7 Gley Soils
7.1 Concept of the Order
Gley Soils are poorly-drained and very poorly-drained soils. In their undrained state, saturation occurs during prolonged periods, oxygen is limited and reducing conditions occur (typically affecting iron, manganese, nitrates, and sometimes sulfates). Low-chroma colours are dominant throughout the solum or to a depth of 90 cm or more.
7.2 Correlation
The order comprises gley soils and gleyed recent soils of the New Zealand Genetic Soil Classification. The soils correlate predominantly with soils exhibiting aquic conditions in Soil Taxonomy, particularly the suborders Aquents, Aquepts, Aquults, and Aquox.
7.3 Occurrence
Gley Soils occur throughout New Zealand, usually in low parts of the landscape where there are high groundwater-tables, or in places where there are seepages. Large areas of Gley Soils have been artificially drained to form productive agricultural land.
7.4 Accessory Properties of the Order
- Segregation of iron and manganese oxides. Particles in reduced parts of the soil are not coated by secondary oxides. Redox segregations of iron and manganese oxides, however, are usually present elsewhere in the soil and may occupy large volumes.
- Commonly formed in alluvial or colluvial parent materials. Soils most frequently occur in relatively low parts of the landscape, in hollows or associated with rushes.
- Wide range of clay minerals. The clay mineralogy commonly reflects the mineralogy of the non-gleyed material from which the soils are derived. Gley Soils cover a wide range of mineralogy classes with Mixed, Illitic, and Smectitic being the most common.
- Poorly or very poorly drained. Topsoils have relatively high levels of organic matter and some are peaty. Subsurface horizons to depth are dominantly grey or bluish grey in colour (i.e., low-chroma colours) with strong brown to dark brown redox segregations.
- High groundwater-tables. Most are affected by high groundwater-tables, at least throughout winter months. Soils with slowly permeable layers may also be subject to perching.
- Shallow potential rooting depth. Potential rooting depth is limited by poor aeration. Even after drainage, root extension may be limited in some horizons.
- Relatively high bulk densities. Bulk densities are likely to be higher than in well drained soils in similar soil materials.
- Limited trafficability. Trafficability is limited in most soils when wet and pugging damage by stock is likely.
- Response to drainage. Crops, not adapted to wetness, respond well to drainage.
- Minimal erosion. Flooding or ponding of water is likely on low-lying sites, especially on floodplains. Deposition of fresh sediment is possible in these sites.
- Nitrogen requirement. Nitrogen requirements are likely to be higher than for associated well drained soils.
7.5 Summary of Gley Soils Hierarchy
| Code | Group | Subgroup | Example Series |
|---|---|---|---|
| GU | Sulfuric | Fluid-saline | Takahiwai |
| Fluid | Takahiwai | ||
| Peaty | Te Kowiwi | ||
| Sandy-saline | Muriwai | ||
| Typic | - | ||
| GT | Tephric | Peaty | - |
| Acidic | - | ||
| Typic | - | ||
| GS | Sandy | Peaty-sulfidic | - |
| Sulfidic | - | ||
| Peaty | - | ||
| Saline | - | ||
| Concretionary | Carnarvon | ||
| Acidic | Berwick | ||
| Typic | Pukepuke | ||
| GX | Oxidic | Nodular | Kapiro |
| Typic | Waipapa | ||
| GR | Recent | Peaty-sulfidic | - |
| Sulfidic | - | ||
| Peaty | Kakawa | ||
| Fluid | - | ||
| Saline | Pāuatahanui | ||
| Calcareous | Ahuriri | ||
| Acidic | Virgin | ||
| Typic | Hastings | ||
| GA | Acid | Peaty | - |
| Granular | Te Hihi | ||
| Ultic | Waikare | ||
| Placic-humose | Flagstaff | ||
| Humose | Flagstaff | ||
| Typic | Dacre | ||
| GO | Orthic | Peaty | Waimairi |
| Saline | - | ||
| Calcareous | Wainui | ||
| Ironstone | Okato | ||
| Melanic | Netherton | ||
| Argillic | Waterton | ||
| Acidic | Bidois | ||
| Typic | Invermay |
7.6 Key to Groups of Gley Soils
GU
Gley Soils that in some part within 90 cm of the mineral soil surface contain sulfuric soil material.
GT
Other Gley Soils in tephric soil material from the mineral soil surface to 30 cm depth or more.
