Glossary:Soil horizon

A layer of soil or soil material approximately parallel to the land surface and differing from adjacent genetically related layers in physical, chemical, and biological properties or characteristics such as color, structure, texture, consistency, kinds and numbers of organisms present, degree of acidity or alkalinity, etc. The following table lists the designations and properties of the major soil horizons. Very few soils, if any, have all of these horizons well developed, but every soil has some of them.

0

Organic horizons of mineral soils.

01

Organic horizons in which essentially the original form of most vegetative matter is visible to the naked eye.

02

Organic horizons in which the original form of most plant or animal matter cannot be recognized with the naked eye.

A

Mineral horizons consisting of (1) horizons of organic-matter accumulation formed or forming at or adjacent to the surface; (2) horizons that have lost clay, iron, or aluminum, with the resultant concentration of quartz or other resistant minerals of sand or silt size; or (3) horizons dominated by (1) or (2) above, but transitional to an underlying B or C.

Ap

The plowed portion of the A horizon.

A1

Mineral horizons, formed or forming at or adjacent to the surface.

A2orE

Mineral horizons in which the feature emphasized is loss of clay.

A3

A transitional horizon between A and B and dominated by properties characteristic of an overlying A1 or A2, but that has some subordinate properties of an underlying B.

AB

A transitional horizon between A and B and dominated by properties of B in which the two parts cannot be conveniently separated into A and B.

A and B

Horizons that would qualify for A2 except for included parts that constitute less than 50% of the volume that would qualify as B.

AC

A horizon transitional between A and C that has subordinate properties of both A and C but that is not dominated by properties characteristic of either A or C.

B and A

Any horizon qualifying as B in greater than 50% of its volume, including parts that qualify as A2.

B

Soil horizon beneath A horizon. Clay and nutrients, etc., have accumulated in this horizon.

B1

A transitional horizon between B and A1 or between B and A2 in which the horizon is dominated by properties of an underlying 82 but has some sub ordinate properties of an overlying A1 or A2.

B2

That part of the B horizon where the properties on which the B is based are without clearly expressed subordinate characteristics, indicating that the horizon is transitional to an adjacent overlying A or an adjacent overlying C or R.

B3

A transitional horizon between B and C or R in which the properties diagnostic of an overlying 82 are clearly expressed but are also associated with clearly expressed properties characteristic of C or R.

C

A mineral horizon or layer, excluding bedrock, that is either like or unlike the material form which the solum is presumed to have formed, relatively unaffected by pedogenic processes, and lacking properties diagnostic of A or B.

R

Underlying consolidated bedrock such as granite, sandstone, or limestone.