SoilChapter 6

Soil Formation & MorphologyPages 159 – 192

Soil formation . . . Parent materials and topography

determine the amount and types of soil formation

Parent materials . . .Unconsolidated materialsSediments of erosion and

weatheringCan vary in mixture and particle size

– rocky, sandy, clayey

CROPT . . . Acronym for soil forming factors . . .

climaterelieforganismsparent materialstime

Climate and soil formation . . .

And climate is?average patterns and conditions of weather (rain, wind, relative humidity, fog, average high and low temperatures, etc.) at a given location over a period of years

Climate and soil formation . . .

vs. weather?current state of the atmosphere with respect to rain, wind, relative humidity, fog, high and low temperatures, etc.

Climate and soil formation . . .

Formation of soils is faster in regions with higher precipitation and higher temperatures . . .

And is slower in more arid regions

Relief and soil formation . . .

How do you spell relief?T-O-P-O-G-R-A-P-H-Y

And topography is?the difference between the high and low areas in a landscape – the natural landscape that is. . . .

Relief and soil formation . . .

Soils develop more quickly and more deeply on terrain with a shallow slope

Rainfall tends to runoff on steep terrain slowing soil development

Rainfall tends to infiltrate on terrain with less slope

Organisms and soil formation . . .

Plants, animals and their residue found in soilalso referred to as biota

Where they exist in large numbers, burrowing animals turn and incorporate materials speeding soil formation

Organisms and soil formation . . .

Microorganisms aid soil development through the decomposition of organic matter

Areas with abundant vegetation contribute humus to the soil

Parent materials and soil formation . . .

Weathering and chemical erosion of parent materials can form secondary minerals or clays

Source of A and B soil horizons

Parent materials and soil formation . . .

Unconsolidated materialsSediments of erosion and weatheringCan vary in mixture and particle size

– rocky, sandy, clayeyWeathering and chemical erosion of

parent materials can form secondary minerals or clays

Time and soil formation . . .Time as related to climate, relief,

organisms and parent materialsSoils can begin to form quickly –

years to decades – as results of river deposits on floodplains

Time and soil formation . . .Glacial sediments may be several

thousand years oldSoil formation can occur rapidly in

warm, humid, forested regions

BajadasMerging and blending of a series of

alluvial fansFormed as alluvium descends

downhillLarge bajadas can take on the form

of gravely plains

AlluviumEroded soil sediments deposited on

land by streamsLarger particles drop out sooner

AlluviumAs particles move further

downslope, particle sizes decrease leaving larger particles behind

AlluviumpH and salinity often increase

moving downslope Carbonates and bicarbonates (of Ca,

Mg) levels increase

Santa Ana RiverDeposits deep alluvial sands and

gravels in Orange County as a result of being watershed of San Bernardino and Riverside Counties

Weathering . . . Chemical action of air and rainwaterBiological action of animals, plants,

fungi, etc. Carbonic acid (H2CO3) formed from

the combination of air and water

Weathering . . .Rocks and minerals decompose and

disintegrate and change characteristics

Clays are formed by weathered or chemically broken-down soils

Basalts are an example of a clay producing mineral

Erosion . . . Wearing away of land surfaces by

wind, water, ice, and other geologic forces

Physically eroded soils produce sands and silts

Serpentine soils . . . Derived from ultramafic rockLow in silica High in magnesium and ironLow calcium to magnesium ratio

Serpentine soils . . .Low in essential nutrients – nitrogen,

phosphorus and potassiumPlants characteristic to serpentine

soils are called serpentine

Organic matter . . .Rich in humusHolds cationsHolds nutrientsReduces pHIncreases water-holding capacitiesIncreases soil porosityChanges structure

Department of repetitive redundancy department . . .In flatter areas with warmer, wetter

weather, soils form faster

In colder, dryer areas with more slope soils form more slowly

Soil horizons . . .

O-Horizon . . .Organic horizon Surface layer

inc. leaves, moss and other plant materials

Rich in organic matter in various stages of decomposition

O-Horizon . . .The O-Horizon can be further divided

into two layers . . .Oi – where organic mater is still

identifiableOa – where organic matter is

becoming highly decomposed

A- Horizon . . .‘Topsoil’ layer Often darker in color and contain more

organic matter than deeper layersMay contain less clay and sesquioxides

(metal oxides)Most biological activity occurs Closely associated with plant root

growth

A- Horizon . . .May be rich in soil organisms: worms,

nematodes, fungi and bacteriaThe term “biomantle” can only be

used if biological activity does not extend deeper into subsequent horizon layers

‘Eluvial’ layer – layer in which materials leach (migrate) from

E-Horizon . . .Leached by waterOrganic matter and

clays may be removed by leaching

‘Eluviated’ layer (migrated out from)

E-Horizon . . .Often pale containing mostly

silicates Only present in older, well-

developed soilsGenerally occurs between the A- and

B-horizons

B-Horizon . . .‘Subsoil’ layer‘Illuviated’ layer (migrated into)Leached minerals may accumulate –

clay minerals like iron or aluminum

B-Horizon . . .Organic materials

may accumulateMay have more

intense colors or a stronger chroma than the a-horizon

C-Horizon . . .Follows the A- and

B-HorizonsMostly unweathered

materials Contains mostly

parent materials

D-Horizon . . .May be recognized by contrasting

pedologic organization between it and overlying layers

Found below layers referred to as ‘solum’ (O-, A-, E-, and B-horizons)

R-Horizon . . .Partially weathered

bedrock at the base of the soil profile

Degradation of Soils . . .Soils with various accumulations of

different minerals

Soils with accumulated salts

Calcic – accumulated carbonatesGypsic - accumulated gypsumSalic – accumulated soluble salts

Soils with accumulated clays

Argillic – accumulated clayKandic – accumulated low-activity

clayNatric – accumulated clay with

sodium

Soils with accumulated humus

Orstein – cemented soils; high in humus and aluminum

Sombric – acidic; high in humus without aluminum

Spodic – acidic; high in humus and aluminum

Soils with hardpansDuripan – silica cemented soilsFragipan – brittle soilsPetrocalcic – soils cemented by

carbonatesPetrogypsum – soils cemented by

gypsumPlacic – soils cemented by iron

Soils showing losses of materials

Albic – leached and light coloredGlossic – leached, degraded clay

layer

Other conditionsAgric – caused by tillageCambic – showing little developmentOxic – excessively weathered PesticidesOther toxic materials