1

Introduction to Soil Descriptions

Part 3 of 3

2

Acknowledgement

This work was supported [in part] by the National Decentralized Water Resources Capacity Development Project with funding

provided by the U.S. Environmental Protection Agency through a Cooperative Agreement (EPA No. CR827881-01-0) with

Washington University in St. Louis. The results have not been reviewed by EPA or Washington University in St. Louis. The

views expressed in this presentation are solely those of NCSU, and University of Arkansas and EPA and Washington University in St.

Louis do not endorse any products or commercial services mentioned in the presentation.

3

Soil Profile Description and

Horizons

Rules .1939, .1941 (a4), .1943, .1944

4

5

6

Soil Profile Descriptions

Soil Horizon - A layer of soil, approximately parallel to the surface, having distinct characteristics produced by soil forming processes.

Soil Profile - A vertical section of the soil extending through all its horizons and into the parent material.

7

8

Soil Profile Descriptions

Soil Profile - A vertical section of the soil extending through all its horizons and into the parent material.Soil Horizon - A layer of soil, approximately parallel to the surface, having distinct characteristics produced by soil forming processes.Soil Layer - A layer in the soil deposited by a geologic force (wind, water, glaciers, oceans, etc.) and not relating to soil forming process.

9

Soil Horizon Nomenclature

Soil horizons designated by a combination of: CAPITAL LETTERS-lower case letters-ARABIC NUMERALSMaster Horizons and Layer: Major breaks in the soil: O, A, E, B, C, and R.Subordinate Distinctions: Lower case letters used as suffixes to designate specific master horizons. i.e. Ap, Bw, CgVertical Subdivisions: A horizon designated by a single combination of letters which needs to be subdivided. i.e. Bw1, Bw2, Cg1, Cg2.

10

Organic (O) Horizons

O horizons or layers: Layers dominated by organic material.Field Criteria

>20% organic matterDark color (never used by itself)Low strength, light dry weight, high fiber contentTypically a surface horizon-if buried may indicate disturbance

Subordinate Distinctions of O HorizonOa-Highly decomposed organic material (Muck)Oe-Moderately decomposed organic material (Mucky Peat)Oi-Slightly decomposed organic material (Peat)

11

Pungo series, organic from top to bottom.

12

A Horizons

Referred to as topsoil and typically ranging from 6-30 centimeters thick. Mineral horizon formed at the surface or below an O horizon. Characterized by an accumulation of humified organic matter intimately mixed with the mineral fraction.Field Criteria

Mineral soil materialMix of well decomposed organic matter and mineral materialSurface mineral horizonTypically dark in color-darker than underlying horizons

Typical Subordinate Distinctions of A HorizonAp-Plowing or other disturbance

13

A Horizon

Cecil series with poorly defined A horizon typical of the piedmont in that it is only slightly darker than the horizons below.

14

Ap Horizon

Ap horizon or plow layer. Note how abrupt the transition is from the Ap to the lower horizon.

15

E Horizons

Mineral horizon in the upper part of the soil typically underlying an O or A horizon. Light colored, leached horizons ranging from not being present to several centimeters thickField Criteria

Zone of eluviation - removal of clays, Fe, Al, and humusLighter in color than over or underlying horizonNear surface, below O or A horizons and above a B horizon

Formed by weak organic acids that strip coatings from sand grains. Light color due to the natural color of the mineral grains.

16

E Horizon

Thick E horizon in a Conetoe Series. The fine reddish brown streaks are lamella composed of Fe and clay. The B horizon shows a slight increase in clay.

17

E Horizon

E horizon in a Aquod (wet Spodosol). Fe and OM have been removed and translocated lower in the profile.

18

A vs. E horizon

A horizons have a high OM contentE horizons have low OM contentE horizons often feel sandier (coarser) than the A horizonE horizons have a higher value

19

E vs. B horizon

E horizons are sandier (coarser) than B horizonsE horizons are higher in value B horizons have more clayB horizons have more Fe

20

B Horizons

Referred to as subsoil. the zone of accumulation with in the soil. In well drained soils it has the brightest colors. May extend over a meter below the surface.

21

B Horizons - Field Criteria

Subsurface horizon formed below an O, A, E horizon and above the C horizonFormed as a result of soil forming processesWeakest expression is color developmentIlluvial concentration-zone of accumulation

22

B Horizons - Subordinate DistinctionsBg-Strong gleying, indicates prolonged periods of saturationBs-Illuvial accumulation of sesquioxides (spodic)Bh-Illuvial accumulation of humus (spodic)Bt-Accumulation of clay

23

Bt Horizon

Bt under a lighter E horizon.

24

Btg Horizon

Btg horizon at 40 cm and continuing to 100 cm. It is a g as it formed under saturated conditions and ha a matrix chroma of 2 or less. It is a t because it has a higher clay content than the Eg horizon above.

25

Bs Horizon

Bh Horizon

Bhs over a Bs horizon in an Aquod. Often a Bhs will overlie a Bs

26

C HorizonReferred to as substratum. These horizons and layers are little affected by soil forming processes (unweathered geologic material).Field Criteria

Little affected by soil-forming processesGeologic layeringLack of color development, color of unweathered geologic material

Subordinate Distinctions of C HorizonsCg-Strong gleying, indicates prolonged periods of saturation

27

28

C horizon that looks more like rock but it offers no resistance to excavations. Note the foliation pattern is still obvious.

29

30

Soil ProfileHorizons

determine if restrictive by rule

Ap

Bw1

2BCd

2Cd

Bw2

Bw3

Restrictive

Trench bottom depth

Soil wetness

Parent material

Most limiting

Tota

l dep

th

Tota

l bel

ow tr

ench

ad

just

ed fo

r wet

ness

Tota

l bel

ow

tren

chUsa

ble

dept

hDepth

to parent materialto soil wetnessto most limiting layerfor trench bottom

Usable depthsUsable depthBelow trench bottomAdjustment for slope and/or wetness condition

Bt under a lighter E horizon.