Soils and Water, Spring 2007

Soil Classification: Outline • • • •

Introduction Soil moisture regimes Soil temperature regimes The Soil Taxonomy – Diagnostic horizons – soil orders

Why Classify? • Organize knowledge in an attempt to find order in soils. • Help dealing with large number of objects/concepts/ideas. • Enable the collection of new knowledge. • Facilitate the optimal use of soils (applied objective). • Understand the distribution and functioning of soils (basic objective)

Precursors of Present Classification Systems • The development of soil classification systems were based on the concepts of Zonal, Intrazonal, and Azonal soils. – Zonal soils reflect the climate and vegetation under which they developed. – Intrazonal soils have properties dominated by some local factor (e.g., drainage, salts). – Azonal soils have properties corresponding to the parent material (young or highly eroded soils).

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Relationship Between Annual Precipitation and C.E.C. for Central USA Modified from Jenny and Leonard (1934, Soil Sci. 38: 363-381)

C.E.C. cmolc/kg

Exchangeable bases Exchangeable H

10

20

30

40

Annual precipitation, inches

Relationship Between N and Rainfall After Jenny and Leonard (1934, Soil Sci. 38: 363-381)

• • • •

medium-textured soils soil sampled from 0-25 cm sampled along a isotherm (11 °C) vegetation: mostly pastures

Relationship Between Organic Carbon and Clay Nichols (1984, Soil Sci. Soc. Am. J. 48: 1382-1384

• Sixty-five pedons from Oklahoma, Texas, and Eastern New Mexico. • % OC could be predicted from % clay and mean annual precipitation (R2=0.90). • %OC increase with an increase of clay content and precipitation.

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Depth to Calcium Carbonate Accumulation as a Function of Precipitation

After Jenny and Leonard (1934, Soil Sci. 38: 363-381)

Depth = 2.5 (R-12) inches

Soil Moisture Regimes • Aquic: soils with a perched or real groundwater table (gley phenomena). • Udic: soils mostly moist. There may be a short dry season. • Ustic: soils that are moist for more than half a year but have a distinct dry season. • Xeric: soils that are moist for more than half a year but have a dry summer. • Aridic: soils that are dry for more than half a year and never moist for long periods.

Soil Moisture Regimes temperature PE

Udic

precipitation

Ustic

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Soil Moisture Regimes temperature

PE

Xeric precipitation

Aridic

Relationship Between N and Temperature After Jenny (1928, J. Amer. Soc. Agronomy 20: 900-921)

• Sampled along a North-South transect of the U.S. Great Plain Region. • Soils developed under prairie grasses.

Temperature Effect on Amount and Type of Clay Minerals

• • •

Sampled from NJ to GA. Clay content increases with increasing temperatures. The ratio silica/alumina decreases from 4 to 2 for the sequence NJ-GA.

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Soil Temperature Regimes • Air and soil temperatures are related. Below depths of 20 m (Alaska), 15 m (mid-latitude), and 10 m (tropics), soil temperature and mean air temperature are similar:

Soil Temperature Regimes • Soil temperature regimes are based on mean annual, mean summer, and mean winter soil temperatures, measured at 50 cm. • There are six soil temperature regimes. Ordered from lower to higher mean annual temperature: Pergelic, Cryic=Frigid, Mesic, Thermic, and Hyperthermic. • Soil temperature regimes are typically used to classify at the lower levels of the Soil Taxonomy.

Types of Soil Classification Systems • Taxonomic systems: based on the natural characteristics of soils, e.g., texture, color, C.E.C., soil temperature and moisture. – Soils are separated into multiple categories (levels) and within each level they are separated into taxa (classes).

• Interpretive systems:based on rearrangement of taxonomic systems for particular uses (e.g., septic tanks, houses with basement, road and streets).

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Soil Taxonomy • Soil Taxonomy was developed in the USA for making and interpreting soil surveys mainly for agricultural purposes. • http://soils.usda.gov/technical/classification/tax_keys/ contains the “Keys to Soil Taxonomy” (2006). • The Soil Taxonomy has six levels, each with a variable number of classes: Order Suborder

12 63

Great group

~250

Subgroup

~1,400

Family

~8,000

Series

~19,000

Soil Orders See http://soils.ag.uidaho.edu/soilorders/ Name

Formative element

Alfisols Andisols Aridisols Entisols Gelisols Histosols Inceptisols Mollisols Oxisols Spodosols Ultisols Vertisols

alf and id ent el ist ept oll ox od ult ert

Vertisols

• Vertisols are clay-rich soils that shrink and swell with changes in moisture content (usually located in areas with a seasonal dry period). • Because of the shrink/swell activity of these soils, they often do not have distinct, well-developed horizons.

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Andisols

• Andisols are soils that have formed in volcanic ash or other volcanic ejecta. • These soils are dominated by poorly crystalline minerals such as allophane and imogolite. • As a result, Andisols possess many unique chemical and physical properties: – very high water-holding capacity – ability to 'fix' (and make unavailable to plants) large quantities of phosphorus.

Histosols

• Soils composed mainly of organic materials. They contain at least 20-30% organic matter by weight and are more than 40 cm thick. • Histosols typically form in settings where poor drainage inhibits the decomposition of plant and animal remains, allowing these organic materials to accumulate over time. Histosols are often referred to as peats and mucks.

Gelisols

• Gelisols are soils of very cold climates that contain permafrost within 2 meters of the surface. • These soils are limited geographically to the polar regions and localized areas at high mountain elevations. • Low soil temperatures cause soil-forming processes to proceed very slowly.

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Aridisols

• Aridisols are soils of arid regions that exhibit at least a minimal amount of subsurface horizon development. They are characterized by being dry most of the year. • Aridisols contain subsurface horizons in which clays, calcium carbonate, silica, salts, and/or gypsum have accumulated.

Mollisols

• Mollisols are the soils of grassland ecosystems. They are characterized by a thick, dark surface horizon (mollic epipedon) and high base saturation. • Mollisols primarily occur in the middle latitudes and are among some of the most important and productive agricultural soils in the world.

Alfisols

• Alfisols are well developed forest soils with a subsurface horizon in which clays have accumulated. • Alfisols are mostly found in temperate humid and subhumid regions of the world. • Alfisols are very productive soils for agriculture and silviculture.

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Ultisols

• Ultisols are acid soils formed under forest vegetation. They are found in humid temperate and tropical areas of the world, typically on old, stable landscapes. • Ultisols exhibit very strong horizon development and have a subsurface horizon in which clays have accumulated. • Like Alfisols but with a Mesic or warmer temperature regime and base saturation of < 35%.

Spodosols

• Spodosols often occur under coniferous forest in cool, moist climates. Usually of coarse texture. • Spodosols are acid soils characterized by a subsurface accumulation of humus and Al & Fe oxides. • Spodosols typically have a light-colored E horizon overlying a reddish-brown spodic horizon.

Oxisols

• Oxisols are very highly weathered soils that are found primarily in the intertropical regions of the world. • These soils contain few weatherable minerals and are often rich in Fe and Al oxide minerals. Extreme low levels of native fertility. • Very productive soils with inputs of lime and fertilizers.

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Inceptisols

• Inceptisols are soils that exhibit minimal horizon development and have diagnostic horizons that form rapidly, e.g.cambic horizon. • Inceptisols occur under a wide range of environmental settings (e.g., steep slopes, young geomorphic surfaces, and on resistant parent materials).

Entisols

• Entisols are soils developed in unconsolidated parent material with usually no genetic horizons except an A horizon. • All soils that do not fit into one of the other eleven orders are Entisols. • The result is that the Entisol order is characterized by great diversity.

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