Transcript Physical Properties of Soil

Physical Properties of
Soils
Subject Outline
Introduction
Soil Color
Soil Texture
Soil Structure
Soil Consistence
Soil Density
Soil Porosity
Soil Tilth and Tillage
Introduction
Soil physical properties relate to the
solid particles of the soil (Soil texture)
and the manner in which they are
aggregated together (soil structure),
creating some spaces within soil (for
soil air and soil water).
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.

Soil color, texture and other physical
properties are used in classifying soil
profiles.
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 subsoil drainage,
organic matter content of surface horizons, and
bulk density.
 Distinguishing soil horizons.
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 that impart color to soils include:
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 to determine the color of soil
Determination of Soil Color
1. Hue = the dominant spectral color either
pure (yellow, green, red) or mixed
(yellow-red).
2. Value = modifies the hue by addition of
gray. 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)
3. Chroma = The amount of gray of a
particular value that is mixed with pure
hue to obtain the actual soil color.
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 to the drainage status of
soils
Color is used to distinguish the different
horizons of a soil profile. Thus, it is
extensively used in classifying soils
Soil Texture
• Soil texture refers to the
relative abundance of sand,
silt, and clay. The particles
are defined by their sizes.
• The classes are on the
United States Dept. of
Agriculture Textural
Classification System
Soil Separates
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.
Specific surface Area
 Adsorbing power (adsorption)
 Swelling
 Plasticity and cohesion

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


High silt


feel smooth and silky when wet
3. High sand
loose and friable when dry
 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 Separate
Clay
Silt
Sand
Textural Class
Soil 1
10%
30%
60%
Sandy Loam
Soil 2
50%
45%
5%
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 different horizons.
Soil Structure
Soil Structure is the grouping or
arrangement of primary soil
separates into secondary
groupings 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
Affects the degree of resistance to erosion.
Formation of a good seedbed for plants
Types of Soil Structure
Soil structure is characterized in terms
of the shape, size, and distinctness.
The four principal Types (shapes) 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
plates.
Plates 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 across campus or in cattle
feedlots.
Blocky Structure
If the intersections were sharp the aggregates would be
angular blocky = angular blocky structure.
Blocky aggregates, as the name implies, look square.
They may have rounded and sharp corners = subangular
blocky aggregates.
Prism-Like StructurePrismlike - (columnar or prismatic subtypes) vertically oriented
prisms or pillars or columns that vary in height in soils, may reach
a diameter of 15 cm or more.
Prismatic Prismatic structure
have pillars that are relatively
angular and flat horizontally. The
aggregates are longer than they
are wide They are typically found
in subsoil horizons with abundant
clay.
Prism-Like Structure- Columnar
Note the shape of the peds
Columnar structures are 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 changes 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) - 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. It may vary from horizon to
horizon with the addition or loss of OM, or
clay. (Average = 2.65 g/cm3).
Bulk Density
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 a soil are
functions of:
1.
2.
3.
Total pore space
distribution between 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 (page 148, Brady
text)
% Pore space = 100 –(BD/PD x 100)
That relationship is important because pore
space in soil is a difficult property to
measure. Thus calculation is necessary.
Tilth and Tillage
Tilth -Soil tilth is the physical condition of the soil in relation to
plant growth. It depends on factors such as: aggregate formation
and stability, bulk density, water holding capacity, aeration,
infiltration rate, and drainage.Soil tilth can change rapidly and
abruptly.
Tillage -The mechanical manipulation of soil for any purpose.
Purposes: weed control, seed bed preparation, incorporation of
organic residues.
Conventional Tillage: lifting, twisting, inverting, and incorporating organic
matter into the plow layer soil. Primary tillage encourages the formation of plow
pans directly beneath the plow layer
Conservation Tillage: Involves minimum tillage. Leaves considerable plant
residues on or near soil surface to protect soil from erosion.