Chapter 7

 Understand categories of the U.S. taxonomic system
 How soil properties help distinguish soil families
 Knowledge of locations of various soil orders
 Investigate how soil temps and moistures are categorized

Soils are named, mapped as a geologic entity or individual
1 st taxonomic system began in 1938
NRCS began extensive use of the system in
1965
12 orders separate all soils
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Pedons are identified to help separate soil orders
(minimum 3.3 ft 2 , & as deep as roots grow)

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Order
 Most general category
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Histosols
 Organic soils
Entisols
 Undeveloped soils
Inceptisols
 Slightly developed
Andisols
 Volcanic material
Vertisols
 Swelling-clay

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Gellisols
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Must have permafrost in the top 6’
Mollisols
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Most extensive soils in the U.S.
Naturally fertile, slightly leached
Can be semiarid to subhumid climates
Alfisols
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Fertile in favorable moisture conditions
Usually very productive
Ultisols
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Leached, acidic
Moderate to low fertility

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Aridisols
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Arid-region soils
Can be very productive
Oxisols
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Hot, wet tropics
Conducive to year-round plant growth
Spodosols
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Found mostly in cool climates
Poorest soils for cultivation
Must have lime & fertilization to grow crops

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Suborder
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Differentiated based on soil properties & horizons
Soil moisture, soil temp, dominating effects of chemical or textural features
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Great Groups
 Differentiated by soil horizons & soil features
 Accumulated clay, iron, humus, hard pans/cement layers
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Subgroup
 Three kinds of subgroups
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Represent the central ( typic ) concept of the soil group
Properties that intergrade towards other groups, etc.

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 Properties that prevent its classification as typic or an intergrade to another category
Family
 Soil properties important to the growth of plants
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Behavior of soils when used for engineering
Important soil properties: texture, mineralogy, pH, avg. soil temp, moisture, permeability, thickness of horizons, structure, consistency
Series
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18,000 soil series identified
Typically named after something local
Differentiated on the basis of observable & mappable soil characteristics

 Must have similar color, texture, structure, consistency, thickness, pH, similar horizon arrangements, similar chemical & mineralogy properties
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Phase
 Not considered to be an official category, but being used to further differentiate, if needed
 Further delineates soils w/in a series

Soil Moisture & Temperature Regimes
 Soil Moisture Regimes
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Attempt to indicate the extent of naturally available water in the soil depth of maximum root proliferation
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Aquic
 Wet w/ anaerobic saturation long enough to produce visual evidence of poor aeration
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Peraquic
 Tidal marsh or inland depression where groundwater is always at or near the surface

Soil Moisture & Temperature Regimes
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Udic
 Usually has adequate water throughout the yr
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Perudic
 Extremely wet, percolation in all months when not frozen
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Ustic
 Moisture is limited but is present during the growing season
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Xeric
 Deficient in water & w/ a dry cropping season
 Most precipitation in the winter

Soil Moisture & Temperature Regimes
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Aridic
 Very water deficient
 Long dry periods, short wet periods
 Soil Temperature Regimes
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Based on the mean annual soil temp (MAST)
Mean summer soil temp
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Mean winter soil temp
Determined at ~2’ depth
Cryic
 MAST 46 ° F

Soil Moisture & Temperature Regimes
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Frigid
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MAST 46
° F
Mean summer temp >11 ° F higher than mean winter temp
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Mesic
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MAST 46 - 59 ° F
Mean summer temp >43 ° than mean winter temp
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Thermic
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MAST 59 - 72
° F
Mean summer temp >11 ° than mean winter temp

Soil Moisture & Temperature Regimes
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Hyperthermic
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MAST >72
° F
Mean summer temp >11 ° than mean winter temp
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Add iso prefix to the classification if the mean summer & winter temps vary <11 ° F

Additional Terminology for Family
Groupings
 Particle-size Classes
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Fragmental
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Mostly stones, cobbles, gravel, etc.
Fine-earth component <10% of soil volume
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Sandy skeletal
 >35% rock fragments
 Fine-earth fraction is sand, sandy loam
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Loamy skeletal
 >35% rock fragments
 Fine-earth fraction loamy

Additional Terminology for Family
Groupings
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Clayey skeletal
 >35% rock fragments
 Fine-earth fraction >35% clay
Sandy
 Texture is sand, loamy sand
Loamy
 Finer than sand or loamy, <35% clay
Clayey
 >35% clay
Very fine
 >60% clay

