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Soils
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Soil Texture = %Sand, Silt & Clay in a soil.
Soil texture is the single most important Soil texture is the single most important
physical property of the soil. Knowing the physical property of the soil. Knowing the
soil texture alone will provide information soil texture alone will provide information
about: about: 1) water flow potential, 1) water flow potential,
2) water holding capacity,2) water holding capacity,
3) fertility potential, 3) fertility potential,
4) suitability for many urban 4) suitability for many urban
uses like bearing capacity uses like bearing capacity
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Texture The Percent of sand, silt, The Percent of sand, silt,
clay in a soil sampleclay in a soil sample Critical for understanding Critical for understanding
soil behavior and soil behavior and managementmanagement
Soil texture is not subject Soil texture is not subject to change in the field but to change in the field but can be changed in can be changed in potting mixes.potting mixes.
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Particle Diameter Size Soil particle diameters Soil particle diameters
range over 6 orders of range over 6 orders of magnitudemagnitude 2 m boulders 2 m boulders Coarse fragments > 2 Coarse fragments > 2
mmmm Sand < 2 mm to 0.05 Sand < 2 mm to 0.05
mmmm Silt < 0.05 mm to 0.002 Silt < 0.05 mm to 0.002
mmmm Clay < 0.002 mClay < 0.002 m
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Sand
Feels gritty Feels gritty Considered non-Considered non-
cohesive – does cohesive – does not stick not stick together in a together in a mass unless it is mass unless it is very wet.very wet.
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Sand
Sand has less nutrients Sand has less nutrients for plants than smaller for plants than smaller particlesparticles
Voids between sand Voids between sand particles promote free particles promote free drainage and entry of airdrainage and entry of air
Holds little water and Holds little water and prone to droughtprone to drought
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Silt
< 0.05 mm to > 0.002 < 0.05 mm to > 0.002 mmmm
Not visible without Not visible without microscopemicroscope
Quartz often dominant Quartz often dominant mineral in silt since mineral in silt since other minerals have other minerals have weathered away.weathered away.
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Silt
Does not feel grittyDoes not feel gritty Floury feel –smooth Floury feel –smooth
like silly puttylike silly putty
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Silt Smaller particles – retains Smaller particles – retains
more water for plants and more water for plants and have slower drainage have slower drainage than sand.than sand.
Easily washed away by Easily washed away by flowing water – highly flowing water – highly erosive.erosive.
Holds more plant Holds more plant nutrients than sand.nutrients than sand.
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Silt is Silt is responsible for responsible for silting over silting over gravel beds in gravel beds in rivers that are rivers that are needed by fish needed by fish for spawning.for spawning.
www.pedrocreek.org/ fishcommittee.html
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Clay
< 0.002 mm < 0.002 mm Flat plates or tiny Flat plates or tiny
flakes flakes Small clay particles Small clay particles
are colloidsare colloids If suspended in water If suspended in water
will not settlewill not settle
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Clay Wet clay is very sticky Wet clay is very sticky
and is plastic or it can and is plastic or it can be molded readily into be molded readily into a shape or rod.a shape or rod.
Easily formed into long Easily formed into long ribbons ribbons
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Clay
Pores spaces are very Pores spaces are very small and convoluted small and convoluted Movement of water and air Movement of water and air
very slowvery slow Water holding capacity Water holding capacity
Tremendous capacity to Tremendous capacity to adsorb water- not all adsorb water- not all available for plants.available for plants.
Chemical adsorption is Chemical adsorption is largelarge
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Textural Triangle
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Sandy Soils
Coarse Coarse texturetexture SandsSands Loamy Loamy
sandssands
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Loamy Soils
Moderately Moderately coarse coarse texturetexture Sandy loamSandy loam Fine sandy Fine sandy
loamloam
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Loamy Soils- Coarse
Medium textureMedium texture Very fineVery fine
sandy loam sandy loam LoamLoam Silt loamSilt loam SiltSilt
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Loamy Soils - Fine
Moderately Moderately fine texturefine texture Sandy Sandy
clay loamclay loam Clay loamClay loam Silty Silty
clay loamclay loam
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Clayey Soils
Fine textureFine texture Silty claySilty clay ClayClay Sandy claySandy clay
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Changing Soil Texture
Soil texture can be Soil texture can be changed only by changed only by mixing with another mixing with another soil with a different soil with a different textural class in small textural class in small quantitiesquantities
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Changing Soil Texture
Adding sand to a clay soil Adding sand to a clay soil creates a cement like creates a cement like substancesubstance
Adding peat or compost to a Adding peat or compost to a mineral soil is not considered mineral soil is not considered changing the texture – since it changing the texture – since it only adds organic matter not only adds organic matter not sand, silt or clay.sand, silt or clay.
