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.
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.
- The podzolic B horizon is at least 10 cm thick and has moist, crushed colors as follows:
- The color is black or the hue is either 7.5YR or redder or 10YR near the upper boundary and becomes yellower with depth.
- The chroma is higher than 3 or the value is 3 or less.
- The accumulation of amorphous material in the podzolic B horizon is indicated by:
- Brown to black coatings on some mineral grains or brown to black microaggregates.
- A silty feel when rubbed wet unless the material is cemented.
- The texture of the podzolic B horizon is coarser than clay.
- 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.
- 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.
- The soils have a B subhorizon (Bf or Bhf) at least 10 cm thick with the following characteristics:
- An organic C content of 0.5% or more.
- 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).
- A ratio of pyrophosphate-extractable Fe+Al to clay (<0.002 mm) of more than 0.05.
- 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:
- 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.
- 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.
- 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:
- 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.
- They have a base saturation, as determined by an unbuffered salt, nearly always below 80% and commonly less than 50%.
- Unless cemented, they have a higher water-holding capacity than non Bf horizons of similar texture.
- They have a high capacity to fix phosphate.
- 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.
- 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.
|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.
|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|