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
2. 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)
Xeric Argidurid subgroup
Fine, smectitic, mesic Xeric Argidurids
Family
Aridisol order
Series: Reno
Durid suborder
Argidurid Great Group
More about families
Cation exchange class (sometimes)
e.g., active, semiactive, superactive
Sandy, mixed, frigid Typic Haplorthods
Depth class (sometimes)
Texture
Mineralogy
(Particle size family)
Soil temperature regime
(Table 7.13 in your book)
Soil MOISTURE REGIME – can be determined from
the taxonomic name (we’ve done this already)
The soil TEMPERATURE REGIME is
all based on the MAST at 50 cm depth
Four most common ones
MAST
Cryic
<8C (but no permafrost)…like central Canada
Frigid
<8C (but warm in summer)…like da UP
Mesic
8 – 15C……………………………like Ohio
Thermic
15 – 22C………………………like Georgia
Hyperthermic >22C…………………………….like Cuba
Cryic
Frigid
Mesic
Thermic
Hyperthemic
Frigid
Mesic
“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
Well drained or drier
Moderately well drained
Somewhat poorly drained
Poorly drained
Very poorly drained
subgroup designation
------- -------------Aquic
Aeric
-------------
-----------------------aqu------------aqu-----------------ist
Drainage class
More practice…
Well drained or drier
Moderately well drained
Somewhat poorly drained
Poorly drained
Very poorly drained
subgroup designation
------- -------------Aquic
Aeric
-------------
-----------------------aqu------------aqu-----------------ist
Fine-silty, siliceous, active, thermic Aeric Albaqualfs
Loamy, mixed, active, mesic Aquic Arenic Hapludults
Clayey, smectitic, shallow Typic Haplocryolls
Coarse-loamy, mixed, active, mesic Aquic Fragiudepts
Loamy-skeletal, mixed, superactive, thermic Cambidic Haplodurids
Coarse-loamy, mixed, superactive, calcareous, mesic Aquic Torriorthents
More about subgroups…
Three kinds:
1. Central concept – the “Typics”
Typic Haplorthods
Typic Dystrudepts
Typic Eutrustox
More about subgroups…
Three kinds:
1. Central concept
2. Intergrades – trending toward another ORDER
Alfic Haplorthods
Spodic Dystrudepts
Ultic Eutrustox
More about subgroups…
Three kinds:
1. Central concept
2. Intergrades
3. Extragrades – with some other important
or limiting attribute
Lamellic Haplorthods
Lithic Dystrudepts
Aquic 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 Mollisols
Ca++ ions common, base saturation >50%
value <3.5 moist, <5.5 dry
chroma <3.5 moist
OC > 0.6%
Generally >25 cm thick
moist 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)
Epipedons – the major ones
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
Histic – mandatory for Histosols
water saturated for 30+ consecutive days (most years)
organic materials dominate
Epipedons – the major ones
Plaggen and Melanic – minor ones, we won’t discuss
Epipedons – the major ones
Ochric: all others
usually light colored, thin
forested and dry soils
Diagnostic subsurface horizons
-can be E or B horizons
Entisols, by definition, lack a diagnostic B horizon
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 present
in 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.
Biscuit tops
Btn
Diagnostic subsurface horizons
Kandic – like argillic, but low CEC clays dominate
has lots of kaolinite and many oxide clays
Bt
Only in Ultisols
Kandiudult
Illuvial clay  ARGILLIC
Oxide clays –
Degraded - Glossic
Na-rich - Natric
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.
Required for Spodosols
Bh, Bs, Bhs, Bsm, Bhsm
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
Required for Oxisols
Bo
Diagnostic subsurface horizons
Calcic - accumulation of secondary CaCO3
If cemented – Petrocalcic
Ifplugged - Bkk
Bk, Bkm
Diagnostic subsurface horizons
Gypsic - rich in secondary gypsum/sulfates
- sea spray and gypsic parent materials are sources
- difficult to differentiate from Bk
If cemented - petrogypsic
By, Bym
Diagnostic subsurface horizons
Salic - soluble salts (more soluble than gypsum)
Bz
Diagnostic subsurface horizons
Sulfuric - pH < 3.5, yellow (jarosite) mottles
- toxic to plants
- in tidal marshes
Diagnostic subsurface horizons
Fragipan – Btx, Bx, Ex
Duripan – silica cementation
- Xeric soil moisture regimes
- Bqm
Diagnostic subsurface horizons
When all else fails…. Cambic
weak "color" B horizon
Bw horizon (some Bg horizons)
must lack geologic structure or have soil structure
not sandy (Bw forms too quickly in sands)
Having a Cambic horizon kicks the soil OUT of
Entisols and (usually) into Inceptisols
Bw