WARNING: This is a lengthy, in-depth dissertation...

For decades, The United States Department of Agriculture (USDA) and Natural Resources Conservation Service (NRCS) have worked with soil scientists from around the world, to increase awareness and expand knowledge concerning the importance of soil and its impact on all aspects of life. While soils differ globally, the ability to apply a system that is universally understood and accepted is a goal shared by many soil scientists.

As the world struggles with ecological issues (such as global warming, climate change, rising sea water, dead spots in the sea caused by massive pollution from agricultural chemical miss-use and increasing topsoil erosion) and other environmental challenges - having a universally accepted method that can be applied when addressing soil problems, will contribute to more successful, much-needed outcomes.

Official soil series description” is a term applied to the description approved by the Natural Resources Conservation Service that defines a specific soil series in the United States.

These 12 official soil series orders are descriptions of the taxa in the series category of the national system of classification.
Alfisols, Andisols?, Aridisols?, Entisols?, Gelisols?, Histosols?, Inceptisols?, Mollisols?, Oxisols?, Spodosols?, Ultisols? and Vertisols?.

Unified Soil Classification System

The Unified Soil Classification System (USCS) is a soil classification system used in engineering and geology disciplines to describe the texture and grain size of a soil. The classification system can be applied to most unconsolidated materials, and is represented by a two-letter symbol. Each letter is described below (with the exception of Pt):

First letter

  • Letter = Definition
    • G = gravel
    • S = sand
    • M = silt
    • C = clay
    • O = organic

Second letter

  • Letter Definition
    • P = poorly graded (uniform particle sizes)
    • W = well graded (diversified particle sizes)
    • H = high plasticity
    • L = low plasticity

AASHTO Soil Classification System

The AASHTO Soil Classification System was developed by the American Association of State Highway and Transportation Officials, and is used as a guide for the classification of soils and soil-aggregate mixtures for highway construction purposes. The classification system was first developed by in 1929, and has been revised several times since.

Usual types of significant constituent materials:

stone fragments, gravel and sand
fine sand, silty or clayey gravel and sand
silty soils,clayey soils

General rating as a subgrade of:

excellent to good
fair to poor

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ASTM International

ASTM International (ASTM), originally known as the American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, and services. The organization's headquarters is in West Conshohocken, Pennsylvania, about 5 miles northwest of Philadelphia. ASTM predates other standards organizations such as BSI (1901), DIN (1917) and AFNOR (1926), but differs from these in that it is not a national standards body, that role being taken in the USA by ANSI. However, ASTM has a dominant role among standards developers in the USA, and claims to be the world's largest developer of standards. Using a consensus process, ASTM supports thousands of volunteer technical committees, which draw their members from around the world and collectively develop and maintain more than 12,000 standards.

The standards produced by ASTM International fall into six categories:

  • the Standard Specification, that defines the requirements to be satisfied by subject of the standard.
  • the Standard Test Method, that defines the way a test is performed. The result of the test may be used to assess compliance with a Specification.
  • the Standard Practice, that defines a sequence of operations that, unlike a test, does not produce a result.
  • the Standard Guide, that provides an organized collection of information or series of options that does not recommend a specific course of action.
  • the Standard Classification, that provides an arrangement or division of materials, products, systems, or services into groups based on similar characteristics such as origin, composition, properties, or use.
  • the Terminology Standard, that provides agreed definitions of terms used in the other standards.

Soil Morphology Characteristics

For soil resources, experience has shown that a natural system approach to classification, i.e. grouping soils by their intrinsic property (soil morphology), behavior, or genesis, results in classes that can be interpreted for many diverse uses.

Differing concepts of pedogenesis, and differences in the significance of morphological features to various land uses can affect the classification approach.

Despite these differences, in a well-constructed system, classification criteria group similar concepts so that interpretations do not vary widely.

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Technical Systems Approach

This is in contrast to a technical system approach to soil classification, where soils are grouped according to their fitness for a specific use and their edaphic characteristics.

Natural Systems Approach

Natural system approaches to soil classification are based on presumed soil genesis. Systems have developed, such as USDA soil taxonomy and the World Reference Base for Soil Resources, which use taxonomic criteria involving soil morphology and laboratory tests to inform and refine hierarchical classes.

Numerical Classification

Another approach is numerical classification, also called ordination, where soil individuals are grouped by multivariate statistical methods such as cluster analysis. This produces natural groupings without requiring any inference about soil genesis.

Soil Survey

In soil survey, as practiced in the United States, soil classification usually means criteria based on soil morphology in addition to characteristics developed during soil formation. Criteria are designed to guide choices in land use and soil management.

As indicated, this is a hierarchical system that is a hybrid of both natural and objective criteria. USDA soil taxonomy provides the core criteria for differentiating soil map units.

