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Ch. 2: Soil Physical Properties 2.1 Important Facts to Know


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Title: Ch. 2: Soil Physical Properties 2.1 Important Facts to Know

Ch. 2 Soil Physical Properties 2.1 Important
Facts to Know
  • Soil properties that can be described by physical
  • Soil separates, textural class, and descriptive
  • Soil structure, stability consistency
  • Mass/volume relationships
  • Soil temperature and radiation
  • Engineering terms relationships

Homework Chapter 2
  • Questions 1, 2, and 8 And
  • a. Assuming a ?s 2.65 g cm-3 and given a
  • cylinder height (h) 7.0 cm, inside diameter
    (d) 5.5 cm, and oven-dry soil weight (Ms) 95
  • calculate total porosity.
  • b. Calculate the per hectare weight of soil to a
    depth of 10 cm for (a) above.
  • _at_ 3pts 15 pts
  • Due 4 September 2008

22 Soil Texture
  • Coarse Fragments (gt2mm)
  • Soil Separates (lt 2mm fine earth fraction)
  • Sands (0.05-2.0 mm) 0.05-0.10 vfs, 0.10-0.25 fs,
    0.25-0.50 ms, 0.50-1.0 cos, 1.0-2.0 vcos
  • Silts (0.002-0.05 mm)
  • Clays (lt0.002mm)
  • Soil Texture
  • Relative proportions of sands (s), silts (si),
    clays (c)

  • Physical properties (s, si)
  • Water intake
  • Water holding capacity
  • Aeration Drainage
  • Physico-Chemical properties (c)
  • Fertility
  • Surface activity
  • Ion exchange

Surface area see Insight on page 27. Smaller
particles have considerably more surface area per
unit mass than larger ones and therefore much
greater potential for surface chemical reactions.
Porosity and Pore Size - Smaller particles also
create smaller pores. Smaller pores will have a
major effect on water holding capacity and plant
available water, as we will see in Chapter 3.
Soil Textural Classes
  • Textural Triangle (Figure 2-1)
  • 4 basic groupings sands, silts, clays loam
  • 12 textural classes
  • Determined from relative proportions of s, si,
  • Loam soils are not equal s, si, c rather
    that the properties and characteristics are
    equally expressed

Textural Triangle
See the example of how to calculate percent sand,
silt and clay on page 30 (Calculation 1) and how
to use this to determine textural class.
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Mechanical Analysis Measurement
Dry or Wet Sieving 0.50 mm (medium sand) 2.0 mm
(very coarse sand) gt2.0 (gravels) Sedimentation
(Stokes Law) Settling rate is a function of
particle size (8711 d2 cm-1 sec-1)
Coarse fragments, gt 2mm, stay on top of the
sieve. They are weighed and added to field
measurements of rocks.
2 mm sieve
Soil, or fine earth fraction Analyzed for texture
and chemistry
Step 2 Determine particle sizes of lt 2mm (fine
earth) fraction
Particle sizes are determined by settling times
using Stokes Law the settling velocity of a
particle is the net difference between its
downward force (gravity) against the bouancy
(resistance to fall) by surface friction and
movement of the water. Assumes spherical shape,
which is a major weakness.
After a given time t, the small particle has
fallen to a much shallower depth in the water
than the large particle. The depth of fall can be
used to calculate the particle size according to
Stokes Law
Typical Fall Rates (rates at which different size
particles settle
Fall rate 8711 d2 cm-1 sec-1, where d
diameter of particle (cm) Med sand (0.05 cm)
22 cm/sec Fine sand (0.05 cm) 3.5 cm/sec Med
silt (0.001 cm) 0.087 cm/sec, or 0.52
cm/min Coarse clay (0.0002 cm) 0.00035 cm/sec,
or 0.021 cm/min, or 1.26 cm/hr Fine clay
(0.00002 cm) 0.0000035 cm/sec, or 0.30 cm/day
23 Rock or Coarse Fragment Content
Very important in wildland soils not so much in
agricultural soils. Shapes rounded or
flat Rounded gravel (3 sizes), cobble, stone,
and boulder Flat channer, flagstone, stone,
boulder Table 2-2 shows specific size classes and
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Adjective used to describe rock fragments lt15
vol no mention 15-35 vol name of the dominant
rock is used (eg, stony loam) 35-60 vol the
word very precedes above (very stony loam) gt60
vol extremely Skeletal soils are those with
gt35 rock.
2.4 Soil Structure (the physical arrangement of
soil particles)
Aggregates secondary units or granules composed
of many soil particles held together by organics,
Fe oxides, carbonates, clays, and/or silica. Very
important for soil aeration and water
properties. Peds Natural aggregates. Clod is a
coherent mass of soil broken into shape by
artificial means, such as ploughing or soil
sampling. Fragment is a piece of broken
ped Concretions (nodules) formed by chemical
precipitation. Deterioration of aggregates can
occur with high Na (sodic soils), excessive
ploughing, removal of OM.
Soil Structure type, class, grade
Structurless (single grain) Massive Grade (stab
ility) Class (size) Type (shape) weak very
fine platy moderate fine prismatic strong me
dium columnar coarse angular blocky very
coarse subangular blocky granular
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2.5 Particle Density and Bulk Density Density (g
cm-3 Mg m-3 1000 kg m-3)
Particle Density - Mass of solids only per unit
volume (?s). Some typical particle densities are
presented on p.35. Water is the reference (1.0 g
cm-3). Standard value for mineral particle
density in soils is 2.65 g cm-3, but this varies
in practice.
Material Density (g
cm-3) Water 1.0 Pine wood 0.7 Loose
sand 1.6 Soil minerals, rocks 2.65 Steel 7.7
Lead 11.3
Bulk density (?B in this text but ?b standard)
is defined as the dry weight of soil per unit of
total volume including pore space.
Values range from 0.2 in organic soils to 1.9 in
heavily compacted soils. Cultivated loam ?b
1.1-1.4 g cm-3 Good for plants below 1.4 for
loam and 1.6 for sand. Potting mixtures of
peat moss, perlite, vermiculite 0.1 to 0.4.
Table 2-4 gives a range of values.
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Methods for determining soil bulk density
  • Core
  • Set volume V p r2 h
  • Dry mass/total volume
  • Clod
  • Thin wax coating
  • Water displacement
  • Quantitative pit
  • Excavation

