Podzols
Concept of the Order
Podzols are acidic soils with low base saturation. Podzols have an horizon of accumulation of aluminium occurring as complexes with organic matter and/or as short-range-order minerals (typically with silicon as allophane/ imogolite). Iron (typically as ferrihydrite) may or may not be accumulated with aluminium. This horizon is usually associated with an overlying eluvial horizon indicating translocation. The eluvial horizon may be missing as a result of erosion, ploughing or bioturbation or it may be masked by organic matter.
Correlation
Podzols correspond to the podzols and some podzolised yellow-brown earths of the NZ Genetic Soil Classification. They mostly correspond with the Spodosols of Soil Taxonomy.
Occurrence
Podzols occur in areas of high precipitation and are usually associated with forest species which produce an acid litter. They are most common in Northland, the West Coast of the South Island, and the high country of the North and South Islands.
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 acid 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 are derived predominantly from felsic rocks such as granite, greywacke, schist or rhyolite.
- 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 cause problems of wetness.
- 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.
- 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 a precipitation of 1400 mm or more and unless sandy, are likely to have a soil water surplus for a considerable part of the year.
Summary of Podzols Hierarchy
| Code | Group | Subgroup | Example Series |
|---|---|---|---|
| ZD | Densipan | Humus-pan | pt. Te Kopuru |
| Ultic-humose | pt. Te Kopuru | ||
| Humose | - | ||
| Ortstein | - | ||
| Typic | Parahaki | ||
| ZP | Perch-gley | Fluid | Hukarere var. |
| Peaty-silt-mantled | pt. Okarito | ||
| Silt-mantled | pt. Okarito | ||
| Humus-pan | Addison | ||
| Humose-ortstein | Rutherglen | ||
| Humose-placic | Rakiura | ||
| Humose | Maimai | ||
| Ortstein | pt. Charleston | ||
| Placic | Maungatua | ||
| Typic | Rowallan | ||
| ZG | Groundwater-gley | Humose | - |
| Typic | - | ||
| ZX | Pan | Humus-pan | Kairua |
| Humose | Tautuku | ||
| Fragic | Nevis | ||
| Ortstein | Spencer | ||
| Placic | Pukepahi | ||
| Firm | - | ||
| ZO | Orthic | Humose | Borland |
| Typic | Shewell |
Key to Groups of Podzols
ZD
Podzols that have a densipan (Ed horizon).
ZP
Other Podzols that have both
- an Eluvial horizon with either the greyish colours of a reductimorphic horizon, a peaty topsoil, or a placic horizon together with redox-segregations in the Eluvial horizon, and
- a slowly permeable layer or perch-gley features.
ZG
Other Podzols that have both
- either an Eluvial horizon with the greyish colours of a reductimorphic horizon or a peaty topsoil, and
- a BC or C horizon that has the greyish 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 skeletan grains.
ZX
Other Podzols 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 Podzols.
Key to Subgroups of Podzols
ZD - DENSIPAN PODZOLS
Densipan Podzols have a high density but uncemented pan within the Eluvial horizon at shallow depths which severely limits root penetration and water movement.
ZDU
Densipan Podzols that have a humus-pan.
Humus-pan 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 Podzols occur in periodically or predominantly saturated sites (unless artificially drained) in which wetness is indicated by grey colours along with brownish or reddish mottles or peaty topsoils. The wetness is caused by the perching of water on a slowly permeable subsurface layer, although a groundwater-table may also be present.
ZPF
Perch-gley Podzols 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 slightly firm or greater moist soil strength, and
- is apedal-massive or has very coarse to extremely coarse prismatic structure, and
- occurs within a layer that is silty through 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 pedality, and occurs within a layer that is silty through a thickness of at least 15 cm.
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 Podzols 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 Podzols that have a Bh horizon thicker than 5 cm.
Humose Groundwater-gley Podzols
ZGT
Other soils.
Typic Groundwater-gley Podzols
ZX - PAN PODZOLS
Pan Podzols 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
ZXH
Other soils that have a Bh horizon, 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 Podzols are moderately well or well drained and do not have a cemented or high density pan, or a firm horizon with massive structure.
ZOH
Orthic Podzols with a Bh horizon thicker than 5 cm.
Humose Orthic Podzols
ZOT
Other soils.
Typic Orthic Podzols