Diagnostic horizons and other differentiae

Diagnostic horizons and other differentiating criteria are defined to facilitate the assignment of soils to classes. The definitions are not intended to represent a comprehensive classification of horizons. A summary of these is given in Table 4.1. Transitional horizons are not included when considering horizon notations for NZSC criteria unless explicitly stated. Note that:

  • “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.
  • “in the major part” means that the requirement must be met over more than half the specified thickness (e.g. from the base of the A horizon to 60 cm from the mineral soil surface). Analyses or estimates should be used from subhorizons that subdivide the thickness, or if the subhorizons are not recognised, then from subsamples of the relevant horizons.

The following diagnostic horizons and soil characteristics have been modified from Soil Taxonomy Soil Survey Staff (1999): argillic horizon, duripan, fragipan, lithic contact, paralithic contact and placic horizon.

Allophanic Soil Material

Allophanic soil material has soil properties dominated by minerals with short-range order, especially allophane, imogolite and ferrihydrite. Other terms such as “amorphous”, “poorly crystalline” and “non-crystalline”, have been, or are, sometimes used for such minerals. Their chief characteristics are reactive variable-charge surfaces, and a very high specific surface area (several hundreds of m2/g).

Allophanic soil material has BOTH

  1. Either
    1. All of the following (in a fresh sample):
      1. Sensitive or strongly sensitive sensitivity class (distinctly greasy or smeary feel except in some sandy soils), and
      2. Very weak or weak unconfined soil strength class (when moist), and
      3. Non-sticky or slightly sticky stickiness class, and
      4. Strong or very strong reactive-aluminium test; OR
    2. P retention of 85% or more; AND
  2. Dry bulk density of the fine-earth fraction (where the volume is determined on a field-moist soil) of less than 0.9 Mg/m3.

Layers meeting the requirements of allophanic soil material may also meet the requirements of vitric soil material.

Accessory chemical properties relate to variable-charge characteristics, P retention, and high organic-matter contents. Accessory physical properties include high total available water capacity and readily available water capacity, and low penetration resistance. In addition, allophanic soil material undergoes irreversible changes upon drying, for example, in plastic and liquid limits and in apparent particle-size distribution. It should be noted that minerals other than allophane (e.g. ferrihydrite) can give rise to allophanic soil material.

Anthropic Soil Material

Anthropic material constitutes human-altered or human-transported soil material that shows evidence of the purposeful modification of soil properties or of earth-surface features by human activity. The field evidence of alteration excludes low impact agricultural practices such as shallow cultivation, digging, ploughing, or the addition of amendments. Artifically drained wetlands are also not included here because it is difficult to unambiguously link artificial drainage to specific, field-identifieable soil indicators. The nature of alteration can comprise profile truncation, mixing, deposition of transported material, and addition of artefacts. Due to the high variability of the morphological or chemical soil properties that can result from human soil modifications, the criteria of Anthropic material are mainly linked to their anthropic genesis, and thus deviate from the principles of the NZSC. Common indicators of Anthropic material in New Zealand are:

  • excavated or constructed landforms,
  • mechanically detached, redistributed, and re-oriented soil material,
  • mechanically abraded and broken rock fragments that do not correspond with the fragments in the underlying soil material (e.g., fractures that cut through rather than between individual minerals),
  • manufactured materials, such as charcoal, and artefacts,
  • relocated soil material overlying bones, shells or manufactured materials,
  • eating and cooking waste and associated charred products,
  • waste products of manufacturing processes, including chemical contaminants.

The relationship between natural soil horizons or parent materials with Anthropic material or truncation is commonly in the form of a lithological discontinuity, indicated by distinct to sharp transitions between horizons. The majority of Anthropic material (including truncation) in NZ is the result of distinct and extensive modification from the mid-19th century coinciding with early European settlement, deforestation, and farming. From the mid-20th century, excavation and construction (e.g., infrastructure, roading, industrial and urban expansion) has increasingly intensified these impacts on our soils. The influence of ‘older’ Māori cultural practice (e.g., soil alteration/modification practices, Māori plaggen soils) is still recognised in many parts of NZ, and can predate European settlement. Distinct soil management alterations from historical (dating back to early Polynesian settlement) to modern can be distinguished at the group or family level. Anthropic material is indicated on a per-horizon basis by the Parent Material Class notation ‘An’. This notation can be appended with ‘m’, ‘e’, or ‘n’ to distinguish pre-modern Māori modifications, pre-modern European modifications, and modern modifications, if desired (e.g ‘An-m’).

