where values are less than 4.8 are at risk. A more precise
measure of potential aluminium toxicity is the amount of
labile aluminium extracted from the soil by potassium
chloride solution. Soils with KCl-extractable Al values more
than about 2 cmol
(+) kg
−1 are toxic to many plants and
values in Podzol Soils are predominantly very high.
Topsoil and E horizon CEC levels in Podzol Soils are
mainly low (Table 11.2) because they are limited by low
clay and low humification of soil organic matter. Subsoil
CECs are medium to high because of accumulated B horizon
allophanic soil material and related humus complexes.
Base saturations in unfertilized soils are very low in A
and E horizons.
Unexpectedly, total P values are commonly very high in
B horizons in Podzol Soils because P anions are entangled in
the leaching process and accumulate in B horizons. P retention values are generally low in the highly leached A and E
horizons, and high or very high in B horizons where allophanic soil material and related metal-humus complexes
accumulate. Thus the P available to plants may not be so
high.
11.4.4 Biological Properties
Podzol Soils are associated with mor-forming forest trees
with acid, low nutrient litter (as described above). They have
low natural fertility including very low natural levels of
nitrogen, phosphorus, potassium, and other nutrients in the
topsoil. Low levels of faunal activity (due to the acidic and
low nutrient conditions) occur with subsequent low rates of
mineralization (conversion of mineral elements held within
organic matter back to plant-available mineral forms). The
lack of soil fauna means there is little soil mixing, hence
horizon boundaries are typically distinct or sharp. Carbon/nitrogen ratios are very high which is also an indicator of
the limitations to organic matter decomposition. The accumulation of weakly decomposed humus and minimal incorporation of humus into the upper mineral soil results in the
mor (relatively undecomposed) humus form (e.g. Fig. 11.5).
11.5 Distinguishing Between Podzol Soils
and Related Soil Orders
A typical Podzol Soil is distinctive in its profile morphology
but in the landscape it can merge into associated soils,
including Brown, Pumice, and Gley Soils, and to Ultic Soils
which may also have an E horizon, making it difficult to
recognise a clear spatial boundary. For this reason, the New
Zealand Soil Classification defines the Podzol Soils using
two alternative characteristic (diagnostic) features. The first
is the presence of a dark-coloured podzolic-B horizon (Bh or
Bs, or both), usually with an overlying pale E horizon, and a
pH less than 5.5. The second feature is the occurrence of an
ortstein-pan (Bsm) with pH < 5.5, or a dark-coloured
humus-pan (Bhm) (both types of pans must exceed 10 mm
in thickness). The Mamaku soil profile (Tables 11.1 and
11.2) has dark reddish brown Bs horizons (the reddish hue,
7.5YR or redder, being important). Although no humus-rich
(Bh) horizons are evident, the presence of the overlying E
horizon helps confirm the Mamaku soil as a Podzol Soil
formed on tephras.
The Gley Soils and Brown Soils both have ‘Acid’ soil
groups, and Ultic Soils have ‘Albic’ soil groups. Such
groups incorporate soils that had previously been identified
as ‘podzolic’ soils in the earlier New Zealand genetic soil
classification. Subgroups within the Acid groups indicate
soils that are currently considered marginal (i.e. intergrading) to the Podzol Soils—for instance, Placic Acid Brown
Soils. Similarly, a few subgroups still use the adjective
‘Podzolic’ (e.g. Podzolic Yellow Ultic Soil and Podzolic
Table 11.3 Correlation
a
between Podzol Soil groups and
equivalent classes of Soil
Taxonomy, World Reference
Base, and the New Zealand
genetic soil classification
NZ Soil Classification
Soil Taxonomy
World Reference
Base
NZ genetic soil
classification
Densipan Podzol Soils
Haplorthods,
Epiaquods
Stagnic Podzol
Gley podzol
Perch-gley Podzol Soils
Epiaquods,
Placorthods
Stagnic Podzol
Gley podzol
Groundwater-gley
Podzol Soils
Endoaquods
Gleyic Podzol
Gley podzol
Pan Podzol Soils
Placohumods,
Durihumods
Placic Podzol
Ortsteinic Podzol
Podzol
Orthic Podzol Soils
Haplorthods,
Haplohumods
Haplic Podzol
Podzol
a The correlations given here are a guide only and for accurate classifications the relevant soil classification
documents should be consulted. The two major international soil classification systems are Soil Taxonomy,
which was developed in the USA, and World Reference Base, which was developed primarily in Europe.
The NZ genetic soil classification was used in NZ prior to 1992.
