Melanic Soils
Concept of the Order
Melanic Soils are well structured soils with very dark A horizons. They commonly have high base saturation combined with weakly alkaline or weakly acidic subsurface horizons. However, more acidified and allophanic variants exist under higher soil moisture regimes. Their parent materials are rich in calcium and/or magnesium.
Correlation
The Melanic Soils include the rendzinas and rendzic intergrades to yellow-grey earths and yellow-brown earths. They also include the weakly weathered and drier brown-granular loams and clays of the NZ Genetic Soil Classification. Most correlate with Mollisols and a few with Vertisols and Inceptisols of Soil Taxonomy.
Occurrence
Melanic Soils are scattered throughout New Zealand in association with calcareous, mafic, and ultramafic rocks.
Accessory Properties of the Order
- Swelling clays. Most soils have smectite, or minerals with interstratifications of smectite, in the clay mineral assemblage. Melanic Soils usually have a Smectitic, Illitic or Kandic mineralogy class.
- High base saturations. Except for acidified variants, base saturations are usually more than 50%, and KCl-extractable aluminium values are usually very low.
- Stable structure. Structural stability of topsoils is high with relatively large amounts of organic carbon intimately associated with clay minerals. The soils are likely to have relatively high resistance to structural damage under heavy cropping unless organic matter is reduced significantly. The porosity is stabilised by divalent ion/organic matter/ clay complexes.
- High shrink/swell. The soil materials are sticky and plastic. They are expected to have significant shrink/swell potential, expressed in high coefficient of linear expandibility values. This is reflected in strong polyhedral, blocky or prismatic pedality. P retention values are moderate to high.
- Fertile. Except for acidified variants, exchangeable calcium and magnesium values are high, particularly at the base of profile.
- Parent materials. The soils are derived from calcareous, mafic or ultramafic rock classes.
- Deep rooting. Except for shallow soils on rock or soils affected by high water-tables, potential rooting depths are relatively large.
- Biologically active. Carbon/nitrogen ratios are low (except in areas with very high precipitation).
Summary of Melanic Soils Hierarchy
| Code | Group | Subgroup | Example Series |
|---|---|---|---|
| EV | Vertic | Mottled-calcareous | - |
| Mottled | pt. Waiareka | ||
| Calcareous | Te Aneraki | ||
| Typic | Waiareka | ||
| EP | Perch-gley | Vertic | Awapuni |
| Argillic | Okoia | ||
| Typic | - | ||
| ER | Rendzic | Peaty | Chalky |
| Mottled | - | ||
| Weathered | Te Matai | ||
| Typic | Oamaru | ||
| EM | Mafic | Magnesic | Dun |
| Mottled | pt.Awapuku | ||
| Typic | Rapaki | ||
| EO | Orthic | Mottled-calcareous | pt. Waikakahi |
| Argillic-calcareous | Kauana | ||
| Pedal-calcareous | pt. Waikakahi | ||
| Calcareous | pt. Pikikiruna | ||
| Mottled-argillic | - | ||
| Mottled | - | ||
| Argillic | Kaihiku | ||
| Typic | Bishopdale |
Key to Groups of Melanic Soils
EV
Melanic Soils that have both
- Either
- cracks at least 4 mm wide in some part, either in the B horizon and infilled with A horizon material, or open to a depth of 30 cm or more from the mineral soil surface, or
- coefficient of linear extensibility of 0.09 or more with moderate or strong blocky or prismatic pedality in the major part of the B horizon, and
- No redox segregations within 30 cm of the mineral soil surface.
EP
Other soils that have both
- a peaty topsoil or a gley profile form, and
- perch-gley features.
ER
Other soils that have limestone or other calcareous material either in the form of a lithic or paralithic contact, or as a layer dominated by rock fragments (70% or more by volume), with an upper boundary at 60 cm or less and which continues to more than 90 cm from the mineral soil surface.
EM
Other soils that, in a subhorizon of the B or BC horizon at 60 cm from the mineral soil surface, or at the base of the B or BC if shallower, have
- matrix colour value 4 or less and chroma 3 or more, or
- 5% (by volume) or more rock fragments that consist mainly of mafic or ultramafic rocks (but not tuffaceous greywacke), or
- an exchangeable calcium/magnesium molar ratio of 0.2 or less and exchangeable magnesium of 1.5 cmolc/kg or more.
EO
Other soils.
Key to Subgroups of Melanic Soils
EV - VERTIC MELANIC SOILS
Vertic Melanic Soils occur in clayey soil materials dominated by clay minerals with high capacity to shrink on drying and swell on rewetting.
