Granular Soils

Concept of the Order

Granular soils are clayey soils in which kaolin-group minerals are dominant, and are usually associated with vermiculite and hydrous-interlayered vermiculite. The soil fabric comprises polyhedral peds with strength characteristics which change rapidly with water content. Consistence is sticky and plastic. The presence of vermiculite gives these soils a moderate buffering capacity. The soils lack the weak strength, friable failure, low plasticity, and low-activity-clay properties which either define or are accessory to Oxidic Soils. Clay coatings where they occur are thin.

Correlation

The order comprises many soils previously classified as brown granular loams and moderately to strongly leached brown granular clays of the NZ Genetic Soil Classification. Most are correlated with the Ultisols but a few with the Alfisols of Soil Taxonomy.

Occurrence

Granular Soils are only known to occur in the northern North Island, particularly in the lowlands of the Waikato and South Auckland regions.

Accessory Properties of the Order

  1. Moderate activity clay. CEC is greater than 16 cmol/kg (clay) and ECEC ranges from about 8 to 16 cmol/kg (clay).
  2. Parent materials. The soils are derived predominantly from strongly weathered tephras mostly older than 50 000 years, but also from basaltic and andesitic rocks with possible additions from aeolian material.
  3. Kandic mineralogy. Granular Soils usually belong to the Kandic mineralogy class.
  4. Slowly permeable. Saturated hydraulic conductivity is slow or marginally slow somewhere in the profile, resulting in periods of perching of water.
  5. Limited root depth. The extension of plant roots in subsoils is commonly limited by either high penetration resistance, wetness or aluminium toxicity.
  6. Limited workability when wet. Workability and trafficability is constrained by stickiness and plasticity after heavy rainfall, particularly in contrast to Oxidic Soils.
  7. Low phosphorus status. Phosphorus fixation may be high, as indicated by high P retention levels.
  8. Strongly weathered with low nutrient reserves. Reserves of phosphorus, potassium and magnesium are low, particularly in the Oxidic group.
  9. Sulphate in B horizons. Sulphate tends to be strongly adsorbed in B horizons.

Summary of Granular Soils Hierarchy

Code

Group

Subgroup

Example Series

NP

Perch-gley

Oxidic

Rangiuru var.

Acidic

Tutamoe

Typic

Kohumaru

NE

Melanic

Mottled

-

Allophanic

pt. Morrinsville

Typic

Morrinsville

NX

Oxidic

Mottled-acidic

Awarua var.

Mottled

pt. Waimatenui

Allophanic

pt. Naike

Acidic

Awarua

Typic

Naike

NO

Orthic

Mottled-acidic

Waipoua var.

Mottled

pt. Hamilton

Allophanic

-

Acidic

Waipoua

Typic

Hamilton

Key to Groups of Granular Soils

NP

Granular soils that have both

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

PERCH-GLEY GRANULAR SOILS

NE

Other Granular soils that have pH of 5.9 or more in the major part of the B horizon to 60 cm from the mineral soil surface.

MELANIC GRANULAR SOILS

NX

Other Granular soils that have a cutanoxidic horizon more than 30 cm thick with an upper boundary at 25 cm or more from the mineral soil surface.

OXIDIC GRANULAR SOILS

NO

Other Granular soils.

ORTHIC GRANULAR SOILS

Key to Subgroups of Granular Soils

NP - PERCH-GLEY GRANULAR SOILS

Perch-gley Granular Soils occur in sites that are periodically saturated (if undrained). Wetness and associated reducing conditions are indicated by grey colours and reddish mottles. The wetness is caused by perching of water on a clay-enriched slowly permeable layer, although a groundwater-table may also be present.

NPX

Perch-gley Granular Soils that have a cutanoxidic horizon more than 30 cm thick with an upper boundary below 25 cm from the mineral soil surface.

Oxidic Perch-gley Granular Soils

NPA

Other soils that have pH 5.1 or less in some part of the B horizon to 60 cm from the mineral soil surface.

Acidic Perch-gley Granular Soils

NPT

Other soils.

Typic Perch-gley Granular Soils

NE - MELANIC GRANULAR SOILS

Melanic Granular Soils are less acid and more fertile than other Granular Soils. Base saturation exceeds 50% in part of the root zone.

NEM

Melanic Granular Soils that have a mottled profile form.

Mottled Melanic Granular Soils

NEL

Soils that have a horizon that is 10 cm or more thick within 60 cm of the mineral soil surface that meets all the requirements of allophanic soil material, but not necessarily the requirement for bulk density.

Allophanic Melanic Granular Soils

NET

Other soils.

Typic Melanic Granular Soils

NX - OXIDIC GRANULAR SOILS

Oxidic Granular Soils have low ECEC (marginal to Oxidic Soils), have low fertility and are acid. Some plants may be susceptible to aluminium toxicity.

NXMA

Oxidic Granular Soils that have both

  1. a mottled profile form, and
  2. pH of less than 5.1 in some part of the B horizon to 60 cm from the mineral soil surface.

Mottled-acidic Oxidic Granular Soils

NXM

Other soils that have a mottled profile form.

Mottled Oxidic Granular Soils

NXL

Other soils that have a layer that meets the requirements of allophanic soil material except for bulk density, and is 10 cm or more thick, within 60 cm of the mineral soil surface.

Allophanic Oxidic Granular Soils

NXA

Other soils that have pH of less than 5.1 in some part of the B horizon to 60 cm from the mineral soil surface.

Acidic Oxidic Granular Soils

NXT

Other soils.

Typic Oxidic Granular Soils

NO - ORTHIC GRANULAR SOILS

Orthic Granular Soils are well, moderately well or imperfectly drained soils that are sticky or very plastic with clay-enriched B horizons. Their fertility is intermediate between Oxidic and Melanic groups.

NOMA

Orthic Granular Soils that have both

  1. a mottled profile form, and
  2. pH of less than 5.1 in some part of the B horizon to 60 cm from the mineral soil surface.

Mottled-acidic Orthic Granular Soils

NOM

Other soils that have a mottled profile form.

Mottled Orthic Granular Soils

NOL

Other soils that have a layer that meets the requirements of allophanic soil material except for bulk density, and is 10 cm or more thick within 60 cm of the mineral soil surface.

Allophanic Orthic Granular Soils

NOA

Other soils that have pH of less than 5.1 in the major part of the B horizon to 60 cm from the mineral soil surface.

Acidic Orthic Granular Soils

NOT

Other soils.

Typic Orthic Granular Soils