Soils

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

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.

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

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.

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

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.

Silt

Does not feel grittyDoes not feel gritty Floury feel –smooth Floury feel –smooth

like silly puttylike silly putty

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.

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

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

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

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

Textural Triangle

Sandy Soils

Coarse Coarse texturetexture SandsSands Loamy Loamy

sandssands

Loamy Soils

Moderately Moderately coarse coarse texturetexture Sandy loamSandy loam Fine sandy Fine sandy

loamloam

Loamy Soils- Coarse

Medium textureMedium texture Very fineVery fine

sandy loam sandy loam LoamLoam Silt loamSilt loam SiltSilt

Loamy Soils - Fine

Moderately Moderately fine texturefine texture Sandy Sandy

clay loamclay loam Clay loamClay loam Silty Silty

clay loamclay loam

Clayey Soils

Fine textureFine texture Silty claySilty clay ClayClay Sandy claySandy clay

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

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?

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.

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.

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

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.

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

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)

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))

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.

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

B horizon

SubsoilSubsoil Clay and many Clay and many

mineralsminerals IronIron AluminumAluminum CalciumCalcium

Leached from layers Leached from layers aboveabove

C horizon

Parent RockParent Rock Can be saturated in Can be saturated in

groundwatergroundwater

Soil Types

Mollisols

Fertile dark soilsFertile dark soils Found: Temperate grassland biomeFound: Temperate grassland biome Best agriculture soilsBest agriculture soils

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

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

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

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