Argillic Horizon

An argillic horizon is a clay-enriched horizon. It is indicated by a “Bt” horizon notation as in Btg, BCt, etc. It has ONE of the following:

  1. It is vertically continuous and is 10 cm or more thick. Clay coatings occur that have a waxy lustre when dry and sufficient thickness to envelop fine sand grains. Coatings occur either on peds (10% or more of the ped surfaces), or in pores (in more than one-third of the observed tubular pores) or as bridges between sand grains (in more than half of the horizon); OR
  2. It is composed of clay-enriched lamellae (clay bands), that within 90 cm of the mineral soil surface have a combined thickness of 15 cm or more; OR
  3. It contains sufficiently more clay than the overlying horizon, as detected by hand texturing (5% or more) , excluding any differences which result from a lithological discontinuity, and either
    1. it is overlain by an eluvial horizon and the upper boundary of texture contrast is abrupt or sharp, or
    2. clay coatings occur on ped or pore surfaces.

Horizons having coatings which do not meet the requirements of an argillic horizon are likely to meet the requirements of a cutanic horizon.

Brittle-B Horizon

A brittle-B horizon is a B or BC horizon that has ALL of the following:

  1. Brittle failure (the horizon may be dominated by rock fragments, but the fine-earth fraction must be sufficiently coherent to allow brittle failure); AND
  2. It is apedal-massive. Extremely coarse or gross prisms may be present, if the interior of the prisms is apedal-massive; AND
  3. Few or less fine roots occur throughout the horizon.

The brittle-B horizon differs from the fragipan by having either lower soil strength or lower penetration resistance. Extremely coarse prism faces, if present, are not defined by low chroma colours as they are in a fragipan. The horizon commonly has some roots throughout, in contrast to the fragipan in which roots are confined to cracks. A brittle-B horizon is usually given the horizon notation suffix (x).

Calcareous Horizon

A calcareous horizon is a horizon in which calcium carbonate occurs in the fine-earth fraction. The concentration may be as low as 1%, but its presence can be detected by effervescent reaction with 10% HCl on samples from a freshly exposed profile. The calcium carbonate may be inherited from a calcareous parent material (‘primary’ carbonates), or it may be formed in the soil and occur as coatings on rock fragments, threadlike deposits in pores, or as nodules (‘secondary’ carbonates). A calcareous horizon is given the notation suffix k (e.g. Btk, Bwk, etc).

Crumb Structure

Crumb structure is defined as an earthy apedal material (Milne et al. 1995) with very friable failure class. Soil with crumb structure in situ has the gross appearance of massive soil material, but when disaggregated or disturbed, microfragments are produced with a superficial resemblance to breadcrumbs. When examined with a ×10 power hand-lens, these prove to be loosely packed aggregates of spheroidal micropeds (<0.5 mm) with packing voids forming a prominent part of the aggregate.

Cutanic Horizon

A cutanic horizon is a B or BC horizon containing translocated material forming dark-coloured coatings on ped faces, in pores or on coarse fragments. The coatings fail to meet the requirements for coatings of an argillic horizon or a Bh horizon.

It meets BOTH of the following:

  1. The coatings do not have a waxy lustre when dry or are not sufficiently thick to envelop fine sand grains. Silt coatings are excluded. (Silt coatings have similar colour to the matrix, or have higher value and/or lower chroma than the matrix. On drying they may be thick enough to meet argillic horizon requirements, and have flow-like surfaces, but they have a matt rather than a waxy lustre.) AND
  2. The coatings have moist colour values of 4 or less, or value 5 and chroma 3 or less. Many soils have horizons with coatings on peds or in pores which are very thin, do not have a waxy lustre when dry and have lower colour value than the matrix. It is difficult in the field to decide whether these coatings are inorganic or organic, and whether they are derived by illuviation from overlying horizons, by movement within horizons or by in-situ weathering. The cutanic horizon is designed to recognise such horizons. A cutanic horizon is usually given the horizon notation suffix (h).

