Soils

Pedogenesis

Pedogenesis is the process of soil formation.

 

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Soils form due to the action and interaction of five distinct and identifiable soil-forming factors (ie. pedogenic factors)

 

1)type of parent material (PM)

2)climate

3)organisms

4)topography

5)time

horizon

1)a soil horizon is an individual, more or les horizontal layer distinguishable from other layer(s) within the soil cross-section

 

2)soil hoizons are distinguishable (from one another) on the basis of:

-physical properties and/or

-chemical properties and/or

-biological properties (usually when one ‘class’ (or set) of properties change, one or both of the other classes of properties change(s) as well….. this often occurs in a cause/effect relationship)

Variable soil properties/characteristics are:

physical properties– include color, texture (S, Si and C content), structure, consistence, pore volume (porosity), water holding capacity, tilth

 

chemical properties- include presence of carbonate minerals (free lime) indicated by reaction with acid, soil reaction (pH), presenceof soluble salts, fertility/nutrient status, etc.

 

biological properties- include organic matter (humus) content, presence activity and proliferation of roots, presence and function of microorganisms, etc

 

 

*******a change in a specific soil property often results in changes in one or more other properties

Definitions of Master Soil Horizons

-soil horizons are divided into 2 fundamental classes, with a miscellaneous 3rd class

 

1)Master Organic Horizons

-the OM content must be >30% (OC>17%) to be identified as an organic horizon

-master organic horions are designated as:

  • L, F, or H for well drained sites (may be used alone or in combination… LF, FH, LFH, etc)
  • Of, Om, and Oh uner poorly drained conditions

-An organic horizon must have a minimum thickness of 5cm to be included in the soil profile description, therefore, organic horizons are commonly found under forest stands (duff) and designated L, F and H; and in poorly drained locations (bogs, muskeg, etc) designated as Of, Om, or Oh.

-although there is some leaf litter associated with grassland soils, these layers do NOT constitute organic horizons (not thick enough)

-also, the organic layer(s) must be >40cm thick overlying water (as in muskeg) to be classified as an organic soil

 

2)Master Mineral Horizons

-are assigned the term mineral because their organic matter content (OM) is <30% (expressed on a mass basis)

-soil organic matter content can also be stated in terms of organic carbon (OC); mineral soils have OC content <17% mass

-master mineral horizons are designated as A, B, and C

-A and B (and sometimes C) master mineral horizons require further lower case suffix modifiers to be completely accurate as mineral horizon designations

-note that the lower case suffix modifier(s) indicate specifically how the horizon has been changed from original parent material (PM) by the soil forming factors

Designation of Mineral Horizons

By definition, mineral horizons have OM content <30% and by implication S, Si and C content is >70%

 

Master mineral horizon designations (labels) include:

-A, B, C (written all in capitals/upper case letters)

-these labels distinguish one major type of master mineral hoizon from the others

-A and B master mineral horizons always require a lower case modifier suffix

-modifier suffixes indicate specifically what kind of master mineral horizon is present and in particular how this horizon has been modified or changed from the original parent material (PM) by the soil forming (pedogenic) factors active at this site.

 

-C master horizons may or may not use/require a modifier suffix, since, by definition, C horizons show negligible change from original PM

-therefore C horizons may stand alone

Examples of Some Common mineral horizon designations

Ah

-h designates a zone of maximum “in situ” accumulation of humus at the surface of the soil profile

-these horizons are darker in olor than the horizons located lower in the soil profile (presence of organic carbon)

-they are typical of soils developed under grassland vegetation

 

Ae

-e indicates eluviation (downward movement or leaching) of clay or other minerals and/or humus, etc

-these horizons are generally lighter in color than the underlying horizons in the soil profile

-they are characteristic of surface horizons in soils formed under forest vegetation

 

Bt

-t indicates the accumulation of clay in a B horizon; the clay having been eluviated (moved down) from the Ae horizon above

-logically a Bt horizon is usually found immediately below an Ae horizon

-they are common in forested soils of western Canada, even after the land has been celared for agriculture or other uses

 

Bf

-f indicates the process of iron and aluminum oxides accumulation below a severely leached (eluviated) Ae horizon

-this is typical of B horizon(s) in soils found under well developed, extremely acidic deciduous or coniferous forest vegetation

-Bf soil horizons have a typical reddish-orange color (which is used as an easy-to-observe visual diagnostic in the field that can be verified with lab analysis to determine enrichment with Fe and Al)

-Bf hoizons are commonly found under highly leached Ae horizons

 

Ck

-k denotes the presence of carbonate minerals (free lime) often located in the lower C horizons(s)

-this is indicated by a characteristic effervenscence when treated with dilute acid

-the presence of carbonate minerals results in an alkalice pH(>7.5)

 

Cca

-indicates the accumulation (increase) of free lime

-note that Ck and CCa horizons are very commonly from the subsoils in soil profiles on the plains of western Canada

 

Csa

-indicates the accumulation of soluble salts and salinity

 

Cg

-indicates gleying as a result of a high water table

 

-also, a subsoil horizon may exist simply as a C horizon, which indicates the starting point or parent material (PM) of the soil profile with no or little modification from the original

Other Master Mineral Horizons and Their Applications

R

-consolidated rock/bedrock; regolith at or near surface

-must be at least 10cm thick to be classified as soil, if less than 10cm, it is R (non-soil)

-soil covered by a veneer of fresh surficial deposit at least 50cm thick is classified as buried soil (a Paleosol)

 

W

-a surface layer

-indicates free water >60cm in depth overlying unconsolidated mineral parent material for most of the calendar year

-indicates organic material in layer(s) <40 cm thick over free water