GS
Other Gley Soils that have a sand or loamy sand texture, and less than 35% (by volume) coarse rock fragments, in all horizons from the base of the A horizon to 60 cm from the mineral soil surface.
GX
Other Gley Soils that have an oxidic horizon more than 30 cm thick with an upper boundary between 20 and 60 cm from the mineral soil surface.
GR
Other Gley Soils that have
- Either of the following
- sedimentary stratification at 60 cm or less, or
- a buried A horizon with its upper surface deeper than 30 cm but within 120 cm of the mineral soil surface, or some other indication of an irregular carbon profile such as sedimentary plant leaf material, AND
- De neither have
- a buried B horizon with an upper surface at 60 cm or less from the mineral soil surface, nor
- a B or BC horizon with base deeper than 30 cm from the mineral soil surface, nor
- a placic horizon nor ortstein-pan, nor
- a horizon with more than 2% concretions or nodules larger than 2 mm within 60 cm of the mineral soil surface.
GA
Other Gley Soils that have pH of 4.8 or less in some part between 20 and 60 cm from the mineral soil surface.
GO
Other Gley Soils.
7.7 Key to Subgroups of Gley Soils
GU - SULFURIC GLEY SOILS
Sulfuric Gley Soils occur in marine or estuarine sites in which sufficient oxidation of ferrous sulfides (e.g., pyrite) has occurred to produce either sulfuric acid, or the mineral jarosite, or both.
GUFQ
Sulfuric Gley Soils that within 60 cm of the mineral soil surface have BOTH
- Moderately fluid or very fluid fluidity class, AND
- Saline soil material.
Fluid-saline Sulfuric Gley Soils
GUF
Other soils that within 60 cm of the mineral soil surface have moderately fluid or very fluid fluidity class.
Fluid Sulfuric Gley Soils
GUO
Other soils that have a peaty topsoil either at the surface or buried with its upper surface within 60 cm of the soil surface.
Peaty Sulfuric Gley Soils
GUSQ
Other soils that are sandy by weighted average to 90 cm from the mineral soil surface and have saline soil material within 60 cm of the mineral soil surface.
Sandy-saline Sulfuric Gley Soils
GUT
Other soils.
Typic Sulphuric Gley Soils
GT - TEPHRIC GLEY SOILS
Tephric Gley Soils occur in unconsolidated sediment of pyroclastic origin of varying composition and grain sizes.
GTO
Tephric Gley Soils that have a peaty topsoil either at the surface or buried with its upper surface within 60 cm of the mineral soil surface.
Peaty Tephric Gley Soils
GTA
Other soils that have pH of less than 5.5 in some part from the base of the A horizon to 60 cm from the mineral soil surface.
Acidic Tephric Gley Soils
GTT
Other soils.
Typic Tephric Gley Soils
GS - SANDY GLEY SOILS
Sandy Gley Soils occur in sand deposits which are usually aeolian, but may also be of alluvial origin. Subsurface horizons are sand or loamy sand.
GSOU
Sandy Gley Soils that have BOTH
- A peaty topsoil either at the surface or buried with its upper surface within 60 cm of the mineral soil surface, AND
- Sulfidic soil material in some part within 90 cm of the mineral soil surface.
Peaty-sulfidic Sandy Gley Soils
GSU
Other soils in which sulfidic soil material is present in some part within 90 cm of the mineral soil surface.
Sulfidic Sandy Gley Soils
GSO
Other soils that have a peaty topsoil either at the surface or buried with its upper surface within 60 cm of the mineral soil surface.
Peaty Sandy Gley Soils
GSQ
Other soils that within 60 cm of the mineral soil surface have saline soil material.
Saline Sandy Gley Soils
GSC
Other soils that have more than 2% by volume concretions in some horizon at a depth of 60 cm or less from the mineral soil surface.
Concretionary Sandy Gley Soils
GSA
Other soils that have pH of less than 5.5 in some part from the base of the A horizon to 60 cm from the mineral soil surface.
Acidic Sandy Gley Soils
GST
Other soils.
Typic Sandy Gley Soils
GX - OXIDIC GLEY SOILS
Oxidic Gley Soils have variable charge properties and contain low-activity clays. The mineralogy is dominated by kaolin group clay minerals. Iron oxides are less common than in the Oxidic Soils.
GXN
Oxidic Gley Soils that have a nodular horizon with an upper boundary within 60 cm of the mineral soil surface.
Nodular Oxidic Gley Soils
GXT
Other soils.
