23  NZSC Family and Sibling

Levels 4 and 5 of the NZSC, the Family and Sibling, provide useful summaries of key whole-profile parameters. As the classification system has evolved slightly since its initial publication Webb and Lilburne (2011), the current version is reproduced in an abridged format here.

23.1 NZSC Family

23.1.1 Soil Profile Material

The soil profile material concept is a brief summary of the overall character of the profile, in particular its mineral and stone content and major erosional/depositional origin. Some information on the likely range of bulk density values is implicit.

Table 23.1 below defines the available classes and should be used as a key. For Organic soils, skip the ‘M’ codes and start at Sl. Choose one code only per profile. Applying the key accurately requires familiarity with the major NZSC Diagnostic Criteria. Links to glossary definitions are provided below, but the source material (Hewitt and MWLR Pedology Staff, 2024) should also be consulted.

Table 23.1: Soil Profile Material

Code	Name	Description
Mf	Fragmental	Rock fragments with minimal fine earth from within 20 cm to >60 cm depth
Ml	Lithic	Other soils with a coherent- or shattered lithic contact within 45 cm
Mp	Paralithic	Other soils with a paralithic contact within 45 cm
Mt	Tephric	Other soils with at least 30 cm of tephric soil material within 100 cm
Mr	Rounded-stony	Other soils with ≥ 35% by volume rounded or sub-rounded rock fragments from within 45 cm to >100 cm.
Ma	Angular-stony	Other soils with ≥ 35% by volume angular or sub-angular rock fragments from within 45 cm to >100 cm.
Mm	Moderately deep on rock	Other soils with a lithic or paralithic contact at <100 cm.
Mg	Moderately deep on rock fragments	Other soils with ≥35% by volume rounded or sub-rounded rock fragments from within 45–90 cm, extending continuously to 100 cm with a cumulative thickness of ≥50 cm (unless over rock).
Md	Stoneless	Other soils with weighted average <3% rock fragments by volume in the top 100 cm.
Ms	Stony	Other mineral soils.
Sl	Organic-lithic	Organic soils with a coherent- or shattered lithic contact within 45 cm.
Sp	Organic-paralithic	Other organic soils with a paralithic contact within 45 cm.
Sd	Organic-deep	Other organic soils with continuous peat throughout the top 100 cm.
So	Organic-other	Other organic soils.

Profile material class places restrictions on other parts of the family and sibling definition. These are discussed as they arise below.

23.1.2 Rock class

23.1.2.1 Rock class of stone and bedrock

Rock class of in-profile stone and of bedrock (where encountered) uses the lithology codes in Table 5.1, with specific restrictions around how they are applied to the whole profile. These are:

• Md soils effectively have no stone or bedrock. Leave the property undefined.
• Rock class is only defined in reference to stones >2 mm and bedrock (where encountered within 100 cm). The rock class of the fine earth fraction is accounted for below.
• The ‘deep and stoneless’ classes (Mt, Md, So and Sd) can still have a total of <3% by volume stones in the top 100 cm.
• Include pumice in rock class only where pumice strength is more than ‘extremely weak’ (Lynn and Crippen 1991, p. 10) - i.e., it can only be broken by hand with difficulty. If the pumice’s lithology is uncertain, use the dominant lithology of the probable volcano of origin (e.g. Rh in the Taupō Volcanic Centre and An around Taranaki).
• Where in-profile stone lithology contrasts with bedrock lithology, two codes separated by ‘/’ can be used e.g. Sc/Gw.
• Where two lithologies are close to co-dominant in a stony profile, two codes separated by ‘+’ can be used e.g. An+Rh.
• More complicated combinations are not allowed, e.g. no An+Rh/Gw.

23.1.2.2 Rock class of fines

The same rock class codes in Table 5.1 can be applied to report the dominant lithology of the fine earth (<2 mm) fraction of the full profile. Again, some restrictions apply:

• Sd soils and Mf soils effectively have no fine earth fraction. Leave the property undefined.
• Where a lithological discontinuity exists in the profile, two codes separated by ‘/’ can be used e.g. Sc/Gw.
• Where two lithologies are close to co-dominant, two codes separated by ‘+’ can be used e.g. An+Rh.
• More complicated combinations are not allowed, e.g. no An+Rh/Sm+Ss/Gw.

23.1.3 Family texture class

The family texture class describes the dominant character of the top 60 cm, or to the profile material class-defining rock or gravel layer contact if that is shallower. Use the codes in Table 23.2. Note that 18% organic carbon corresponds to approximately 30% organic matter.