So why add peat or compost?So why add peat or compost?
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Changes in soil texture
Over long periods Over long periods (1000’s yrs) (1000’s yrs) pedologic processes pedologic processes alter soil horizon alter soil horizon textures.textures.
As soils get older As soils get older sand weathers to silt sand weathers to silt and silt weathers to and silt weathers to clay….therefore old clay….therefore old soils have more clay.soils have more clay.
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Soil Texture
Soil texture can also Soil texture can also be determined by be determined by feeling the soil. feeling the soil.
This procedure takes This procedure takes practice but practice but eventually eventually a person can a person can become become very very proficient and will be proficient and will be able to able to estimate the % estimate the % clay clay within 3% of within 3% of the the actual actual value.value.
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Determining Soil Texture - Feel Method
Wet soil in handWet soil in hand Make ribbonMake ribbon Length of ribbon Length of ribbon
indicates clay contentindicates clay content Grit or lack of grit Grit or lack of grit
indicates sand or siltindicates sand or silt Smoothness indicates Smoothness indicates
siltsilt
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Soil Profiles in different biomes
You should now know plants, animals You should now know plants, animals and soils in the different biomes.and soils in the different biomes.
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Weak humus-mineral mixture
Mosaicof closelypackedpebbles,boulders
Dry, brown toreddish-brown, with variable accumulationsof clay, calciumcarbonate, andsoluble salts
Desert Soil(hot, dry climate)
Grassland Soil(semiarid climate)
Alkaline,dark,and richin humus
Clay,calciumcompounds
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Acidiclight-coloredhumus
Iron andaluminumcompoundsmixed withclay
Forest litterleaf mold
Humus-mineralmixture
Light, grayish-brown, silt loam
Dark brownfirm clay
Acid litterand humus
Humus andiron andaluminumcompounds
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 layerTopmost layer High % of dead organic High % of dead organic
matter. matter. Ie: leaves, stems, fruits, Ie: leaves, stems, fruits,
seeds, pine needlesseeds, pine needles
Formed from Formed from decomposition of decomposition of organic matter. (organic matter. (humushumus))
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A horizon
Known as topsoilKnown as topsoil Mixture of soil from Mixture of soil from
below and the humus below and the humus above.above.
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E horizon
mineral horizon mineral horizon upper part of the soil upper part of the soil
(also called (also called zone of eluviation)
Typically present only Typically present only in forested areas it in forested areas it underlies an O or A underlies an O or A horizon. horizon.
It is a light colored, It is a light colored, leached horizonleached horizon
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B horizon
SubsoilSubsoil Clay and many Clay and many
mineralsminerals IronIron AluminumAluminum CalciumCalcium
Leached from layers Leached from layers aboveabove
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C horizon
Parent RockParent Rock Can be saturated in Can be saturated in
groundwatergroundwater
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Soil Types
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Mollisols
Fertile dark soilsFertile dark soils Found: Temperate grassland biomeFound: Temperate grassland biome Best agriculture soilsBest agriculture soils
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Oxisols
Found: Tropical, Subtropical rain forestsFound: Tropical, Subtropical rain forests Most organic material is found in living plantsMost organic material is found in living plants
Infertile soilInfertile soil
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Alfisols
Moderately weathered forest soilsModerately weathered forest soils Found: Moist temperate forest biomesFound: Moist temperate forest biomes
Most organic material is found in living plantsMost organic material is found in living plants Adequate for agriculture if suppplemented Adequate for agriculture if suppplemented
with ferilizeror organic materialwith ferilizeror organic material
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Aridisols
Thin ligh colored and contain a lot of sand.Thin ligh colored and contain a lot of sand. Found: Dry lands and desertsFound: Dry lands and deserts Susceptible to salinizationSusceptible to salinization
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Be sure to review as these all tie together as we move towards May. Soil degradationSoil degradation
ErosionErosion DesertificationDesertification OvergrazingOvergrazing SalinizationSalinization
Soil conservationSoil conservation Sustainable agricultureSustainable agriculture Fertilizers & PesticidesFertilizers & Pesticides SubsidiesSubsidies
Rock CycleRock Cycle