Additional Terminology for Family
Groupings
 Soil Mineralogy Classes
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Ferritic: >40% iron oxide in fine-earth fraction
Kaolinitic: >50% kaolinite & other 1:1 or nonexpanding clay
Carbonatic: >40% carbonates plus gypsum
Magnesic: >40% magnesium-silicate minerals
Smectitic: clayey soil w/ more smectite than any other clay
Siliceous: >90% silica minerals
Mixed: not dominated by any mineral type

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Soil orders diverse & unevenly distributed
Some land areas don’t correspond to any order
 See inside front & back covers for distributions of soil orders

 Occur in tundra regions
 Cold & relatively barren
 May have exposed rock intermixed
 Yukon & Northwest Territories of Canada, northern 2/3 of Alaska

 Organic soils formed in cold or wet regions
 Can occur almost anywhere
 Found in FL, LA, GA, some in the Great
Lakes states
 Large area found in Canada

 Lack horizons due to being a young soil, or weathering is ineffective
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Widely distributed in the U.S.
Include river floodplains, rocky soils, mountainous areas, barren islands of East &
Gulf coasts, beach sands
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Found on all continents
Can be excellent ag soil, but may be very unproductive

 Weakly developed soils
 More development than Entisols
 Mostly found in Middle Atlantic & Pacific states, northern Rockies
 Develop in many climates
 Largest area globally found in China

 Weakly to moderately developed
 Most from volcanic materials
 Extensively found in the Pacific Ocean &
Hawaii
 Some also in the northwestern U.S.

 Long dry periods, short periods of wetness
 Found in the U.S. primarily in the western mountain states & Pacific states
 Low rainfall, scattered grasses, desert shrubs
 Rank second worldwide in area to Entisols

 Dark-colored soils of grasslands & some hardwood forests
 Deep, dark-colored, fertile A horizon (mollic epipedon)
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Mollic epipedon extends from surface to ~2’ depth
 Properties & Classification of Mollisols
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Large number of suborders
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Naturally fertile
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Easily managed

 Management of Mollisols
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Formed under grasses, forests
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Tend to be most fertile soil
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High humus content
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High concentration of N
In wetter climates don’t need irrigation, but in dryer areas can produce highly
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May become acidic
Black soil colors to depth of 23’

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1/5 of the U.S. soils
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Can withstand much variation in cropping
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Limited leaching
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Quite fertile, even w/out fertilization
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Little to no lime needed
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Only Alfisols may have high natural productivity
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Texture, depth, climate make it ideal for cropping and highly valuable

 Distribution of Mollisols
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Found in the Great Plains region of the U.S. extending north into Canada, south to Gulf of
Mexico
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Most extensive of U.S. soil orders

 Self-mixing soils, >30% shrink/swell clays
 Found mostly in central & southeastern TX & along lower Mississippi River

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Usually enough precipitation to move clays downward & form an argillic (clay accumulation) horizon
Generally have high CEC’s, usually fairly fertile
Properties & Classification of Alfisols
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Medium to high supply of basic cations
 Evidence of mild leaching
Water is adequate for plant growth for 3+ warm season mos.

 Management of Alfisols
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If topography & climate are favorable; alfisols can be very productive
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Most are leached of lime, and can have an acidic zone
 If leached enough, forms an E horizon
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Erosion exposing clays at the surface not favorable for plant growth
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Most naturally productive soils w/out fertilization or irrigation

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Usually will require lime amendments
Alfisols & Mollisols generally located in a region’s breadbasket
 Distribution of Alfisols
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North-central states & mountain states
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Occur on all continents
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Corn belt of IN, OH, MI, WI, IL
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Woodland soils in TX, CO

 High sand content
 High rainfall w/ easy leaching
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Humus, colloids leached
 Cold, wet climates are common
 Mostly found in Cascade Mountains in WA,
OR, New England, & Great Lakes states
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Also found in FL

 Warm, humid regions
 Usually too acidic to be classified as a
Mollisol/Alfisol, not weathered enough to be an Oxisol
 Located in southern Atlantic states, eastern south-central states, Pacific sates

 Most extensively weathered soils
 Typically found on old landforms in tropical, subtropical climates
 Found only in HA, Puerto Rico, Guam
 Extensive in South America, Africa