Table 23.2: Family texture classes

Code	Name	Organic Carbon %	Silt %	Clay %
c	Clayey	<18	-	≥35
s	Sandy	<18	<40	<8
l	Loamy	<18	<40	≥8–<35
z	Silty	<18	≥40	≥8–<35
p	Peaty	≥ 18	-	-

23.1.4 Permeability class

This class estimates the permeability of the top 100 cm in units of mm/hr. Use the codes in Table 23.3 and follow the rules in Figure 23.1 to assign permeability class.

Table 23.3: Family permeability classes

Code	Name	Description
s	Slow	<4 mm/hr
m	Moderate	≥4–<72 mm/hr
r	Rapid	≥72 mm/hr

Follow the flowchart to assign family permeability class:

%%{ init: { 'flowchart': { 'curve': 'natural' } } }%%
graph TD
  A["Identify the uppermost horizon with the slowest permeability, or rock above 100 cm. This is Horizon X."]-->B["Does Horizon X have > 30 cm of contrasting permeability above?"]
  B--->|Yes|C("Use **m/s**, **r/s**, **r/m**")
  B--> |No|D["Does the profile have a horizon ≥ 20 cm thick below Horizon X with a different permeability class?"]
  D-->|Yes|E("Use **s/m**, **s/r** or **m/r**")
  D-->|No|F("Use **s**, **m**, or **r**")
  

Figure 23.1: NZSC Family permeability flowchart

Some additional rules apply for particular soil profile material classes:

• For soils with an S* profile material class, assess family permeability class from 20–100 cm or to rock (i.e. ignore the surface 20 cm).
• Soils with profile material class Ml or Mp and any soil with an M root barrier must use m/s or r/s.
  • Exception: Ml with F barrier can’t be */s.

23.1.5 Parent material origin

23.2 NZSC Sibling

These codes help define level 5 of the NZSC, the soil sibling.

23.2.1 Soil Depth

Soil depth from a sibling perspective is the depth to where digging becomes difficult, e.g.

• A horizon with ≥ 35% stones by volume (V or X functional horizons)
• A soft- or hard-rock surface (M rooting barrier, Section 23.2.5.2)
• A shattered-lithic contact (F rooting barrier, Section 23.2.5.2)
• A pan (Q functional horizons)
• Firm strength with massive or coarse structure (*Cf functional horizons)

Use the codes in Table 23.4 to assign a depth class to a profile.

Table 23.4: Sibling depth classes

Code	Name	Description
d	Deep	>100 cm
md	Moderately deep	≥45–<100 cm
s	Shallow	≥20–<45 cm
vs	Very Shallow	<20 cm

23.2.2 Topsoil stoniness

Topsoil stoniness assesses the amount of rock fragments (as % by volume) in the top 20 cm of the soil profile, including those resting on the soil surface.

A weighted average calculation is required if the top 20 cm contains more than one horizon. As an example,

Table 23.5

Horizon Thickness (cm)	Relevant thickness (cm)	Horizon proportion of top 20 cm	Horizon stone (%)	Weighted average topsoil stone (%)
12	12	0.6	15	9
20	8	0.4	25	10
				19

Use the codes in Table 23.6 to assign stoniness.

Table 23.6: Sibling topsoil stoniness classes

Code	Name	Description
1	Stoneless	<1 %
2	Slightly stony	≥1–< 5 %
3	Moderately stony	≥5–< 35 %
4	Very stony	≥35 %

23.2.3 Sibling texture classes

These classes are assessed over the top 100 cm, or to rock or gravel layer contact if those are shallower. Note that this is deeper than the family-level texture control section. The following rules apply:

• Mf, Mr, Ma, Ml, and Mp soils must match their family texture code (c, s, l, or z).
• Sl and Sp soils must use one of Tp, Tc, Tl or Ts to align with their family texture code of p.
• Sd soils must use Tp.
• Upper and lower textures can be defined for other soil profile classes, but only the dominant two are recorded. Both contributing layers must be at least 20 cm thick.
• When there are more than two texture layers within the control section, then the texture profile is identified according to the uppermost texture contrast.
  • For example, if a sandy horizon occurs at 80 cm depth and a clayey horizon overlays a silty horizon at 40 cm, then the texture contrast is reported as clayey over silty (c/z) and the sandy layer is ignored.
• Silty and loamy horizons are not considered contrasting and can be added together where contiguous.
  • For example, a profile with silty (0–15 cm) over loamy (15–25 cm) over sandy (25-90 cm) is considered to have a contrasting upper layer of 25 cm, and is identified as silty over sandy (z/s). The texture specification uses the uppermost of the silty or loamy layers together with the lower layer.
• Skeletal horizons (k) may only be used for Ms, Mm, Mt and So soils. The skeletal material must be non-tephric.
• Skeletal horizons in moderately deep gravelly soils (Mg) are ignored in favour of describing the overlying fine sediments.

