Transcript Physical Properties of Soil

Soil Physical Properties
Introduction
Soil physical properties relate to:
the solid particles of the soil (Soil texture)
the manner in which the particles are
aggregated together (soil structure)
creating some spaces within soil (for soil air
and soil water).
Introduction
Soil Physical Properties include the following:
Soil Color
Soil Texture
Soil Structure
Soil Consistence
Soil Density
Soil Porosity
Introduction
Soil physical properties influence how
soils function in an ecosystem and how
they are managed.

E.g. texture and structure determine how
much water the soil can hold and how
water can move around in soil.

Soil physical properties (e.g., soil color, soil
texture and other physical properties) are
used in soil classification.
Soil Color
Why is soil color an important soil property?
 Color directly affects the absorption of solar
radiation and therefore is a factor in determining
soil temperature.
 Color is indirectly related to other soil properties
and can provide information on subsoil drainage,
organic matter content of surface horizons, and
bulk density.
 Distinguishing soil horizons and soil
classification.
Causes of Soil Color
Organic matter or humus
1.


imparts a brown to black color to soil.
The higher the organic content the darker the soil
Oxidation status of Iron (Fe) containing compounds
2.


Fe[III] (ferric, oxidized form) imparts a red or yellow color to
soil.
Fe[II] (ferrous, reduced form) imparts neutral gray colors to
soils and allows the colors of other metal-containing
compounds to predominate.
Other soil components (Soil Minerals):
3.



Manganese oxide (black)
Glauconite (green)- not very common
Carbonates (whitish)- aridand semiarid soils
Determination of Soil
Color
Soil color is determined by comparison of
color of soil with standard colors on a soil
color chart called the Munsell Soil Color
Chart
Munsell soil color chart uses three
variables (hue, chroma, and value) to
determine the color of soil
Determination of Soil Color
1.
2.
3.
Hue = the dominant spectral color of an object
defined in terms of either pure (green, red,
yellow, and blue) or mixed (yellowness or
redness of soil).
Value = relative blackness or whiteness of soil.
Value is made by mixing white and black.
Therefore, a value of 5 is an equal mixture of
white and black (10=white and 0=black)
Chroma = the purity of the color. The amount
of gray of a particular value that is mixed with
pure hue to obtain the actual soil color. As
chroma increases, color is more brilliant.
Determination of Soil Color
Interpreting Soil Color
Color gives some indication of OM
status of soils
Color gives some indication of soil
mineralogy
Color gives insights into the drainage
status of soils
Soil Texture
Soil Separates
• Soil texture refers to the
relative abundance of sand,
silt, and clay. The particles
are defined by their sizes.
• The classes are based on
the United States Dept. of
Agriculture Textural
Classification System
Particle
Size (m)
Gravel
> 2000
Sand
2000 - 50
Silt
50 - 2
Clay
<2
Many physical, chemical, and mineralogical
properties of a soil particle are a function of
size. E.g.
 Surface Area (Specific Surface= surface area/g)
 Water retention
 Adsorption of gases and chemicals
 Weathering of particles
 Electromagnetic charges
 Microbial reactions
Therefore soil particle size
distribution influences water
retention, chemical adsorption,
microbial reactions, weathering, etc
Soil Textural Analysis
(2 methods of determination)
1. Feel Method (For quick field analysis)
 High clay
Feel hard and cloddy when dry
 sticky and plastic when wet ~ rolls into a ribbon


High silt
feel smooth and silky when wet
 little stickiness or resistance to deformation


High sand
Rough and gritty ~ makes grinding noise
 little or no plasticity or stickiness when wet