Cutanoxidic Horizon

A cutanoxidic horizon (Wilson 1987) is a strongly weathered, clayey, low-activity-clay horizon. The dominant clays are kaolin group minerals, and clay coatings occur on less than 10% of ped faces. Exchangeable aluminium, as a percentage of CEC, is usually greater than 25% (and is frequently more than 50% in some part of the horizon). It has ALL the following characteristics:

  1. It meets the requirements of a cutanic horizon, AND
  2. It is a B horizon that is clayey and has fine polyhedral peds; AND
  3. Soil materials lack the characteristic friable failure over a wide range in moisture contents that is exhibited by oxidic horizons. The horizon has a failure class of friable only at water contents close to field capacity. Small changes in water content from field capacity result in large changes in soil strength and failure. Semi-deformable failure occurs at water contents wetter than field capacity. Very firm or stronger ped strength with brittle failure occurs at soil water matric potentials drier than about 30 kPa; AND
  4. Soil materials are sticky and very plastic, in comparison to oxidic materials which are only slightly sticky in relation to their clay content; AND
  5. Peds are larger than 2 mm and have smooth faces with silt-sized aggregates of iron oxide crystallites which give the ped faces a dusty appearance when dry. The latter property in particular distinguishes this horizon from horizons developed in well-drained Brown Soils.

The significance of this horizon lies in the combination of low ECEC, clay illuviation, acidity and “non-oxidic” physical properties.

Densipan

A densipan is a non-cemented eluvial horizon, denoted with the suffix d (e.g. Ed), of very high soil strength and bulk density. It meets ALL the following requirements:

  1. It is an eluvial horizon; AND
  2. Either
    1. Unconfined strength is hard or very hard at soil water states from near wet to dry, or
    2. Soil penetration resistance exceeds 4000 kPa at soil water states from near wet to dry; AND
  3. Moist and dry samples slake in water.

Densipans occur in soils with felsic parent materials. The strength is due to a close-fit arrangement of sand and silt-sized quartz particles. It differs from a duripan by lack of cementation.

Distinct Topsoil

A distinct topsoil (modified from Avery 1980) is normally designated an A horizon and has BOTH of the following:

  1. Moist colour value and/or chroma is less than that of the horizon below; AND
  2. Thickness is 5 cm or more (including any F, H or O topsoil layer). The distinct topsoil is used to distinguish minimal soil development in the distinction between Recent Soils and Raw Soils.

Duripan

A duripan is a subsurface horizon that is cemented by silica or other opaque or uncoloured material. It has ALL of the following requirements, but does not meet the requirements of the calcareous horizon:

  1. Dry fragments do not slake in water even during prolonged wetting; AND
  2. It does not react visibly with 10% HCl; AND
  3. The average lateral distance between any fractures is 100 mm or more.

The duripan is recognised in Pallic Soils where the cementing materials are apparently related to the presence of siliceous tephra in the parent material or high exchangeable sodium in the soil. A duripan is given the horizon notation suffix q (e.g. Bq).

Eluvial horizon

An eluvial horizon is a mineral horizon that, as a result of downward or lateral movement, contains less organic matter, pedogenic oxides, clay, or a combination of these than the horizon immediately below. The primary notation E is used for these horizons. Relative to an overlying H, O or A horizon, it contains less organic matter. The colour of the horizon can be that of uncoated sand and silt grains but E horizons are also recognised where coats of iron oxides or other minerals mask the colour of the primary particles. It has all the following characteristics:

  1. A moist colour value of 4 or more, or a dry colour value of 5 or more, with or without skeletans; AND
  2. Both
    1. A higher colour value or lower chroma combined with less well developed pedality or coarser texture than an underlying B horizon; AND
    2. A higher colour value than an overlying H, O or A horizon.

Fluvial Features

The intention of the term ‘fluvial features’ is to recognise soils with parent materials that result from transportation, sorting and deposition by water. Fluvial features are used to differentiate Recent Soils, Raw Soils and parent material classes of soil series that occur on landforms formed through fluvial processes.

Relevant landforms include floodplains, estuarine surfaces, lacustrine surfaces, aggregating fan surfaces, levees, backplains, bars, channels, deltas, floodplain benches, outwash plains and swamps (all defined by Milne et al. (1995)).

Confirming soil characteristics include:

  1. A buried A horizon or some other field indication of an irregular change in carbon with depth (such as sedimentary plant-leaf material).
  2. Sedimentary bedding in C (or transitional C) horizons, indicating deposition in water (such as scoured surfaces, cross-stratification, sedimentary laminations, current ripples or foreset beds).
  3. An unripened horizon with fluid, or very fluid, fluidity class in some layer with an upper surface within 120 cm of the soil surface.
  4. In tephric soil materials
    1. disturbance or overthickening of the regional sequence of tephras;
    2. rounded or subrounded rock fragments;
    3. presence of non-volcanic rock fragments.

