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Soil classification and soil maps. Why a classification system for soils? -mapping -taxation -inventory -management/use. Pedon – basic unit of soil classification. Classification system must be: 1. based on quantifiable characteristics of soils - PowerPoint PPT Presentation
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Soil classification and soil maps
Why a classification system for soils?-mapping-taxation-inventory
-management/use
Pedon – basic unitof soil classification
Classification system must be:1. based on quantifiable characteristics of soils2. open-ended (we do not know about every soil…)3. hierarchical (like biological systems)
Also – use new names (avoid old meanings and baggage)
The “old” (1938) system of soil classification
-based on interpretation and assumed genesis-grouped soils based on climate/vegetation assemblages
Three ORDERS in old system
1. Zonal or “normal” soils-climate inputs dominate a soil’s genesis-soil is in equilibrium with the climate/veg
2. Intrazonal soils -salts, wetness or limestone bedrock
overwhelm the soil’s genesis
3. Azonal soils-too young, dry or sandy to have developed
into Zonal soils
Zonal soils – examples
Podzols – cool climates, coniferous forest
Brunizems – tallgrass prairies
Sierozems – desert soils
Laterite soils – red tropical soils
Intrazonal soils – examples
Rendzinas – shallow to limestone bedrock
Gley soils – wet soils
Peat soils
Azonal soils – examples
Dry, sandy soils
Shallow-to-bedrock soils
Alluvial soils (young parent materials)
The new system of SOIL TAXONOMY-adopted in 1975, revisions ever since
Order – highest level (12)
Suborder – (>60)
Great Group – (about 300)
Subgroup – (about 1600)
Family – (>8000)
Series – (>14,000)
The 12 soil orders
• Gelisols – cold soils, permafrost (-el)• Histosols – organic soils, usually Very PD (-ist)• Spodosols – podzolization, cool, moist forested areas (-od)• Andisols – volcanic ash parent materials (-and)• Oxisols – old, weathered soils of the humid tropics, oxide clays (-ox)• Vertisols – shrink-swell clays, wet-dry climate, deep cracks (-ert)• Aridisols – desert soils, some evidence of B horizon formation (-id)• Ultisols – 1:1 clays dominate, red, weathered soils but not like Oxisols (-
ult)• Mollisols – thick, dark A horizon, grassland soils (-oll)• Alfisols – minimally weathered soils, lessivage, not highly acidic (-alf)• Inceptisols – weak B horizon development, some pedogenesis (-ept)• Entisols – most weakly developed soils, no diagnostic B horizon (-ent)
Fine, smectitic, mesic Xeric Argidurids
Aridisol order
Durid suborder
Argidurid Great Group
Xeric Argidurid subgroup
Family
Series: Reno
Sandy, mixed, frigid Typic Haplorthods
More about families
Texture(Particle size family)
Mineralogy Soil temperature regime (Table 7.13 in your book)
Cation exchange class (sometimes) e.g., active, semiactive, superactive
Depth class (sometimes)
Soil MOISTURE REGIME – can be determined from the taxonomic name (we’ve done this already)
MASTCryic <8C (but no permafrost)…like central CanadaFrigid <8C (but warm in summer)…like da UPMesic 8 – 15C……………………………like OhioThermic 15 – 22C………………………like GeorgiaHyperthermic >22C…………………………….like Cuba
The soil TEMPERATURE REGIME is all based on the MAST at 50 cm depth
Four most common ones
Frigid
Mesic
Thermic
Hyperthemic
Cryic
FrigidFrigid
MesicMesic
MesicMesic
MesicMesic
“Iso-” minimal seasonal variation in soil temperature islands, and the tropics
It’s usually possible to estimate the drainage class of a soil, knowing only it’s taxonomic subgroup. Here’s how:
Drainage class subgroup designation
Well drained or drier ------- --------------Moderately well drained Aquic ----------------Somewhat poorly drained Aeric --------aqu-----Poorly drained ------- --------aqu-----Very poorly drained ------- -------------ist
More practice…
Fine-silty, siliceous, active, thermic Aeric Albaqualfs
Coarse-loamy, mixed, superactive, calcareous, mesic Aquic Torriorthents
Clayey, smectitic, shallow Typic Haplocryolls
Coarse-loamy, mixed, active, mesic Aquic Fragiudepts
Loamy-skeletal, mixed, superactive, thermic Cambidic Haplodurids
Loamy, mixed, active, mesic Aquic Arenic Hapludults
Drainage class subgroup designation
Well drained or drier ------- --------------Moderately well drained Aquic ----------------Somewhat poorly drained Aeric --------aqu-----Poorly drained ------- --------aqu-----Very poorly drained ------- -------------ist
More about subgroups…