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Bulk density used to calculate total soil mass to
a given depth.
1 ha 10,000 m2 100,000,000 (108) cm2 Let ?b
1.3 g cm-3 or 1,300 kg m-3 Let soil depth 15
cm Soil volume (0.15 m)(10,000 m2) 1,500
m3 Soil mass (1,500 m3)(1,300 kg m-3)
1,950,000 kg So we have 1,950,000 kg ha-1 - 15
cm-1 This is often used in agriculture and
ecology to calculate water and nutrient storage
per ha for plant growth or hydrology.
2.6 Soil Porosity and Permeability
Pores (voids) are spaces not occupied by soil
solids (mineral or organic). Matrix pores
spaces between particles Non-matrix pores
outside of aggregates by worms, root channels,
cracks, etc. Classes Very fine
lt0.5mm Fine 0.5 - 2.0 mm Med 2-5 mm
Very coarse pores are also called macropores in
some cases. Tortuous pathways include dead end
pores and/or convoluted channels. Water drains
from pores larger than 0.03 to 0.06 mm by
gravity the rest can be stored. Thus, pore
size distribution is very important for water
holding capacity and is more important than total
porosity for plant growth.
Total Porosity (pore space)
Ø (1 ?b/?s) x 100 total
porosity Assuming ?s is constant at 2.65 g
cm-3 Porosity is an inverse relationship to
?b Ø a (vol. air content) ? (vol. water
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2.7 Soil Air
Atmospheric air we breathe is composed mainly
of Nitrogen (N2) 79 Oxygen (O2)
20.9 Carbon dioxide (CO2) 0.037 (says 0.035
in book, but it is increasing all the time and
is now 0.037) Water vapor (relative
humidity) 20-90.
Soil air composition
Because O2 is consumed and CO2 released during
respiration, soil air is depleted in O2 and
enriched in CO2. Soil also has higher relative
humidity in most cases. Gas Atmosphere
Surface soil air Subsoil air N2 79
79 79 O2 20.9 14-20.6 7-18 CO2 0.036
0.5-0.6 3-10
Oxygen diffusion rate (ODR) rate at which O2
diffuses into (and conversely CO2 diffuses out
of) soil.
O2 and CO2 exchange 1 for 1 in soil atmosphere
N2 does not vary. ODR depends highly on
porosity, pore size distribution, tortuosity, and
water content. When reduced by high water
content, diffusion is restricted and CO2 can
build up (O2 is depleted) plant root growth is
inhibited (except for aquatic plants) and microbe
are strongly affected.
Rates of Oxygen exchange
Aeration is then poor, and redox potential (the
availability of electrons for reduction) is
increased. In poorly aerated or anaerobic soils
(as opposed to well-aerated, aerobic soils), O2
is not available as an electron acceptor for
oxidation of carbon compounds (energy). Other
components then become reduced.
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2.8 Soil Consistence
Soils response to mechanical forces - important
for engineering purposes. Resistance to rupture
Plasticity Stickiness
2.9 Soil Color
Can be related to soil physical and chemical
properties. Dark soils are often (but not
always) enriched in OM. White colors can be
associated with E horizons or salts or
carbonates. Spots of different color (often
rust colored) called mottles can indicate
periods of poor drainage. Bluish, grayish, or
greenish colors in subsoils (gleying)
indicated prolonged periods of poor drainage.
Munsell Color Charts
Hue the dominant spectral color (red, yellow,
blue, green) Value Relative blackness or
whiteness the amount of reflected light Chroma
Purity of color (increases as color is more pure
and grayness decreases). 10YR 7/1 Hue 10YR,
Value 7, Chroma 1
