16  Secondary features

Modified

September 26, 2025

Secondary soil features are bodies formed within the soil through pedogenic (natural soil-forming) processes. They contain a higher amount of a particular substance (or association of substances) and thus contrast with the surrounding material. Many of these features create barriers to water movement or alter chemistry in a way that affects both soil biota and plant root growth.

16.1 Concentrations

Concentrations are secondary features that are not associated with specific surfaces within the soil. Some concentrations contrast sharply with the surrounding material due to differences in colour, hardness, or composition, while others may be more subtle.

For rapid assessment, record concentration form and abundance percentage using Table 16.1.

For detailed assessment, note form, type, abundance in percentage, and size range in millimetres. Colour is not required as it is usually determined by type and may have already been recorded as a colour pattern (Section 14.3). Abundance and size can be classified using Table 21.1 and Table 21.4 respectively.

Example: NO F 10% 6-8 mm for scattered small ironstone nodules.

Table 16.1: Concentration forms
Code Name Description
CC Concretion Hard concentrations with a layered internal structure, usually rounded
NO Nodule Hard concentrations with no internal structure, usually irregular
CT Crystal A mineral arranged as interlocking or twinned macroscopic crystals or as a single macroscopic crystal, e.g., gypsum.
SS Soft masses A non-cemented concentration that can seldom be removed from the soil as a discrete unit
Table 16.2: Concentration types (adapted from National Committee on Soil and Terrain (2024))
Code Name Description
E Earthy Loamy or silty texture within a sandy or clayey matrix
A Argillaceous Clayey texture within a lower-clay matrix
F Ferrugineous Iron-dominated minerals
M Manganiferous Manganese-dominated minerals
N Ferromanganiferous Dominated by a mix of iron and manganese
K Calcareous Dominated by calcium carbonate (CaCO3)
Y Gypseous Dominated by calcium sulfate (gypsum, CaSO4)
Z Salty Dominated by salts more soluble than Calcium carbonate or calcium sulfate
S Sulfurous Dominated by sulfur-bearing minerals, e.g. jarosite in acid sulfate soils
O Organic Organic texture in a low-organics matrix
UK Unknown Composition cannot be reliably determined

16.2 Coatings

Coatings are layers of material that form on the surfaces of other soil features, e.g. grains, peds, or the walls of voids. Some coatings are formed by chemical precipitation, similar to concentrations, while others form as a result of selective removal processes, where the material left behind accumulates. Some coatings may be biological in origin (e.g. some ant species line their burrow tunnel walls with fine material).

To record coatings, note their type using the options in Table 16.3 and their abundance as percentage of soil surfaces covered. For rapid assessment, only record the most abundant or well-developed coating.

e.g. OG 80% for well-developed organic coats covering most ped surfaces.

Table 16.3: Types of surface coating
Code Name Description
CC Clay coats Appear to have more clay than the soil matrix. They are often different in colour from the adjacent matrix and are usually recognisable in sandy or loamy soils, but can be difficult to recognise in clayey soils.
ZC Silt coats Silt grains concentrated at surfaces; they may be residual concentrations resulting from clay removal, or may have moved in from horizons above
SC Sand coats Sand grains concentrated at surfaces; they may be residual concentrations resulting from silt and/or clay removal, or may have moved in from horizons above
OG Organic coats Surface features that have a moist colour value of 4 or less and are rich in organic matter compared to the interior of the solid.
SQ Sesquioxide coats Films of sesquioxides on interfaces or impregnating interfaces. They vary in colour depending on the composition and degree of oxidation and hydration. They are difficult to identify chemically, and the different kinds cannot normally be distinguished in the field. Ferri-manganiferous coats, however, are normally very dark brown or black and effervesce vigorously with hydrogen peroxide (H2O2)
MN Manganese coats Dark to black coatings of precipitated manganese oxides, reactive to H2O2.
CB Carbonate coats Precipitated calcium carbonate (CaCO3). Usually pale-coloured, and reactive to HCl.
SI Silica coats Precipitated silica (SiO2). Usually pale-coloured, and non-reactive to HCl.

16.3 Pans

Pans form when particular chemical compounds precipitate out of the soil solution and cement a layer of mineral particles (perhaps as a further accumulation and joining together of coatings and/or concentrations), or when particular particle size distributions and/or shape range become very tightly packed. Many pans are diagnostic layers in the New Zealand Soil Classification, because they can form a barrier to plant root growth or water movement.

