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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.
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Soil Texture
Three separates: Sand, Silt, ClayImportance: indicator of pore size, surface area, water movement, reactivityThere are 12 textural classes based on relative abundance of sand, silt, clayFlorida soils tend to range from sandy to sandy clay texturesTexture-by-feel assesses grittiness, smoothness, plasticity to estimate textureLaboratory analysis relies on sedimentation of soil particles in waterStokes’ Law determines the settling rates of particles from waterLarge particles (sand) settle quickly, small particles slowly.A hydrometer measures the density of a soil suspensionBased on the density, the mass of particles remaining suspended is determined
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Particle Size Large/coarse Medium Fine/Small
SandLoamy SandSandy Loam
Sandy clay LoamSilty clay Loam
Silt LoamLoam
Clay LoamSandy ClaySilty Clay
Clay
Pore Size Large Medium Small
Reactivity Weak Moderate Strong
Texture
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Texture and Civilization?
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Tigris-Euphrates
Nile
Earliest Civilizations
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Civilization
Year-round supply of waterEnduring SunlightTrustworthy harvestsBuilding materials
Population increasesSubstantial homesRelative Safety, peace
Neolithic Founder CropsWheatBarleyFlaxChick PeaLentil
cows, goats, sheep, and pigs
NileJordanTigrisEuphrates
Periodic Flooding
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Flooding
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Flooding and Soil Texture
Sand 2.0-.05 mmSilt 0.05 – 0.002 mmClay < 0.002 mm
Clay
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Sand 1 mm V = 112 cm/secSilt 0.05 mm V = 0.281 cm/secClay 0.002 mm V = 0.0004 cm/sec
Stokes’ Law: V = kD2
K = 11,241 cm-1 sec-1
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Flooding slows flow
River channel
Sedimentation
Sand Clays/Silts
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Flood peaks in mid-September
Blue NileThe Nile
KenyaUgandaTanzania
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Mesopotamia
Euphrates
Alluvial Plain
Flood: March through June
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Agriculture and IrrigationIrrigationCanalsDikesWeirsReservoirschannels
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History and Soil Texture(knowledge from clay and stone)
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Stone and Clay
Egypt
Sumer
Stone
Clay
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Architecture and Sculpture
Egypt
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The Language of Power
sacred, ceremonial, literary, and scientific language
Sumerian
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Soil Structure, Density, Porosity
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Soil StructureSoil Structure
Arrangement or grouping of individual soil particles into secondary units.
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Soil StructureSoil Structure
PedsAggregates
Units of soil Structure
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Chemical Processes –
electrostatic attraction between clayparticles and between organic particles.
Biological Processes-macro-organisms (burrowing, tunneling,wastes)
-roots, fungal hyphae (compression, fibers, exudates)
-microorganisms (organic residues)
Formation
Soil StructureSoil Structure
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Importance
Effects on porosity, water retention, water movement
Small pores
Large pores
Intra-aggregate pores
Inter-aggregate pores
(within)
(between)
Water moves easily and is poorly retained in inter-aggregate poresWater moves slowly and is strongly retained in intra-aggregate pores
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Soil Structure is Desirable
Poor soil structure caninhibit infiltration of water, water movement,growth of roots.
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Structure is the arrangement or grouping of individual soil particles into larger secondary units.
Clays and organic matter possess natural electricalCharge which can electrostatically bind particles together.
Macro-organisms and microorganisms can aid inbinding of individual particles into larger aggregates..
Aggregation allows for both movement and retentionof water via macropores and micropores, respectively..
Poor soil structure can inhibit infiltration of water, water movement, growth of roots.
Summary
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Soil Density
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DensityDensity
Density = Mass Volume
gcm3( )
2.65 g/cm3
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Soil Bulk DensitySoil Bulk Density
Density of soil including the particles and Density of soil including the particles and the pore spacesthe pore spaces
xy
z
Volume = xyz
B.D. ranges between 1.1 and 1.6 g/cm3
(for mineral soil with 1 – 5% organic matter)
BD = mass OD soil volume solids + pores
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Bulk vs. Particle DensityBulk vs. Particle DensityBulk density measures the mass of the soil solids in relation to the volume of the soil solids and the soil pores.
xy
zParticle density =
Mass of particle
Volume of particle
No pores
Bulk density = mass solids
volume xyz
BD = 1.6 g/cm3 PD = 2.65 g/cm3
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Sampling for Density
Known Volume
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Known Volume
Dry and Weigh: mass/volume = Bulk Density
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Factors Affecting Bulk DensityFactors Affecting Bulk Density
• organic matter• aggregation• arrangement of particles• compaction• depth in profile
Porosity (pores are weightless)
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Organic MatterOrganic Matter
Typical 1-5% organic matterMineral Soil:
bulk density = 1.1 –1.6 g/cm3
Organic Soils: > 20% organic matter
bulk density = 0.1 – 0.6 g/cm3
Factors Affecting Bulk DensityFactors Affecting Bulk Density
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AggregationAggregation
One sand grain One aggregate ofClay sized particles(Zero porosity)(50% porosity)
Aggregation generally increases overall porosity, decreases density
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Packing ArrangementPacking ArrangementDiscrete particle size classesContinuum of particle sizes
Particle Size
Arrangement(compaction)
Sorting
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Depth in ProfileDepth in Profile
Lower organic matterFewer rootsCompaction from above
=> Higher bulk density
Aggregation can mitigateSome of these effects.
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Summary Summary
Bulk densities of typical mineral soilsrange between 1.0 and 1.6 g/cm3.
Organic matter increases porosity, decreases BDOrganic 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
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Porosity
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Bulk Density and Total PorosityBulk Density and Total Porosity
0 2.65Bulk Density (g/cm3)
100%Porosity
)( BDPD
1 - % Porosity = [ ] X 100
2.65 g/cm3
Bulk density high porosity lowBulk density low porosity high
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A
E
B
BD = 1.1 g/cm3
BD = 1.15 g/cm3
BD = 1.6 g/cm3
P = 59 %
P = 57%
P = 40 %
Bulk Density and Porosity
A
EE
B
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Pore Size DistributionPore Size Distribution
Macropores > 0.8 mm in diameterlarge, freely draining
sands, inter-aggregate pores
Micropores < 0.8 mm in diametersmall, storage of water
clays, intra-aggregate pores
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The effect of total porosity and of pore size distribution is largelyrelated to the movement and retention of water as well asthe 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
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