Use the codes in Table 23.7 to assign sibling level texture class(es).

Table 23.7: Sibling texture classes

Code	Name	Organic Carbon %	Silt %	Clay %	Stone %
k	Skeletal				≥35
Tp	Peat or litter	≥30			<35
Tc	Clayey peat	≥18–<30		≥35	<35
Tl	Loamy peat	≥18–<30		≥8–<35	<35
Ts	Sandy peat	≥18–<30	<40	<8	<35
c	Clayey	<18		≥35	<35
s	Sandy	<18	<40	<8	<35
l	Loamy	<18	<40	≥8–<35	<35
z	Silty	<18	≥40	<3	<35

23.2.4 Drainage

Profile drainage is determined from horizon depths and per-horizon drainage ratings completed according to Section 21.1. A single drainage rating is selected for the sibling.

23.2.5 Rooting barriers

Rooting barriers may be identified in some soils, potentially determining their Family and Sibling codes. Where present, nominate the most limiting barrier using the codes in Table 23.8 or Table 23.9. Otherwise use N to confirm no barrier.

23.2.5.1 Horizon barriers

Table 23.8: Horizon-specific root barriers (Lilburne et al. 2021)

Code	Name	Description
C	Chemical	Chemical toxicity affecting pasture growth, that is not readily alleviated through management. Not salinity.
D	Densely packed gravels	Has very dense particle packing (4) Requires a V*d FH Requires s in family permeability
G	Extremely gravelly	Has at least 70% gravels by volume and a sandy matrix, or is a very stony horizon with a clean sand matrix Requires ≥85% sand and FH with VAl, XA, or Xx |
L	Extremely dense	Penetration resistance of >3000 kpa, or packing density of >1.85 mg/m3 | Requires FH with d, YC, VLc, VAc , or z Requires s in family permeability
P	Pan - continuous	Meets the diagnostic criteria for any NZSC pan Requires FH with: Q, YC, VAc, VAd, LCf, VLc, VLd Requires NZSC to mention pan status (cemented, placic, ironstone, duric, ortstein subgroups) Requires s in family permeability
S	Clean sand	Requires ≥85% sand and FH with Aa or Al – can be S but not V or X |

NoteNotes

These barriers must be connected to a soil horizon, signified with a number, e.g. P(3) for a pan in Horizon 3.

23.2.5.2 Other barriers

Table 23.9: Whole-profile root barriers (Lilburne et al. 2021)

Code	Name	Description
A	Anoxic	Air-filled porosity <5% |
F	Fractured rock	Lithic contact present within top 100 cm Fractures are <30 cm apart |
M	Massive Rock	Lithic or paralithic contact within top 100 cm If fractures are present, they are >30 cm apart |

NoteNotes

Many water tables are not anoxic and many plant species’ roots are able to live under oxygenated water at least for short periods. Anoxic water tables generally occur in loamy or clayey horizons, are tightly packed, or have significant organic matter (> 1% carbon).

23.2.6 Functional horizons

Functional horizons provide a shorthand for groups of soil morphology properties that have been linked to soil physical behaviour. Functional horizons finalise the definition of a soil sibling. Each functional horizon has an associated permeability class. Functional and conventional horizons align in terms of depth limits. but adjacent conventional horizons with the same functional horizon may be treated as one larger horizon (e.g. a Bw1 over a Bw2 may comprise a single thicker LFs horizon).

Valid functional horizon names are listed in Table 23.10, along with their default and allowable permeability classes (see Section 23.2.6.2). Details of how the functional horizon codes are constructed is below in Section 23.2.6.1.

• The t prefix can be added to any of these.
• The z or b prefix can be added to any of these, except for Oh, Or, J, and Q.
• A good reason is needed to deviate from the default permeability (e.g. an Lw with r permeability would need a low clay content).

