Brown Soils

Concept of the Order

Brown Soils usually contain 2:1 clay minerals. Secondary iron oxides tend to be evenly dispersed through the soil and give a yellowish brown colour to the upper part of the B horizon. Base saturation values are usually moderate to very low.

Correlation

The order comprises moderately and weakly weathered yellow-brown earths, yellow-brown sands, southern brown-granular loams and clays, and intergrades from yellow-brown earths to yellow-grey earths, podzols, brown-granular soils, and recent soils, as well as associated steepland soils of the NZ Genetic Soil Classification. The soils predominantly correlate with the Dystrochrepts of Soil Taxonomy, except for some stony and sandy soils which are Ustochrepts or Psamments.

Occurrence

Brown Soils occur in places in which summer dryness is uncommon and that are not waterlogged in winter. They are the most extensive New Zealand soils.

Accessory Properties of the Order

  1. Dispersed secondary oxides. Secondary iron and aluminium oxides are dispersed throughout the soil mass. The soil is brunified (i.e. iron and aluminium oxides form coatings around phyllosilicate clay particles and form bridges between these particles and humus). P retention is moderate to very high.
  2. Low to moderate base saturation. Base saturation values in subsoils are usually less than 50%, and KCl-extractable aluminium levels are usually more than 1.5 cmol(+)/kg except where clay contents are relatively low.
  3. Parent materials are mostly weakly weathered. Mafic Brown Soils are derived from weakly weathered intermediate or basic igneous rocks (e.g. phonolite and basalt). Other groups are derived dominantly from acid quartzo-feldspathic sedimentary rocks (schist and greywacke) or acid igneous rocks (e.g. rhyolites and granites).The alteration status of gravel or hard rock substrates is usually fresh to moderately weathered and occasionally highly weathered.
  4. Mica/illite and vermiculite are common clay minerals. Profiles tend to be mineralogically uniform with depth. Brown soils cover a wide range of mineralogy classes. Mixed, Illitic, Vermiculitic and Clay-mineralic (involving vermiculite and mica-vermiculite) are common. Some Allophanic Brown Soils have an Amorphic mineralogy class.
  5. Good Drainage. No poorly drained or very poorly drained soils are included. Macroporosity is generally moderate (10–14%) except in subsurface horizons of Firm Brown Soils.
  6. Biologically active. Except in soils that are limited by coldness or acidity, cast spheroidal peds are common in topsoils and C/N ratios are moderate to low. The roots of native plants penetrate deeply.
  7. Relatively stable topsoils. Aggregates are not readily dispersed.
  8. Moist climate or low available-water capacity. Most soils occur in areas with mean annual precipitation more than 1000 mm. Others have low available-water capacity (usually less than 75 mm as in some Stony Brown Soils and Sandy Brown Soils), or are in sites with low evapotranspiration.

Summary of Brown Soils Hierarchy

Code

Group

Subgroup

Example Series

BL

Allophanic

Mottled

-

Acidic

Tekoa

Firm

Te Anau

Acidic-mafic

Stewart

Typic

Craigieburn

Acidic-pedal

Kaiuma

Pedal

Levin

BS

Sandy

Mottled-Placic

-

Mottled

Awahou

Acidic

Koputaroa

Pallic

pt. Halkett

Pan

ToeToes

Typic

Foxton

BX

Oxidic

Typic

-

BM

Mafic

Mottled-magnesic

Croisilles var.

Magnesic

pt. Dun

Mottled

-

Acidic

Cargill

Typic

Pipikaretu

BA

Acid

Peaty

pt. Spenser

Mottled-placic

Lammerlaw

Mottled

Mackley

Placic

pt. Tautuku

Pan

Whiterig

Typic

Carrick

BF

Firm

Mottled-acidic

-

Mottled-cemented

Harwarden

Mottled-weathered

-

Mottled-Pallic

-

Mottled

Mahinerangi

Acidic-cemented

Whiterig

Cemented

Steward

Acidic-allophanic

Judgeford

Allophanic

Belmont

Weathered

-

Pallic

Pinelheugh

Acidic

Porteous

Typic

Waikiwi

BO

Orthic

Mottled-weathered

-

Mottled-acidic

-

Mottled

-

Humose

Pukaki

Immature

Grassmere

Pallic

-

Acidic

Pelorus

Weathered

-

Calcareous

-

Typic

Ruahine

Key to Groups of Brown Soils

BL

Brown soils that have within the B horizon a subhorizon that meets the requirements of allophanic soil material but not necessarily the requirement for bulk density, and that is 10 cm or more thick and occurs with its upper surface at 60 cm or less from the mineral soil surface.

