Unit 4: Soil Taxonomy

Chapter 7

Objectives

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

U. S. System of Soil TaxonomySoils are named, mapped as a geologic entity

or individual

1st taxonomic system began in 1938

NRCS began extensive use of the system in 1965

12 orders separate all soils Pedons are identified to help separate soil orders

(minimum 3.3 ft2, & as deep as roots grow)

U. S. System of Soil Taxonomy

Order Most general category Histosols

Organic soils Entisols

Undeveloped soils Inceptisols

Slightly developed Andisols

Volcanic material Vertisols

Swelling-clay

U. S. System of Soil Taxonomy

Gellisols Must have permafrost in the top 6’

Mollisols Most extensive soils in the U.S. Naturally fertile, slightly leached Can be semiarid to subhumid climates

Alfisols Fertile in favorable moisture conditions Usually very productive

Ultisols Leached, acidic Moderate to low fertility

U. S. System of Soil Taxonomy

Aridisols Arid-region soils Can be very productive

Oxisols Hot, wet tropics Conducive to year-round plant growth

Spodosols Found mostly in cool climates Poorest soils for cultivation Must have lime & fertilization to grow crops

U. S. System of Soil Taxonomy

Suborder Differentiated based on soil properties & horizons Soil moisture, soil temp, dominating effects of chemical

or textural features Great Groups

Differentiated by soil horizons & soil features Accumulated clay, iron, humus, hard pans/cement layers

Subgroup Three kinds of subgroups

Represent the central (typic) concept of the soil group Properties that intergrade towards other groups, etc.

U. S. System of Soil Taxonomy

Properties that prevent its classification as typic or an intergrade to another category

Family Soil properties important to the growth of plants Behavior of soils when used for engineering Important soil properties: texture, mineralogy, pH, avg.

soil temp, moisture, permeability, thickness of horizons, structure, consistency

Series 18,000 soil series identified Typically named after something local Differentiated on the basis of observable & mappable

soil characteristics

U. S. System of Soil Taxonomy

Must have similar color, texture, structure, consistency, thickness, pH, similar horizon arrangements, similar chemical & mineralogy properties

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

Attempt to indicate the extent of naturally available water in the soil depth of maximum root proliferation

Aquic Wet w/ anaerobic saturation long enough to produce

visual evidence of poor aeration Peraquic

Tidal marsh or inland depression where groundwater is always at or near the surface

Soil Moisture & Temperature Regimes

Udic Usually has adequate water throughout the yr

Perudic Extremely wet, percolation in all months when not frozen

Ustic Moisture is limited but is present during the growing

season Xeric

Deficient in water & w/ a dry cropping season Most precipitation in the winter

Soil Moisture & Temperature Regimes

Aridic Very water deficient Long dry periods, short wet periods

Soil Temperature Regimes Based on the mean annual soil temp (MAST) Mean summer soil temp Mean winter soil temp Determined at ~2’ depth Cryic

MAST 46° F

Soil Moisture & Temperature Regimes

Frigid MAST 46° F Mean summer temp >11° F higher than mean winter

temp Mesic

MAST 46 - 59° F Mean summer temp >43° than mean winter temp

Thermic MAST 59 - 72° F Mean summer temp >11° than mean winter temp

Soil Moisture & Temperature Regimes

Hyperthermic MAST >72° F Mean summer temp >11° than mean winter temp

Add iso prefix to the classification if the mean summer & winter temps vary <11° F

Additional Terminology for Family Groupings Particle-size Classes

Fragmental Mostly stones, cobbles, gravel, etc. Fine-earth component <10% of soil volume

Sandy skeletal >35% rock fragments Fine-earth fraction is sand, sandy loam

Loamy skeletal >35% rock fragments Fine-earth fraction loamy

Additional Terminology for Family Groupings

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

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

Distribution of Soil Orders

Soil orders diverse & unevenly distributed Some land areas don’t correspond to any

order See inside front & back covers for

distributions of soil orders

Gellisols

Occur in tundra regions Cold & relatively barren May have exposed rock intermixed Yukon & Northwest Territories of Canada,

northern 2/3 of Alaska

Histisols

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

Entisols

Lack horizons due to being a young soil, or weathering is ineffective

Widely distributed in the U.S. Include river floodplains, rocky soils,

mountainous areas, barren islands of East & Gulf coasts, beach sands

Found on all continents Can be excellent ag soil, but may be very

unproductive

Inceptisols

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

Andisols

Weakly to moderately developed Most from volcanic materials Extensively found in the Pacific Ocean &

Hawaii Some also in the northwestern U.S.

Aridisols

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

Mollisols

Dark-colored soils of grasslands & some hardwood forests

Deep, dark-colored, fertile A horizon (mollic epipedon) Mollic epipedon extends from surface to ~2’ depth

Properties & Classification of Mollisols Large number of suborders Naturally fertile Easily managed

Mollisols

Management of Mollisols Formed under grasses, forests Tend to be most fertile soil High humus content High concentration of N In wetter climates don’t need irrigation, but in

dryer areas can produce highly May become acidic Black soil colors to depth of 2-3’

Mollisols

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

and highly valuable

Mollisols

Distribution of Mollisols Found in the Great Plains region of the U.S.

extending north into Canada, south to Gulf of Mexico

Most extensive of U.S. soil orders

Vertisols

Self-mixing soils, >30% shrink/swell clays Found mostly in central & southeastern TX &

along lower Mississippi River

Alfisols

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 Medium to high supply of basic cations

Evidence of mild leaching Water is adequate for plant growth for 3+ warm

season mos.

Alfisols

Management of Alfisols If topography & climate are favorable; alfisols can

be very productive Most are leached of lime, and can have an acidic

zone If leached enough, forms an E horizon

Erosion exposing clays at the surface not favorable for plant growth

Most naturally productive soils w/out fertilization or irrigation

Alfisols

Usually will require lime amendments Alfisols & Mollisols generally located in a region’s

breadbasket Distribution of Alfisols

North-central states & mountain states Occur on all continents Corn belt of IN, OH, MI, WI, IL Woodland soils in TX, CO

Spodosols

High sand content High rainfall w/ easy leaching

Humus, colloids leached Cold, wet climates are common Mostly found in Cascade Mountains in WA,

OR, New England, & Great Lakes states Also found in FL

Ultisols

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

Oxisols

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

Assignment