13 Podzols
13.1 Concept of the Order
Podzols are acidic soils with low base saturation. Podzol Soils have a horizon of aluminium accumulation occurring as complexes with organic matter and/or as nanocrystalline minerals including allophane (with some imogolite) that result from the translocation of silicon as well as aluminium in solution as proto-imogolite. Iron (typically as ferrihydrite) may or may not accumulate with aluminium and silicon. This podzolic-B horizon is usually associated with an overlying eluvial horizon indicating translocation. The podzolic-B horizon can develop into a placic horizon or ortstein-pan if cementation occurs. The eluvial horizon may be missing due to erosion, ploughing, or bioturbation, or it may be masked by organic matter.
13.2 Correlation
Podzols correspond to the podzols and some podzolised yellow-brown earths of the New Zealand Genetic Soil Classification. They mostly correspond with the Spodosols of Soil Taxonomy.
13.3 Occurrence
Podzols occur in areas of high precipitation and are usually associated with forest species which produce an acidic litter. They are most common in Northland, the West Coast of the South Island, Stewart Island and generally at higher elevations.
13.4 Accessory Properties of the Order
- Secondary oxides strongly differentiated between horizons. In A and E horizons, sand and silt grains are uncoated. In B horizons they are coated, and in some soils cemented, by short-range-order minerals and/or organic matter.
- Low base saturation. Base saturations are very low, and the soils are extremely acidic, in A and E horizons. KCl-extractable-aluminium levels are high, and aluminium in soil solution may be toxic to some plants.
- Parent materials. The soils occur mainly in materials from felsic rocks such as granite, greywacke, schist or rhyolite, and derivatives including loess and tephra deposits.
- Mineralogically differentiated. Mica-smectite or smectite often occur in A and E horizons, and hydroxy-coated or interlayered minerals or allophane often occur in the B horizon. In some Northland soils, silica is dominant in B horizons. Podzols cover a wide range of mineralogy classes.
- Limited rooting depth. The rooting depth is limited for many introduced plants by low pH or aluminium toxicity, or by pans which often impede drainage.
- Low biological activity. Low levels of faunal activity occur with a low rate of mineralisation. Carbon/nitrogen ratios are very high. The accumulation of weakly decomposed humus and minimal incorporation of humus into the upper mineral soil results in a mor-like humus form.
- Mor-forming vegetation. The soils have been associated with vegetation producing an acid litter of low nutrient content (mainly Agathis australis, Nothofagaceae, Podocarpaceae).
- Infertile. The soils have very low natural fertility with high nitrogen, phosphorus, potassium and lime requirements.
- Wet climate. The soils occur mainly in areas with an average annual precipitation of 1400 mm or more and, unless sandy, are likely to have a soil water surplus for a considerable part of the year.
13.5 Summary of Podzols Hierarchy
| Code | Group | Subgroup | Example Series |
|---|---|---|---|
| ZD | Densipan | Humus-pan | Te Kopuru |
| Argillic-humose | Te Kopuru | ||
| Ultic-humose | Te Kopuru | ||
| Humose | - | ||
| Ortstein | Onahau | ||
| Typic | Parahaki | ||
| ZP | Perch-gley | Fluid | Hukarere |
| Peaty-silt-mantled | Okarito | ||
| Silt-mantled | Okarito | ||
| Humus-pan | Addison | ||
| Humose-ortstein | Rutherglen | ||
| Humose-placic | Rakiura | ||
| Humose | Maimai | ||
| Ortstein | Charleston | ||
| Placic | Maungatua | ||
| Typic | Rowallan | ||
| ZG | Groundwater-gley | Humose | - |
| Typic | - | ||
| ZX | Pan | Humus-pan | Kairua |
| Argillic-humose | - | ||
| Humose | Tautuku | ||
| Fragic | Nevis | ||
| Ortstein | Spencer | ||
| Placic | Pukepahi | ||
| Firm | - | ||
| ZO | Orthic | Humose | Borland |
| Argillic-humose | - | ||
| Typic | Shewell |
13.6 Key to Groups of Podzols
ZD
Podzols that have a densipan (Ed horizon).
ZP
Other Podzols that have BOTH
- An eluvial horizon with the low-chroma colours of a reductimorphic horizon, a peaty topsoil, or a placic horizon with redox-segregations in the eluvial horizon, AND
- A slowly permeable layer or perch-gley features.
ZG
Other Podzol Soils that have BOTH
- Either an Eluvial horizon with the low-chroma colours of a reductimorphic horizon or a peaty topsoil, AND
- A BC or C horizon that has the low-chroma colours of a reductimorphic horizon, that either
- extends to more than 90 cm from the mineral soil surface, or
- has redox segregations that occur mainly as coatings on voids and/or skeletans.