ALLOPHANIC BROWN SOILS

BS

Other Brown Soils that from the base of the A horizon to 60 cm from the mineral soil surface, have

  1. Sand or loamy sand texture and less than 35% gravel (by volume), in all horizons (except for sandy loam laminations that do not meet the requirements of an argillic horizon), and
  2. do not have a placic horizon.

SANDY BROWN SOILS

BX

Other Brown Soils that in some part of the B horizon within 60 cm of the mineral soil surface, have

  1. matrix colour value 4 or less, and
  2. friable or very friable unconfined failure from very moist to dry, and
  3. fine or finer polyhedral peds.

OXIDIC BROWN SOILS

BM

Other Brown Soils that, in a subhorizon of the B 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 moderately or strongly pedal polyhedral peds (20 mm or less in size), or
  2. 5% (by volume) or more gravel that consists 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 BROWN SOILS

BA

Other Brown Soils that have either

  1. pH of 4.8 or less in some part between 20 and 60 cm from the mineral soil surface, or
  2. a placic horizon.

ACID BROWN SOILS

BF

Other Brown Soils that have a fragipan, or an apedal subhorizon with a slightly firm or stronger moist soil strength in the B horizon, with an upper boundary within 90 cm of the mineral soil surface.

FIRM BROWN SOILS

BO

Other Brown Soils.

ORTHIC BROWN SOILS

Key to Subgroups of Brown Soils

BL - ALLOPHANIC BROWN SOILS

Allophanic Brown Soils occur in soils that have a horizon with properties dominated by the presence of minerals with short-range order and aluminium-humus complexes. Such horizons are weak in strength, sensitive, and have low bulk density. They occur in quartzo-feldspathic and tuffaceous (greywacke) sandstone and argillite, and in volcanic-ash parent materials.

BLM

Allophanic Brown Soils that have a mottled profile form.

Mottled Allophanic Brown Soils

BLA

Other soils that have, in some part1 of the B horizon to 60 cm from the mineral soil surface, both

  1. crumb (or earthy) structure, or bulk density of the fine-earth fraction less than 1.1 Mg/m3 with weakly pedal or apedal fabric, and
  2. pH less than 5.5.

Acidic Allophanic Brown Soils

BLF

Other soils that have both

  1. In some part of the B horizon to 60 cm from the mineral soil surface, crumb (or earthy) structure, or bulk density of the fine-earth fraction less than 1.1 Mg/m3, and
  2. An underlying layer that meets the requirements of a fragipan except for pedality, or an apedal subhorizon with a slightly firm or stronger moist soil strength, with an upper boundary within 90 cm of the mineral soil surface.

Firm Allophanic Brown Soils

BLAM

Other soils that have both

  1. pH less than 5.5 in some part of the B horizon to 60 cm from the mineral soil surface, and
  2. in a subhorizon at 60 cm from the mineral soil surface, or at the base of the B horizon if shallower, either
    1. matrix colour value 4 or less and moderately or strongly pedal polyhedral fabric, or
    2. 5% (by volume) or more gravel consisting mainly of mafic rocks.

Acidic-mafic Allophanic Brown Soils

BLT

Other soils that have, in some part of the B horizon to 60 cm depth from the mineral soil surface, crumb (or earthy) structure, or bulk density of the fine-earth fraction less than 1.1 Mg/m3 with weakly pedal or apedal fabric.

Typic Allophanic Brown Soils

BLAD

Other soils that have pH less than 5.5 in some part of the B horizon at 60 cm or less from the mineral soil surface.

Acidic-pedal Allophanic Brown Soils

BLD

Other soils.

Pedal Allophanic Brown Soils

BS - SANDY BROWN SOILS

Sandy Brown Soils occur in sand deposits which are usually of aeolian origin, but may also be of alluvial origin. Subsurface horizons are sand or loamy sand.

