Exam Schedule
Exam I: Friday, May 22nd
Exam II: Friday June 5th
Exam III: Friday June 19th
Review sessions will be on the Thursday preceding the exam.
Soil Texture
Three separates: Sand, Silt, Clay
Importance: indicator of pore size, surface area, water movement, reactivity
There are 12 textural classes based on relative abundance of sand, silt, clay
Florida soils tend to range from sandy to sandy clay textures
Texture-by-feel assesses grittiness, smoothness, plasticity to estimate texture
Laboratory analysis relies on sedimentation of soil particles in water
Stokes’ Law determines the settling rates of particles from water
Large particles (sand) settle quickly, small particles slowly.
A hydrometer measures the density of a soil suspension
Based on the density, the mass of particles remaining suspended is determined
Texture
Particle Size
Large/coarse
Sand
Loamy Sand
Sandy Loam
Medium
Fine/Small
Sandy clay Loam
Silty clay Loam
Silt Loam
Loam
Clay Loam
Sandy Clay
Silty Clay
Clay
Pore Size
Large
Medium
Reactivity
Weak
Moderate
Small
Strong
Texture and Civilization?
Earliest Civilizations
Tigris-Euphrates
Nile
Civilization
Year-round supply of water
Enduring Sunlight
Trustworthy harvests
Building materials
Nile
Jordan
Tigris
Euphrates
Neolithic Founder Crops
Wheat
Population increases
Barley
Flax
Substantial homes
Pea
Relative Safety,Chick
peace
Lentil
cows, goats, sheep, and pigs
Periodic Flooding
Flooding
Flooding and Soil Texture
Sand 2.0-.05 mm
Silt
0.05 – 0.002 mm
Clay < 0.002 mm
Clay
Stokes’ Law: V = kD2
K = 11,241 cm-1 sec-1
Sand
Silt
Clay
1 mm
0.05 mm
0.002 mm
V = 112
cm/sec
V = 0.281 cm/sec
V = 0.0004 cm/sec
Sedimentation
Sand
Clays/Silts
River channel
Flooding slows flow
The Nile
Blue Nile
Kenya
Uganda
Tanzania
Flood peaks in mid-September
Mesopotamia
Alluvial Plain
Euphrates
Flood: March through June
Agriculture and Irrigation
Irrigation
Canals
Dikes
Weirs
Reservoirs
channels
History and Soil Texture
(knowledge from clay and stone)
Stone and Clay
Egypt
Stone
Sumer
Clay
Architecture and Sculpture
Egypt
Sumerian
The Language of Power
sacred, ceremonial, literary, and scientific language
Soil Structure, Density, Porosity
Soil Structure
Arrangement or grouping of individual soil particles into secondary units.
Soil Structure
Units of soil Structure
Peds
Aggregates
Soil Structure
Formation
Chemical Processes –
electrostatic attraction between clay
particles and between organic particles.
Biological Processes
-macro-organisms (burrowing, tunneling,wastes)
-roots, fungal hyphae (compression, fibers, exudates)
-microorganisms (organic residues)
Importance
Effects on porosity, water retention, water movement
Large pores
Inter-aggregate pores
(between)
Small pores
Intra-aggregate pores
(within)
Water moves easily and is poorly retained in inter-aggregate pores
Water moves slowly and is strongly retained in intra-aggregate pores
Soil Structure is Desirable
Poor soil structure can
inhibit infiltration of
water, water movement,
growth of roots.
Summary
Structure is the arrangement or grouping of
individual soil particles into larger secondary units.
Clays and organic matter possess natural electrical
Charge which can electrostatically bind particles together.
Macro-organisms and microorganisms can aid in
binding of individual particles into larger aggregates..
Aggregation allows for both movement and retention
of water via macropores and micropores, respectively..
Poor soil structure can inhibit infiltration of
water, water movement, growth of roots.
Soil Density
Density
Density = Mass
Volume
g
( cm3)
2.65 g/cm3
Soil Bulk Density
y
BD =
mass OD soil
volume solids + pores
x
z
Volume = xyz
Density of soil including the particles and
the pore spaces
B.D. ranges between 1.1 and 1.6 g/cm3
(for mineral soil with 1 – 5% organic matter)
Bulk vs. Particle Density
Bulk density measures the mass of the soil solids in
relation to the volume of the soil solids and the soil pores.
Bulk density = mass solids
volume xyz
x
y
Particle density =
z
Mass of particle
Volume of particle
No pores
BD = 1.6 g/cm3
PD = 2.65 g/cm3
Known Volume
Sampling for Density
Known Volume
Dry and Weigh: mass/volume = Bulk Density
Factors Affecting Bulk Density
Porosity
(pores are weightless)
• organic matter
• aggregation
• arrangement of particles
• compaction
• depth in profile
Factors Affecting Bulk Density
Organic Matter
Typical
Mineral Soil:
1-5% organic matter
bulk density = 1.1 –1.6 g/cm3
Organic Soils:
> 20% organic matter
bulk density = 0.1 – 0.6 g/cm3
Aggregation
One sand grain
(Zero porosity)
One aggregate of
Clay sized particles
(50% porosity)
Aggregation generally increases overall porosity, decreases density
Packing Arrangement
Discrete particle size classes
Continuum of particle sizes
Arrangement
(compaction)
Particle
Size
Sorting
Depth in Profile
Lower organic matter
=>
Fewer roots
Compaction from above
Aggregation can mitigate
Some of these effects.
Higher bulk density
Summary
Bulk densities of typical mineral soils
range between 1.0 and 1.6 g/cm3.
Organic matter increases porosity, decreases BD
Organic soils can have BD as low as 0.1 g/cm3.
Compaction decreases porosity, increases BD
Aggregation increases porosity, decreases BD
Depth in profile decreases porosity, increases BD
Porosity
Bulk Density and Total Porosity
[
% Porosity = 1 -
( BD
)
]
PD
X 100
2.65 g/cm3
Porosity
100%
Bulk Density (g/cm3)
0
Bulk density high
Bulk density low
2.65
porosity low
porosity high
A
Bulk Density and Porosity
E
B
A
BD = 1.1 g/cm3
P = 59 %
E
BD = 1.15 g/cm3
P = 57%
B
BD = 1.6 g/cm3
P = 40 %
Pore Size Distribution
Macropores
> 0.8 mm in diameter
large, freely draining
sands, inter-aggregate pores
Micropores
< 0.8 mm in diameter
small, storage of water
clays, intra-aggregate pores
The effect of total porosity and of
pore size distribution is largely
related to the movement and
retention of water as well as
the movement of soil gases
Knowledge of texture, structure, bulk density,
and porosity allow deduction of the patterns
of movement of water and gases in soils
Next: Water in Soils