Horizons

The action and interaction of soil-forming processes as influenced by soil-forming factors gives rise to distinct soil horizons. These layers are assigned distinctive alphabetic symbols as a form of shorthand for their characteristics. The horizon description system begins by splitting soil horizons into two distinct groups: organic and mineral horizons. Organic horizons are those that contain 17% or more organic carbon; mineral horizons have less than 17% organic carbon.

The major symbols used in describing mineral soil layers in Canada are shown in the following tables. The assignment of mineral soil layers to each horizon is done by comparing the properties of the horizons in the field to a list of distinctive characteristics, called diagnostic properties.

Table 1: Basic description of mineral soil horizons in the Canadian System of Soil Classification (Agriculture Canada Expert System on Soil Survey, 1987).

Master Horizon	Suffixes	Basic Description
A		Mineral horizon formed at or near the soil surface
	Ah	Accumulation of soil organic matter (SOM)
	Ae	Removal of clay, SOM, iron, or aluminum
B		Horizon formed by accumulation of material removed from Ae horizon or by alteration of the parent material
	Bh	Accumulation of SOM
	Bf	Accumulation of iron and/or aluminum
	Bss	Presence of slickensides (smooth clay coating caused by stress in high clay soils)
	Bv	Vertic horizon caused by turbation (mixing) of material in high clay soils
	Bt	Accumulation of clay
	Bn	Strong soil structure and sodium accumulation
	Bg	Mottling and gleying due to water saturation
	Bm	Slight colour or structural changes from the parent material
C	C	Horizon with little evidence of pedogenic activity
	Cca	Accumulation of Ca and Mg carbonates
	Cs	Accumulation of soluble salts
	Ck	Presence of original Ca and Mg carbonates
	Css	Presence of slickensides
	Cg	Mottling and gleying due to water saturation
R		Consolidated bedrock
W		Water layer

Table 2: Basic description of organic soil horizons as classified in the Canadian System of Soil Classification. Reference as in Table 1.

Master Horizon	Suffixes	Basic Description
O		An organic horizon developed mainly from bog vegetation; it is more commonly called peat. These materials are usually water saturated.
	Of	Composed of fibrous materials of readily recognizable origin
	Om	Organic materials in an intermediate (or mesic) stage of decomposition; some have a recognizable form, but the remainder is highly decomposed
	Oh	Organic material which is highly decomposed (in a humic state); the origin of the material is unrecognizable
L, F, H		Organic materials that occur from the accumulation of leaves, twigs and woody materials and which overlies a mineral soil; commonly found in well to imperfectly drained forest environments.
	L	Leaf litter, readily recognizable
	F	Partially decomposed leaf and twig material (folic material)
	H	Humic material; decomposed organic materials with no original structures evident