| Code | Name | Horizon Names | Typical Moisture | Main colours | Redox concentrations |
|---|---|---|---|---|---|
| v | Very poor | O*, *r | Saturated or wet | Low-chroma | Few to none |
| p | Poor | *r, *g | Wet | Low-chroma | Few to none, mottles more often on ped surfaces than internals |
| i | Imperfect | *(g) | Wet to Moist | Low-chroma patterns on brighter matrix | Fe and/or Mn-rich segregations, nodules and pans |
| m | Moderately Well | *(f) | Wet to Dry | Colour patterns but no low chroma colours | Mottles more often in ped interiors |
| w | Well | No redox suffixes | Moist to Dry | No low chroma colours | Few to none |
19 Profile behaviour
Soil profile descriptions can be used to identify behavioural characteristics of a soil that are relevant to ecosystem composition and to productivity.
19.1 Drainage
Soil drainage is an assessment of how fast water leaves a soil profile relative to supply. This characteristic can be controlled by morphological characteristics that occur in a range of combinations.
19.1.1 Horizon drainage
Drainage should first be assessed on a per-horizon basis. These interpretations may then be summarised a profile level (see Section 19.1.2). Table 19.1 below highlights the common characteristics associated with each horizon drainage class.
19.1.1.1 Classifying topsoil drainage
Morphological features associated with drainage status can be hard to observe accurately in A horizons, mostly due to their darker colour. When otherwise in doubt, a well developed A horizon should be assigned a drainage class one better than the horizon below it (usually a B, E, or C horizon). A poorly developed A horizon should receive the same class assignment as the horizon below. Use the ‘distinct topsoil’ NZSC diagnostic criteria to identify well developed A horizons.
The reasoning behind this rule of thumb is that the lack of topsoil development may be partially explained by lack of contrast in the drainage regime between the topsoil and the layer below.
19.1.2 Profile Drainage
The drainage status of the profile as a whole is determined using horizon depths and per-horizon drainage ratings completed according to Section 19.1.1. Note that ‘perch-gley profiles’, where a zone of impeded drainage rests above a low-permeability layer that appears less affected by redox processes, are treated the same as any other soil profile. The depth to the shallowest zone of impeded drainage is used to determine the profile drainage class.
Workers classifying drainage for NZSC Sibling construction have no need to subdivide within the profile ratings in Table 19.2. For other types of work e.g. agricultural suitability assessment, it may be useful to recognise subgroups – e.g. for a deep-rooted, wetness-intolerant crop, there is a difference between an imperfectly drained profile that is at its worst imperfectly drained in the top 1 m, and one that has poor drainage below 60 cm.
| Code | Name | Description |
|---|---|---|
| VP | Very Poor | Profiles with very poorly drained horizons starting <10 cm from the land surface |
| PO | Poor | Profiles with poorly drained horizons starting <30 cm from the land surface |
| IP | Imperfect | Profiles with poorly drained horizons starting 30–60 cm from the land surface, or Profiles with imperfectly drained horizons starting <30 cm from the land surface |
| MW | Moderately Well | Profiles with moderately well-drained horizons starting <90 cm from the surface, or Profiles with poorly-drained horizons starting 60–90 cm from the surface, or Profiles with imperfectly drained horizons starting 30–90 cm from the surface |
| WE | Well | All horizons starting within 90 cm of the surface are well-drained. |
19.1.2.0.1 Natural vs Artificial drainage
Profile drainage is often modified by human activity. Many of these modifications require ongoing maintenance. Use the codes in Table 19.3 to signify the degree of drainage modification.
| Code | Name | Description |
|---|---|---|
| N | Natural | No evidence of human impacts on the local drainage regime |
| M | Minor | Drainage regime passively altered by e.g. conversion to or from forest; or ambiguous evidence of any human impacts beyond clearing |
| A | Major | Clear evidence of directed human alteration to the drainage regime e.g. drain or dam construction adjacent to site, flow diversions, infill, levelling or contouring works |
19.2 Permeability
Permeability is the measured or estimated rate at which water moves through the soil under ‘free-draining’ conditions - i.e., in the absence of barriers and in an unsaturated state. Like drainage, permeability is assessed first on a per-horizon basis before the profile as a whole is considered.
Permeability can be measured directly, and while this data is always preferred, the measurement process is time-consuming and labour intensive. As a result, a great deal of research effort has been directed towards developing reliable estimates of permeability for New Zealand soils based on soil morphology (McLaughlin and Watt 1989; Griffiths 1991; Webb and Lilburne 2011). This section describes a method of permeability estimation based on that work.