The emphasis here on genetic landform criteria is not consistent with the principle that soils should be classified on the basis of similarity of measurable soil properties rather than presumed genesis (see Introduction). Measurable soil properties that will group together the required soils have not been recognised. The confirmatory soil properties, however, will aid class assignment decisions in many cases.

Fragipan

A fragipan is an apparently non-cemented subsoil horizon that has high bulk density (usually ≥ 1.5 Mg/m3) with high strength when dry. It is given the horizon notation suffix x (e.g. Bwx, BCx). It has ALL of the following:

  1. An air-dried clod must slake when fully immersed in water; AND
  2. Brittle failure when moist (the horizon may be dominated by rock fragments but the fine-earth material must be Sufficiently coherent to allow brittle failure); AND
  3. It has at least slightly firm moist soil strength; AND
  4. Either
    1. Extremely coarse or gross prismatic peds: the prisms have apedal-massive interiors, or break to secondary peds with horizontal dimensions of 100 mm or more, and the prism faces are defined by colours of chroma 3 or less, or
    2. The horizon is apedal-massive throughout, or has extremely coarse or gross prismatic peds, and the moist soil strength is very firm; AND
  5. If roots are present, they are confined predominantly to planar voids between prisms or to worm burrows; AND
  6. Penetration resistance under moist conditions is 3100 kPa or more; AND
  7. It does not occur within an Eluvial horizon.

Gley Profile Form

A gley profile form is defined by the presence of a reductimorphic horizon with an upper boundary within either 15 cm of the base of the A horizon or 30 cm of the mineral soil surface.

Soils with a gley profile form have usually been recognised as poorly or very poorly drained soils in the soil drainage classification of Taylor and Pohlen (1962).

Humus-Pan

A humus-pan is a B horizon that is 10 mm or more thick and is normally given the horizon designation Bhm.

It has ALL of the following requirements:

  1. It is apedal (massive); AND
  2. It has either firm or stronger moist soil strength, brittle failure when moist, or moist penetration resistance of 3100 kPa or more; AND
  3. It has dominant moist colour value in the matrix of 3 or less, or moist colour value of 4 if the chroma is 2; AND
  4. It contains more than 1.0% organic carbon.

Ironstone-Pan

An ironstone-pan is an indurated B horizon that is given the notation Bsm. It is dominantly composed of iron oxides with or without manganese oxides. It has ALL of the following characteristics:

  1. The upper boundary is distinct, abrupt or sharp; AND
  2. It is weakly or strongly indurated; AND
  3. Fresh fracture surfaces are black and have a metallic lustre; AND
  4. It forms a continuous horizon, or it is fractured into blocks of 100 mm (in horizontal dimension) or more; AND
  5. It is 10 mm or more thick.

Ironstone-pans commonly occur at a textural discontinuity, in the fluctuating zone of a water-table. It is likely that the iron has been precipitated from iron-rich groundwater moving laterally. In Taranaki (Childs et al. 1990), the pans are porous and some appear to have formed as iron-oxide rhizomorphs, which have been progressively infilled and welded together by further precipitations of iron oxide. In these pans the mineralogy is dominated by varying proportions of goethite and ferrihydrite.

Ironstone-pans are not usually associated with eluvial horizons and do not occur in Podzols. They differ from ortstein-pans and placic horizons which are often associated with eluvial horizons in Podzols or Brown Soils, and which have high organic carbon levels. Ironstone pans meet the requirements of the material below a petroferric contact of Soil Taxonomy (Soil Survey Staff 1999).

The pans are a barrier to plant roots. Heavy machinery is required to break them up for the installation of drains. The permeability of the pans is likely to be slow.

Lithic Contact

A lithic contact occurs at the contact of soil with underlying rock. The rock is hard or very hard and is impracticable to dig with a spade.

In situ rocks in New Zealand are commonly jointed at intervals of less than 100 mm, and consequently the lithic contact definition of Soil Taxonomy (Soil Survey Staff 1999) often fails to apply (Laffan 1979). The lithic contact is defined here as follows:

At a lithic contact, rock fragments accommodate one another with non-random orientation with respect to any geological structure that may be present, and cracks or joints are mostly less than 5 mm wide.

The lithic contact may be subdivided into coherent-lithic or shattered-lithic materials.

Coherent-lithic materials are equivalent to materials beneath the lithic contact of Soil Taxonomy. Cracks or joints occur at horizontal intervals of more than 100 mm. They often cause drainage impedance.