ZX
Other Podzol Soils that have a subhorizon in the B horizon that is BOTH
- Massive, AND
- Has firm or greater moist soil strength or has moist or wet penetration resistance of 3100 kPa or more.
ZO
Other Podzol Soils.
13.7 Key to Subgroups of Podzols
ZD - DENSIPAN PODZOLS
Densipan Podzol Soils have a high density but uncemented pan within the eluvial horizon at shallow depths which severely limits root penetration and water movement.
ZDU
Densipan Podzol Soils that have a humus-pan.
Humus-pan Densipan Podzols
ZDJH
Other soils that have an argillic horizon beneath the Bh horizon.
Argillic-humose Densipan Podzols
ZDYH
Other soils that have a Bh horizon more than 5 cm thick that occurs within the top of a clayey prismatic B horizon.
Ultic-humose Densipan Podzols
ZDH
Other soils that have a Bh horizon 5 cm or more thick.
Humose Densipan Podzols
ZDQ
Other soils that have an ortstein-pan.
Ortstein Densipan Podzols
ZDT
Other soils.
Typic Densipan Podzols
ZP - PERCH-GLEY PODZOLS
Perch-gley Podzol Soils occur in periodically, or predominantly, saturated sites (unless artificially drained) in which wetness is indicated by low=chroma colours along with brownish or reddish mottles or peaty topsoils. The wetness is caused by the perching of water on a slowly permeable layer, although a groundwater-table may also be present.
ZPF
Perch-gley Podzol Soils that have a moderately fluid or very fluid failure in some part above the podzolic-B horizon.
Fluid Perch-gley Podzols
ZPOZ
Other soils that have BOTH
- an eluvial horizon that has all of the following
- a slightly firm or greater moist soil strength, and
- an apedal-massive or very coarse to extremely coarse prismatic structure, and
- is part of a silty layer that has a thickness of at least 15 cm, AND
- A peaty topsoil.
Peaty-silt-mantled Perch-gley Podzols
ZPZ
Other soils that have an eluvial horizon that has slightly firm or greater moist soil strength, is apedal massive or has very coarse to extremely coarse prismatic structure, and occurs within a layer that is silty and at least 15 cm thick.
Silt-mantled Perch-gley Podzols
ZPU
Other soils that have a humus-pan.
Humus-pan Perch-gley Podzols
ZPHQ
Other soils that have a Bh horizon thicker than 5 cm, and an ortstein-pan.
Humose-ortstein Perch-gley Podzols
ZPHP
Other soils that have a Bh horizon thicker than 5 cm, and a placic horizon.
Humose-placic Perch-gley Podzols
ZPH
Other soils that have a Bh horizon thicker than 5 cm.
Humose Perch-gley Podzols
ZPQ
Other soils with an ortstein-pan.
Ortstein Perch-gley Podzols
ZPP
Other soils that have a placic horizon.
Placic Perch-gley Podzols
ZPT
Other soils.
Typic Perch-gley Podzols
ZG - GROUNDWATER-GLEY PODZOLS
Groundwater-gley Podzol Soils occur in periodically or predominantly saturated sites (unless artificially drained) in which the wetness is indicated by brownish or reddish mottles or peaty topsoils. The wetness is caused by a groundwater-table.
ZGH
Groundwater-gley Podzol Soils that have a Bh horizon thicker than 5 cm.
Humose Groundwater-gley Podzols
ZGT
Other soils.
Typic Groundwater-gley Podzols
ZX - PAN PODZOLS
Pan Podzol Soils have a firm or indurated layer which restricts the penetration of roots and reduces permeability.
ZXU
Pan Podzols that have a humus-pan.
Humus-pan Pan Podzols
ZXJH
Other soils that have a Bh horizon that also have an argillic horizon beneath the ortstein-pan or placic horizon.
Humus Pan Podzols
ZXH
Other soils that have a Bh horizon that and either an ortstein-pan or placic horizon.
Humus Pan Podzols
ZXX
Other soils that have a fragipan.
Fragic Pan Podzols
ZXQ
Other soils that have an ortstein-pan.
Ortstein Pan Podzols
ZXP
Other soils that have a placic horizon.
Placic Pan Podzols
ZXF
Other soils.
Firm Pan Podzols
ZO - ORTHIC PODZOLS
Orthic Podzol Soils are well or moderately well drained, and do not have a pan or a firm horizon with massive structure.
ZOH
Orthic Podzol Soils with a Bh horizon thicker than 5 cm.
Humose Orthic Podzols
ZOJH
Other soils with an argillic horizon beneath a Bh horizon thicker than 5 cm.
Humose Orthic Podzols
ZOT
Other soils.
Typic Orthic Podzols