water to the environment as well as making plant nutrients
available for reuse.
Generally, there are three parts to the decomposition
process:
assimilation or uptake of organic material by microbial
decomposers, whereby plant or animal material becomes
part of the microbial biomass;
mineralisation, which is the conversion of organic molecules
to inorganic (or mineral) molecules, which are then available
for plant uptake, or may be released to the wider environment, predominantly as carbon dioxide and water, and
immobilisation, which is the conversion of mineral elements
to organic matter (in Organic Soils particularly to stable
forms of humus that do not break down readily), thus
making the nutrients unavailable for plant uptake.
The main conditions for decomposition of plant material
are favourable temperature, pH, moisture, and available
oxygen and nutrients. Therefore, if an Organic Soil is drained
and has fertiliser and lime added, to increase the ability of the
soil to support plant growth, microbial growth is also
enhanced and biodegradation of the organic material will
greatly increase. The decomposition process will ultimately
lead to breakdown of the organic matter, with the main
products being water and carbon dioxide which escape from
the soil. Biodegradation of the organic matter thus leads to
losses of organic material to the atmosphere and water table,
as well as further surface subsidence because of shrinkage
and consolidation of the remaining organic material.
8.3 Soil-Landscape Relationships
8.3.1 Introduction
Organic Soils occur in a wide range of wet environments.
Nine wetland types are widely recognised (Table 8.1). The
wetland types have different water and nutrient sources and
varying plant assemblages, with a range of resulting soils
formed on them. Organic Soils are most commonly formed
on raised and blanket bogs, fens, and swamps.
Domed or raised bogs, dominated by restiad species,
occur predominantly in the Waikato and Northland regions.
It takes an abundant water supply (and sufficiently frequent
rain-days) to produce an accumulation of peat. Most Organic
Soils form in places where the soil water table is close to the
land surface, as occurs with fens. However, a dome-shaped
raised bog is special because the only water source is from
rainfall (referred to as ombrogenous). Many such sites have
now been drained and developed, primarily for pastoral
agriculture.
Blanket bogs occur in cool, wet climates on flattish to
rolling uplands (at relatively high altitude), such as the
Lammerlaw Range of east Otago. Blanket bogs are formed
from restiad plants (including wire rush) and mosses
Table 8.1 Wetland types in New Zealand and the inferred relationship to the NZ Soil Classification
Wetland type
Description
Likely Organic Soils and associated soils
Bog (raised
or blanket)
Peatland that receives its water solely from precipitation
Fibric Organic Soils, Acid Mesic
Organic Soils
Fen
Peatland that receives water from the surrounding land. Water table is near the
land surface. Low to moderate pH
Mellow subgroups of Mesic or Humic
Organic Soils
Peaty Orthic Gley Soils
Swamp
Mineral or peaty wetland in topographically low sites, slightly acid to near
neutral pH
Fluvial Gley Soils. Mellow Mesic
Organic Soils
Marsh
Mineral wetland, moderate pH, fed by groundwater, surface fluctuates from wet
to inundated, often at margins of water bodies
Fluvial Gley Soils
Seepage
Wet patches on hillsides (often footslopes) releasing modest volumes of
groundwater by slow leakage to the surface
Orthic Gley Soil, Mellow Mesic Organic
Soils
Shallow
water
Standing water most of the time, less than a few metres deep, rivers, lake
margins, streams
Gley Raw Soils
Ephemeral
wetland
Closed depressions intermittently wet in winter and spring
Gley Raw Soils, Recent Gley Soils
Pakihi
Extremely low fertility formed under high (metres/year) rainfall
Acid Gley Soils
Perch-Gley Podzols, Acid Fibric Organic
Soils
Saltmarsh
Tidal influence, including saline mudflats
Gley Raw Soils, Fluvial Raw Soils,
Saline, Sandy, or Orthic Gley Soils
8.2 Soil Profile Genesis
117
available for reuse.
Generally, there are three parts to the decomposition
process:
assimilation or uptake of organic material by microbial
decomposers, whereby plant or animal material becomes
part of the microbial biomass;
mineralisation, which is the conversion of organic molecules
to inorganic (or mineral) molecules, which are then available
for plant uptake, or may be released to the wider environment, predominantly as carbon dioxide and water, and
immobilisation, which is the conversion of mineral elements
to organic matter (in Organic Soils particularly to stable
forms of humus that do not break down readily), thus
making the nutrients unavailable for plant uptake.
The main conditions for decomposition of plant material
are favourable temperature, pH, moisture, and available
oxygen and nutrients. Therefore, if an Organic Soil is drained
and has fertiliser and lime added, to increase the ability of the
soil to support plant growth, microbial growth is also
enhanced and biodegradation of the organic material will
greatly increase. The decomposition process will ultimately
lead to breakdown of the organic matter, with the main
products being water and carbon dioxide which escape from
the soil. Biodegradation of the organic matter thus leads to
losses of organic material to the atmosphere and water table,
as well as further surface subsidence because of shrinkage
and consolidation of the remaining organic material.
8.3 Soil-Landscape Relationships
8.3.1 Introduction
Organic Soils occur in a wide range of wet environments.
Nine wetland types are widely recognised (Table 8.1). The
wetland types have different water and nutrient sources and
varying plant assemblages, with a range of resulting soils
formed on them. Organic Soils are most commonly formed
on raised and blanket bogs, fens, and swamps.
Domed or raised bogs, dominated by restiad species,
occur predominantly in the Waikato and Northland regions.
It takes an abundant water supply (and sufficiently frequent
rain-days) to produce an accumulation of peat. Most Organic
Soils form in places where the soil water table is close to the
land surface, as occurs with fens. However, a dome-shaped
raised bog is special because the only water source is from
rainfall (referred to as ombrogenous). Many such sites have
now been drained and developed, primarily for pastoral
agriculture.
Blanket bogs occur in cool, wet climates on flattish to
rolling uplands (at relatively high altitude), such as the
Lammerlaw Range of east Otago. Blanket bogs are formed
from restiad plants (including wire rush) and mosses
Table 8.1 Wetland types in New Zealand and the inferred relationship to the NZ Soil Classification
Wetland type
Description
Likely Organic Soils and associated soils
Bog (raised
or blanket)
Peatland that receives its water solely from precipitation
Fibric Organic Soils, Acid Mesic
Organic Soils
Fen
Peatland that receives water from the surrounding land. Water table is near the
land surface. Low to moderate pH
Mellow subgroups of Mesic or Humic
Organic Soils
Peaty Orthic Gley Soils
Swamp
Mineral or peaty wetland in topographically low sites, slightly acid to near
neutral pH
Fluvial Gley Soils. Mellow Mesic
Organic Soils
Marsh
Mineral wetland, moderate pH, fed by groundwater, surface fluctuates from wet
to inundated, often at margins of water bodies
Fluvial Gley Soils
Seepage
Wet patches on hillsides (often footslopes) releasing modest volumes of
groundwater by slow leakage to the surface
Orthic Gley Soil, Mellow Mesic Organic
Soils
Shallow
water
Standing water most of the time, less than a few metres deep, rivers, lake
margins, streams
Gley Raw Soils
Ephemeral
wetland
Closed depressions intermittently wet in winter and spring
Gley Raw Soils, Recent Gley Soils
Pakihi
Extremely low fertility formed under high (metres/year) rainfall
Acid Gley Soils
Perch-Gley Podzols, Acid Fibric Organic
Soils
Saltmarsh
Tidal influence, including saline mudflats
Gley Raw Soils, Fluvial Raw Soils,
Saline, Sandy, or Orthic Gley Soils
8.2 Soil Profile Genesis
117