Shattered-lithic materials are similar except that joints or cracks may occur at intervals of less than 100 mm. Shattered-lithic materials differ from fragmental or skeletal materials in which there is no continuity of geological structure between adjacent rock fragments, and rock fragment faces do not accommodate one another. Shattered-lithic materials are more permeable than coherent-lithic materials, and offer a significant rooting volume.

Mottled Profile Form

A mottled profile form is defined by EITHER

  1. A redox-mottled horizon with an upper boundary within 15 cm of the base of the A horizon, or within 30 cm of the mineral soil surface; OR
  2. A reductimorphic horizon with an upper boundary between 30 and 60 cm of the mineral soil surface.

Soils with a mottled profile form have usually been recognised as imperfectly drained soils in the soil drainage classification of Taylor and Pohlen (1962). Redox mottles are formed as a result of the reduction and solubilisation of iron and/or manganese, their translocation and concentration, and their oxidation and precipitation in the form of oxides. Mottles that have originated in some other way (e.g. rock colour patterns or skeletans) are excluded.

Nodular Horizon

A nodular horizon has more than 15% (by volume) nodules, as segregations of iron or aluminium oxyhydroxides, with some kaolinite, in a layer more than 10 cm thick. Nodular horizons are given the horizon notation suffix i (e.g. Boi, BCi).

The nodules are common features in Oxidic Soils and some Granular Soils (Wilson 1987). The frequency distribution of nodules is clustered in the < 2% range and the > 15% range. Few profiles are known to lie in between.

The nodular horizon limit is intended to exclude thin layers of rewashed nodules on colluvial footslopes, and also infrequent localised concentrations in soils with characteristically few nodules.

Organic Soil Material

Organic soil material is soil material dominated by organic matter, excluding fresh litter (L horizons) and living plant material. Organic soil material usually has at least 18% organic carbon (approximately 30% organic matter) but it is defined here using morphology and simple analyses for easier recognition. (For most New Zealand soils, organic carbon may be estimated by total carbon.)

Organic soil material has EITHER

  1. All of the following:
    1. Colour value moist of 3 or less (after exposure to air) and colour value dry of 4 or less, AND
    2. Deformable failure, AND
    3. Weight loss of 65% or more by oven-drying a field-saturated sample; OR
  2. More than 20% (by volume) unrubbed fibre content; OR
  3. More than 35% (by weight) loss on ignition except in materials dominated by allophanic soil material or by limestone; OR
  4. 18% or more total carbon.

Unrubbed fibres are pieces of plant tissue large enough to be retained on a 100-mesh (0.15 mm) sieve, except for wood fragments that cannot be crushed or shredded in the hand and are larger than 2 cm in the smallest dimension. Rubbed fibre is the fibre that remains after rubbing a wet sample 10 times between the thumb and forefinger, or kneading a ball in the palm 10 times using firm pressure.

Organic soil materials that have been accumulated under wet conditions are subdivided into three classes, based on evidence of decomposition (Clayden and Hewitt 1989). These classes are used to distinguish soil groups of Organic Soils.

Fibric soil material (Of horizon) consists mainly of well preserved plant remains that are readily identifiable in terms of botanical origin. The fibre content after rubbing is at least 75% by volume.

Mesic soil material (Om horizon) consists mainly of partially decomposed plant remains (semi-fibrous peat or hemic soil material) and does not meet the requirements of either fibric soil material or humified soil material.

Humified soil material (Oh horizon) consists of strongly decomposed organic material (humified peat or sapric soil material) with few or no identifiable plant remains other than resistant woody fragments > 20 mm that cannot be reduced to fibres by crushing and shredding between the fingers. The fibre content is less than 15% after rubbing.

Ortstein-Pan

An ortstein-pan is a B horizon that is normally given the horizon notation Bsm. It has ALL of the following requirements:

  1. Thickness of more than 10 mm; AND
  2. The upper boundary is sharp or abrupt; AND
  3. It is massive and has either firm or stronger moist soil strength, or has moist penetration resistance of 3100 kPa or more; AND
  4. It does not meet the requirements of a humus-pan, and does not have the metallic lustre of fresh fracture surfaces of an ironstone-pan.

