Recent Soils

Concept of the Order

Recent Soils show only incipient marks of soil-forming processes because of youthfulness, truncation of an older solum or, less commonly, because the soil material is resistant to alteration. Soil formation has been sufficient to develop a distinct topsoil, or as in the case of fine-textured wetland soils, to have ripened sufficiently that fluid layers are not close to the surface. A continuous cover of vascular plants is normally well established. The concept of the order relates predominantly to weak soil development rather than to the length of time of soil formation.

Correlation

The order correlates with the recent soils, but not the gleyed-recent soils, of the NZ Genetic Soil Classification, and includes Entisols and some Inceptisols and Andisols of Soil Taxonomy.

Occurrence

Recent soils occur throughout New Zealand in young landscapes, including alluvial floodplains, unstable steep slopes and slopes mantled by young volcanic ash.

Accessory Properties of the Order

  1. Weak soil development. Soil development is mostly confined to topsoils, with B horizon colours and/or pedality occurring in some soils.
  2. Base saturations are generally high. Base saturations are usually high except in very humid areas.
  3. Gravel or rock is not strongly altered. The alteration status of gravel or rock usually ranges from fresh to moderately weathered.
  4. Illitic mineralogy. The soil mineralogy reflects the mineralogy of the parent material. Most Recent Soils have an Illitic mineralogy class except for those formed in tephra which are usually Glassy.
  5. Deep rooting. Although textures vary, potential rooting depths are mostly large. Subsurface horizons are usually apedal or weakly pedal and penetration may be limited in some soils.
  6. Good drainage. Poorly drained or very poorly drained soils are not included.
  7. Low P retention. P retentions are likely to be either low or very low unless the soils occur in mafic materials.
  8. High fertility. Natural fertility is generally high.
  9. Subject to erosion or sedimentation. The soils are susceptible to erosion and/or sedimentation.

Summary of Recent Soils Hierarchy

Code

Group

Subgroup

Example Series

RH

Hydrothermal

Inactive

-

RX

Rocky

Peaty-acidic

pt. Titiraurangi

Acidic

-

Typic

pt. Cadzow

RS

Sandy

Mottled

Waiowhiro

Acidic

Poison Bay

Tephric

Hangatahua

Typic

pt. Selwyn

RF

Fluvial

Mottled-acidic

Waiwhetu

Mottled-saline

pt. Motukarara

Saline

-

Mottled-weathered

Eweburn

Mottled

Hari Hari

Acidic-weathered

pt. Manawatu

Weathered

Waimakariri

Acidic

pt. Seaforth

Typic

pt. Selwyn

RT

Tephric

Mottled

-

Buried-pumice

Matahina

Buried-allophanic

-

Typic

Ngauruhoe

RO

Orthic

Mottled-pallic

-

Mottled

-

Acidic-weathered

-

Weathered

-

Acidic

-

Typic

Barhill

Key to Groups of Recent Soils

RH

Recent Soils that have either

  1. mean annual soil temperature at 30 cm from the mineral soil surface of at least 2.5°C more than the mean annual air temperature, or
  2. pH of 4.8 or less in some part of a subsurface horizon at less than 60 cm from the mineral soil surface.

HYDROTHERMAL RECENT SOILS

RX

Other Recent Soils that have a lithic or paralithic contact either directly beneath the A horizon or have a weathered-B or unconsolidated C horizon that is less than 10 cm thick.

ROCKY RECENT SOILS

RS

Other Recent Soils that have sand or loamy sand texture and have less than 35% (by volume) gravel, in all horizons between the base of the A horizon and 60 cm from the mineral soil surface. Lamellae of sandy loam (of insufficient thickness to meet the requirements of an argillic horizon) may be present.

SANDY RECENT SOILS

RF

Other Recent Soils that have fluvial features and are not buried by more than 30 cm of non-fluvial soil material.

FLUVIAL RECENT SOILS

RT

Other Recent Soils in tephric soil material from the mineral soil surface to 30 cm depth or more.

TEPHRIC RECENT SOILS

RO

Other Recent Soils.

ORTHIC RECENT SOILS

Key to Subgroups of Recent Soils

RH - HYDROTHERMAL RECENT SOILS

Hydrothermal Recent Soils occur adjacent to active hydrothermal areas or areas that were formerly active. The soils indicate active or former hydrothermal activity by either elevated soil temperatures or extreme acidity, respectively.

RHI

(only one subgroup recognised).

Inactive Hydrothermal Recent Soils

RX - ROCKY RECENT SOILS

Rocky Recent Soils have rock at shallow depths, severely restricting the soil volume available to roots.