174
11 Podzol Soils
measure of potential aluminium toxicity is the amount of
labile aluminium extracted from the soil by potassium
chloride solution. Soils with KCl-extractable Al values more
than about 2 cmol
(+) kg
−1 are toxic to many plants and
values in Podzol Soils are predominantly very high.
Topsoil and E horizon CEC levels in Podzol Soils are
mainly low (Table 11.2) because they are limited by low
clay and low humification of soil organic matter. Subsoil
CECs are medium to high because of accumulated B horizon
allophanic soil material and related humus complexes.
Base saturations in unfertilized soils are very low in A
and E horizons.
Unexpectedly, total P values are commonly very high in
B horizons in Podzol Soils because P anions are entangled in
the leaching process and accumulate in B horizons. P retention values are generally low in the highly leached A and E
horizons, and high or very high in B horizons where allophanic soil material and related metal-humus complexes
accumulate. Thus the P available to plants may not be so
high.
11.4.4 Biological Properties
Podzol Soils are associated with mor-forming forest trees
with acid, low nutrient litter (as described above). They have
low natural fertility including very low natural levels of
nitrogen, phosphorus, potassium, and other nutrients in the
topsoil. Low levels of faunal activity (due to the acidic and
low nutrient conditions) occur with subsequent low rates of
mineralization (conversion of mineral elements held within
organic matter back to plant-available mineral forms). The
lack of soil fauna means there is little soil mixing, hence
horizon boundaries are typically distinct or sharp. Carbon/nitrogen ratios are very high which is also an indicator of
the limitations to organic matter decomposition. The accumulation of weakly decomposed humus and minimal incorporation of humus into the upper mineral soil results in the
mor (relatively undecomposed) humus form (e.g. Fig. 11.5).
11.5 Distinguishing Between Podzol Soils
and Related Soil Orders
A typical Podzol Soil is distinctive in its profile morphology
but in the landscape it can merge into associated soils,
including Brown, Pumice, and Gley Soils, and to Ultic Soils
which may also have an E horizon, making it difficult to
recognise a clear spatial boundary. For this reason, the New
Zealand Soil Classification defines the Podzol Soils using
two alternative characteristic (diagnostic) features. The first
is the presence of a dark-coloured podzolic-B horizon (Bh or
Bs, or both), usually with an overlying pale E horizon, and a
pH less than 5.5. The second feature is the occurrence of an
ortstein-pan (Bsm) with pH < 5.5, or a dark-coloured
humus-pan (Bhm) (both types of pans must exceed 10 mm
in thickness). The Mamaku soil profile (Tables 11.1 and
11.2) has dark reddish brown Bs horizons (the reddish hue,
7.5YR or redder, being important). Although no humus-rich
(Bh) horizons are evident, the presence of the overlying E
horizon helps confirm the Mamaku soil as a Podzol Soil
formed on tephras.
The Gley Soils and Brown Soils both have ‘Acid’ soil
groups, and Ultic Soils have ‘Albic’ soil groups. Such
groups incorporate soils that had previously been identified
as ‘podzolic’ soils in the earlier New Zealand genetic soil
classification. Subgroups within the Acid groups indicate
soils that are currently considered marginal (i.e. intergrading) to the Podzol Soils—for instance, Placic Acid Brown
Soils. Similarly, a few subgroups still use the adjective
‘Podzolic’ (e.g. Podzolic Yellow Ultic Soil and Podzolic
Table 11.3 Correlation
a
between Podzol Soil groups and
equivalent classes of Soil
Taxonomy, World Reference
Base, and the New Zealand
genetic soil classification
NZ Soil Classification
Soil Taxonomy
World Reference
Base
NZ genetic soil
classification
Densipan Podzol Soils
Haplorthods,
Epiaquods
Stagnic Podzol
Gley podzol
Perch-gley Podzol Soils
Epiaquods,
Placorthods
Stagnic Podzol
Gley podzol
Groundwater-gley
Podzol Soils
Endoaquods
Gleyic Podzol
Gley podzol
Pan Podzol Soils
Placohumods,
Durihumods
Placic Podzol
Ortsteinic Podzol
Podzol
Orthic Podzol Soils
Haplorthods,
Haplohumods
Haplic Podzol
Podzol
a The correlations given here are a guide only and for accurate classifications the relevant soil classification
documents should be consulted. The two major international soil classification systems are Soil Taxonomy,
which was developed in the USA, and World Reference Base, which was developed primarily in Europe.
The NZ genetic soil classification was used in NZ prior to 1992.
174
11 Podzol Soils