Table 23.10: Functional horizon names and allowed permeability classes. Default permeabilities are bolded

Code	Name	Allowed permeabilities	Topsoil allowed permeabilities
J	Fluid	xs	xs
Or	Fibric organic	r m	r m
Oh	Humic organic	m ms s	m ms
OhA	Sandy humic organic	m ms	m ms
OhL	Loamy humic organic	ms s	m ms
OhY	Clayey humic organic	s	m ms s
Q	Indurated pan (massive, hard)	xs	xs
P	Placic horizon (non-indurated pan)	ms, s	s
Xx	Extremely stony fragmental	r	r
XA	Extremely stony sandy	r	r
XL	Extremely stony loamy	r m	r m
XY	Extremely stony clayey	s	m ms s
VAl	Very stony sandy loose	r	r
VAc	Very stony sandy compact	m ms	r m
VAd	Very stony sandy dense	s	ms s
SAl	Stony sandy loose	r	r
SAw	Stony sandy weak	r	r
SAs	Stony sandy slightly firm	m r	r
SAf	Stony sandy firm	m ms s	m ms
Aa	Sandy loose (dune sand)	r	r
Al	Sandy loose	r	r
Aw	Sandy weak	r	r
As	Sandy slightly firm	m r	r
Af	Sandy firm	m ms	m ms
VLl	Very stony loamy loose	r m	r m
VLc	Very stony loamy compact	m ms s	m ms
VLd	Very stony loamy dense	s xs	s
SLw	Stony loamy weak	m r	m r
SLFs	Stony loamy fine, slightly firm	m ms	m ms
SLFf	Stony loamy fine, firm	ms s	m ms
SLCs	Stony loamy coarse, slightly firm	m ms s	m ms
SLCf	Stony loamy coarse, firm	s xs	ms s
LEw	Loamy earthy weak	r m	r m
Lw	Loamy week	r m	r m
LFs	Loamy fine slightly firm	m ms	m ms
LFf	Loamy fine firm	ms s	m ms
LCs	Loamy coarse slightly firm	m ms s	m ms
LCf	Loamy coarse firm	s xs	ms s
LK	Loamy platy (> weak)	s	ms s
VYl	Very stony clayey loose	r m ms	r m
VYc	Very stony clayey compact	ms s	m ms
VYd	Very stony clayey dense	s xs	s
SYw	Stony clayey fine weak	r m	r m
SYFs	Stony clayey fine slightly firm	m ms	r m ms
SYFws	Stony clayey fine weak + slightly firm	r m ms	r m ms
SYFf	Stony clayey fine firm	ms s	m ms
SYC	Stony clayey coarse	s	ms s
Yw	Clayey weak	r m	r m
YFs	Clayey fine slightly firm	m ms	r m ms
YFws	Clayey fine weak + slightly firm	r m ms	r m ms
YFf	Clayey fine firm	ms s	m ms
YC	Clayey coarse firm	s	m ms s

23.2.6.1 Code Components

Each functional horizon is built from a set of components - topsoil status, tephric character, generalised stone and texture content, degree of structural development and consistence.

23.2.6.1.1 Topsoil

Table 23.11: Functional horizon - topsoils

Code	Name	Description
t	Topsoil	Surface functional horizon(s) - conventional A, AB or O designation. L/F/H horizons are ignored. Must have more than 25% roots by volume.
leave blank	Subsoil	Non-topsoil functional horizons. Conventional designation generally B, E, or C.

The topsoil code can be prefixed to any horizon - it is not restricted to the first functional horizon in the profile. This enables buried topsoils or multi-horizon topsoils to be adequately described.

23.2.6.1.2 Tephric character

Table 23.12: Functional horizon - tephric materials

Code	Name	Description
z	Acidic tephric	Acidic tephra (rhyolite-sourced, or similar) that qualifies as allophanic or vitric soil material
b	Basic tephric	Basic or intermediate tephra (basalt or andesite-sourced, or similar) that qualifies as allophanic or vitric soil material
leave blank	Non-tephric	non-volcanic materials, or volcanic materials that do not qualify as allophanic or vitric soil material

Tephric codes cannot be used in combination with O*, J or Q functional horizon textures (see below).

23.2.6.1.3 Stone content

Use the volumetric stone content assessments made in Section 13.2.1 to classify stone content of functional horizons.

Table 23.13: Functional horizon - stone content

Code	Name	Description
S	Stony	>5%–35%
V	Very stony	>35%–70%
X	Extremely stony	>70%
leave blank	Non-stony	≤5%

23.2.6.1.4 Texture

Table 23.14: Functional horizon - texture class

Code	Name	Description
A	Sandy	Sandy texture classes
L	Loamy or Silty	Loamy and Silty texture classes
Y	Clayey	Clayey texture classes
O	Organic	Organic soil materials
J	Fluid	Moderately or very fluid materials
Q	Pan	Indurated pan
P	Placic	Placic horizon

• Texture J is used where fluid behaviour is observed (generally in saturated loose fine sediments below the permanent water table) and is not used in concert with any other codes.
• Texture Q is used where a pan is observed and is not used in concert with any other codes.
• Texture P is used where a placic horizon is observed and is not used in concert with any other codes.

