soils
The soil profile
A vertical section of soil showing the various horizons from the surface to the unaffected parent material
A horizon is “A layer, approximately parallel to the surface of the soil that is distinguishable from adjacent layers by a distinctive set of properties produced by the soil process
O horizon - organic material not mixed with minerals
Oi - decomposed organic matter
Oe - moderately decomposed organic matter
Oa - highly decomposed organic matter
A horizon, highest density of root growth, leading to organic matter deposition from roots
E horizon - zone of eluviation, or leaching must be underlain by B horizon, but can be in place of an A horizon
B horizon - a zone of illuviation - or accumulation due to the leaching from above horizons of: Fe and Al oxides, Ca carbonate and Ca sulfate
C horizon - less affected by soil forming processes + outside zone of major biological influence; may have accumulation of Ca, MG, carbonates, sulfates
R/ (Regolith) = unaltered parent material - likely rock
Why do E horizons occur in forest but not in grassland soils
Possible reasons:
Greater rainfall in forests -> greater effective precipitation for leaching
More acidic leaf litter in forests (compared to more neutral pH roots of grasslands -> more organic acids in leaching processes
Organic inputs more stratified to O horizon (from leaf fall) rather than distributed throughout surface soil ( as in grasslands) → fewer high-activity surfaces in surface soil
Time as a soil forming factor
Warm humid climate residual parent igneous material
Soils generally develop more prominent layers over time
Warm, subhumid climate
Calcareous loess parent material
Transition horizons and subdivisions within horizon
Transitional horizons that combine properties of two horizons, dominant listed first
Subdivisions within horizons
Used to differentiate differences in structure or color within a master horizon.
Sub Horizons - More specific designations of master horizons but their formation can be pronounced
EX;
b - buried horizon
Ap - ploughed horizon
k - accumulation of carbonates
kk- engulfment of carbonates
t - accumulation of silicate clays
w - Distinctive color or structure without clay accumulation
Soil Classification 1
Humans like to organize things - taxonomic classes of plants/animals
World reference Base (WRB)
An international system for soil classification, supported by FAO, UNESCO and other orgs
Soil Taxonomy
Developed by USDA, most commonly used in U.S. because we have to be different…
6 Taxonomic levels in USDA soil
Criteria for classification encompass chemical, physical, and biological processes
Temperature and moisture status throughout year
Color, texture, structure of soil
Contents of organic matter, aFe, Al, SIlicate clays, salts, the pH
Precise soil classification using this taxonomy may be expensive or time consuming
Cation Exchange Capacity (CEC)
Fundamental soil property
Plant nutrient availability and retention
Total quantity of negative surface charges
Sum of cations: base cations + acid Cations
(Ca+Mg+K+Na)+(H+Al)
Base saturation
Percent of CEC occupied by base cations
Base saturation (%)
=(Ca+Mg+K+Na)/CEC
Increases as pH increases
Diagnostic horizons
Horizons whose presence or absence indicates a soil’s location in the taxonomy
Epipdeon
Surface diagnostic horizon
A,E, or sometimes upper part of B
Subsurface diagnostic horizon
Usually B, sometimes E or C
Mollic Epipedon
From latin mollis, Meaning soft
>0.6% organic C
Generally >25cm thick
Softness even when dry
High base saturation (>50%)
Moist at least 3 months of the year
Characteristic of grasslands
Umbric epipedon
From latin Umbra, meaning shade
Similar to the mollic epipedon EXCEPT
%base saturation is lower
Develops in higher rainfall areas and w parent materials low in Ca and Mg
Ochric epipedon
From greek ochros, pale
Too thin
Too light in color or
Too low in organic matter to be mollic or umbric
Due to low organic matter content, may be hard and massive when dry
The “wimpy” a
Histic epipedon
Greek Histos tissue
A thick surface organic horizon that is 20-60 cm thick overlying mineral soil
Formed in wet areas, it is a layer of peat or much with very dark color.
Diagnostic subsurface horizons
Important for illuvial material to have been transported
Entisols
Recent soils: mineral soils with little to no evidence of pedogenic horizons
No diagnostic horizon other than ochric epipedon
Found on landscapes where soil parent material is in no place long enough to pedogenetic processes to act
Inceptisols
More strongly developed soil profiles than those of entisols, but too weakly developed to meet criteria for any other soil horizon
Ochric or umbric epipedon, cambic horizon
Gelisols
Greek gelic, very cold
Permafrost present within specified depth
Over permafrost, gaelic materials “mineral or organic soil materials that show evidence of cryoturbation (frost churning) or ice segregation
May have diagnostic horizons
Cryoturbation
Mixing of materials form various horizons, down to bedrock, due to churning actions of repeated freeze/thaw
Andisols
Jap. ando, black soil
Soil developing pon volcanic ejecta
Dominated by allophane and “al-humus” complexes
Accumulation of organic matter complex with Al
May have diagnostic horizon and occur at any temp, or moisture, or elevation
Aridisols
Latin aridus - dry
Dry environment: no period of 90 consecutive days when moisture is contoniusly available for plant growth
Ochric epipedon, sometimes argillic or natric horizon
Histosols
Gk, histos , tissue
Accumulation of organic material due to wetness (in absence of permafrost)
Organic material is more than half of top 80cm
Can occur in any climate but most prevalent in cold
Precursor to coal
Usually bog like
Vertisols
L. Verto, turn
Dark swelling and cracking clay (>30% clay)
Problematic for constriction of any sort
Challenging for agricultural use: “24 hour soils”
Shrink/swell can impede infiltration in wet vertisols
Alfisols
Occur in cool to hot humid areas, usually forested
Diagnostic sub horizons: Bt or Btn, >35% base saturation
Overlain by ochric or umbric epipedon
trees
Mollisols
L, Mollis, soft,
Accumulation of organic matter form grassland root systems; rich in Ca
Have a mollic epipedon
May have argillic, natric, or cambic subsurface horizons, but NOT oxic or spodic horizons
Prairies with old grasses with deep roots
Ultisols
L. Ultimus, last
Weathering of clays + leaching of base cations
Old land surfaces in moist warm climates
B horizon with <35% base cations
FOund on land surfaces that have been recently exposed - hillsides
Found predominantly in southeastern us
Spodosols
Gk, spodos, wood ash
Intensive acid leaching of coarse textured parent material
Diagnostic horizon : spodic horizon - the illuvial accumulation of Fe,Al oxides and/or organic matter
Often overlain by E horizon
Often in forests because of all of the plant material that lives on the topsoil
Spodosols do derive their name from wood ash, but the E horizon is not diagnostic of the spodosol, it is actually the Bh/Bs
Oxisols
Fr, Oxide - oxide
Most highly weathered soils in the taxonomy
Currently or historically humid and warm
Diagnostic horizon Bo
Epipedon : ochric or umbric
Usually uniform subsurface