sand | % silt | % clay |
| 1 | 10 | 20 | 70 |
| 2 | 30 | 40 | 30 |
| 3 | What is the % sand, silt, clay of a sandy loam? (don't give a range, give specific #'s) |
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| 4 | What is the % sand, silt, clay of a silty clay loam? (don't give a range, give specific #'s) |
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workability, CEC, water movement (permeability) and water holding capacity (porosity)?
CEC:
Sandy soils: Sandy textured (light) soils will have . These soils allow roots to easily penetrate the soil in search of water and nutrients. Surface area is so CEC is also low. While water movement is generally good in sandy textured soils, total water holding capacity is giving these soils a reputation as being droughty. Total pore space is ________________.
Clayey soils: Clayey textured (heavy) soils will have . The soils often hinder roots from penetrating the soil in search of water and nutrients. Surface area is very high so is also also high. While water movement is generally poor, total water holding capacity is great, allowing these soils to resist better than sandy soils.
How are toxic compounds retained for a sandy vs. clayey soil?
What is the relationship between soil separate particle size and surface area? Just how much surface area does clay have?
What 2 properties does humus share in common with clays?
particle size analysis - Determination of the various amounts of the different soil
separates
in a soil sample, usually by
 | method |
| method |
Structure: the combination or arrangement of soil particles into
or ________________.
In other words, how the soil separates are "put together."
Structure is important because it determines the size and continuity of
_____________
spaces
between soil particles and peds. This then relates to
permeability and aeration.
Structure, unlike texture, is modified
by human practice.
Soil structure is characterized in three ways:
- soil structure type - A classification of soil structure based on the shape of the aggregates or peds in the profile.
- soil structure grade - A grouping or classification of soil structure on the basis of
inter-
and intra-aggregate adhesion, cohesion, or stability. Four grades of structure are
recognized as follows: ____________________, weak, moderate, strong - __________________________-
Figure: Courtesy of the Soil Science Society of America
Image: Courtesy of NASA, Soil Science Education Home Page http://ltpwww.gsfc.nasa.gov/globe/pvg/prop1.htm, accessed 01/21/2005.
Why is structure an important characteristic of soils?
| a granular soil structure would allow for
and water penetration. How does one promote a granular structure? |
| A coarse blocky or platy soil with few cracks would retard root and water penetration. Because so little water is reaching lower soil horizons, water holding capacity would be . |
What are two types of structure-less soils?
Group 1: Form a group of 4. Place the name of your group on the left side of the front board. Move to the hall or to a different part of the classroom. Using the text and your own knowledge answer the following:
Describe how the following affects soil temperature:
a. soil color.
b. soil moisture.
c. surface mulch of leaf litter and wood chips.
After 5-10 minutes you will share your answers with another Group 1 set of students, reach consensus, and then teach a Group 2 set of students.
Create 1 multiple choice question from your discussion. Hand in this problem.
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Group 2: Form a group of 4. Place the name of your group on the right side of the front board. Move to the hall or to a different part of the classroom. Using the text and your own knowledge answer the following:
Explain how the following soil characteristics vary with depth for a Michigan
loam:
a. texture.
b. color.
c. porosity
After 5-10 minutes you will share your answers with another Group 2 set of students, reach consensus, and then teach a Group 1 set of students.
Create 1 multiple choice question from your discussion. Hand in this
problem.
Bulk Density
Bulk density - The mass of dry soil divided by its
The value is
expressed as Mg
per cubic meter, Mg/m3.
Bulk density is always less than particle density because for bulk density the volume
of
the solids and the voids are considered.
Particle density--The density of the soil particles, the dry mass of the particles being
divided by
the solid (not pore space voids) volume of the particles, in contrast with bulk density.
Assumption used in this class : the particle density of this soil is 2.65 Mg/m³
(2.65 g/cm³)
How is bulk density determined? Bulk density is determined by removing a 200-250 g
clod of soil, drying it in an oven
to remove water, and then weighing the oven dried clod. Volume is determined by
recording the amount of water displaced when the clod is placed in a beaker of water.
Because soils are porous, they must be sealed with a waterproofing compound.
Usually the weight of this compound can be ignored in the computation.