BSMP

Sandy Brown Soils that have a mottled profile form and a placic horizon.

Mottled-placic Sandy Brown Soils

BSM

Other soils that have a mottled profile form.

Mottled Sandy Brown Soils

BSA

Other soils that have pH less than 5.5 in some part of the B horizon, to its base, or to 60 cm from the mineral soil surface (whichever is shallower).

Acidic Sandy Brown Soils

BSP

Other soils that have either

  1. an argillic horizon composed of lamellae, or
  2. eluvial features, including an E horizon, or skeletans as apparent segregations of relatively higher colour value in B or BC horizons (“two-tone”).

Pallic Sandy Brown Soils

BSX

Other soils that have either a placic horizon or ortstein-pan.

Pan Sandy Brown Soils

BST

Other soils.

Typic Sandy Brown Soils

BX - OXIDIC BROWN SOILS

Oxidic Brown Soils occur in strongly weathered soil materials similar to those of Oxidic Soils except that Oxidic Brown Soils have more weatherable minerals and higher values of reserve magnesium. They usually occur in association with Oxidic Soils, Ultic Soils or Granular Soils but on younger land surfaces.

BXT

Oxidic Brown Soils (only one subgroup).

Typic Oxidic Brown Soils

BM - MAFIC BROWN SOILS

Mafic Brown Soils occur in soil materials weathered from ultrabasic, basic or intermediate igneous rocks or tuffs. They have relatively high proportions of dark magnesium and iron-rich (mafic) silicate minerals, and have relatively large contents of iron oxides.

BMMG

Mafic Brown Soils that have both

  1. a mottled profile form, and
  2. either
    1. 5% (by volume) or more gravel consisting mainly of ultramafic rocks, or
    2. have an exchangeable calcium/magnesium ratio of 0.2 or less in some part of the B horizon to 60 cm from the mineral soil surface.

Mottled-magnesic Mafic Brown Soils

BMG

Other soils that have either

  1. 5% (by volume) or more gravel consisting mainly of ultramafic rocks, or
  2. have an exchangeable calcium/magnesium ratio of 0.2 or less in some part of the B horizon to 60 cm from the mineral soil surface.

Magnesic Mafic Brown Soils

BMM

Other soils that have a mottled profile form.

Mottled Mafic Brown Soils

BMA

Other soils that have pH less than 5.5 in some part of the B horizon to 60 cm from the mineral soil surface.

Acidic Mafic Brown Soils

BMT

Other soils.

Typic Mafic Brown Soils

BA - ACID BROWN SOILS

Acid Brown Soils are strongly or extremely acid soils, many of which occur in very moist or cold mountain environments. Many have a placic horizon.

BAO

Acid Brown Soils that have a peaty topsoil.

Peaty Acid Brown Soils

BAMP

Acid Brown Soils that have both

  1. a placic horizon at 60 cm or less from the mineral soil surface, and
  2. a mottled or gley profile form.

Mottled-placic Acid Brown Soils

BAM

Other soils that have a mottled profile form.

Mottled Acid Brown Soils

BAP

Other soils with a placic horizon.

Placic Acid Brown Soils

BAX

Other soils that in the B have a subhorizon, that meets the strength requirements of an ortstein pan, within 90 cm of the mineral soil surface.

Pan Acid Brown Soils

BAT

Other soils.

Typic Acid Brown Soils

BF - FIRM BROWN SOILS

Firm Brown Soils have an apedal subsurface horizon with strong moist soil strength which shares some of the characteristics of a fragipan or an ortstein-pan. The soils occur on relatively stable sites and are most commonly on flat, rolling or moderately hilly slopes.

BFMA

Firm Brown 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 Firm Brown Soils

BFMC

Other soils that have both

  1. a mottled profile form, and
  2. a horizon that is cemented to the degree that it is at least weakly indurated, within 90 cm of the mineral soil surface.

Mottled-cemented Firm Brown Soils

BFMW

Other soils that have both

  1. a mottled profile form, and
  2. gravel that in the majority is weathered to the extent that clasts may easily be broken by hammer or spade.

