A soil horizon is a layer developed by soil forming processes with morphological properties different from layers below or above it.
Soil horizons are designated by a capital letter followed by numerals and lower case letters for subdivisions of various kinds. Whereas interminable and surprisingly heated debates flourish over the naming and definition of soil layers, the terminology used here is now well accepted. It has been adapted from the Australian Soil and Land Survey Field Handbook.
The major types of horizons.
A horizons
These are mineral horizons at or near the surface (topsoils) that have some accumulation of organic materials. (but less than purely organic material horizons).
Typically, A horizons are darker than underlying horizons but they may also be horizons with lighter colours or lower contents of clay compared to underlying horizons. There are three types of A horizon.
A1 horizon
This is a mineral horizon at or near the soil surface that has some accumulation of organic matter. It is usually darker than the underlying horizons and it is the zone of maximum biological activity.
A2 horizon
This is a mineral horizon that has less organic matter, oxides or clay than the horizons above or below. It is a pale horizon and is common throughout Australia. Various degrees of bleaching are recognised with white or near-white layers being referred to as sporadically or conspicuously bleached, depending on its extent.
A3 horizon
This is a transitional horizon between A and B horizons but it is more like the A horizon.
B horizons
These subsoil horizons have one or more of the following:
- a concentration of clay, iron, aluminium, or organic material in the layer
- a structure or consistence unlike the overlying A horizon and different from the horizons below
- stronger colours than the horizons above or below
The BI horizon is a transitional layer between the A and B horizons but it is more like the B horizon. The B2 horizon exhibits strongest development of the features listed on the previous screen, whereas the B3 horizon (not shown) is a transitional layer to the underlying material. The soil shown on the left illustrates these horizons.
C horizons
These are layers below the A and B horizons composed of consolidated or unconsolidated materials. These materials are usually partially weathered and geological features are often evident. When moist, C horizons can be dug by hand.
D horizons
These are soil layers below the A, B and C horizons that differ in general character but are not C horizons. They cannot be reliably described as buried soils but they do have a contrasting pedological organisation to the overlying horizons.
R horizons
These are continuous masses of rock usually too strong to dig with hand tools.
O horizons
These horizons are dominated by organic matter that has accumulated on the surface of the soil. O horizons are subdivided according to the degree of organic material decomposition. These horizons are not common in Australia and are mostly restricted to moist or cool environments (e.g. alpine areas, swamps, wet forests).
P horizons
These horizons are dominated by organic materials, in various stages of decomposition, which have accumulated either under water or in very wet areas. They are often referred to as peat layers.
