Soils: Chapter 14:
 Soils Classification


Below the line at which the land appears to meet the sky, there are other horizons.

These “hidden” horizons are the descending layers within a soil. Horizons are the visible evidence of soil-forming factors at work:

  • climate
  • organisms
  • relief (topography)
  • parent materials
  • time


Scientists group soils based on the arrangement and properties of horizons.

Soils of the same type share horizons with similar properties such as:

  • color
  • texture
  • types of minerals
  • organic content


In some soils, the boundaries between layers may be sharp; in others, the composition of the layers gradually changes.

Not all soils have horizons, but by “reading” soil layers, along with observing moisture content and temperatures, scientists can classify or describe soils.


Soil Horizons

  • O horizon:
    • the organic layer of newly deposited debris and partially decomposed matter on the surface.
  • A horizon:
    • a mineral layer at or near the surface, with more soil organic matter than lower layers, also called topsoil.
  • E horizon:
    • a layer of loss, as water carries clay, iron, aluminum and organic matter into lower levels.
  • B horizon:
    • a layer of gain, as minerals and organic matter wash in from higher layers or form in place from weathering.
  • C horizon:
    • a mineral layer that represents the condition of the soil when it first started to form; also called parent material, it is relatively unaffected by biological activity or the processes of loss and gain in higher layers.





Soil Classification



Soil Order Song


Soil Orders:

  • Alfisols: moderately leached soils with a subsurface zone of clay accumulation and ≥35% base saturation*
  • Andisols: soils formed in volcanic ash
  • Aridisols: CaCO3-containing soils of arid environments with subsurface horizon development
  • Entisols: soils with little or no development
  • Gelisols: soils with permafrost within 2m of the surface.
  • Histosols: organic soils
  • Inceptisols: soils with weakly developed subsurface horizons
  • Mollisols: grassland soils with high base status
  • Oxisols: intensely weathered soils of tropical and subtropical climates
  • Spodisols: acid forest soils with a subsurface accumulation metal-humus complexes
  • Ultisols: strongly leached soils with a subsurface zone of clay accumulation and <35% base saturation*
  • Vertisols: clayey soils with high shrink/swell capacity




Cation Exchange Capacity (CEC): Any element with a positive charge is called a cation and, for agricultural purposes.

  • it refers to the basic cations, calcium (Ca+2), magnesium (Mg+2), potassium (K+1) and sodium (Na+1)
  • the acidic cations, hydrogen (H+1) and aluminum (Al+3).
  • The CEC refers to the total amount of these positively charged elements that a soil can hold.
  • The cations are held on "exchange sites" where one cation can be exchanged for the same type or a different cation.
  • The CEC is expressed in milliequivalents per 100 grams (meq/100g) of soil.
  • The larger this number, the more cations the soil can hold.
  • A clay soil will have a larger CEC than a sandy soil.
  • In the Southeast, where we have highly weathered soils, the dominant clay type is kaolinite, which has very little capacity to hold cations compared to other clays.
  • A typical CEC for a soil in the coastal plains region is about 2.0 meq/100g of soil, and the typical CEC for a soil in the piedmont is about 5.0 meq/100g of soil.
  • The CEC gives an indication of the soil's potential to hold plant nutrients.
  • Increasing the organic matter content of any soil will help to increase the CEC since it also holds cations like the clays.
  • Organic matter has a high CEC, but there is typically small amounts of organic matter in our soils.




* Percent Base Saturation: tells what percent of the exchange sites are occupied by the basic cations.

  • If calcium has a base saturation value of 50% and magnesium has a base saturation value of 20% as shown above, then calcium occupies half of the total exchange sites (CEC) and magnesium occupies one-fifth of the total exchange sites (CEC).
  • In this example, where the soil has a CEC of 5 meq/100g, 2.5 meq/100g of the CEC is occupied by calcium and 1 meq/100g of the CEC is occupied by magnesium.
  • If all the exchangeable bases (Ca, Mg, K and Na) total 100%, then there is no exchangeable acidity.