Three kinds:
1. Central concept – the “Typics”
Typic HaplorthodsTypic DystrudeptsTypic Eutrustox
More about subgroups…
Three kinds:
1. Central concept2. Intergrades – trending toward another ORDER
Alfic HaplorthodsSpodic DystrudeptsUltic Eutrustox
More about subgroups…
Three kinds:
1. Central concept2. Intergrades3. Extragrades – with some other important
or limiting attribute
Lamellic HaplorthodsLithic DystrudeptsAquic Eutrustox
Soil Taxonomy a quantitative system of soil classification, largely based on diagnostic horizons
Diagnostic horizons at the surface – epipedons (all soils have one, and only one)Diagnostic subsurface horizons – soils may have none, one, or several
Epipedons – the major ones
Mollic – mandatory for MollisolsCa++ ions common, base saturation >50%value <3.5 moist, <5.5 drychroma <3.5 moistOC > 0.6%Generally >25 cm thickmoist for 3+ months at a time thru the year
Epipedons – the major ones
Anthropic: resembles mollic, but can be dry high in phosphorous (due to long-term agriculture
or human occupation)
Umbric: same as mollic, but base sat. <50% mountainous or hilly regions moist, acidic soils where OM
decomposition is slowed due to high water table or cold temps
Epipedons – the major ones
Epipedons – the major ones
Histic – mandatory for Histosols water saturated for 30+ consecutive days (most years) organic materials dominate
Plaggen and Melanic – minor ones, we won’t discuss
Epipedons – the major ones
Epipedons – the major ones
Ochric: all others usually light colored, thin forested and dry soils
Entisols, by definition, lack a diagnostic B horizon
Diagnostic subsurface horizons-can be E or B horizons
Diagnostic subsurface horizons
Albic – E horizons (must meet thickness and color requirements)
Diagnostic subsurface horizons
Argillic – clay-enriched Bt horizons - illuvial silicate clays (NOT formed in place)
indicates a stable surface, because illuviation is slow, and therefore it must exceed rate of argillan destruction if argillans are to be presentin sands, clay bridging is acceptable
Required for Alfisols, Ultisols
Diagnostic subsurface horizons
Glossic -degradation of an argillic (or kandic or natric) horizon -E/Bt or Bt/E -tongues of E material penetrating B material
Diagnostic subsurface horizons
Natric: same as argillic, but has either: (1) prisms/columns in upper part, or (2) high Na content
Bn or Btn
The poop on Na+
At low electrolyte concentrations, Na+ favors dispersion, just like other +1 cations
(Na in upper profile can favor lessivage)
At high electrolyte concentrations, as is typical of most desert and dryland soils, Na enhances flocculation.Btn becomes very clay-rich, and is virtually impermeable when wet.
Btn
Biscuit tops
Diagnostic subsurface horizons
Kandic – like argillic, but low CEC clays dominate has lots of kaolinite and many oxide clays
BtKandiudult
Only in Ultisols
Illuvial clay ARGILLIC
Na-rich - Natric
Degraded - Glossic
Oxide clays – Kandic
Diagnostic subsurface horizons
Spodic - amorphous Al and humus, with or without Fe - sandy soils, little clay to inhibit podzolization - rapidly forming, under forest or heath veg.
Bh, Bs, Bhs, Bsm, Bhsm
Required for Spodosols
Diagnostic subsurface horizons
Placic - thin pan, dark, cemented by Fe, Mn, and/or OC - 2-10 mm thick, brittle
Has many similarities to Spodic horizon
Diagnostic subsurface horizons
Oxic - mineral horizon in advanced stage of weathering- clay minerals are 1:1 or oxides- residual accumulation of Al, Ti, Fe and Mn- very old, stable surfaces- tropical/subtropical, wet/dry climates
Bo
Required for Oxisols
Diagnostic subsurface horizons
Calcic - accumulation of secondary CaCO3
If cemented – Petrocalcic
Bk, Bkm
Ifplugged - Bkk
Diagnostic subsurface horizons
Gypsic - rich in secondary gypsum/sulfates - sea spray and gypsic parent materials are sources - difficult to differentiate from Bk
By, Bym
If cemented - petrogypsic
Diagnostic subsurface horizons
Salic - soluble salts (more soluble than gypsum)
Bz
Sulfuric - pH < 3.5, yellow (jarosite) mottles - toxic to plants - in tidal marshes
Diagnostic subsurface horizons
Diagnostic subsurface horizons
Fragipan – Btx, Bx, Ex
Duripan – silica cementation - Xeric soil moisture regimes - Bqm
Diagnostic subsurface horizons
When all else fails…. Cambicweak "color" B horizon Bw horizon (some Bg horizons)must lack geologic structure or have soil structurenot sandy (Bw forms too quickly in sands)
Having a Cambic horizon kicks the soil OUT of Entisols and (usually) into Inceptisols
BwBw