10 Organic Soils
10.1 Concept of the Order
Organic Soils are soils that occur in the partly decomposed remains of wetland plants (peat) or forest litter. Mineral soil material is commonly present but organic soil material is dominant. The soils occur in sites where rates of organic matter decomposition are balanced or exceeded by rates of plant biomass production and accumulation.
10.2 Correlation
The order corresponds with the organic soils group of the New Zealand Genetic Soil Classification. It correlates with the Histosols of Soil Taxonomy.
10.3 Occurrence
Organic Soils occur in wetlands in most parts of New Zealand or under forest-produced acidic litter in areas with high precipitation.
10.4 Accessory Properties of the Order
- Low bulk density. Due to the high organic matter content, bulk densities are very low, usually in the range of 0.03 to 0.4 Mg/m3. Too few data are yet available to allow mineralogy classes to be stated.
- High cation exchange capacity. The organic components of Organic Soils have high surface area, and high negative charge that varies markedly with pH. CEC values are very high, mostly ranging from 40 to 170 cmolc/kg.
- High carbon/nitrogen ratios. C/N ratios range from 18 to as high as 70 in unfertilised and uncultivated Organic Soils.
- Low bearing strength. Construction of buildings or roads requires special foundation design.
- Very low thermal conductivity. Soils warm and cool slowly. Bare soil surfaces, however, have high radiance.
- High shrinkage potential. The soils shrink markedly upon drying, and lose organic matter due to oxidation. Consequently, following drainage, the classification may change.
- High total available water capacity. While total available water capacity is high, plant-available water capacity may only be moderate, due to limits in rooting depth (anoxic conditions, species-specific limitations).
- Common nutrient deficiencies. The major nutrients nitrogen, phosphorus, potassium and sulfur, and the trace elements copper, selenium and molybdenum, are frequently deficient for crops and pasture.
- Peats are very poorly drained, litters are variable. In their natural state, Organic Soils formed in peats are very poorly drained and those formed from litters may range from well drained to very poorly drained. Most Organic Soils from peat in New Zealand have been artificially drained, leading to drastic shifts towards drier soil moisture conditions.
10.5 Summary of Organic Soils Hierarchy
| Code | Group | Subgroup | Example Series |
|---|---|---|---|
| OL | Litter | Buried-podzol | Waitutu |
| Buried-gleyed | - | ||
| Orthic | Waipoua | ||
| OF | Fibric | Sphagnic | Kaherekoau |
| Sulfidic | - | ||
| Acid | Rukuhia | ||
| Mellow | Otanomomo | ||
| OM | Mesic | Sulfidic | - |
| Acid | Otautau | ||
| Mellow | Kaipaki | ||
| OH | Humic | Sulfidic | - |
| Acid | Ardmore | ||
| Mellow | Pukehina |
10.6 Key to Groups of Organic Soils
OL
Organic Soils that occur entirely in partly or fully decomposed forest litter and are not almost continually saturated in the natural state (F and H horizons).
OF
Other soils that, from the upper surface of the organic soil material to 60 cm depth, or to the base of the organic soil material if shallower, have horizons dominated (by accumulated thickness) by fibric organic soil material (Of horizons).
OM
Other soils that, from the upper surface of the organic soil material to 60 cm depth, or to the base of the organic soil material if shallower, have horizons dominated (by accumulated thickness) by mesic organic soil material (Om horizons).
OH
Other soils that, from the upper surface of the organic soil material to 60 cm depth, or to the base of the organic soil material if shallower, have horizons dominated (by accumulated thickness) by humified organic soil material (Oh horizons).
Key to Subgroups of Organic Soils
OL - LITTER ORGANIC SOILS
Litter Organic Soils occur under forest beneath a canopy of acidic litter or mor-forming species. The organic matter is derived predominantly from leaves and twigs and is normally not saturated except for a few days following heavy rain.
OLBZ
Litter Organic Soils in which an underlying mineral soil, with an upper boundary at less than 90 cm from the soil surface, has a podzolic-B or placic horizon.
Buried-podzol Litter Organic Soils
OLBG
Other soils in which a reductimorphic horizon occurs at less than 30 cm beneath the upper surface of the underlying mineral soil.
Buried-gley Litter Organic Soils
OLO
Other soils.
Orthic Litter Organic Soils
OF - FIBRIC ORGANIC SOILS
Fibric Organic Soils occur in sites that are saturated to the surface for extended periods (or in sites that have been artificially drained) in which the peat materials are only weakly decomposed. The wetland plant constituents are so little decomposed that their botanic origin may be readily determined, and fibres are not destroyed by rubbing.
OFS
Fibric Organic Soils in which the organic fibres, to a depth of 60 cm from the soil surface, or to the base of organic soil material if shallower, are more than 70% Sphagnum species.
Sphagnic Fibric Organic Soils
OFU
Other soils in which sulfidic soil material is present in some part within 90 cm of the soil surface.
Sulfidic Fibric Organic Soils
OFA
Other soils in which the organic soil material to a depth of 60 cm from the soil surface, or to its base if shallower, has pH of 4.5 or less throughout the major part.
Acid Fibric Organic Soils
OFM
Other soils.
Mellow Fibric Organic Soils
OM - MESIC ORGANIC SOILS
Mesic Organic Soils occur in very wet sites (or in sites that have been artificially drained) in which the peat materials are moderately decomposed. The remains of up to two-thirds of the original wetland plants that make up the bulk of the soil are unrecognisable or are largely destroyed by rubbing between the fingers.
OMU
Mesic Organic soils in which sulfidic soil material is present in some part within 90 cm of the soil surface.
Sulfidic Mesic Organic Soils
OMA
Other soils in which the organic soil material to a depth of 60 cm from the soil surface, or to its base if shallower, has pH of 4.5 or less throughout the major part.
Acid Mesic Organic Soils
OMM
Other soils.
Mellow Mesic Organic Soils
OH - HUMIC ORGANIC SOILS
Humic Organic Soils occur in very wet sites (or in sites that have been artificially drained). The peat materials are strongly decomposed to the extent that the original wetland plant remains cannot be recognised throughout most of the soil profile.
OHU
Humic Organic soils in which sulfidic soil material is present in some part within 90 cm of the soil surface.
Sulfidic Humic Organic Soils
OHA
Other soils in which the organic soil material to a depth of 60 cm from the soil surface, or to its base if shallower, has pH of 4.5 or less throughout the major part.
Acid Humic Organic Soils
OHM
Other soils.
Mellow Humic Organic Soils