Melanic Soils

Concept of the Order

Melanic Soils are soils with high base saturations, well structured, very dark A horizons, and with weakly alkaline or weakly acid subsurface horizons. 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 or basaltic rocks.

Accessory Properties of the Order

  1. 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.
  2. High base saturations. Base saturations are usually more than 50%, and KCl-extractable aluminium values are usually very low.
  3. 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.
  4. 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. Pretentions are moderate to high.
  5. Fertile. Exchangeable calcium and magnesium values are high, particularly at the base of profile.
  6. Parent materials. The soils are derived from calcareous rocks or from mafic or ultramafic rocks (e.g. basalt or peridotite).
  7. Deep rooting. Except for shallow soils on rock or soils affected by high water-tables, potential rooting depths are relatively large.
  8. 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

  1. Either
    1. 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
    2. 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
  2. No redox segregations within 30 cm of the mineral soil surface.

VERTIC MELANIC SOILS

EP

Other soils that have either

  1. a peaty topsoil or a gley profile form, and
  2. perch-gley features.

PERCH-GLEY MELANIC SOILS

ER

Other soils that have limestone or other calcareous material either in the form of a lithic or paralithic contact, or as an extremely gravelly layer (70% or more by volume) in the form of rock rubble, with an upper boundary at 60 cm or less and which continues to more than 90 cm from the mineral soil surface.

RENDZIC MELANIC SOILS

EM

Other soils that, in a subhorizon of the B horizon at 60 cm from the mineral soil surface, or at the base of the B if shallower, have

  1. matrix colour value 4 or less and chroma 3 or more, or
  2. 5% (by volume) or more gravel that consist mainly of mafic or ultramafic rocks (but not tuffaceous greywacke), or
  3. an exchangeable calcium/magnesium ratio of 0.2 or less and exchangeable magnesium of 1.5 cmol/kg or more.

MAFIC MELANIC SOILS

EO

Other soils.

ORTHIC MELANIC 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

  1. redox segregations within 60 cm of the mineral soil surface, and
  2. 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

  1. 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
  2. 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 ultrabasic, basic or intermediate igneous rocks or tuffs. They have relatively high proportions of dark-coloured magnesium- and iron-rich (mafic) silicate minerals.

EMG

Mafic Melanic Soils that have, in some part of the B horizon to 60 cm from the mineral soil surface, either

  1. 5% (by volume) or more gravel that consist mainly of ultramafic rocks, or
  2. have an exchangeable calcium/magnesium ratio of 0.2 or less and exchangeable magnesium of 1.5 cmol/kg or more.

Magnesic Mafic Melanic Soils

EMM

Other soils that have a mottled profile form.

Mottled 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