8.4.2 Soil Composition
Organic Soils are, by definition, composed of organic, predominantly plant, material in various stages of decay. It is
the plant species, hydrological setting (including rainfall and
rain-days), and landscape position that strongly influence the
composition of Organic Soils. Any plant material can
potentially become an Organic Soil parent material if it is
prevented from biodegradation, usually as a consequence of
saturated (anaerobic) conditions. The effect of plant species
is important as the nature of the peat fabric is conditioned by
the structure and composition of the plant material. For example, sphagnum bogs of the east Otago uplands are highly
porous with very low bearing strength, whereas peat from
sedges may have greater density and moderate load-bearing
strength. Woody materials may be slower to decompose
because of the presence of lignin which has a particularly
high C/N ratio and so nitrogen, as well as oxygen, availability limits microbial biodegradation. Thus relatively ‘intact’ wood material, including large tree stumps and logs,
may be found within some Organic Soils.
Layers of mineral (sand, silt, and clay) material may
occur within an Organic Soil, especially in areas where river
flood deposits, or tephra-fallout, occur. The composition of
the mineral matter depends on the source of the material and
mode of deposition. The mineralogy of the sand, silt, and
clay within an Organic Soil is likely to be similar to that of
associated Recent and Gley Soils in the same environment.
8.4.3 Physical Properties
The depth of Organic Soils is highly variable, both spatially
and temporally. If conditions remain favourable for extended
periods (thousands of years), the organic material can
gradually accumulate. At some sites, the depth of organic
material may run to tens of metres (for example, Kopouatai
bog is up to 14 m deep, the base being below sea level).
However, Organic Soils are vulnerable to loss by oxidation
and biodegradation which converts the organic matter predominantly to carbon dioxide and water. Fire or drainage
may lead to rapid loss, or consolidation, of peat material, and
thus a rapid decrease in the soil depth.
The rate of carbon loss on farmed soils is faster in the
initial years following development but continues at a lesser
rate while the peat resource remains. Dave Campbell and
co-workers reported a net loss of about 300 g C m
−2 yr
−1 on
Waikato dairy farms on Organic Soils with a water table at
about 50 cm depth. Louis Schipper, Malcolm McLeod, Jack
Pronger, and others, have estimated a long-term loss of
370 ± 130 g C m
−2 yr
−1 on farmed peat. They reported
surface subsidence rates of about 3–3.5 cm yr
−1
, over the
40 years following initial development and about 2 cm yr
−1
between 2000 and 2014. About two-thirds of the subsidence
was attributed to consolidation with the balance due to
carbon loss.
Most of the Organic Soils in the NZ Soil Data Repository
contain some clay material (Fig. 8.9), most commonly
derived from tephra or alluvium. Soil dry bulk densities are
very low, usually in the range of 0.03–0.4 t m
−3 . With the
low dry bulk density comes an exceptionally high porosity
and water holding capacity (Fig. 8.9, Table 8.2), with the
soil often containing a much greater weight of water than
soil dry matter, thus often giving gravimetric moisture
contents much greater than 100%. The volumetric water
content of Organic Soil can be as high as 90% and walking
on the surface of a peat bog can be unnerving as the ‘ground’
may quiver like jelly.
The high water holding capacity of Organic Soils is due
to the ‘sponge’ effect of both sphagnum moss and the root
system of the restiads. The water holding capacity in moss is
provided by the close-packed leaves, whereas in the restiads
it is the dense root mat that traps and contains water.
Although total available-water capacity is high, in drained
Organic Soils the plant-available-water capacity may only be
moderate. This is because the coarse pores in the Organic
Soils, which supply readily available water, may be lost as
the soil compacts following drainage. Organic Soils shrink
markedly upon drying, and may also lose organic matter
through oxidation. Consequently, following drainage, the
soil water holding capacity, and indeed the soil classification
group, may change.
Organic Soils formed in peats are usually described as
very poorly drained. However, Litter Organic Soils, formed
on forest floors, may range from well drained to very poorly
drained. Organic Soils warm and cool slowly due to the high
heat capacity of the large volume of water held within the
soil.
8.4.4 Chemical Properties
The chemical properties of Organic Soils reflect the predominance of organic material within the soil profile. Strong
acidity (strongly and extremely acid) is common in New
Zealand Organic Soils with pH often at or below 4.5
(Fig. 8.10, Table 8.3). In the Kopouatai bog, Peter de Lange
(with Peter Hodder) reported pHs as low as 3 in the upper
3 m of the bog, increasing to 4 from 3 to 8 m depth, and to
pH 5 from 8 to 10 m. Mellow subgroups have pHs of more
than 4.5 and are usually associated with calcareous sediments in the catchment or calcic mineral material in the soil
profile.
