Soil Classification Soil Classification Introduction Why we classify soils Some concepts of soil Examples of soil classification systems Basis for classifying soils Diagnostic horizons Soil Orders of the US Soil Taxonomy Suborders, Greatgroups, Subgroups, Family, Series

Generally, we classify things so that we can make sense out of them. So we may study just a few similar groups rather than many

individuals.

In the past, classification of soils did not provide enough information for use and management of the soils. For instance, names such as black cotton seed soils, rice

soils, olive soils, limestone soils, piedmont soils, alluvial soils etc, some of which are still in common use today are problematic.

In present day classification, soils are classified as natural bodies on the basis of: The soil profile characteristics; and the suitability of the

soils for different kinds of use.

Introduction

Reasons for Classifying Soils

1. To organize our knowledge of soils

2. To remember the properties of the soils

3. To understand relationships among soil individuals and classes

4. To establish subdivisions of soils groupings in a manner useful for practical and applied purposes

• predicting soil behavior• identifying their best uses• and estimating their productivity.

Examples of Soil Classification systems

Russian Soil Classification System French Soil Classification System Belgium Soil Classification British System of Classification Soil Classification of Canada Australian Soil Classification System Brazilian Soil Classification System FAO/UNESCO Soil Map of the World US Soil Taxonomy

Concept of individual soils In classifying soils, soil is perceived as being composed

of many individual soil units (pedons) or natural bodies called soils.

Pedon is the smallest volume that can be considered as soil. Pedon refers to the smallest three-dimensional body of soil

layers large enough to illustrate the nature and arrangement of soil horizons and their variability. It varies in size from 1-10 m2.

A group of very similar pedons that are closely associated in the field is called polypedons.

The concept of “pedon”

0.8 m

5m

1 m 1.4 m

smallest three-dimensional body of soil layers large enough to illustrate the nature and arrangement of soil horizons and their variability

It varies in size from 1-10 m2

Natural body concept of soils A soil unit in a landscape

consists of a group of very similar pedons (polypedon).

A polypedon is a soil individual.

Soil individuals that have in common a suite of soil profile properties and horizons that fall within a particular range are said to belong to the same soil series.

Soil TaxonomySoil Taxonomy

The Soil Taxonomy was adopted as the official soil classification system of the United States in 1965.

Since then, it has gained recognition as a possible universal system for classifying soils.

However, to date, no system of classification has world wide acceptance.

Bases for Soil Classification in Soil Taxonomy

The physical, chemical and mineralogical, and biological properties of soils as they are today (that can be objectively observed and measured), are the basis for classifying soils in the Soil Taxonomy.

E.g. moisture, temperature, color, texture, structure, organic matter, pH, % base saturation, soil depth, etc. are important criteria for classification.

Some of these properties are observed in the field, but others require precise measurements in the lab.

These precise measurements are used to define certain diagnostic soil horizons, the presence or absence of which determine the place of a soil in the classification system.

Diagnostic Horizons

Soil Taxonomy makes use of diagnostic soil horizons for putting a soil into a taxonomic class. Diagnostic horizons have specific characteristics that are

indicative of certain classes of soils

There are two types of diagnostic horizons

• Diagnostic soil surface horizons (Epipedons)• Diagnostic sub surface horizons

Diagnostic Soil Surface Horizons (Epipedons)

Mollic epipedon (A) Umbric (A) Histic (O) Ochric (A)

MelanicAnthropic (A)Plaggen (A)

An epipedon is a diagnostic horizon that forms at the surface.

There can be only one epipedon for mineral soils.

Mollic

Mollic/UmbricHistic Melanic

Anthropic/Plaggen

Ochric

Soil subsurface diagnostic horizons

Argillic (Bt) Albic (E) Spodic (Bhs) Cambic (B) Kandic Oxic (Bo) Natric (Btn) Agric (A or B) Calcic (K)

Duripan (m) Fragipan (x) Gypsic (y) Salic (z) Petrocalcic Petrogypsic Placic Sombric Sulfuric

Diagnostic subsurface horizons form below the soil surface. Usually, they are B horizons but diagnostic subsurface horizons may include parts of A or E horizons. Some soils do not have a diagnostic subsurface horizon.