Typic Oxidic Gley Soils
GR - RECENT GLEY SOILS
Recent Gley Soils occur on young land surfaces, usually in fluvial or estuarine sediments. In many sites there is a significant flood risk.
GROU
Recent Gley Soils that have BOTH
- a peaty topsoil either at the surface or buried with its upper surface within 60 cm of the soil surface, AND
- Sulfidic soil material in some part within 90 cm of the mineral soil surface.
Peaty-sulfidic Recent Gley Soils
GRU
Other soils in which sulfidic soil material is present in some part within 90 cm of the mineral soil surface.
Sulfidic Recent Gley Soils
GRO
Other soils that have a peaty topsoil either at the surface or buried with its upper surface within 60 cm of the soil surface.
Peaty Recent Gley Soils
GRF
Other soils that within 60 cm of the mineral soil surface have moderately fluid or very fluid fluidity class.
Fluid Recent Gley Soils
GRQ
Other soils that within 60 cm of the mineral soil surface have saline soil materials.
Saline Recent Gley Soils
GRC
Other soils that have a calcareous horizon or a shelly layer within 60 cm of the mineral soil surface.
Calcareous Recent Gley Soils
GRA
Other soils that have pH less than 5.5 in some part from the base of the A horizon to 60 cm of the mineral soil surface.
Acidic Recent Gley Soils
GRT
Other soils.
Typic Recent Gley Soils
GA - ACID GLEY SOILS
Acid Gley Soils occur on relatively stable land surfaces, and have been subject to a fluctuating groundwater-table, or a deep layer of perched water. Plants grown are susceptible to aluminium toxicity.
GAO
Acid Gley Soils that have a peaty topsoil at the surface or buried within 60 cm of the surface.
Peaty Acid Gley Soils
GAG
Other soils that have a clayey, moderately or strongly pedal cutanic or argillic horizon and polyhedral peds 20 mm or less in the major part of the B horizon to 60 cm of the mineral soil surface.
Granular Acid Gley Soils
GAY
Other soils that have a B horizon which has in the major part, BOTH
- 10% or more clay or humus coatings on ped faces, AND
- Less silt than clay.
Ultic Acid Gley Soils
GAPH
Other soils that have both a placic horizon and a B horizon with 10% or more dark-coloured coats on ped faces, in pores or on coarse rock fragments, with moist colour value 4 or less or colour value 5 and chroma 3.
Placic-humose Acid Gley Soils
GAH
Other soils that have a B horizon with 10% or more dark-coloured coats on ped faces, in pores or on coarse rock fragments, with moist colour value 4 or less or colour value 5 and chroma 3.
Humose Acid Gley Soils
GAT
Other soils.
Typic Acid Gley Soils
GO - ORTHIC GLEY SOILS
Orthic Gley Soils occur on relatively stable land surfaces in sites affected by groundwater. Sediment deposition is unlikely if flooding occurs. They are neither strongly acidic, sandy nor sulfuric and have no oxidic horizon.
GOOU
Orthic Gley Soils that have BOTH
- A peaty topsoil either at the surface or buried with its upper surface within 60 cm of the mineral soil surface, AND
- Sulfidic soil material in some part within 90 cm of the mineral soil surface.
Peaty-sulfidic Orthic Gley Soils
GOU
Other soils in which sulfidic soil material is present in some part within 90 cm of the mineral soil surface.
Sulfidic Orthic Gley Soils
GOO
Other soils that have a peaty topsoil either at the surface or buried with its upper surface within 60 cm of the soil surface.
Peaty Orthic Gley Soils
GOQ
Other soils that within 60 cm of the mineral soil surface have saline soil materials.
Saline Orthic Gley Soils
GOC
Other soils that have a calcareous horizon at 60 cm or less from the mineral soil surface.
Calcareous Orthic Gley Soils
GOI
Other soils that have an ironstone-pan at 90 cm depth or less from the mineral soil surface.
Ironstone Orthic Gley Soils
GOE
Other soils that have a B horizon, in which the major part to 60 cm from the mineral soil surface or to its base (whichever is less)
- Has moderate or strong pedality, AND
- Is moderately sticky or very sticky, AND
- has pH of 5.9 or more.
Melanic Orthic Gley Soils
GOJ
Other soils that have an argillic horizon.
Argillic Orthic Gley Soils
GOA
Other soils that have pH of less than 5.5 in some part from the base of the A horizon to 60 cm from the mineral soil surface.
Acidic Orthic Gley Soils
GOT
Other soils.
Typic Orthic Gley Soils