EVMC
Vertic melanic soils that have both
- redox segregations within 60 cm of the mineral soil surface, and
- a calcareous horizon within 90 cm of the mineral soil surface.
Mottled-calcareous Vertic Melanic Soils
EVM
Other soils that have redox segregations within 60 cm of the mineral soil surface.
Mottled Vertic Melanic Soils
EVC
Other soils with a calcareous horizon within 90 cm of the mineral soil surface.
Calcareous Vertic Melanic Soils
EVT
Other soils.
Typic Vertic Melanic Soils
EP - PERCH-GLEY MELANIC SOILS
Perch-gley Melanic Soils occur in sites that are periodically saturated (unless artificially drained). Wetness and associated reducing conditions are indicated by grey colours in horizons subjacent to the topsoil, and is caused by perching of water on a slowly permeable subsurface layer, although a groundwater-table may also be present.
EPV
Perch-gley Melanic Soils that have either
- cracks at least 4 mm wide in some part, either in the B horizon and infilled with A horizon material, or open to a depth of 30 cm or more, from the mineral soil surface, or
- coefficient of linear expandability of 0.09 or more with moderate or strong, blocky or prismatic pedality in the major part of the B horizon.
Vertic Perch-gley Melanic Soils
EPJ
Other soils with an argillic horizon.
Argillic Perch-gley Melanic Soils
EPT
Other soils.
Typic Perch-gley Melanic Soils
ER - RENDZIC MELANIC SOILS
Rendzic Melanic Soils occur in soils in which limestone or calcareous sedimentary rocks or rock debris occur at shallow depths.
ERO
Rendzic Melanic Soils that have a peaty topsoil.
Peaty Rendzic Melanic Soils
ERM
Rendzic Melanic Soils that have a mottled profile form.
Mottled Rendzic Melanic Soils
ERW
Other soils that have a weathered-B or cutanic horizon 10 cm or more thick.
Weathered Rendzic Melanic Soils
ERT
Other soils.
Typic Rendzic Melanic Soils
EM - MAFIC MELANIC SOILS
Mafic Melanic Soils occur in soil materials weathered from mafic and ultramafic rocks or tuffs (but not tuffaceous greywacke). They have relatively high proportions of dark-coloured magnesium- and iron-rich silicate minerals.
EMG
Mafic Melanic Soils that have, in some part of the B or BC horizon to 60 cm from the mineral soil surface, either
- 5% (by volume) or more rock fragments that consist mainly of ultramafic rocks, or
- have an exchangeable calcium/magnesium molar ratio of 0.2 or less and exchangeable magnesium of 1.5 cmolc/kg or more.
Magnesic Mafic Melanic Soils
EMM
Other soils that have a mottled profile form.
Mottled Mafic Melanic Soils
EML
Other soils that have P-retention 85% or more, or strong or very strong reactive-aluminium test, in some part of the B horizon to 90 cm or less from the mineral soil surface.
Allophanic Mafic Melanic Soils
EMA
Other soils that have pH less than 5.5 in some part of the B horizon to 60 cm or less from the mineral soil surface.
Acidic Mafic Melanic Soils
EMT
Other soils.
Typic Mafic Melanic Soils
EO - ORTHIC MELANIC SOILS
Orthic Melanic Soils occur in soil materials containing calcium carbonate or that have high calcium contents.
EOMC
Orthic Melanic Soils that have both a calcareous horizon with an upper surface at 90 cm or less from the mineral soil surface, and a mottled profile form.
Mottled-calcareous Orthic Melanic Soils
EOJC
Other soils that have an argillic horizon and a calcareous horizon with an upper surface at 90 cm or less from the mineral soil surface.
Argillic-calcareous Orthic Melanic Soils
EODC
Other soils that have a moderately or strongly pedal weathered-B horizon 15 cm or more thick occurring immediately beneath the A horizon, and a calcareous horizon with an upper surface at 90 cm or less from the mineral soil surface.
Pedal-calcareous Orthic Melanic Soils
EOC
Other soils that have a calcareous horizon with an upper surface at 90 cm or less from the mineral soil surface.
Calcareous Orthic Melanic Soils
EOMJ
Other soils that have a mottled profile form, and an argillic horizon.
Mottled-argillic Orthic Melanic Soils
EOM
Other soils that have a mottled profile form.
Mottled Orthic Melanic Soils
EOJ
Other soils that have an argillic horizon.
Argillic Orthic Melanic Soils
EOT
Other soils.
Typic Orthic Melanic Soils