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:
- relief (topography)
- parent materials
Scientists group soils based on the arrangement and
properties of horizons.
Soils of the same type share horizons with similar
properties such as:
- 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.
- 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
- E horizon:
- a layer of loss,
as water carries clay, iron, aluminum and organic matter into lower
- 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
Soil Order Song
Alfisols: moderately leached
soils with a subsurface zone of clay accumulation and ≥35% base
formed in volcanic ash
soils of arid environments with subsurface horizon development
Entisols: soils with little
or no development
Gelisols: soils with
permafrost within 2m of the surface.
Inceptisols: soils with
weakly developed subsurface horizons
Mollisols: grassland soils
with high base status
weathered soils of tropical and subtropical climates
forest soils with a subsurface accumulation metal-humus complexes
Ultisols: strongly leached
soils with a subsurface zone of clay accumulation and <35% base
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
- 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
- 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.