For rapid assessment, only record pans that are strongly developed enough to be classification-relevant. A classification-relevant pan is one that is developed enough to meet all of the relevant diagnostic requirements listed in the New Zealand Soil Classification. Record the pan type using Table 16.4.

Many (but not all) pans have dedicated conventional horizon names. The full definitions of those horizons are available in Section 20.1.

Pans can be described in detail in terms of type, thickness, continuity, structure, strength and cementation. Pans comprise distinct horizons, so their thickness is defined using horizon boundaries. Their strength and cementation are assessed using Table 17.1 and Table 17.9. Codes for type, continuity and structure are given below in Table 16.4, Table 16.5, and Table 16.6. Note that continuity can only be observed in large pits or exposures.

Table 16.4: Types of pan
Code Name Description Horizon
C Cultivation pan Pan comprising compressed soil, induced by cultivation (e.g. use of heavy farm machinery)
D Densipan Silty layers with a very high bulk density and extremely slow permeability *d (E horizons only)
H Humus-pan Cemented by aluminium and organic matter. Common in some Podzol Soils; sometimes known as ‘coffee rock’ Bhm
F Iron pan Interlocking nodules of Fe(/Mn) precipitates (bog iron), often at a texture contrast boundary
K Calcareous pan Created by the precipitation of calcium carbonate (CaCO3) *km (B or BC horizons)
O Ortstein pan Pan cemented by iron and organic matter. Common in some Podzol Soils Bsm
P Placic A very thin (< 10 mm), dark, iron-dominated pan with sharp boundaries Bfm
Q Silica pan Created by the precipitation of silica (SiO2) *qm (not A horizons)
X Fragipan An unstructured, uncemented but high-density, erosion-resistant layer common in many Pallic Soils *x (B or BC horizons)
UK Unknown Pan origin cannot be clearly determined
Table 16.5: Pan continuity (National Committee on Soil and Terrain 2024)
Code Name Description
C Continuous Extends as a layer with little or no break across 1 m or more.
D Discontinuous Broken by cracks but original orientation of fragments is preserved.
B Broken Broken by cracks and fragments are disoriented
Table 16.6: Pan structure (adapted from National Committee on Soil and Terrain (2024))
Code Name Description
L Solid Either no recognisable structure, or close-packed peds > 100 mm
S Vesicular Sponge-like structure having large pores, which may or may not be filled with softer material
C Concretionary Spheroidal concretions cemented together
N Nodular Irregular nodules cemented together
P Platy Plate-like units cemented together

16.4 Stress features

Stress features develop along fissures or on ped faces in soils that shrink and swell as water content changes. Stress features are usually shiny but do not differ in colour from the soil matrix. Magnification may be required to observe them clearly. Stress features will not develop in sandy soil textures (see Table 15.2).

Stress features can be optionally noted during a detailed soil description. Describe type using Table 16.7 and abundance as integer percentage of ped faces covered.

Example: SL 30% in a well-developed Vertic Melanic soil.

Table 16.7: Types of pressure face
Code Name Description
SL Slickensides Smooth faces with linear grooves, from peds sliding past each other.
PF Pressure faces Smooth planar faces from peds pressing against each other without sliding.

16.5 Biological features

Biological secondary features are mostly created by plant roots as they grow into the soil, and soil fauna as they create and infill burrows.

For rapid assessment, note type(s) using the codes in Table 16.8.

For detailed assessment, also note the areal percentage of the exposed horizon face occupied by the feature.

Example: T 30% for infilled earthworm burrows creating an A/Bw horizon.

Table 16.8: Types of secondary biological feature
Code Name Description
T Tunnel Infilled earthworm or insect burrow, < 20 mm in diameter
B Burrow Infilled former macrofauna burrow, > 20 mm in diameter
C Casts Earthworm wastes forming balls of fine soil material

16.6 Cryofeatures

Cryoturbation refers to the physical disturbance and mixing of soil caused by repeated freezing and thawing. It commonly occurs in soils exposed to seasonal or historic freeze and thaw cycles. Affected soils may show discontinuous horizon boundaries and physical mixing as a result of intense shrink-swell activity caused by ice formation. Freezing and thawing can also accelerate downslope movement (solifluction), often incorporating angular, poorly sorted rock fragments into the soil. These features can be recognised by examining horizon boundary characteristics (Section 12.3, Section 12.4), colour patterns from mixing (Section 14.3.1), and rock fragment characteristics (Section 13.2.1).