1. Orders (12) (surface and subsurface diagnostic horizons)

2. Suborders (55) (Soil temperature. and moisture regimes)

3. Great group (238) (subsurface diagnostic horizon)

4. Subgroup (1243) (drainage, lithic contact, PM, clay type)

5. Family (7504) (Texture of diagnostic surface horizon)

6. Series (about 19,000) in U.S.

Categories of the Soil Taxonomy

Soil Orders Every soil in the world is assigned to one of 12 orders that

reflect major course of development. In the orders, there is considerable emphasis placed on the

presence or absence of major diagnostic horizons. Bold letters in the soil order names indicate formative

element used as ending for lower taxa.

1. Entisols (-recent)

(Little if any profile development)

• No diagnostic subsurface horizons.

• Very recent or young soil• Little if any profile development

• Form on • resistant P.M., • mine spoils, • steep slopes, • floodplains

2. Inceptisols (Latin, beginning)

(inception of B Horizon)

• Slightly more development than Entisols

• Young soils but beginning of profile development is evident.• Well-defined profile

characteristics of mature soils are yet to be developed.

• May have a cambic horizon

3. Mollisols mollis (Latin, soft)

(Dark, soft soils of grasslands)

• Typically form under grasslands. • E.g., Central US

• Soils with a mollic epipedon:• Thick humus-rich surface horizon High % base saturation

throughout profile Slightly leached Very fertile soils

4. Alfisols pedalfer

(Argillic or natric soils, moderately leached)

• Do have an argillic horizon• About 35% base saturation• Develop in humid, temperate

regions• Vegetation is usually

deciduous (forests, savanna)• Gray to brown surface horizon• Good for grain production

6. Ultisols -ultimate (Latin, last)

(Argillic horizon, low bases)

• Have Argillic or Kandic with <35% Base saturation.

• Common in S.E. US, humid tropical, and subtropical regions

• Warm and humid environs.

• Old, highly weathered, high degree of development (ult)

• Low fertility

5. Spodosols -spodos (Greek, wood ashes)

(Acid, sandy, forest soils, low bases)

• Have a spodic horizon

• Form in humid, cool climates and occur most often in conifer forests (New England, Mich., Canada)

• Form in acid, coarse, quartz (sandy) bearing P.M.

• Low fertility

7. Oxisols -oxide (French, oxide) (Oxic horizon, highly

weathered)• Have an oxic horizon.

• Dominated by residual accumulations of Fe and Al oxides and kaolinite

• Most extensively weathered soils (occur in tropical regions).

• Intense red or yellow (high in oxidized iron).

• Low % Base Saturation.• extensively leached

• Low fertility.

8. Histosols -histos (Greek,

tissue) (Organic soils)• Organic soils (peats and mucks)

• Mainly found in cold climates (Alaska, Canada, Finland, Russia)

• Low-lying swampy areas• Virtually all Histosols

occur in wetland environments

• Have very low bulk densities and very high water holding capacity

9. Vertisols -invert (Latin, turn)

(Dark swelling and cracking clays)

• High clay soils (>30% sticky shrink-swell clays).

• Deep cracks upon drying

• Materials from soil surface fill cracks and become part of the subsurface when the cracks close (inverted soil)

10. Aridisols -arid (Latin, dry)

(Dry soils)

• Form in aridic moisture regimes (dry)

• Dry at least 6 months• Low OM• Little leaching

• High base sat.• Can be very productive if

irrigated

11. Andisols -ando (Jap., black soil) (Volcanic ash soils)

• Form from P.M. of volcanic origin• Soil forms by rapid weathering of

volcanic ash to produce poorly crystallized aluminosilicates (allophane & imogolite)

• Andisols are young soils ( and they have high OM)

• In dry climates, can be susceptible to wind erosion

• Can have unusually low bulk densities

12. Gelisols -gelid (Greek, cold)

(Permafrost and frost churning soils)

• Presence of permafrost layer within 100 cm of soil surface defines this soil class

• Young soils with little profile development• Cold temperatures and frozen

conditions for much of the year slow the process of soil formation.

• May show evidence of cryoturbation• physical disturbance of soil

material caused by freezing and thawing

Simplified Key to the 12 Soil Orders of The Soil Taxonomy

Soil map of North America showing distribution of 12 soil orders of the Soil Taxonomy