RXOA

Rocky Recent Soils that have a peaty topsoil and a pH less than 5.5 in some part below 20 cm from the mineral soil surface.

Peaty-acidic Rocky Recent Soils

RXA

Other soils that have pH less than 5.5 in some part below 20 cm from the mineral soil surface.

Acidic Rocky Recent Soils

RXT

Other soils.

Typic Rocky Recent Soils

RS - SANDY RECENT SOILS

Sandy Recent Soils occur in sand deposits which are dominantly aeolian, and sometimes alluvial in origin. Subsurface horizons are sandy.

RSM

Sandy Recent Soils that have a mottled profile form.

Mottled Sandy Recent Soils

RSA

Other soils that have pH less than 5.5 in some part between 20 and 60 cm from the mineral soil surface.

Acidic Sandy Recent Soils

RSK

Other soils in tephric soil material from the mineral soil surface to 30 cm depth or more.

Tephric Sandy Recent Soils

RST

Other soils.

Typic Sandy Recent Soils

RF - FLUVIAL RECENT SOILS

Fluvial Recent Soils occur in sediments deposited by flowing water. Many occur on land surfaces that are susceptible to flooding.

RFMA

Fluvial Recent Soils that have both

  1. a mottled profile form, and
  2. pH less than 5.5 in some part between the base of the A horizon and within 60 cm of the mineral soil surface.

Mottled-acidic Fluvial Recent Soils

RFMQ

Other soils that have both

  1. a mottled profile form, and
  2. have electrical conductivity of 0.8 mS/cm or more within 60 cm of the mineral soil surface at some time of the year.

Mottled-saline Fluvial Recent Soils

RFQ

Other soils that have electrical conductivity of 0.8 mS/cm or more within 60 cm of the mineral soil surface at some time of the year.

Saline Fluvial Recent Soils

RFMW

Other soils that have both

  1. a mottled profile form, and
  2. a weathered-B horizon with its lower boundary at 30 cm or more from the mineral soil surface.

Mottled-weathered Fluvial Recent Soils

RFM

Other soils that have a mottled profile form.

Mottled Fluvial Recent Soils

RFAW

Other soils that have both

  1. a weathered-B horizon with its lower boundary at 30 cm or more from the mineral soil surface, and
  2. pH less than 5.5 in some part of the subsurface horizons within 60 cm of the mineral soil surface.

Acidic-weathered Fluvial Recent Soils

RFW

Other soils that have a weathered-B horizon with its lower boundary at 30 cm or more from the mineral soil surface.

Weathered Fluvial Recent Soils

RFA

Other soils that have pH less than 5.5 in some part of subsurface horizons within 60 cm of the mineral soil surface.

Acidic Fluvial Recent Soils

RFT

Other soils.

Typic Fluvial Recent Soils

RT - TEPHRIC RECENT SOILS

Tephric Recent Soils occur in unconsolidated sediments of volcanic origin including ash, cinders, lapilli, pumice, and other pyroclastics.

RTM

Tephric Recent Soils that have a mottled profile form.

Mottled Tephric Recent Soils

RTBP

Other soils that have a buried soil, with its upper surface within 60 cm or less from the mineral soil surface that has a B horizon that meets the requirements of vitric soil material.

Buried-pumice Tephric Recent Soils

RTBL

Other soils that have a buried soil within 60 cm or less from the mineral soil surface to the upper surface of the buried soil, and have a B horizon that meets the requirements of allophanic soil material, but not necessarily the requirements for bulk density.

Buried-allophanic Tephric Recent Soils

RTT

Other soils.

Typic Tephric Recent Soils

RO - ORTHIC RECENT SOILS

Orthic Recent Soils occur on land that is being eroded or has received sediment that has been deposited mainly as a result of slope processes.

ROMP

Other soils that have both

  1. a mottled profile form, and
  2. meets part 2(b) of the requirements for Pallic Soils.

Mottled-pallic Orthic Recent Soils

ROM

Orthic Recent Soils that have a mottled profile form.

Mottled Orthic Recent Soils

ROAW

Other soils that have both

  1. a weathered-B horizon with its lower boundary at 30 cm or more from the mineral soil surface, and
  2. pH less than 5.5 in some part of the subsurface horizons within 60 cm of the mineral soil surface.

Acidic-weathered Orthic Recent Soils

ROW

Other soils that have a weathered-B horizon with its lower boundary at 30 cm or more from the mineral soil surface.

Weathered Orthic Recent Soils

ROA

Other soils that have pH less than 5.5 in some part between the base of the A horizon and within 60 cm of the mineral soil surface.

Acidic Orthic Recent Soils

ROT

Other soils.

Typic Orthic Recent Soils