Chapter 10: Podzolic Order

A diagrammatic representation of profiles of some subgroups of the Podzolic order is shown in Figure 37. Individual subgroups may include soils that have horizon sequences different from those shown. In the description of each subgroup, presented later in this chapter, a common horizon sequence is given; diagnostic horizons are underlined and some other commonly occurring horizons are listed.

Figure 37: Diagrammatic horizon pattern of some subgroups of the Podzolic order.

Figure 37 is a diagrammatic horizon pattern of some subgroups of the Podzolic order. Orthic Humic Podzol (O.HP) Common horizon sequence: O or LFH, Ae, Bh, Bfgj, BCgj, Cg. These soils have the general properties specified for the Podzolic order and the Humic Podzol great group. They are identified by the following properties: 1. They have a Bh horizon at least 10 cm thick. 2. They do not have an ortstein horizon at least 3 cm thick, a placic horizon, a duric horizon, or a fragipan. Usually Orthic Humic Podzols have L, F, and H or O horizons and an Ae horizon. Commonly they have a Bhf or Bf horizon underlying the Bh horizons. They may have mottling that indicates gleying at any depth within the control section. Parts of the Bhf or Bf may be cemented but do not meet the requirements of an ortstein horizon. Gleyed Ferro-Humic Podzol (GL.FHP) Common horizon sequence: LFH or O, Aegj, Bhf, Bfgj, BCg, Cg. These soils have the general properties specified for the Podzolic order and the Ferro-Humic Podzol great group. They differ from Orthic Ferro-Humic Podzols by having distinct or prominent mottles that indicate gleying within 1 m of the surface. Usually they have thick L, F, and H or O horizons. They do not have ortstein, placic, duric, or Bt horizons, a fragipan, or an Ah horizon at least 10 cm thick. Placic Ferro-Humic Podzol (P.FHP) Common horizon sequence: LFH or O, Ae, Bhf, Bhfc or Bfc, Bf, BC, C. These soils have the general properties specified for the Podzolic order and the Ferro-Humic Podzol great group. They differ from Orthic Ferro-Humic Podzols by having a placic horizon within the control section. A placic horizon (Bhfc, Bfc, Bfgc) consists of a single thin layer (commonly 5 mm or less thick) or a series of thin layers that are irregular or involute, hard, impervious, often vitreous, and dark reddish brown to black. These thin horizons are apparently cemented by Fe-organic complexes, hydrated Fe oxides, or a mixture of Fe and Mn oxides. The placic horizon, or thin iron pan, may occur in any part of the B horizon and commonly extends into the BC horizon. Placic Ferro-Humic Podzols usually have L, F, and H or O horizons and an Ae horizon. They do not have an ortstein horizon but may have a duric, Ah, or Bt horizon, or a fragipan. Evidence of gleying in the form of dull colors or mottling is commonly apparent especially above depressions in the placic horizon. These soils occur most commonly in coarse textured deposits in perhumid maritime climates. Duric Ferro-Humic Podzol (DU.FHP) Common horizon sequence: LFH or O, Ae, Bhf, BCc, C. These soils have the general properties specified for the Podzolic order and the Ferro-Humic Podzol great group. They differ from Orthic Ferro-Humic Podzols by having a duric horizon within the control section. A duric horizon is a strongly cemented horizon that does not satisfy the criteria of a podzolic B horizon. It usually has an abrupt upper boundary to an overlying podzolic B horizon and a diffuse lower boundary at least 50 cm below. Cementation is usually strongest near the upper boundary, which occurs commonly at a depth of 40-80 cm from the mineral surface. Usually the color of a duric horizon differs little from that of the parent material. As well the structure is usually massive or very coarse platy. Moist clods at least 3 cm thick usually cannot be broken in the hands. Air-dry clods of the material do not slake when immersed in water. Some duric horizons may meet the requirements of a Bt horizon (Btc). Duric Ferro-Humic Podzols usually have L, F, and H or O horizons. They do not have an ortstein or a placic horizon but may have an Ah horizon and mottles that indicate gleying