A Word About

Soil Taxonomy

This is classification of soils based on the different horizon types they exhibit. Again here, as is usual in classification systems, are a number of systems in use (some are FAO - United Nations Food and Agriculture Organization, Canada uses a different system, and also the USDA Capability Classification differs). The general classification of soils is thought to have began in the second half of the 19th century in Russia by Dokuchaev.

There are 11 major orders of soils, which are divided into suborders. The division goes through great groups, subgroups, family, to series. There are over 16,000 series of soils in the USA alone.

The Orders in Brief

The following diagram show an approximate relationship between Water-Temperature and the different soil orders. Please note that this is a simplified graph (therefore the borders are fuzzy). Soil temperature is for mean annual °C and the "E I H" stands for Entisols, Inceptisols, and Histosols.

Alfisols are productive soils with argillic or natric horizons (silicate clay or silicate clay with more than 15% sodium accumulation). Medium to high amount basic cations. More weathered than Inseptisols but less than Spodosols. World examples are: SW Australia, Italy, central Spain, central California, Texas, and New Mexico as Xeralfs. SE Australia, central Europe, England, Ohio, Indiana, Minnesota as Udalfs. End syllable -alf (soil names ending on -alf are Alfisols). See U.S.A. distribution.

Andisols are not highly weathered soils from volcanic ejecta. This order has been only recently established. They have been under Inceptisols classification. Upper layers are high in silicate minerals allophane and imogolite and are of low bulk density. They are found in Colombia, Equador, Indonesia, and Japan and in the USA in Washington, Idaho, and Oregon. End syllable -and. See U.S.A. distribution.

Aridisols are soils of arid regions. Usually high electric conductivity due to salts. Under proper care these soils are very productive albeit not from the soil itself but from the climate in which these soils are found. World examples: Argids found in S California, Arizona, New Mexico. Aridisols are present in large areas of the Gobi and Taklamakan desert, central Australia and parts of S Australia, in the Middle East and SW Africa. End syllable -id. See U.S.A. distribution.

Entisols have none or little developed horizons due to being young soils. Some can be very productive but have limited clay content and depth. World examples: Sahara desert, Saudi Arabia, north central Australia, southern Africa, Georgia, Alaska, Siberia, SW Texas, and Alabama. End syllable -ent. See U.S.A. distribution.

Histosols are known as peat or bog soils. These are organic soils as opposed to mineral soils. This book deals with mineral soils; organic soils have a completely different buildup and use. Histosols have more than 30% organic matter and most have over 80% O.M. Depending on their development most are good producing soils. Histosols comprise less than 1% of the world soils. End syllable -ist. See U.S.A. distribution.

Inceptisols are older than Entisols, have some developed horizons. These are found through out the world (NW Australia, SW India, along the Amazon, France, Germany, Chile, N Africa, E China, E Ohio, W Pennsylvania, West Virginia). End syllable -ept. See U.S.A. distribution.

Mollisols are good producing soils with crumbly structure and often border on desert soils. These are the most important agricultural soils, with high base saturation. Found in the great Plains, Utah, parts of Idaho, Washington, Oregon, California, Mongolia, N China, Paraguay, Uruguay, and N Argentina. End syllable -oll. See U.S.A. distribution.

Oxisols have horizons of iron or aluminum hydrous oxides. These are old soils with deep weathering structure. Usually not well productive. Most weathered of all. They occur mostly in the tropics. South America, central Africa, Malaysia, Indonesia. End syllable -ox. See U.S.A. distribution.

Spodosols have spodic horizons (accumulation horizon like forest soils). These have low soil fertility. Only in moist to wet areas. NE USA, N Michigan, Florida, N Europe, Siberia, S South America, End syllable -od. See U.S.A. distribution.

Ultisols highly weathered alfisols. These soils are poor producers with low cation exchange capacity. Usually with aluminum toxicity. Less acidic than Spodosols. Need good management practices. Found in SE USA, W California, SE Asia, S Brazil. End syllable -ult. See U.S.A. distribution.

Vertisols are characterized by high amounts of expanding clays (>30%). In dry seasons these soils shrink and leave wide deep cracks; this shrinking and swelling produces soil movement, a problem for house structures. Sticky when wet hard when dry. Relatively high producing soils, especially with addition of organic matter and good management. Found in Alabama, E central and SE Texas, India, Ethiopia, Sudan, N and E Australia Venezuela, Bolivia, Mexico. End syllable -ert. See U.S.A. distribution.