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Soils

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Soil Texture = %Sand, Silt & Clay in a soil.

Soil texture is the single most important

physical property of the soil. Knowing the

soil texture alone will provide information

about:

1) water flow potential,

2) water holding capacity,

3) fertility potential,

4) suitability for many urban

uses like bearing capacity

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Texture The Percent of sand, silt,

clay in a soil sample

Critical for understandingsoil behavior andmanagement

Soil texture is not

subject to change in thefield but can be changedin potting mixes.

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Particle Diameter Size

Soil particle diametersrange over 6 orders of

magnitude2 m boulders

Coarse fragments > 2 mm

Sand < 2 mm to 0.05 mm

Silt < 0.05 mm to 0.002mm

Clay < 0.002 m

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Sand

Feels gritty

Considered non-cohesive doesnot stick togetherin a mass unless it

is very wet.

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Sand

Sand has less nutrients forplants than smaller particles

Voids between sandparticles promote freedrainage and entry of air

Holds little water and proneto drought

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Silt

< 0.05 mm to > 0.002 mm

Not visible without

microscope Quartz often dominant

mineral in silt since

other minerals haveweathered away.

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Silt

Does not feel gritty Floury feel smooth like

silly putty

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Silt Smaller particles retains

more water for plants andhave slower drainage thansand.

Easily washed away byflowing water highlyerosive.

Holds more plant nutrients

than sand.

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Silt isresponsible forsilting over

gravel beds inrivers that areneeded by fishfor spawning.

www.pedrocreek.org/ fishcommittee.html

http://www.pedrocreek.org/fishcommittee.htmlhttp://www.pedrocreek.org/fishcommittee.html

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Clay

< 0.002 mm Flat plates or tiny flakes Small clay particles are

colloidsIf suspended in water will

not settle

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Clay

Wet clay is very stickyand is plastic or it can

be molded readily into ashape or rod.

Easily formed into long

ribbons

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Clay

Pores spaces are very smalland convolutedMovement of water and air

very slow

Water holding capacityTremendous capacity to

adsorb water- not all availablefor plants.

Chemical adsorption is large

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Textural Triangle

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Sandy Soils

CoarsetextureSands

Loamy sands

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Loamy Soils

ModeratelycoarsetextureSandy loam

Fine sandyloam

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Loamy Soils- Coarse

Medium texture Very fine

sandy loam

Loam

Silt loam

Silt

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Loamy Soils - Fine

Moderately

fine textureSandy

clay loam

Clay loam

Siltyclay loam

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Clayey Soils

Fine textureSilty clay

Clay

Sandy clay

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Changing Soil Texture

Soil texture can bechanged only by mixingwith another soil with adifferent textural class

in small quantities

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Changing Soil Texture

Adding sand to a clay soilcreates a cement like substance

Adding peat or compost to amineral soil is not consideredchanging the texture since itonly adds organic matter notsand, silt or clay.

So why add peat or compost?

Ch i il t t

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Changes in soil texture

Over long periods(1000s yrs) pedologicprocesses alter soilhorizon textures.

As soils get older sandweathers to silt andsilt weathers toclay.therefore oldsoils have more clay.

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Soil Texture

Soil texture canalso be determinedby feeling the soil.

This proceduretakes practice buteventuallya person canbecome

very proficient andwill be able toestimate the % claywithin 3% of theactual value.

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Determining Soil Texture - Feel Method

Wet soil in hand

Make ribbon

Length of ribbonindicates clay content

Grit or lack of grit

indicates sand or silt Smoothness indicates

silt

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Soil Profiles in different biomes

You should now know plants,

animals and soils in the differentbiomes.

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Weak humus-

mineral mixture

Mosaic

of closely

packed

pebbles,boulders

Dry, brown to

reddish-brown, with

variable accumulationsof clay, calcium

carbonate, and

soluble salts

Desert Soil

(hot, dry climate)

Grassland Soil

(semiarid climate)

Alkaline,

dark,

and rich

in humus

Clay,

calcium

compounds

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Acidiclight-coloredhumus

Iron andaluminumcompounds

mixed withclay

Forest litter

leaf mold

Humus-mineral

mixture

Light, grayish-

brown, silt loam

Dark brown

firm clay

Acid litterand humus

Humus andiron and

aluminumcompounds

Light-coloredand acidic

Tropical Rain Forest Soil(humid, tropical climate)

Deciduous Forest Soil(humid, mild climate)

Coniferous Forest Soil(humid, cold climate)

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O horizon

Topmost layer

High % of dead

organic matter. Ie: leaves, stems, fruits,

seeds, pine needles

Formed from

decomposition oforganic matter.

(humus)

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A horizon

Known as topsoil

Mixture of soil from

below and the humusabove.

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E horizon

mineral horizon

upper part of the soil

(also called zone ofeluviation)

Typically present

only in forested areas

it underlies an O or A

horizon.

It is a light colored,

leached horizon

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B horizon

Subsoil

Clay and many

minerals Iron

Aluminum

Calcium

Leached from layers

above

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C horizon

Parent Rock

Can be saturated in

groundwater

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Soil Types

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Mollisols

Fertile dark soils

Found: Temperate grassland biome

Best agriculture soils

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Oxisols

Found: Tropical, Subtropical rain forests

Most organic material is found in living plants

Infertile soil

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Alfisols

Moderately weathered forest soils

Found: Moist temperate forest biomes

Most organic material is found in living plants

Adequate for agriculture if suppplemented

with ferilizeror organic material

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Aridisols

Thin ligh colored and contain a lot of sand.

Found: Dry lands and deserts

Susceptible to salinization

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Be sure to review as these all tie together as we move towards

May.

Soil degradation

Erosion

Desertification

Overgrazing

Salinization

Soil conservation

Sustainable agriculture

Fertilizers & Pesticides

Subsidies

Rock Cycle