Soil classification and soil maps

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


Three kinds:

1. Central concept2. Intergrades – trending toward another ORDER

Alfic HaplorthodsSpodic DystrudeptsUltic Eutrustox


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


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



Histic – mandatory for Histosols water saturated for 30+ consecutive days (most years) organic materials dominate

Plaggen and Melanic – minor ones, we won’t discuss



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)


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


Glossic -degradation of an argillic (or kandic or natric) horizon -E/Bt or Bt/E -tongues of E material penetrating B material


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


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


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


Placic - thin pan, dark, cemented by Fe, Mn, and/or OC - 2-10 mm thick, brittle

Has many similarities to Spodic horizon


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


Calcic - accumulation of secondary CaCO3

If cemented – Petrocalcic

Bk, Bkm

Ifplugged - Bkk


Gypsic - rich in secondary gypsum/sulfates - sea spray and gypsic parent materials are sources - difficult to differentiate from Bk

By, Bym

If cemented - petrogypsic


Salic - soluble salts (more soluble than gypsum)

Bz

Sulfuric - pH < 3.5, yellow (jarosite) mottles - toxic to plants - in tidal marshes



Fragipan – Btx, Bx, Ex

Duripan – silica cementation - Xeric soil moisture regimes - Bqm


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

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Soil classification and soil maps