Feel Method
(For quick field analysis)
The gritty, non-cohesive
appearance and short
ribbon of a sandy loam
The smooth, dull
appearance and
crumbly ribbon of a silt
loam.
The smooth, shiny
appearance and
long, flexible ribbon
of a clay
Soil Textural Analysis
2. Laboratory Method
(Sedimentation techniques based on
Stokes Law)
 Prepare oven dry sample (approx.
50-100 g)
 Use mechanical dispersion to
prepare a soil suspension
 Determine sand, silt and clay
fractions by settling method using a
Buoycos Hydrometer (or by the
pipette method).
Textural Triangle
Soil Particles
Soil 1
Soil 2
Clay
10%
50%
Silt
30%
45%
Sand
60%
5%
Textural Class
Sandy Loam
Silty Clay
Some Influences of Soil Separates on
Some Properties and Behavior of Soils
Sandy Soils
Silty Soils
Low water holding
Medium-high WHC
capacity (WHC)
More aeration, drainage Moderate aeration,
drainage
Rapid decomposition of Moderate decomp. of
OM. Thus, low OM
OM. Medium OM
Resists compaction
Easily compacted
Poor supply and low
Good supply and
capacity to hold plant
medium capacity to
nutrients
hold nutrients
Little or no swelling
and shrinking
Mineralogy is mainly
quartz and other
weathering resistant
minerals
Clayey Soils
High WHC
Poor aeration and
drainage
Slow decomp of OM.
High to medium OM
More easily compacted
Medium to excellent
supply and high
capacity to hold
nutrients
Little swelling and
Moderate to high
shrinking
shrinking and swelling
Mainly quartz and other Mineralogy vary widely
primary minerals
from simple silicates to
complex silicates
Changes in Soil Texture
Texture of a given soil can only be changed
by mixing with another soil of a different
textural class.
This type of textural modification is not very
common in the field.
However, over very long periods of time,
pedologic processes such as erosion,
deposition, illuviation, and weathering alter
textures of soils.
Soil Structure
Soil Structure is the grouping or
arrangement of primary soil
particles into secondary units
called aggregates or peds.
Soil structure defines the pattern of
pores which in turn influences other
physical properties of soil.
Importance of Soil Structure
1.
2.
3.
4.
Aeration - to allow oxygen in and carbon
dioxide out.
Drainage - water infiltration and its
relationship to runoff
Formation of a good seedbed for plants
Affects the degree of resistance to erosion.
Types of Soil Structure
Soil structure is characterized in terms
of the shape, size, and distinctness.
The four principal shapes (Types) of
soil structure are:
1.
2.
3.
4.
Spheroidal
Platy
Blocky
Prismatic
Spheroidal Structure
Spheroidal structures are characteristic of
many surface soils (A horizons).
Two types of spheroidal structures are:


Granular- when spheroidal peds are separated
from one another in a loosely packed
arrangement.
Crumb- when spheroidal peds have very
abundant pores
Platy Structure
Characterized by thin horizontal peds or
aggregates.
The peds develop due to processes related
to soil formation:


inherited from the initial arrangement of the
parent material or
due to disturbance (compaction).

May be found in foot-paths or in cattle feedlots.
Blocky Structure
Blocky aggregates, as the name implies, look square.
If the intersections are sharp the aggregates would be
angular blocky = angular blocky structure.
If the intersections have some rounded and some sharp
corners = subangular blocky structure.
Prism-Like Structures
Prism-like structures are vertically oriented prisms or columns
that vary in height in soils. The aggregates are longer than they are
wide. There are 2 subtypes of prism-like structures ~Prismatic or
Columnar subtypes.
Prismatic Prismatic structure have pillars that are relatively
angular and flat horizontally. They are typically found in subsoil
horizons with abundant clay.
Prism-Like Structure- Columnar
Columnar
Columnar structures are also longer than they are wide
and they have rounded tops.
Both prismatic and columnar often occur where large
amounts of sodium are present in arid soils, or in
poorly drained areas in humid regions.
Structure Grades
(Distinctness)
Weak- poorly formed, barely
observable.
Moderate- well formed, moderately
durable.
Strong- very evident, durable peds.
Structure Classes (size)
Structure size class depends on the type e.g.
Angular and subangular have the following classes:
very fine
fine
medium
coarse
very coarse
<5mm
5 - 10 mm
10 – 20 mm
20 – 50 mm
> 50 mm
Some soils are structureless:
Single Grain - each soil particle is independent of
all others like a sandy beach.
Massive - The entire soil mass clings together with
no definite signs of weakness. Usually indicative of
large amounts of clay.
Some soils have had their structure destroyed, these
soils are called puddled. When soils are plowed
when too wet, trampled, or otherwise mashed, the
large pores collapse and the soil is left in an
undesirable condition.
Soil Consistence
Soil Consistence describes the relative
resistance of soil to mechanical stress
and its ability to be molded or changed in
shape. Consistence is usually described
when the soil is dry or when it is moist.
When dry, soil consistence could be:

Loose, soft, hard or rigid
When moist, soil consistence could be:

Loose, friable, firm, or rigid
Soil Density
Particle Density (PD)
Particle density is the weight per unit volume
of individual soil particles (Mg/m3 = g/cm3),
calculated as PD = Ws / Vs
Particle density is not affected by particle size
and pore space (texture and structure).
In general the mean particle density (PD) for
mineral soil varies in a narrow range from 2.6
to 2.7 g/cm3. (Average = 2.65 g/cm3).
Bulk Density (BD)
Bulk Density (BD) - weight of soil per unit volume in
its natural state (Mg/m3 = g/cm3), calculated as
BD = Ws / (Vs + Vp)
Where:
Ws = weight of soil solids
Vs = volume of soil solids
Vp = volume of soil pores
Bulk density is highly variable and can range
from 1.6 or higher for sandy soils to less
than 1.0 g/cm3 for soils high in organic
matter.
Porosity
Air and water flow through soil are
functions of:
1.
2.
3.
Total pore space
Pore size distribution (macro and mesopores)
Continuity of macropores
There is a relationship between BD, PD,
and Pore space in soil. That relationship is
shown with a calculation (see Brady text)
% Pore space = 100 –(BD/PD x 100)
That relationship is important because pore
space in soil is a difficult property to
measure.