19.2.1 Horizon permeability
For each horizon, in the absence of direct measurements, use texture group, structure, stone content, organic matter content and unconfined strength at field condition to rate permeability using the charcteristics described in Table 19.4.
Note that for each permeability class, topsoil horizons and soils with tephric parent materials are generally considered to be one class more permeable than described (e.g., a clayey topsoil horizon with <20 mm peds would be assigned moderate permeability rather than moderately slow).
| Code | Name | Permeability | 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 |
|
19.2.2 Profile permeability
Summarising permeability at a profile level involves identifying the layer with the slowest permeability, and then identifying any significant deviations from that rating within the profile. First, simplify the horizon ratings according to Table 19.5 then follow Figure 19.1 to determine the profile permeability.
| Code | Name | Description | Horizon permeability class |
|---|---|---|---|
| SL | Slow | <4 mm/hr | Extremely slow, slow |
| MO | Moderate | ≥4–<72 mm/hr | Moderately slow, moderate |
| RA | Rapid | ≥72 mm/hr | Rapid |
%%{ 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**, or **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**")
19.3 Rooting depth restrictions
Some soils contain barriers to plant root growth that can affect some or all species, depending on the natural size of their root systems and their tolerance for particular conditions. Such barriers can include physical properties (soil firmness, buried rock or concrete), chemical properties (pH, salinity, macro and micro-nutrient levels, presence of pollutants), biological properties (pathogens, pests) and hydrological properties (excessive wetness or dryness).
Rooting depth barriers can be identified most clearly by the behaviour of living roots in the profile. Absence of roots beyond a certain depth where vegetation is present at the surface may indicate a barrier, assuming the species present normally have a deep-rooting growth habit. Roots may also be observed to grow downwards until encountering a barrier and then turn to grow horizontally, sometimes forming subsurface mats. Root density may decrease suddenly at a horizon boundary without disappearing entirely, perhaps becoming sparse and confined to voids. Significant barriers may also affect the character of vegetation at the surface, causing e.g., stunted growth.
Where rooting behaviour appears to be impaired, evidence of a likely cause may be inferred from the profile description, particularly:
- Field tests for pH, salinity or reducing conditions (Chapter 17)
- Strength, penetration resistance and induration tests (Chapter 16)
- Structural characteristics, particularly the absence of peds or inter-ped spaces (Section 12.1)
- Relative abundance of rock fragments, where excess rock might be a physical barrier and also limit water retention and movement (Section 12.2)
- Soil texture, where high clay content may cause waterlogging or high sand content may limit water retention (Chapter 14)
- Identification of pans blocking root growth, or concentrations that may suggest a chemical issue (Chapter 15)
Where these features are observed, to be considered as a potential rooting depth barrier their occurrence must usually coincide with the zone where root behaviour changes, or roots disappear. Only a few barriers (e.g. bedrock) can be considered definite regardless of whether any roots are actually present in the profile. Where no obvious restricting features are present but impeded root growth is still observable, laboratory testing may be required to investigate the cause.
Record rooting barrier information as type using Table 19.6 and depth encountered in cm, e.g. Br 30 cm for a shallow profile over bedrock. Rooting depths should usually coincide with a horizon boundary.
| Code | Name | Description |
|---|---|---|
| Ac | Acid | pH below 5.5 |
| Ak | Alkaline | pH above 8.0 |
| Na | Saline | EC above 0.6 dS/cm |
| Ax | Anoxic | Presence of reduced iron, or a very poor horizon drainage rating |
| Tx | Toxic | Presence of a human-made pollutant at levels impacting plant growth |
| Nx | Nutrient | Lack of a macro- or micro-nutrient required for plant growth |
| Fm | Firm | Unconfined soil strength firm or greater |
| Pp | Physical pan | Presence of a well-developed densipan or fragipan, or a compacted layer caused by cultivation, livestock trampling, or machinery traffic |
| Pc | Chemical pan | Presence of a well-developed chemical pan e.g. duripan, iron-pan, calcrete |
| Sk | Sandy | Soil texture is coarse sandy or sandy-skeletal with loose packing |
| Cl | Clayey | Soil texture is clayey and structure is apedal massive |
| Rk | Profile rock | Rock fragments occupy over 35% of the horizon by volume, either in the soil or in regolithic material |
| Ak | Artefact | A buried layer of anthropogenic origin is present (e.g. geofabric) |
| Bf | Fractured substrate | Saprolithic, paralithic or lithic material with notable cracking is present |
| Bm | Massive substrate | Paralithic or lithic material with no or mininimal cracking is present |