This is a substantial revision of the 1938 USDA soil taxonomy which was a strictly natural system. Soil taxonomy based soil map units are additionally sorted into classes based on technical classification systems. Land Capability Classes, hydric soil, and prime farmland are some examples.

Vernacular Soil Classification

In addition to scientific soil classification systems, there are also vernacular soil classification systems. Folk taxonomies have been used for millennia, while scientifically based systems are relatively recent developments
The French, Australian and Canadian governments has designed their own soil classification systems.

United Nations FAO

The Food and Agriculture Organization of the United Nations (FAO) developed a supra-national classification, also called World Soil Classification, which offers useful generalizations about soils pedogenesis in relation to the interactions with the main soil-forming factors.

It was first published in form of the UNESCO Soil Map of the World (1974) (scale 1 : 5 M.). Many of the names offered in that classification are known in many countries and do have similar meanings.

Originally developed as a legend to the Soil Map of the World, the classification was applied by United Nations sponsored projects. Many countries modified this system to fit their particular needs.
The Soil Units (106) were mapped as Soil Associations, designated by the dominant soil unit:

  • with soil phases (soil properties, such as saline, lithic, stony),
  • with three textural classes (coarse, medium, and fine)
  • three slopes classes superimposed (level to gently undulating, rolling to hilly, and steeply dissected to mountainous)

Soil Units form 26 World Classes:
Acrisols?, Andosols?, Arenosols?, Cambisols?, Chernozems?, Ferralsols?, Fluvisols?, Gleysols?, Greyzems?, Gypsisols?, Histosols?, Kastanozems?, Lithosols?, Luvisols?, Nitosols?, Phaeozems?, Planosols?, Podzols?, Podzoluvisols?, Rankers?, Regosols?, Rendzinas?, Solonchaks?, Solonetz?, Vertisols? and Yermosols?.

The FAO soil map was a very simple classification system with units very broad, but was the first truly international system, and most soils could be accommodated on the basis of their field descriptions. The FAO soil map was intended for mapping soils at a continental scale but not at local scale. In 1998 this system was replaced by the World Reference Base for Soil Resources.

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World Reference Base for Soil Resources

The World Reference Base for Soil Resources (WRB) is the international standard taxonomic soil classification system endorsed by the International Union of Soil Sciences (IUSS).

It was developed by an international collaboration coordinated by the International Soil Reference and Information Centre (ISRIC) and sponsored by the IUSS and the FAO via its Land & Water Development division. It replaces the previous FAO soil classification.

The WRB borrows heavily from modern soil classification concepts, including USDA soil taxonomy, the legend for the FAO Soil Map of the World 1988, and the Référentiel Pédologique and Russian concepts.

The classification is based mainly on soil morphology as an expression pedogenesis. A major difference with USDA soil taxonomy is that soil climate is not part of the system, except insofar as climate influences soil profile characteristics.

As far as possible, diagnostic criteria match those of existing systems, so that correlation with national and previous international systems is as straightforward as possible.

The WRB is meant for correlation of national and local systems. The level of detail corresponds to USDA soil taxonomy subgroups, without the soil climate information. It is not detailed enough for mapping at scales larger than about 1:200k, although proposal have been made to couple WRB with substrate information to map at 1:50k in regional studies.

There are 30 WRB soil groups: AC Acrisol?(AC), AlbeluvisolAB?), Alisol?(AL), Andosol?(AN), Anthrosol?(AT), Arenosol?(AR), Calcisol?(CL), CambisolCM?), ChernozemCH?), Cryosol?(CR), Durisol?(DU), Ferralsol?(FR), Fluvisol?(FL), Gleysol?(GL), Gypsisol?(GY), Histosol?(HS), Kastanozem?(KS), Leptosol?(LP), Lixisol?(LX), Luvisol?(LV), Nitisol?(NT), Phaeozem?(PH), Planosol?(PL), Plinthosol?(PT), Podzol?(PZ), Regosol?(RG), Solonchak?(SC), Solonetz?(SN), UmbrisolUM?), Vertisol?(VR).

So you can see that there are many ‘accepted’ soil classification systems worldwide – some of which do not relate well with each other. As yet, many of the world’s governments and academia have not “come to terms” with the need for a truly universal system by which soil is classified – and probably won’t, anytime soon. Such is human diversification - and nature of the international scientific/academic community.

Therefore, unless you are (or desire to become) a soil scientist – it is probably best to limit your interest and studies to regional terrain, and concern yourself with plantings that thrive within the scope of your local soil environment and climate – focusing on methods to improve the soil you have.

For example, in the Coastal Bend of Texas where this author lives, the single most beneficial activity – regardless of whether the soil is sand or clay-based – is the regular amendmening of organic matter (as compost), to increase rainwater percolation and incorporate microbe populations that improve soil quality - and change it into DIRT, to support vigorous plant life.

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