in some part of the control section. These soils occur most commonly in coastal southwestern British Columbia. Orthic Humo-Ferric Podzol (O.HFP) Common horizon sequence: LFH, Ae, Bf, BC, C. These soils have the general properties specified for the Podzolic order and the Humo-Ferric Podzol great group. They are identified by the following properties: They have a podzolic B horizon at least 10 cm thick (Bf or thin Bhf and Bf). They do not have a Bh horizon at least 10 cm thick, a Bhf horizon at least 10 cm thick, an ortstein horizon at least 3 cm thick, a placic horizon, a duric horizon, a fragipan, a Bt horizon, an Ah horizon at least 10 cm thick, nor evidence of gleying in the form of distinct or prominent mottles within 1 m of the surface. Usually Orthic Humo-Ferric Podzols have L, F, and H or O horizons and an Ae horizon. Parts of the Bf may be cemented, but it does not meet the requirements of an ortstein horizon. Ortstein Humo-Ferric Podzol (OT.HFP) Common horizon sequence: LFH, Ae, Bfc, Bfj, C. These soils have the general properties specified for the Podzolic order and the Humo-Ferric Podzol great group. They differ from Orthic Humo-Ferric Podzols by having an ortstein horizon at least 3 cm thick. An ortstein horizon in this subgroup is a Bhf or Bf horizon that is strongly cemented and occurs in at least one-third of the lateral extent of the pedon. Ortstein horizons are generally reddish brown to very dark reddish brown in color. Usually Ortstein Humo-Ferric Podzols have L, F, and H or O horizons and an Ae horizon. They may have faint mottling, and placic, duric, Ah, or Bt horizons, or a fragipan. Fragic Humo-Ferric Podzol (FR.HFP) Common horizon sequence: LFH, Ae, Bf, BCx, C. These soils have the general properties specified for the Podzolic order and the Humo-Ferric Podzol great group. They differ from Orthic Humo-Ferric Podzols by having a fragipan within the control section. A fragipan (Bx or BCx) is a subsurface horizon of high bulk density that is firm and brittle when moist and hard to extremely hard when dry. Usually it is of medium texture. Commonly it has bleached fracture planes separating very coarse prismatic units. The secondary structure is platy. Usually the fragipan has a color similar to that of the parent material but differs in structure and consistence and sometimes in bulk density. The upper boundary of a fragipan is usually either abrupt or clear, but the lower boundary is usually diffuse. Commonly it is necessary to dig to about 3 m to expose clearly the material beneath the lower boundary of the fragipan. Air-dry clods of fragipans slake in water. A fragipan may have clay skins and meet the limits of a Bt horizon (Btx). Fragic Humo-Ferric Podzols usually have L, F, and H horizons and an Ae horizon. They have neither ortstein, placic, nor duric horizons but may have an Ah horizon and mottles that indicate gleying at some depth within the control section. Luvisolic Humo-Ferric Podzol (LU.HFP) Common horizon sequence: LFH, Ae, Bf, Bt, C. These soils have the general properties specified for the Podzolic order and the Humo-Ferric Podzol great group. They differ from Orthic Humo-Ferric Podzols by having a Bt horizon of which the upper boundary is at a depth of more than 50 cm from the mineral surface. If the upper boundary of the Bt horizon is ≤50 cm from the surface, the soil is classified in the Luvisolic order. Luvisolic Humo-Ferric Podzols usually have L, F, and H horizons and an Ae horizon. They may also have an Ah horizon. They have neither ortstein, duric, nor placic horizons, nor a fragipan but may have mottles that indicate gleying at some depth within the control section. Sombric Humo-Ferric Podzol (SM.HFP) Common horizon sequence: LFH, Ah, Ae, Bf, BC, C. These soils have the general properties specified for the Podzolic order and the Humo-Ferric Podzol great group. They differ from Orthic Humo-Ferric Podzols by having an Ah horizon at least 10 cm thick. Usually Sombric Humo-Ferric Podzols have L, F, and H horizons and may have an Ae horizon. They have neither ortstein, placic, duric, nor Bt horizons, nor a fragipan, nor distinct or prominent mottles that indicate gleying. .