Oxidic Horizon

The oxidic horizon is a strongly weathered B horizon consisting of mixed crystalline iron and aluminium oxides and kaolin group minerals, with low activity clay properties. An oxidic horizon is given the notation suffix o (e.g. Bo). It has ALL the following requirements:

  1. Weak or very weak primary ped strength and soil strength as determined by the unconfined resistance-to-crushing test at moist to dry soil water states; AND
  2. Unconfined failure is friable or very friable over very moist to dry soil water states. Materials fail to predominantly 3 mm or smaller peds comprising silt- and sand-sized polyhedra and spheroids; AND
  3. Primary peds slake rapidly in water to stable microaggregates which show no dispersion or slight dispersion after 100 inversions using the method of McQueen (1981); AND
  4. Non-reactive or very weakly reactive to the reactive-aluminium test (Fieldes and Perrott 1966). Oxidic horizons are clayey, with measured clay contents commonly exceeding 60%. The measured clay percentage is usually larger than in overlying A horizons, but clay increase is not a defining criterion because of the problem of quantifying clay contents in materials that are frequently difficult to disperse.

Materials are only slightly sticky and plastic in relation to clay content. Clay coatings are either visually absent or only present at frequencies of about 1%. The oxidic horizon has low activity clay accessory properties with ECEC and CEC less than 12 and 16 cmolc/kg clay respectively.

Paralithic Contact

A paralithic contact is the upper surface of rock or regolith material that has ALL of the following requirements:

  1. It can be cut with difficulty with a spade; AND
  2. Wet penetration resistance exceeds 2600 kPa; AND
  3. Roots if present are few and confined to cracks; AND
  4. If the overlying horizon is a reductimorphic or redox-mottled horizon, low chroma or high chroma mottles are less common below the contact.

The paralithic contact meets the definition of a paralithic contact of Soil Taxonomy (Soil Survey Staff 1999), but without the restrictive requirement for spacing of cracks. The horizon beneath the contact is given the horizon designation CR. Paralithic contacts may occur either on weakly weathered or unweathered rocks which are not strongly lithified, or on saprolites which have become soft by strong weathering.

Peaty Topsoil

A peaty topsoil is 10 cm or more thick and is saturated for 30 or more consecutive days in most years (unless it is artificially drained), and has EITHER

  1. Peat, sandy peat or loamy peat texture, OR
  2. Slightly peaty texture (17–30% organic matter) if the clay content is less than 18%.

In some subgroups a peaty topsoil may be buried by a surface mantle of new material of up to 60 cm in thickness.

Perch-Gley Features

Perch-gley features are the morphologic indicators of saturation and reducing conditions caused by a water-table perched on a slowly permeable layer within the soil profile.

A horizon with perch-gley features EITHER

  1. Has redox-segregations that occur mainly within peds, or in the case of an apedal soil, mainly within the soil matrix. Macro-void surfaces, either partings or pores, are dominated by greyish colours (moist chroma 2 or less, or moist chroma 3 and value 6 or more). Iron and manganese precipitates occur either adjacent to the greyish void surfaces as a selvedge or as discrete mottles within the soil mass; OR
  2. Overlies a horizon that is less gleyed (e.g. less redox-segregations) and that has a matrix not dominated by greyish colours.

Placic Horizon

A placic horizon is a thin iron pan that is normally designated Bfm. It has ALL of the following:

  1. It is 10 mm or less thick; AND
  2. It is at least weakly indurated, and is black to reddish brown or dark red in colour. A black upper part can often be distinguished from a reddish brown lower part; AND
  3. The upper and lower boundaries are sharp, and may be smooth, wavy or convolute in shape.

The placic horizon usually occurs as a single pan but in places can be bifurcated. It is equivalent to the placic horizon of Soil Taxonomy (Soil Survey Staff 1999) except that New Zealand iron pans are enriched in iron and organic matter without significant accumulations of manganese (Clayden et al. 1990).

Podzolic-B Horizon

A podzolic-B horizon is a B horizon that meets one of the following:

  1. It meets the requirement of a Bh horizon (because it has colour value and chroma of 3 or less, or value 4 and chroma 2, dominant in the matrix, and contains more than 1% organic carbon). The fabric has sand- or silt-size pellet-like aggregates, coats on mineral grains, or both; OR
    1. It is associated with an overlying (but not necessarily immediately overlying) Eluvial horizon in which weathered films on sand and silt particles are either absent, very thin or discontinuous, so that the colour of the horizon is mainly determined by the colours of the uncoated grains. The moist colour value of the Eluvial horizon is 4 or more (or a dry colour value is 5 or more). It has higher colour value or lower chroma and less well developed pedality than an underlying B horizon; AND
    2. The B horizon (or some part of it) is 5 cm or more thick and meets the requirements of a Bs (or Bs(g) or Bs(f)) horizon because it has a strong or very strong reactive-aluminium test, and at least one of the following:
      1. Reddish hue and highest chroma at the top of the horizon; or
      2. Earthy apedal with fine spheroids, or weakly developed blocks or polyhedra; or
      3. Very weak or weak soil strength when moist or dry; or
      4. Sand- or silt-sized pellet-like aggregates; OR
  3. It meets the definition of a Bs horizon (part 2(b) above) and has in addition, coatings of value 4 or less either
    1. On 50% or more ped faces, or
    2. As patches covering 20% or more of cut faces.

