Podzols are mineral soils characterized by an accumulation of amorphous organic matter, Al and Fe oxides in the B horizon and subsequent leaching of these substances from the A horizon. Podzols are known in the American Soil Classification Systems as Spodosols. They occupy approximately 4% of the earth’s total land mass, and they are the most common soils in British Columbia.
Generally, their existence predominates in regions with cool soil temperatures, but they do occur in tropical climates as well, as long as the precipitation levels facilitate the existence of a per-humid or humid climate. Typically, Podzols form from medium-to coarse-textured acid parent materials.
Conditions needed for formation of Podzols
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.
The process of Podzol development is a combination of several soil processes and is referred to collectively as podzolization. Podzolization is characterized by various chemical and biological transformations, resulting in transformation (weathering) of primary minerals and decomposition of organic matter. Soluble organic matter and mobile compounds of Al and Fe are readily leached from the A horizon into the B horizon where they accumulate and form a discrete horizon called a podzolic B (Bh, Bhf, Bf). Note that the podzolic B horizon must be at least 10 cm thick, have a texture coarser than clay, and meet specific requirements for colour, organic C, and Fe and Al content.
Podzols form under forest or heath vegetation, specifically coniferous or mixed coniferous/deciduous forest, or ericaceous shrubs. Vegetation that can supply mobile and sesquioxide-mobilizing organic compounds favors the formation of Podzols:
- production of humic and fulvic acids (low molecular weight organic acids),
- vegetation with high cellulose contents / acidic, low-base litter, which decomposes slowly, and
- the release of phenolic and carboxylic compounds in throughfall and stemflow that are able to chelate or immobilize Fe an Al.
The parent materials under which Podzols form are generally low in Ca and Mg minerals but are rich in quartz, such as granite and sandstone. Podzols cannot be formed on parent material with clay content above 35%.
Podzols will form under quite varied topography from low-lying sites to steeply sloped sites, however, in depressions the soil matrix becomes water-logged and Podzol formation is uncommon. In those environments either Gleysols or Organic soils develop.
The length of the Podzolic development can vary, but on average most Podzols take between 3,000 and 10,000 years to develop. Under optimal conditions, Podzols can develop in several hundred years, though this is not common.
Canadian system of soil classification
The Canadian system of soil classification has five levels of classification (orders, great groups, subgroups, families, and series). For more information on classification criteria, please review the Canadian System of Soil Classification, 3rd ed. The broadest class is called order and Podzolic order is one of 10 orders in the Canadian system. Podzolic order is subdivided into three great groups and 25 subgroups.
|Great Groups of Podzolic order||Description|
|Humic Podzol||Bh horizon >10 cm in thickness|
Organic C > 1%
Pyrophosphate Fe < 0.3%
Organic C : pyrophosphate Fe ≥ 20
|Ferro-Humic Podzol||Bhf horizon >10 cm in thickness|
Organic C > 5%
Pyrophosphate Al+Fe > 0.6% (> 0.4% for sands)
|Humo-Ferric Podzol||Bf, or thin Bhf+Bf ≥ 10 cm thick|
Organic C = 0.5-5%
Pyrophosphate Al+Fe = 0.6% (> 0.4% for sands)
Using knowledge of Podzol’s morphological characteristics, soil identification and classification (at the soil order level) can done in the field, while chemical properties are needed for more detailed classification of specific great groups within this order.
Distribution of Podzols
British Columbia has a wealth of soil types representative of all 10 soil orders (Regosols, Brunisols, Cryosols, Chernozems, Gleysols, Luvisols, Solonetz, Podzols, Vertisols, and Organic soils) of the Canadian soil classification system. At the University Endowment Lands (UEL) and the Pacific Spirit Park, the following soil orders can be found: Podzols, Gleysols, Brunisols, Regosols, and Organic soils, but the most common are the Podzols.
Most soils on the UEL lands belong to the great group of Humo-Ferric Podzols. The Humo-Ferric Podzols have a podzolic B horizon at least 10 cm thick (but do not have Bh or Bhf horizons at least 10 cm thick). A Bf horizon contains 0.5-5% organic C and 0.6% or more pyrophosphate-extractable Fe+AI (>0.4% for sands). Humo-Ferric Podzols are generally strongly acidic and have less than 50% base saturated (neutral salt). Humo-Ferric Podzols are divided into 10 subgroups based on the kind and sequence of the horizons.
The most common soil series on the UEL is the Bose Humo-Ferric Podzol. This soil series developed on marine lag or glaciofluvial deposits, which rest upon glacial till or glaciomarine sediments. The texture of the original parent material was moderate to very stony, creating gravelly soil, which ranges from sandy loam to loamy sand in texture. The glacial till underlying the A and B horizons are often strongly cemented in the upper 25-50 cm and slowly pervious to water. Link to download of the Soils of the Langley-Vancouver Map Area, vol. 3, which includes detailed description of soil series for this region, including the Bose series.
History of Podzolic soil order
Podzols were named and identified first by the Russian soil scientist V.V. Dokuchaev. He did not initiate the use of the term Podzol, but he simply used the vernacular of his time. The term “Podzol” in Russian means “wood ash”. Another explanation of the name of this soil order was offered by Muir who believed that the “pod” in Podzol refers to the Greek word “pedon” or Latin word “peda,” indicating that it simply refers to “soil.”
Many Russian forests under which Podzols have formed were occasionally burned by natural forest fires, therefore referral to the ash is not without merit. Early soil scientists thought that the Ae horizon was a layer of wood ash. Later soil scientists discovered that the Ae horizon was actually a layer in which Al and Fe oxides and organic acids had been leached and deposited into the lower horizon, leaving behind white or gray silicates. Many Podzols do not have the bleached Ae horizon and the presence of a podzolic B horizon is a diagnostic feature of this soil order.
To learn more about Podzols, please visit Soils of Canada web site