Soils of the Podzolic order have B horizons in which the dominant accumulation product is amorphous material composed mainly of humified organic matter combined in varying degrees with Al and Fe. Typically Podzolic soils occur in coarse- to medium-textured, acid parent materials, under forest or heath vegetation in cool to very cold humid to perhumid climates. However, some occur under soil environmental conditions outside this range. For example, minor areas of Podzolic soils occur in wet sandy sites in areas of subhumid climate. Other Podzolic soils have formed in parent materials that were once calcareous.

Podzolic soils can usually be recognized readily in the field. Generally they have organic surface horizons that are commonly L, F, and H but may be Of or Om and have a light-colored eluvial horizon, Ae, which may be absent. Most Podzolic soils have a reddish brown to black B horizon with an abrupt upper boundary and lower B or BC horizons with colors that become progressively yellower in hue and lower in chroma with depth, except in reddish-colored parent materials.

Soils of the Podzolic order are defined based on a combination of morphological and chemical criteria of the B horizons. Soils of the order must meet all the following morphological limits and those specified under either 1 or 2 of the chemical limits.

Morphological Limits

  1. The podzolic B horizon is at least 10 cm thick and has moist, crushed colors as follows:
    1. The color is black or the hue is either 7.5YR or redder or 10YR near the upper boundary and becomes yellower with depth.
    2. The chroma is higher than 3 or the value is 3 or less.
  2. The accumulation of amorphous material in the podzolic B horizon is indicated by:
    1. Brown to black coatings on some mineral grains or brown to black microaggregates.
    2. A silty feel when rubbed wet unless the material is cemented.
  3. The texture of the podzolic B horizon is coarser than clay.
  4. The soil either has no Bt horizon or the upper boundary of the Bt horizon is at a depth greater than 50 cm from the mineral soil surface.

Chemical Limits

  1. The soils have a B subhorizon (Bh) at least 10 cm thick, with both color value and chroma (moist) of 3 or less, that contains more than 1% organic C, less than 0.3% pyrophosphate-extractable Fe, and has a ratio of organic C to pyrophosphate-extractable Fe of 20 or more.
  2. The soils have a B subhorizon (Bf or Bhf) at least 10 cm thick with the following characteristics:
    1. An organic C content of 0.5% or more.
    2. A pyrophosphate-extractable Fe+Al content of 0.6% or more in textures finer than sand and of 0.4% or more in sands (coarse sand, sand, fine sand, and very fine sand).
    3. A ratio of pyrophosphate-extractable Fe+Al to clay (<0.002 mm) of more than 0.05.
    4. A ratio of organic C to pyrophosphate-extractable Fe of less than 20, or pyrophosphate-extractable Fe of at least 0.3% or either color value or chroma of more than 3.

A Bf horizon contains 0.5-5% organic C, and a Bhf horizon contains more than 5% organic C.

Any B horizon that satisfies the specified morphological and chemical requirements is a podzolic B horizon. In the following cases, the color criteria of a podzolic B horizon are waived for the following: Ap horizons that meet the chemical limits; and B subhorizons that meet the chemical limits specified. Some Bh, Bhf, and Bf horizons do not qualify as podzolic B horizons because they are too thin. To determine whether a pedon meets Podzolic order criteria, it is necessary to sample only the 10 cm of B horizon that are most strongly expressed. If all of the B horizon looks the same, it may be desirable to take three samples (top, middle, and bottom 10 cm). Similarly, to determine whether a pedon has a Bhf or a Bh horizon, it is necessary to sample a subhorizon only 10 cm thick that appears most likely to meet the limits of Bhf or Bh. Average samples should consist of 10 cm of subhorizon taken uniformly from an exposure 50 cm wide or wider. For many studies it is desirable to sample all subhorizons.

Some soils that are not Podzolic will satisfy the minimum morphological limits specified. However, these limits are thought to be useful to exclude from the order certain soils having horizons that satisfy the chemical limits specified but otherwise do not resemble Podzolic soils. To be classified as Podzolic, a borderline soil must meet both the morphological and the chemical limits specified.

Some acid Ah horizons satisfy the morphological and chemical criteria of podzolic B horizons. These are commonly associated with volcanic ash. No specific criteria have been developed to distinguish these horizons from Bhf or Bf horizons. The following guidelines are useful:

  1. Such Ah horizons are generally black and underlain by brown or dark brown B horizons. For example, the moist color of the A horizon may be 10YR 2/2 and that of the B 7.5YR 4/4.
  2. The ratio of humic to fulvic acid in these Ah horizons is greater than 1:2 (usually 1:1 or higher) and in the underlying B horizon it is less than 1:2.
  3. Less than 50% of the total organic C is extracted from these A horizons by alkali pyrophosphate and more than 50% is extracted from the associated B horizons.