Reactive-Aluminium Test

This test indicates the presence of reactive hydroxy-aluminium groups, as occur for example in allophane and aluminium-humus complexes (Milne et al. 1995).

Using the procedure of Fieldes and Perrott (1966), 1 drop of saturated sodium fluoride (NaF) solution is placed on a small test sample of soil, which has been smeared on to a filter paper treated with phenolphthalein indicator. The soil sample must be field moist. For classification, the reactivity of the soil sample is placed into one of the following classes.

Table 4.2: Sodium fluoride reaction ratings
Code Name Description Colour
N No reaction No colour change Circle
L Slight reaction Paper turns faint pink within two minutes Circle
M Moderate reaction Paper turns moderate pink, usually within 30 seconds Circle
H Strong reaction Paper turns dark pink, usually instantly Circle

Redox-Mottled Horizon

A redox-mottled horizon is a horizon affected in parts by reducing conditions as indicated by the presence of redox-segregations. These usually indicate intermittent saturation of the soil by water. A redox-mottled horizon is given the notation suffix (f) if no low-chroma colours are present, or (g) if they are (e.g. Bw(f), B(g), BC(g)).

A redox-mottled horizon has 2% or more redox segregations. If low chroma colours (moist chroma 2 or less, or moist chroma 3 with value 6 or more) occur, they must occupy less than 50% of the matrix exposed in a cut face of the horizon and are not dominant on ped faces.

The intermittent wetness may be caused by intermittent perched water, or by the fluctuating upper limits of deeper more prolonged groundwater. A redox-mottled horizon may represent more prolonged saturation and reduction in parent materials that are predominantly andesitic or basaltic compared with other parent materials.

Redox Segregations

Redox segregations are mottles or concretions formed as a result of the reduction and solubilisation of iron and/or manganese, their translocation, concentration, and their oxidation and precipitation in the form of oxides Clayden and Hewitt (1989). They may occur as low or high chroma colours, or both.

The nature of the water table is indicated by the association of low and high chroma colours. If subject to reduction by perched water, the low chroma colours are likely to be at ped or pore surfaces and the high chroma colours are likely to be within the soil matrix. If the soil is subject to reduction by groundwater, the low chroma colours are likely to be within the soil matrix and the high chroma colours are likely to be at ped or pore surfaces. Reducing conditions may also be indicated by the presence of sufficient ferrous iron to give a positive reaction to α,α’-dipyridyl (Childs 1981).

Reductimorphic Horizon

A reductimorphic horizon has a slightly peaty texture class, or low chroma colours (moist chroma 2 or less, or moist chroma 3 with value 6 or more) that occupy 50% or more of the matrix exposed in a cut face of the horizon or are dominant on ped faces. A reductimorphic horizon includes any subjacent layers or interlayers of peaty soil material.

A reductimorphic horizon is a horizon strongly affected by reducing conditions as indicated by greyish colours consistent with long saturation by water. The prolonged wetness may be caused by a water-table perched on a slowly permeable layer within the soil profile or by a groundwater-table. A reductimorphic horizon is given the horizon notification g (if more than 2% mottles are present, e.g. Bg, BCg, Cg) or r (if the horizon is almost or completely reduced, e.g. Br, BCr).

Saline soil material

Soil salinity refers to the accumulation of water-soluble salts, mainly of sodium, calcium and magnesium. These can severely affect plant growth and land use, and increase soil erosion (Hazelton and Murphy 2016, p. 95).

Most soils generate some salt as a product of chemical weathering, with composition and concentration determined by the parent material lithology. Soils may also receive additional salt at their surface, transported by wind and/or rain. Some soils are affected by saline groundwater from marine or hydrothermal sources.

