Podzols

Concept of the Order

Podzols are acid 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 E horizon indicating translocation. The E 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, North Island high country and the West Coast and high country of the South Island.

Accessory Properties of the Order

  1. 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.
  2. 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.
  3. Parent materials. The soils occur mainly in materials from silica-rich rocks such as granite, greywacke, schist or rhyolite.
  4. 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.
  5. 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.
  6. 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.
  7. Mor-forming vegetation. The soils have been associated with vegetation producing an acid litter of low nutrient content.
  8. Infertile. The soils have very low natural fertility with high nitrogen, phosphorus, potassium and lime requirements.
  9. 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).

DENSIPAN PODZOLS

ZP

Other Podzols that have both

  1. an E horizon with either the greyish colours of a reductimorphic horizon, a peaty topsoil, or a placic horizon together with redox-segregations in the E horizon, and
  2. a slowly permeable layer or perch-gley features.

PERCH-GLEY PODZOLS

ZG

Other Podzols that have both

  1. either an E horizon with the greyish colours of a reductimorphic horizon or a peaty topsoil, and
  2. a BC or C horizon that has the greyish colours of a reductimorphic horizon, that either
    1. extends to more than 90 cm from the mineral soil surface, or
    2. has redox segregations that occur mainly as coatings on voids and/or skeletan grains.

GROUNDWATER-GLEY PODZOLS

ZX

Other Podzols that have a subhorizon in the B horizon that is both

  1. massive, and
  2. as slightly firm or greater moist soil strength or has moist or wet penetration resistance of 3100 kPa or more.

PAN PODZOLS

ZO

Other Podzols.

ORTHIC PODZOLS

Key to Subgroups of Podzols

ZD - DENSIPAN PODZOLS

Densipan Podzols have a high density but uncemented pan within the E 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

  1. an E horizon that
    1. has slightly firm or greater moist soil strength, and
    2. is apedal massive or has very coarse to extremely coarse prismatic pedality, and
    3. occurs within a layer that is silty through a thickness of at least 15 cm, and
  2. a peaty topsoil.

Peaty-silt-mantled Perch-gley Podzols

ZPZ

Other soils that have an E 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 horizon.

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.

ZOH

Orthic Podzols with a Bh horizon thicker than 5 cm.

Humose Orthic Podzols

ZOT

Other soils.

Typic Orthic Podzols