Mottled-weathered Firm Brown Soils

BFMP

Other soils that have both

  1. a mottled profile form, and
  2. that have in some part of the B or BC horizon to 90 cm in from the mineral soil surface, either matrix hue of 2.5Y or yellower or matrix hue of 10YR and chroma 4 or less with either
    1. non-reactive, very weak or weak reactive-aluminium test, or
    2. a cutanic horizon, or
    3. P-retention less than 30%.

Mottled-pallic Firm Brown Soils

BFM

Other Soils that have a mottled profile form.

Mottled Firm Brown Soils

BFAC

Other soils have both

  1. pH less than 5.5 in some part between the base of the A horizon and within 60 cm of the mineral soil surface, and
  2. a horizon that is cemented to the degree that it is at least weakly indurated, within 90 cm of the mineral soil surface.

Acidic-cemented Firm Brown Soils

BFC

Other soils that have a horizon that is cemented to the degree that it is at least weakly indurated, within 90 cm of the mineral soil surface.

Cemented Firm Brown Soils

BFAL

Other soils that have in some part of the B horizon to 90 cm or less from the mineral soil surface both

  1. P retention 85% or more, or strong or very strong reactive-aluminium test, and
  2. pH of less than 5.5.

Acidic-allophanic Firm Brown Soils

BFL

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 Firm Brown Soils

BFW

Other soils in which the majority of the gravel is weathered to the extent that clasts may easily be broken by hammer or spade.

Weathered Firm Brown Soils

BFP

Other soils that have in some part of the B or BC horizon to 90 cm from the mineral soil surface, either

  1. matrix hue of 2.5Y or yellower, or
  2. matrix hue of 10YR and chroma 4 or less with either
    1. non-reactive, very weak or weak reactive-aluminium test, or
    2. a cutanic horizon, or
    3. P-retention less than 30%.

Pallic Firm Brown Soils

BFA

Other soils that have in some part of the B horizon above the apedal horizon with a slightly firm or stronger strength class, either

  1. pH less than 5.5, or
  2. a subhorizon with 10% or more humus or humus-clay coatings of moist colour value 4 or less or colour value 5 and chroma 3.

Acidic Firm Brown Soils

BFT

Other soils.

Typic Firm Brown Soils

BO - ORTHIC BROWN SOILS

Orthic Brown Soils have B horizon peds or have weak or very weak soil strength to depth. They most commonly occur on hilly or steep slopes, or on Holocene land surfaces.

BOMW

Orthic Brown Soils that have both

  1. a mottled profile form, and
  2. gravel that in the majority is weathered to the extent that clasts may easily be broken by hammer or spade.

Mottled-weathered Orthic Brown Soils

BOMA

Other 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 Orthic Brown Soils

BOM

Other soils that have a mottled profile form.

Mottled Orthic Brown Soils

BOH

Other soils that have both

  1. colour value of the matrix 4 or less and hue 2.5Y or redder, or 10% or more coatings of colour value 4 or less in the greater part of the B horizon, and
  2. 10% or less clay within 90 cm of the mineral soil surface.

Humose Orthic Brown Soils

BOI

Other soils that have either

  1. a buried A horizon within 120 cm of the mineral soil surface, or
  2. a weathered-B horizon 30 cm or less thick that, throughout,
    1. has chroma less than 6, and
    2. is either apedal massive or apedal single-grain and has weak or very weak moist soil strength.

Immature Orthic Brown Soils

BOP

Other soils that have in some part of the solum within 90 cm from the mineral soil surface either

  1. matrix hue of 2.5Y or yellower, or
  2. matrix hue of 10YR and chroma 4 or less with either
    1. non-reactive, very weak or weak reactive-aluminium test, or
    2. a cutanic horizon, or
    3. P-retention less than 30%.

Pallic Orthic Brown Soils

BOA

Other soils that have pH less than 5.5 in some part of the B horizon to 60 cm from the mineral soil surface.

Acidic Orthic Brown Soils

BOW

Other soils with gravel that in the majority is weathered to the extent that clasts may easily be broken by hammer or spade.

Weathered Orthic Brown Soils

BOC

Other soils that have in part of the B horizon a calcareous horizon within 60 cm of the mineral soil surface.

Calcareous Orthic Brown Soils

BOT

Other soils.

Typic Orthic Brown Soils

  1. “in some part” means that the requirement must be met in some subhorizon or some subsample that is 10 cm or more thick within the specified thickness.↩︎