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 Taxonomy 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 orders separate all soils  Pedons are identified to help separate soil orders (minimum 3.3 ft 2, & 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 ° F Mean summer temp >43° than mean winter temp  Thermic MAST ° 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

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