8
Organic Soils
Down in the swamp where we get wet feet
Organic Soils are formed in peat.
Plants may die but they’re not decomposed
unless fertiliser and drains are imposed.
Abstract
Organic Soils form from partly decomposed remains of
plants in peat-forming wetlands, or in deep accumulations
of forest litter. Organic Soils comprise about 1% of New
Zealand with the largest areas in the Waikato, on the high
rainfall South Island West Coast, and in some moist
mountainous areas where soils are often saturated.
Organic Soils are acidic (pH
*4), have a low dry
bulk density, high C:N ratios, high shrinkage potential,
and exceptionally high soil water holding capacity.
Organic Soils have been developed mainly for pasture,
maize, potato, and blueberry production, and are also
mined to extract peat. Organic Soils have naturally low
nutrient contents so need inputs of fertiliser and lime,
along with drainage, for productive farming. Such inputs
hasten the biodegradation of Organic Soils. To slow the
rate of degradation, water tables need to be kept high to
prevent oxygen from entering the soil. Although about
70% of Organic Soils have been heavily impacted by
drainage and development for farming, or by peat mining,
some natural areas remain in reserves. The Whangamarino wetland and the rainfed Kopouatai and Moanatuatua bogs are recognised as wetlands of international
importance with unique indigenous vegetation, including
the tall cane rush (Sporodanthus ferrugineus).
8.1 Important Features of Organic Soils
8.1.1 Concept and Key Features of the Soil
Order
Organic Soils (Fig. 8.1) are formed where organic matter is
dominant in the soil profile in both wetland and forest
environments. Organic Soils occur where the partly
decomposed remains of wetland plants (peat) is ! 30 cm in
thickness (within 60 cm of the soil surface), having accumulated under wet, anaerobic (oxygen-depleted) conditions.
In forests, Organic Soils form in acidic and/or nitrogen-poor,
partly or well decomposed, forest litter that is ! 40 cm
deep. Organic Soil profiles comprise organic soil materials
(defined as having ! 18% organic carbon) that may range
from undecomposed plant remains through to humic material where the original plant material is no longer recognisable. All parts of the plant may be biodegraded and
incorporated into Organic Soils including roots, trunks,
branches, leaves, flowers, and seeds. Organic soil material,
used to define Organic Soils, specifically excludes fresh litter
(L horizons) and living plant material.
The predominant plants at a site influence the structure and
nature of the soil. For instance, some peats are formed
exclusively from sphagnum moss while others may form from
a range of wetland plants including restiads (jointed-rush
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
A. E. Hewitt et al., The Soils of Aotearoa New Zealand, World Soils Book Series,
https://doi.org/10.1007/978-3-030-64763-6_8
113
Organic Soils
Down in the swamp where we get wet feet
Organic Soils are formed in peat.
Plants may die but they’re not decomposed
unless fertiliser and drains are imposed.
Abstract
Organic Soils form from partly decomposed remains of
plants in peat-forming wetlands, or in deep accumulations
of forest litter. Organic Soils comprise about 1% of New
Zealand with the largest areas in the Waikato, on the high
rainfall South Island West Coast, and in some moist
mountainous areas where soils are often saturated.
Organic Soils are acidic (pH
*4), have a low dry
bulk density, high C:N ratios, high shrinkage potential,
and exceptionally high soil water holding capacity.
Organic Soils have been developed mainly for pasture,
maize, potato, and blueberry production, and are also
mined to extract peat. Organic Soils have naturally low
nutrient contents so need inputs of fertiliser and lime,
along with drainage, for productive farming. Such inputs
hasten the biodegradation of Organic Soils. To slow the
rate of degradation, water tables need to be kept high to
prevent oxygen from entering the soil. Although about
70% of Organic Soils have been heavily impacted by
drainage and development for farming, or by peat mining,
some natural areas remain in reserves. The Whangamarino wetland and the rainfed Kopouatai and Moanatuatua bogs are recognised as wetlands of international
importance with unique indigenous vegetation, including
the tall cane rush (Sporodanthus ferrugineus).
8.1 Important Features of Organic Soils
8.1.1 Concept and Key Features of the Soil
Order
Organic Soils (Fig. 8.1) are formed where organic matter is
dominant in the soil profile in both wetland and forest
environments. Organic Soils occur where the partly
decomposed remains of wetland plants (peat) is ! 30 cm in
thickness (within 60 cm of the soil surface), having accumulated under wet, anaerobic (oxygen-depleted) conditions.
In forests, Organic Soils form in acidic and/or nitrogen-poor,
partly or well decomposed, forest litter that is ! 40 cm
deep. Organic Soil profiles comprise organic soil materials
(defined as having ! 18% organic carbon) that may range
from undecomposed plant remains through to humic material where the original plant material is no longer recognisable. All parts of the plant may be biodegraded and
incorporated into Organic Soils including roots, trunks,
branches, leaves, flowers, and seeds. Organic soil material,
used to define Organic Soils, specifically excludes fresh litter
(L horizons) and living plant material.
The predominant plants at a site influence the structure and
nature of the soil. For instance, some peats are formed
exclusively from sphagnum moss while others may form from
a range of wetland plants including restiads (jointed-rush
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
A. E. Hewitt et al., The Soils of Aotearoa New Zealand, World Soils Book Series,
https://doi.org/10.1007/978-3-030-64763-6_8
113