Organic Soils can contain 35–100% organic matter, and
thus comprise about 30–60% organic carbon (Fig. 8.10,
Table 8.3). The organic material provides a large surface
8.4 Key Soil Properties
123
Organic Soils are, by definition, composed of organic, predominantly plant, material in various stages of decay. It is
the plant species, hydrological setting (including rainfall and
rain-days), and landscape position that strongly influence the
composition of Organic Soils. Any plant material can
potentially become an Organic Soil parent material if it is
prevented from biodegradation, usually as a consequence of
saturated (anaerobic) conditions. The effect of plant species
is important as the nature of the peat fabric is conditioned by
the structure and composition of the plant material. For example, sphagnum bogs of the east Otago uplands are highly
porous with very low bearing strength, whereas peat from
sedges may have greater density and moderate load-bearing
strength. Woody materials may be slower to decompose
because of the presence of lignin which has a particularly
high C/N ratio and so nitrogen, as well as oxygen, availability limits microbial biodegradation. Thus relatively ‘intact’ wood material, including large tree stumps and logs,
may be found within some Organic Soils.
Layers of mineral (sand, silt, and clay) material may
occur within an Organic Soil, especially in areas where river
flood deposits, or tephra-fallout, occur. The composition of
the mineral matter depends on the source of the material and
mode of deposition. The mineralogy of the sand, silt, and
clay within an Organic Soil is likely to be similar to that of
associated Recent and Gley Soils in the same environment.
8.4.3 Physical Properties
The depth of Organic Soils is highly variable, both spatially
and temporally. If conditions remain favourable for extended
periods (thousands of years), the organic material can
gradually accumulate. At some sites, the depth of organic
material may run to tens of metres (for example, Kopouatai
bog is up to 14 m deep, the base being below sea level).
However, Organic Soils are vulnerable to loss by oxidation
and biodegradation which converts the organic matter predominantly to carbon dioxide and water. Fire or drainage
may lead to rapid loss, or consolidation, of peat material, and
thus a rapid decrease in the soil depth.
The rate of carbon loss on farmed soils is faster in the
initial years following development but continues at a lesser
rate while the peat resource remains. Dave Campbell and
co-workers reported a net loss of about 300 g C m
−2 yr
−1 on
Waikato dairy farms on Organic Soils with a water table at
about 50 cm depth. Louis Schipper, Malcolm McLeod, Jack
Pronger, and others, have estimated a long-term loss of
370 ± 130 g C m
−2 yr
−1 on farmed peat. They reported
surface subsidence rates of about 3–3.5 cm yr
−1
, over the
40 years following initial development and about 2 cm yr
−1
between 2000 and 2014. About two-thirds of the subsidence
was attributed to consolidation with the balance due to
carbon loss.
Most of the Organic Soils in the NZ Soil Data Repository
contain some clay material (Fig. 8.9), most commonly
derived from tephra or alluvium. Soil dry bulk densities are
very low, usually in the range of 0.03–0.4 t m
−3 . With the
low dry bulk density comes an exceptionally high porosity
and water holding capacity (Fig. 8.9, Table 8.2), with the
soil often containing a much greater weight of water than
soil dry matter, thus often giving gravimetric moisture
contents much greater than 100%. The volumetric water
content of Organic Soil can be as high as 90% and walking
on the surface of a peat bog can be unnerving as the ‘ground’
may quiver like jelly.
The high water holding capacity of Organic Soils is due
to the ‘sponge’ effect of both sphagnum moss and the root
system of the restiads. The water holding capacity in moss is
provided by the close-packed leaves, whereas in the restiads
it is the dense root mat that traps and contains water.
Although total available-water capacity is high, in drained
Organic Soils the plant-available-water capacity may only be
moderate. This is because the coarse pores in the Organic
Soils, which supply readily available water, may be lost as
the soil compacts following drainage. Organic Soils shrink
markedly upon drying, and may also lose organic matter
through oxidation. Consequently, following drainage, the
soil water holding capacity, and indeed the soil classification
group, may change.
Organic Soils formed in peats are usually described as
very poorly drained. However, Litter Organic Soils, formed
on forest floors, may range from well drained to very poorly
drained. Organic Soils warm and cool slowly due to the high
heat capacity of the large volume of water held within the
soil.
8.4.4 Chemical Properties
The chemical properties of Organic Soils reflect the predominance of organic material within the soil profile. Strong
acidity (strongly and extremely acid) is common in New
Zealand Organic Soils with pH often at or below 4.5
(Fig. 8.10, Table 8.3). In the Kopouatai bog, Peter de Lange
(with Peter Hodder) reported pHs as low as 3 in the upper
3 m of the bog, increasing to 4 from 3 to 8 m depth, and to
pH 5 from 8 to 10 m. Mellow subgroups have pHs of more
than 4.5 and are usually associated with calcareous sediments in the catchment or calcic mineral material in the soil
profile.
Organic Soils can contain 35–100% organic matter, and
thus comprise about 30–60% organic carbon (Fig. 8.10,
Table 8.3). The organic material provides a large surface
8.4 Key Soil Properties
123