Under higher rainfall regimes and under good drainage conditions, these salts are leached from the soil (Hewitt et al. 2021). Where rainfall is too low to remove salt (or a profile is in a low-lying, water-receiving location), salts can accumulate and may ultimately precipitate as crystals in the profile and/or on the soil surface.

Soil salinity is somewhat difficult to measure in the field in ways that can be strongly related to plant growth (DERM 2011). As a compromise, the electrical conductivity (EC) of a 1:5 solution of soil to deionised water is measured. Results exceeding 0.8 mS/cm within in the top 60 cm of the soil profile are considered significant enough to be considered in the classification of Gley, Recent and Semi-Arid soil orders. Exceedences within 30 cm of the surface are recognised in the classification of Raw soils.

Note that salinity at any given location (including within the profile) is expected to be highly variable across seasons, and will generally be at its maximum towards the end of a dry summer.

Slowly Permeable Layer

A slowly permeable layer is one in which the vertical saturated hydraulic conductivity is less than 4 mm/h (1.0 × 10–6 m/s) as measured by a standard method. If no measurement is available then the horizon can be identified by the following morphological characteristics (Griffiths 1991):

A slowly permeable layer meets EITHER

    1. The soil material is pedal; AND
    2. More than half of the peds are coarser than 10 mm (mean of the x and y axes in a horizontal plane) and meet one of the following:
      1. Peds 20 to 50 mm, with degree of packing at least extremely high; or
      2. Peds 50 to 100 mm with degree of packing at least very high; or
      3. Peds 100+ mm with degree of packing at least high; OR
  2. Either
    1. The soil material is sand or loamy sand and has an extremely high degree of packing; or
    2. The soil material has a particle-size group other than sandy and has at least a high degree of packing.

A slowly permeable layer is significant for the land use, genetic and hydrological understanding of soils. It may overlap a number of other diagnostic horizons — for example, the fragipan, argillic horizon, densipan, humus-pan or ortstein horizons and lithic or paralithic contacts.

Sodic Features

A horizon with sodic features has significant exchangeable sodium. This is not necessarily related to a high soluble salt content. A sodic horizon is given the notation suffix n (e.g. Btn, BCn). It has BOTH

  1. Either
    1. Exchangeable sodium percentage of 6% or more (or exchangeable sodium is 0.7 cmolc/kg or more); or
    2. When a 10 mm diameter sample which has been air-dried is placed in distilled water or salt free water a cloud of dispersed clay will form within 10 minutes around the sample. This test will not apply if the soil is in any degree cemented; AND
  2. Either
    1. Clay or clay/organic coatings have colour value 4 or less; or
    2. Prismatic or blocky peds; or
    3. It may be overlain by an Ew, Ew(g) or Ew(f) horizon, or have skeletans (visible on dry ped faces) near the top of the horizon.

Tephric Soil Material

Tephric soil material occurs in or below the soil solum. It includes:

  1. Tephra — unconsolidated, primary pyroclastic products of volcanic eruptions (Froggatt and Lowe 1990) such as ash, cinders, lapilli, pumice, scoria, blocks, or volcanic bombs, and
  2. Tephra deposits — material derived at least partly from tephra that has been reworked and mixed with material from other sources. They include tephric loess, tephric blown sand and volcanogenic alluvium. As a general guide, tephric deposits from andesitic sources have more than 10% volcanic glass in the sand fraction and those from rhyolitic sources have more than 40% volcanic glass in the sand fraction.

Tephric soil material may include soil materials that meet the requirements of allophanic soil material or vitric soil material. It is used to distinguish soil groups of the Raw Soils and Recent Soils, and parent material classes at soil series level.

Vitric Soil Material

Vitric soil material (Parfitt 1984) has more than 35% coarse-fraction (2 mm or greater, by volume) of which 60% is pumice or cinders, or there is more than 40% sand of which more than 30% is volcanic glass (or crystals coated with glass) (Eden 1992).

Weathered-B Horizon

A weathered-B horizon shows evidence of chemical alteration of the original parent material and is given the notation suffix w (e.g. Bw). It has at least ONE of the following:

  1. Redder hue or higher chroma than an underlying horizon in similar materials; OR
  2. Have spheroidal, blocky, polyhedral, tabular, prismatic, columnar or platy pedality which distinguish the horizon from a BC or C horizon below; OR
  3. Evidence of either partial or complete decalcification, i.e. less CaCO3 than the underlying horizon which may contain redeposited carbonates.

A weathered-B horizon may also meet the requirements of a redox-mottled horizon, argillic horizon, cutanic horizon, or brittle-B horizon.