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, 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

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

Texture and Civilization?

Tigris-Euphrates

Nile

Earliest Civilizations

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

Flooding

Flooding and Soil Texture

Sand 2.0-.05 mmSilt 0.05 – 0.002 mmClay < 0.002 mm

Clay

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

Flooding slows flow

River channel

Sedimentation

Sand Clays/Silts

Flood peaks in mid-September

Blue NileThe Nile

KenyaUgandaTanzania

Mesopotamia

Euphrates

Alluvial Plain

Flood: March through June

Agriculture and IrrigationIrrigationCanalsDikesWeirsReservoirschannels

History and Soil Texture(knowledge from clay and stone)

Stone and Clay

Egypt

Sumer

Stone

Clay

Architecture and Sculpture

Egypt

The Language of Power

sacred, ceremonial, literary, and scientific language

Sumerian

Soil Structure, Density, Porosity

Soil StructureSoil Structure

Arrangement or grouping of individual soil particles into secondary units.

Soil StructureSoil Structure

PedsAggregates

Units of soil Structure

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

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

Soil Structure is Desirable

Poor soil structure caninhibit infiltration of water, water movement,growth of roots.

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

Soil Density

DensityDensity

Density = Mass Volume

gcm3( )

2.65 g/cm3

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

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

Sampling for Density

Known Volume

Known Volume

Dry and Weigh: mass/volume = Bulk Density

Factors Affecting Bulk DensityFactors Affecting Bulk Density

• organic matter• aggregation• arrangement of particles• compaction• depth in profile

Porosity (pores are weightless)

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

AggregationAggregation

One sand grain One aggregate ofClay sized particles(Zero porosity)(50% porosity)

Aggregation generally increases overall porosity, decreases density

Packing ArrangementPacking ArrangementDiscrete particle size classesContinuum of particle sizes

Particle Size

Arrangement(compaction)

Sorting

Depth in ProfileDepth in Profile

Lower organic matterFewer rootsCompaction from above

=> Higher bulk density

Aggregation can mitigateSome of these effects.

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

Porosity

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

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

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

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