Some associated properties of podzolic B horizons

Besides the properties specified as diagnostic, podzolic B horizons have a number of associated properties that may be useful in distinguishing them from other B horizons. Some of these associated properties are as follows:

  1. They have a high pH-dependent cation exchange capacity (CEC). The difference (ΔCEC) between CEC measured at pH 7 and at the pH of the soil is usually at least 8 cmol kg-1.
  2. They have a base saturation, as determined by an unbuffered salt, nearly always below 80% and commonly less than 50%.
  3. Unless cemented, they have a higher water-holding capacity than non Bf horizons of similar texture.
  4. They have a high capacity to fix phosphate.
  5. Although they commonly contain appreciably more clay than the overlying Ae horizon (if present), usually very little of the clay occurs as oriented coatings on particles or peds.
  6. They give a strongly alkaline reaction in NaF. As a field test a 2% suspension of soil in 1 M NaF gives a pH of 10.3 or more for most podzolic B horizons. Volcanic ash samples also commonly give a high pH with this test, thus the NaF test is not useful for identifying podzolic B horizons in materials containing volcanic ash.

Distinguishing Podzolic Soils from Soils of Other Orders

Guidelines for distinguishing Podzolic soils from soils of other orders with which they might be confused are as follows:

Luvisolic Some Podzolic and some Luvisolic soils have Ae, Bf, and Bt horizons. These soils are classified as Podzolic if the upper boundary of the Bt horizon is at a depth below 50 cm and as Luvisolic if it is at a depth of 50 cm or less.

Brunisolic In the continuum of soils in nature many pedons have properties close to the arbitrary boundary line between Podzolic soils and acid Brunisolic soils. If the B horizon meets the requirements of a podzolic B, the soils are classified as Podzolic.

Gleysolic A podzolic B horizon takes precedence over gley features. Thus a soil having both a podzolic B horizon and gley colors as specified for soils of the Gleysolic order is classified as Podzolic.

Organic Some soils have podzolic B horizons underlying a thick layer of peat or folic materials. The soil is classified as Organic if the peat layer is either 60 cm or more in depth and consists of fibric materials, or 40 cm or more in depth if it consists of mesic, humic, or folic materials.

The Podzolic order is divided into three great groups: Humic Podzol, Ferro-Humic Podzol, and Humo-Ferric Podzol based on the organic C content and the organic C to pyrophosphate-extractable Fe ratio of the podzolic B horizon as shown in the Pozolic order chart.

Podzolic Order
  Humic Podzol Ferro-Humic Podzol Humo-Ferric Podzol
Diagnostic horizons are underlined
B horizon Bh ≥ 10 cm thick Bhf ≥ 10 cm thick Bf, or thin Bhf+Bf ≥ 10 cm thick
Organic C >1% >5% = 0.5-5%
Other pyrophosphate Fe <0.3%, organic C/pyrophosphate Fe ≥ 20 pyrophosphate Al+Fe ≥0.6% (≥0.4% for sands) pyrophosphate Al+Fe ≥0.6% (≥0.4% for sands)

Subgroups are separated based on the kind and sequence of the horizons indicating conformity with the central concept of the great group, the presence of additional horizons, or intergrading to soils of other orders. Some features formerly recognized taxonomically at the subgroup level are now recognized at either the family (lithic, some cryic) or series (turbic) level. These features may also be recognized as phases of subgroups, great groups, or orders.

Great Group Subgroup
Humic Podzol Orthic Humic Podzol O.HP
Ortstein Humic Podzol OT.HP
Placic Humic Podzol P.HP
Duric Humic Podzol DU.HP
Fragic Humic Podzol FR.HP
Ferro-Humic Podzol Orthic Ferro-Humic Podzol O.FHP
Ortstein Ferro-Humic Podzol OT.FHP
Placic Ferro-Humic Podzol P.FHP
Duric Ferro-Humic Podzol DU.FHP
Fragic Ferro-Humic Podzol FR.FHP
Luvisolic Ferro-Humic Podzol LU.FHP
Sombric Ferro-Humic Podzol SM.FHP
Gleyed Ferro-Humic Podzol GL.FHP
Gleyed Ortstein Ferro-Humic Podzol GLOT.FHP
Humo-Ferric Podzol Orthic Humo-Ferric Podzol O.HFP
Ortstein Humo-Ferric Podzol OT.HFP
Placic Humo-Ferric Podzol P.HFP
Duric Humo-Ferric Podzol DU.HFP
Fragic Humo-Ferric Podzol FR.HFP
Luvisolic Humo-Ferric Podzol LU.HFP
Sombric Humo-Ferric Podzol SM.HFP
Gleyed Humo-Ferric Podzol GL.HFP
Gleyed Ortstein Humo-Ferric Podzol GLOT.HFP
Gleyed Sombric Humo-Ferric Podzol GLSM.HFP

Source: The Canadian System of Soil Classification (Third Edition)

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