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2.10 Soil Temperature
Temperature of the earth is determined by the
energy balance RN RS - RR -RL RN net
energy RS short wave radiation coming in
from the sun RR short wave radiation
reflected (albedo) RL long wave
(infrared) radiation emitted from the earth
Long wave radiation emitted by earth, some
absorbed by greenhouse gases and reflects back to
Short wave radiation -penetrates atmosphere
Fig 2-10 A more complex version
RN G H LE RL G heat absorbed by the
ground H hear absorbed by the air LE latent
heat used to evaporate water RN RL, we hope,or
the earth will heat up or cool down. Because of
rising CO2, we now worry that the earth is
heating up.
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Soil Temperatures
Standard Soil Temperature is measured at 50 cm or
at rock/hardpan interface. Mean Annual Soil
Temperature (MAST) measured at 10 m
approximately equal to mean annual air temp plus
1oC. Measurements at 2 m are close. Subsoil
temperatures fluctuate much less than surface
soils because of buffering by upper soil layers.
Soil Temperature fluctuations dampen with depth
and become close to mean annual temp 1 oC at 2
m. Mulching or litter layer dampens fluctuations
at the surface.
Without mulch
Soil Depth (m)
With mulch
Factors Affecting Soil Temperature
Water Heat Capacity of the soil is the amount
of heat (calories) needed to raise 1 g of soil 1
degree Celsius. It is a function of texture,
organic matter and moisture content. Finer
textured soils (e.g., clays, clay loams) have
higher heat capacity than coarse textured soils
(e.g., sand). Heat Capacity is highly dependent
upon water content Heat Capacity of water is
(1.0 cal g-1) about 5 times greater than that of
the soil itself (0.2 cal g-1).
Thermal conductivity of soil refers to the
movement or penetration of thermal energy into
the soil profile. Conductance is also strongly
affected by texture (increases with finer
texture), organic matter (lowers with increasing
organic matter), and water content (increases
with increasing water content). When soil water
content is high enough to bridge gaps between
particles, further increases in soil moisture
have little effect upon conductance.
Because water has higher heat capacity and
higher thermal conductivity than soil minerals,
wet soils are harder to heat up initially, but
heat to deeper depths than dry soils. Dry soils
tend to get very hot at surface. Moist soils are
usually cooler than dry soils because of their
high specific heat, even though conductance is
also greater in moist soils. Rain and
irrigation water can also cool or warm soil
quickly, depending upon the temperature of the
incoming water compared to the soil.
2.12 Soil Physical Properties Engineering
American Association of State Highway
Transportation Officials Mechanical analysis
(Table 2.6, 2.7), liquid limit plasticity
(Atterberg Limits) Liquid limit is the water
content in the soil at which the soil will flow
under standardized agitation (Fig. 2-16). Plastic
limit is the minimum water content at which the
mixture acts as a plastic solid. Plasticity index
is the difference between the LL and the PL.
Unified Engineering Classification System Coarse,
Fine Grained, Organic (Fig. 2-17) Classified
into 15 groups G Gravel O Organic S
Sand W Well-graded M Nonplastic or
low-plasticity fines P Poorly Graded C Plastic
fines L Low liquid limit Pt Peat, humus,
swamp soils H High liquid limit ML
(good)gtCLgtOLgtMHgtCHgtOHgtPt (very poor)