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Physical Properties of Soils

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Physical Properties of Soils Subject Outline Introduction Soil Color Soil Texture Soil Structure Soil Consistence Soil Density Soil Porosity Soil Tilth and Tillage ... – PowerPoint PPT presentation

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Title: Physical Properties of Soils


1
Physical Properties of Soils
2
Subject Outline
  • Introduction
  • Soil Color
  • Soil Texture
  • Soil Structure
  • Soil Consistence
  • Soil Density
  • Soil Porosity
  • Soil Tilth and Tillage

3
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).

4
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.

5
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.

6
Causes of Soil Color
  • Organic matter or humus
  • imparts a brown to black color to soil.
  • The higher the organic content the darker the
    soil
  • Oxidation status of Iron (Fe) containing
    compounds
  • FeIII (ferric, oxidized form) imparts a red or
    yellow color to soil.
  • FeII (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
  • Manganese oxide (black)
  • Glauconite (green)- not very common
  • Carbonates (whitish)- aridand semiarid soils

7
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

8
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 (10white and 0black)
  3. Chroma The amount of gray of a particular value
    that is mixed with pure hue to obtain the actual
    soil color.

9
Determination of Soil Color
10
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

11
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

12
  •  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

13
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

14
Feel Method (For quick field analysis)
The smooth, dull appearance and crumbly ribbon of
a silt loam.
The gritty, non-cohesive appearance and short
ribbon of a sandy loam
The smooth, shiny appearance and long, flexible
ribbon of a clay
15
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.

16
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17
  • Some Influences of Soil Separates on Some
    Properties and Behavior of Soils 

18
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.

19
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.

20

Importance of Soil Structure
  1. Aeration - to allow oxygen in and carbon dioxide
    out.
  2. Drainage - water infiltration and its
    relationship to runoff
  3. Affects the degree of resistance to erosion.
  4. Formation of a good seedbed for plants

21
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
  • Spheroidal
  • Platy
  • Blocky
  • Prismatic

22
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

23
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.

24
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.

25
Prism-Like Structure-
Prismlike - (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.
26
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.
Prism-Like Structure- Columnar
27
Structure Grades (Distinctness)
  • Weak- poorly formed, barely observable.
  • Moderate- well formed, moderately durable.
  • Strong- very evident, durable peds.

28
Structure Classes (size)
Structure size class depends on the type e.g.
Angular and subangular have the following classes
  • very fine lt5mm
  • fine 5 - 10 mm
  • medium 10 20 mm
  • coarse 20 50 mm
  • very coarse gt 50 mm

29
  • 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.

30
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

31
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).

32
  • 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.

33
Porosity
  • Air and water flow through a soil are functions
    of
  • 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)
  • That relationship is important because pore space
    in soil is a difficult property to measure. Thus
    calculation is necessary.

Pore space 100 (BD/PD x 100)
34
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.
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