Characterizing Soil HorizonsCharacterizing Soil Horizons

Physical Properties of Soils

SolidsVoids

AvenuesStorageDistributionMovement

Interactive MediaMineralsOrganic matterReactivity

Idealized Surface Soil

Parent Material to Soil

Parent Material

Bedrock

Additions

Losses

Translocations

Transformations

Bedrock

Differentiation

Soil Horizons

Roughly parallel layers in the soil with varying composition and properties

Criteria for Characterizing Criteria for Characterizing Soil HorizonsSoil Horizons

ColorColor

TextureTexture

DensityDensity

StructureStructure

Organic matterOrganic matter

MineralogyMineralogy

ChemistryChemistry

Value

Chroma

Hue

Soil ColorSoil Color

dominant spectral color; related to the wavelengthof light. Related to the proportions of red to yellow.

related to total amount of light reflected. measure of the strength

of spectral color

Hue = 10 YRValue = 6Chroma = 3

Munsell Color 10 YR 6/3

Physical Criteria for Delineating HorizonsPhysical Criteria for Delineating Horizons

ColorColor

TextureTexture

StructureStructure

DensityDensity

The Soil Mineral Component: Texture

the size of soil particles

Soil texture refers to the relative amounts of three distinct sizeseparates comprising the soil

mineral component.

Sand Silt Clay

Sizes classes of particles

Soil TextureSoil TextureDiameter

Sand (2.0 – 0.05 mm) Quartz

Silt (0.05 – 0.002 mm) Quartz /Feldspars/mica

Clay (<0.002 mm) Secondary minerals

Dominant Minerals

sandsiltclay

100%Quartz Distribution

Class

Importance of Soil Texture

Soil PorosityParticle Surface Area

(Distribution of particle sizes)

Water/Gas MovementReactivity

Soil Porosity

Porosity – the total volume of soil pores- the distribution of pore sizes

Sand Silt Clay

Texture, Pore Sizes, and Water

Large particles yield large pore spacesSmall particles yield small pore spaces

Water moves rapidly and is poorly retained in Coarse-textured sandy soils.

Water moves slowly and is strongly retained inFine-textured, clayey soils.

Sands

Clays/iron

Rapid

Slow

Water Retained

Poor Retention

Surface Area and Particle Size

Each face is 4 mm2 Each face is 1 mm2

6 faces x 4 mm2 = 24 mm2 6 faces x 1mm2 x 8 cubes = 48 mm2

If each of the resulting cubes was divided similarly, the surface area would increase 16 times more

2 mm 2 mm

2 mm

1 mm

1 mm

1 mm

Surface AreaSurface Area

Specific Surface Area = Surface Area mass

cm2

g

units

Interface with the environment

nutrients

gassesO.M.

water

microorganisms

Specific Surface Area

Separate SSA (cm2/g)Sand 30Silt 1500Clay 3,000,00016 g of clay

Soil A

100 g soil

Soil B

95 g sand 4 g silt 1 g clay

90 g sand 5 g silt 5 g clay

95 g sand x 30 g/cm2 = 2850 cm2

4 g silt x 1500 g/cm2 = 6000 cm2

1 g clay x 3 M g/cm2 = 3 M cm2

Total = 3,008,850 cm2

90 g sand x 30 g/cm2 = 2700 cm2

5 g silt x 1500 g/cm2 = 7500 cm2

5 g clay x 3 M g/cm2 = 15 M cm2

Total = 15,010,200 cm2

Soil Horizons

Texture

Clay Content

Surface Area

Potential Reactivity

Soil Textural Classes

Soil Textural Triangle

Sand <10%Loamy sand 10 – 15%Sandy loam 15 – 20%Sandy clay loam 20 – 35%Sandy clay 35 – 55%Clay > 50%

Florida Soils

clay

70% sand, 20% silt, 10% clay70% sand, 20% silt, 10% clay

60% sand, 10% silt, 30% clay60% sand, 10% silt, 30% clay

Measuring Soil Texture

Texture-by-Feel

Relative amounts of 3 soil separates:

Sand, Silt, and Clay

Gritty smooth plastic

Texture-by-FeelSoil

SandNo Ball

Loamy SandNo Ribbon

Grittiness, Smoothness

Low Clay Medium Clay High Clay

Increasing Ribbon Length

Field Analysis

Texture by Feel

Sand = Gritty

Silt = Smooth

Clay = Sticky, Plastic

Laboratory Analysis of Soil Texture

Laboratory AnalysisLaboratory Analysis

Sedimentation – Sand, Silt, and Clay Fraction

gravity

dragdrag

SandSilt

Clay

SandSilt

Clay

sandsilt

Sedimentation

Quantifying Sedimentation Rates

Stokes’ LawStokes’ Law

Velocity V(cm/s) = g (dp-dL) D2

18ų

g = gravitydp = density of the particledL = density of the liquid

ų = viscosity of the liquid

KV = D2

K = 11,241 cm-1 sec-1

1 cm · sec

Stokes’ LawStokes’ Law

V = K D2

K = 11,241 cm-1 sec-1

Sand: D = 1 mm 0.1 cm

V = 11,241 x (0.1)2

= 112.4 cm/sec

1 cm · sec X cm2

Stokes’ LawStokes’ Law

V = K D2

K = 11,241 cm-1 sec-1

clay: D = 0.002 mm 0.0002 cm

V = 11,241 x (0.0002)2

= 0.00045 cm/sec

1 cm · sec X cm2

050

100150200250

300350400

450500

0 0.5 1 1.5 2 2.5

Particle Diameter (mm)

Set

tlin

g V

elo

city

(cm

/sec

)

sandsilt

clay

SedimentationSedimentation

1 minute Sand settles out

4 hours Silt settles out

sand1 min.silt 4 hr.

suspension

The density of a soil suspension decreasesas particles settle out.

Stem

Scale

Bulb

weight

t = 0 t = 1 min

Hydrometer MethodHydrometer Method

hydrometer

0 g/L

Pure distilled water (18o C) = 0 g/L

Hydrometer MethodHydrometer Method

Time = 0 sec density = 40 g/L

Add 40 g soil to 1 liter of water

Time = 1 min. density = 10 g/L

Sand settled = 40 g– 10 g= 30 g

Sand (%) = 30 g sand = 0.75 = 75%

40 g soil

Time = 4 hrs density = 4 g/L

Hydrometer MethodHydrometer Method

What is being measured?

Clay content = 4 g

Silt % = 100% - (75% + 10%) 100% - 85%

= 15 %

Clay % = 4 g clay = 0.10 = 10% 40 g soil

Sand = 75%Silt = 15%Clay = 10%

Sandy Loam

Hydrometer MethodHydrometer Method

Reactivity, Water Movement

Next: Density, Structure, Porosity