Examples of some typical bulk density soil values are
Table 3.6. General relationship among texture, bulk density and porosity of soils |
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| Textural Class | Bulk Density (Mg/m³) | Porosity (%) |
| Sand | 1.55 | 42 |
| Sandy loam | 1.40 | 48 |
| Fine sandy loam | 1.30 | 51 |
| Loam | 1.20 | 55 |
| Silt loam | 1.15 | 56 |
| Clay loam | 1.10 | 59 |
| Clay | 1.05 | 60 |
| Aggregated clay | 1.00 | 62 |
table from: Juma, N.C. http://www.soils.rr.ualberta.ca/Pedosphere/content/section03/page03_03.cfm,
J
an 22, 1999
Calculation Problem: A moist soil core from field had a mass of 47.8 g and a volume of
33.49 cm³. The oven-dry mass of the soil core was 36.5 g. Calculate the bulk density and
percent pore space of the soil core.
Assumption: The particle density of this soil is 2.65 g/cm³.
Solution:
Bulk density = (mass of oven dry soil core/volume of the soil core)=
36.5 g/33.49 cm³ = 1.09 g/cm³
Pore space (%) = (1 - (bulk density/average particle density))*100= [1 - (1.09/2.65)]*100= 58.9 %
problem from: Juma, N.C. http://www.soils.rr.ualberta.ca/Pedosphere/content/section03/page03_03.cfm,
Jan 22, 1999
Compaction due to tillage, trampling, and construction activity will
bulk density and
the amount of pore space.
| How will an increase in bulk density affect a construction site? |
| How will a reduction in bulk density affect a permeability on a farm? |
| How will a increase in bulk density affect contaminant behavior at a spill? |
How do you improve the structure of compacted soils?
humus
tillage
Pore Space
Pores result from the irregular shape of soil particles or from
pushing and aggregation forces.
When pore space is very small, soil charges retain water and impede drainage,
resulting in poor aeration. Pore size and pore space connectivity in
the form of micro-joints is more important than total pore
space.
What is the relationship between packing
Images
courtesy of Michigan Tech University, Packing,
http://techalive.mtu.edu/meec/module06/Packing.htm 21 Jan. 2005.
Image courtesy of Profile Products, 2002, PROFILE Improves Any
Soil Structure For Superior Performing Designs,
http://www.profileproducts.com/landscaping/lands_2.html 20 Jan. 2003.
The most productive soils usually have of smaller pores for water retention and larger pores for water transmission.
What is the total pore space for each of the four soils portrayed in this animation on permeability?
Soil Color
Munsell color system
- A color designation system that specifies three variables of color:
hue - spectral (rainbow) color indicating a soils relation to red, yellow, green, blue, or
purple. Most soils are in the red and yellow hues.
On the Muncell Chart--Within each color range (R, YR, Y) the color becomes more yellow and less red as the numbers increase.
value - the lightness of a soil. The degree of
of a color in relation to a neutral gray scale. On a neutral gray scale, value extends
from pure black to pure white; one of the three variables of color. On the Muncell Chart--value, found on the vertical axis, extends from 0 for absolute
black to 10 for absolute white.
chroma - The relative of a color; directly related to the dominance of the determining wavelength of the light and inversely related to grayness; On the Muncell Chart-- chroma, found on the horizontal axis, extending from neutral grays.
The order for writing a Muncell notation is hue, vlaue, and chroma with a space between the hue letter and the two numbers. A "/" appears between value and chroma.
Example: 10YR 5/6
How does soil color indicate soil physical, chemical, and biological properties?
, iron oxides, and carbonates are responsible for most soils colors
| color | properties | ||||||||||
| black |
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| red |
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| brown |
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| white |
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| blue-grey |
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Soil Temperature
How does soil texture, structure and water content affect thermal conductivity?
| Water's specific heat: it takes a lot of energy to raise the temperature of water;
conversely, water retains its heat. |
| A waterlogged soil will heat up
in the spring delaying planting. Soils may become waterlogged from a strong, coarse
blocky structure or a clayey soil that would not allow water downward.
normally allow water downward but these mini-joints can become clogged with clay particles
further retarding permeability. |
| A well drained sandy soil or a soil with a
structure will heat more quickly and allow for earlier planting. |
How is it possible for a sandy soil to be waterlogged?
localized barriers to the downward movement of water found above the usual level of the water table
Web Resources to Supplement these Objectives
- Determination of Soil Texture in the Field: booklet from the University of Florida by R.B. Brown to assist in the determination of soil texture in the field using the touch test.