23.2.6.1.5 Texture suffixes

Table 23.15: Functional horizon - texture class modifiers

Code	Name	Description
a	Dune sand	Clean, medium to coarse sand
x	Fragmental	> 85% rock fragments with minimal interstitial material
h	Humified	Non-fibrous peats - humic or mesic peat textures; conventional horizons Om, Oh
r	Fibrous	Fibrous peats and thick litter layers - fibric peat or humose textures; conventional horizons Of, H, F

• Texture suffix a can only be used with Sandy textures (e.g. Aa)
• Texture suffix x can only be used where extremely stony (e.g. Xx)
• Texture suffixes r and h can only be used with Organic texture (e.g. Oh)

23.2.6.1.6 Structure

Table 23.16: Functional horizon - structure classes

Code	Name	Description
C	Coarse	At least 50% peds of size >20 mm (‘coarse’), or apedal massive structure
F	Fine	At least 50% peds of size <20 mm (‘coarse’), including horizons where coarse structure breaks to finer nested peds
E	Earthy	Weak soil strength with very weak ped strength
K	Platy	At least 50% peds of any size, with a platy shape

• Structure codes (C, F) can only be used with L or Y texture codes, and only where consistence is not w.
• Structures E and K can only be used with texture L.

23.2.6.1.7 Consistence

Table 23.17: Functional horizon - consistence classes

Code	Name	Description
w	weak	Fails under gentle force (strength class 1–2)
ws	weak + slightly firm	Contains a mix of weak and slightly firm peds
s	slightly firm	Fails under moderate force (strength class 3)
f	firm	Fails under strong force (strength class 4+)
l	loose	Easily dislodged by spade or pick; does not maintain a vertical face, >35% stone and/or sandy
c	compact	Intermediate between loose and dense, >35% stone
d	dense	Difficult to dislodge except with spade and by removing individual fragments; maintains a stable vertical face, >35% stone

• Consistence ws can only be used with texture Y.
• Consistence w, ws, s, and f can only be used with non-stony or stony (S) codes. V and X codes use consistence l, c, or d instead.

23.2.6.2 Permeability

The permeability class represents saturated hydraulic conductivity (K_sat) measured as the rate of water infiltrating into a soil horizon when water is ponded at 10 mm deep at the top of the horizon. Assign one of the classes in Table 23.18 to each horizon. Note that ‘xs’ represents effective impermeability. The classes have been slightly simplified from those in Griffiths (1991) and Griffiths et al (1999).

Permeability is very difficult to assess in the field, but considerable work has been done to associate particular ranges of values with the components of the functional horizon. As such, each functional horizon has a default permeability class and usually one or more additional allowable classes. Use the functional horizon name to narrow down the appropriate choice of permeability rating, per Table 23.10 below.

Table 23.18: Horizon-level permeability classes

Code	Name	Description
xs	Extremely slow	<0.1 mm/h
s	Slow	0.1–4 mm/h
ms	Moderately slow	4–18 mm/h
m	Moderate	18–72 mm/h
r	Rapid	>72 mm/h

Griffiths E 1991. Assessing soil permeability class from soil morphology. [Internet]. Havelock North, New Zealand. https://doi.org/10.7931/6G9W-AG26

Griffiths E, Webb TH, Watt JPC, Singleton PL 1999. Development of soil morphological descriptors to improve field estimation of hydraulic conductivity. Soil Research [Internet]. 37(5):971. https://doi.org/10.1071/SR98066

Hewitt AE, MWLR Pedology Staff, 2024. New Zealand soil classification (draft 4th edition). [Internet]. 4th ed. Wellington, New Zealand: Manaaki Whenua Landcare Research. https://soils.landcareresearch.co.nz/topics/soil-classification/nzsc/nzsc-wg

Lilburne LR, Webb TH, Hewitt AE, Lynn IH, De Pauw B, Vickers S 2021. S-map manual - data definitions. Lincoln, Canterbury, New Zealand.

Lynn IH, Crippen TF 1991. Rock type classification for the New Zealand Land Resource Inventory. [Internet]. Lower Hutt, New Zealand: DSIR Land Resources. https://doi.org/10.7931/DL1-DLRSR-10

Webb TH, Lilburne LR 2011. Criteria for defining the soil family and soil sibling : The fourth and fifth categories of the New Zealand Soil Classification. [Internet]. 2nd ed. Christchurch, New Zealand: Manaaki Whenua Press. https://doi.org/10.7931/DL1-LRSS-3-2011