10  Soil surface

A soil profile description is placed in its landform and landscape context using the system described in Section 9.3. The land surface is then described primarily in terms of its slope, aspect, and surface cover. If present, surface water and microrelief features should also be recorded. Ongoing or recent erosion and/or deposition events can be briefly noted or described in more detail.

10.1 Slope

Slope is the inclination of the land surface relative to the horizontal plane. Slope steepness affects many soil processes, notably erosion, water run-off, and infiltration behaviour.

Direct measurements of slope should be taken over a minimum distance of 20 m. Stand 10 m upslope from the point of observation and measure downhill along the steepest gradient that passes through the point of observation. Record slope in degrees, with a maximum precision of 0.1.

Record ND if slope cannot be reliably determined. Record NR if no measurement was taken. Ignore the influence of microrelief features smaller than the site extent while measuring slope (see Section 10.7.2; e.g. steep, short slopes from track cuttings).

The degree of precision needed will vary with slope and survey purpose; whole number degrees are sufficient on any slope over about 5 degrees, but finer gradations can be useful for flatter locations. Classification options for slope measurements are presented in Section D.1.1. Record the equipment used to measure slope using Table 10.1.

For example: use 5° E for an estimated 5 degree slope.

Table 10.1: Slope measurement methods

Code	Name	Description
E	Estimate	Estimated by eye
C	Clinometer	Hand-held clinometer or Abney level
T	Tripod	Survey-grade equipment mounted on a tripod
D	DEM	Calculated from a DEM with suitable resolution and vertical accuracy

10.2 Aspect

Aspect is the direction a slope faces. Slope aspect helps determine exposure to sun, rain and prevailing winds, and so influences the biome growing in a particular soil. In some environments it also helps determine how much new material is received by deposition.

Aspect should be measured at the same time and over the same transect as slope. Record aspect in whole degrees clockwise from north, using the range 0–359° (360° = 0°). Use ND for flat ground (see Note 10.1). Use NR if no measurement was taken. Classification options for aspect measurements are presented in Section D.1.2. Record the equipment used to measure aspect using Table 10.2.

For example: use 135° C for a southeast-facing slope, recorded using a compass.

Table 10.2: Aspect measurement methods

Code	Name	Description
E	Estimate	Estimated by eye
C	Compass	Hand-held magnetic compass
D	DEM	Calculated from a DEM with suitable resolution and vertical accuracy

Note 10.1: Defining ‘flat’ in soil survey

Truly flat land is extremely rare over distances of more than a few metres. The cutoff slope value for effective flatness is not always provided in soil description standards (e.g. Soil Survey Staff (2024)) and is not consistent when it is, e.g.

• 0.58° (~1%) for level ground in the Australian Soil and Land Survey Field Handbook
• 2.86° (5%) in Hurst et al. (2022).

The slope measurement method in use can restrict the recordable limit of flat land. For instance, a hand-held clinometer may be difficult to use consistently and accurately at slopes of less than 3°, whereas survey-grade equipment (e.g. a tripod-mounted laser level) will have higher accuracy.

Slope calculated from a DEM also has inherent limitations. For example, a DEM with a cell size of 20 m and a vertical resolution of 0.5 m cannot discriminate slope increments of less than ~1.43° (\(tan^{-1}(0.5/20)\)).

For localised aspect measurement, a flatness threshold of 1° (1.75%) is acceptable. This equates to a ~35 cm fall over 20 m.

Note 10.2: Equipment for measuring slope and aspect

Slope and aspect are conventionally measured with analogue tools: a clinometer for slope, and a compass for aspect. Digital versions of these tools are increasingly available, usually embedded in smartphones. Smartphones can sense slope and magnetic direction using a combination of accelerometers, magnetometers, and gyroscopes, assisted by software. Reliability and accuracy vary by device make and model (Tavani et al. 2022), so any device being used for this task should be assessed first. If the device proves sufficiently reliable, it can be recorded as type C in both Table 10.1 and Table 10.2.

Note 10.3: Extracting slope and aspect from DEMs

Slope data can be extracted from a DEM, provided some conditions are met.

• Cell size should be less than 10 m; larger cells systematically underestimate local slope.
• DEM source should ideally be of type D for acceptable accuracy (see Table 2.2).
• Extract the median slope within a 10 m radius, not the single value directly under the recorded point.

Review and, if necessary, adjust slopes extracted from locations described on exposures. These may be overestimates (e.g. where a track cuts into a slope, the values upslope behind the profile would better represent the soil-forming environment). Profile descriptions located close to landform or landscape boundaries may also need review. For example, for a description made in the base of a narrow valley, the slope of the valley sides will differ from the slope of the valley floor. If the extraction window includes both the valley floor and sides, the median slope most relevant to the profile will be overestimated.

Aspect data are subject to similar constraints, with two additional issues to consider.

• Aspect calculated from a DEM will correspond to ‘map north’, while aspect measured directly will correspond to magnetic north. Adjustments for declination will be required.
• Aspect data should be generalised to the site scale before extracting an exact value; as circular data, this requires more complex calculations than a simple median.
• Software used to calculate aspect sometimes defaults to reporting in degrees anticlockwise from east (consistent with trigonometry rather than orienteering conventions), and can vary in terms of whether and how it signifies flat areas.

10.3 Surface cover

Surface cover helps visualise the area around a profile description and provides some information on infiltration behaviour. Surface cover, specifically ‘the number of days in a year that the soil (agricultural land) is covered with vegetation’, is a key agricultural performance indicator (OECD 2001).

Estimate and record the generalised surface cover components as a percentage composition of the site (Section 9.1), using the codes in Table 10.3. Consider the surface cover from the surface to a height of 1.35 m, for consistency with Hurst et al. (2022). The lithology of surface rock fragments and/or outcrop can be recorded per Section 4.2. Surface rock cover can also be described in detail according to Section 12.2.1.

For example: use BE 10%, GR 90% for a pasture with near-complete ground coverage.

Table 10.3: Surface cover types

Code	Name	Description
BE	Earth	Bare unconsolidated sediment or soil, <2 mm
BG	Small fragments	Rock fragments, shells, or bones, 2–200 mm
BB	Large fragments	Large rock fragments >200 mm
BR	Rock	Bare rock outcrop
WT	Water	Open water, ice or snow
PT	Peat	Plant material decomposing under saturated conditions
LI	Litter	Plant material decomposing under intermittently wet and dry conditions
GR	Small plants	Grasses, ferns, herbs, moss, sedges, annual crops
TR	Large plants	Shrubs, trees and treeferns (stems and roots), perennial crops
SL	Sealed	Building, concrete, asphalt and similar

10.4 Surface disturbances

Disturbances have the potential to influence the soil as well as its cover. Noting these events can provide context for the parameters recorded, and some are especially useful when interpreting laboratory results. Table 10.4 provides a list of disturbance categories. Free-text notes can be used if additional detail is needed.

Only record recent events directly affecting the profile. ‘Recent’ is a relatively flexible concept which highlights the reasons some parameters observed might be locally unusual. Some of these events recur regularly (e.g. tillage) so only need to be recorded if the are very recent (e.g. the previous week).

If the time since the event is uncertain, record an estimate using one of the options in Table 10.5.

For example: use FL S for a flood estimated to have occurred within the past 6 months.

If the time since the event is known with high certainty, record days since the event.

For example: use FL 10 for a well-documented recent flood.

Table 10.4: Recent profile-affecting events

Code	Name
MC	Mechanical disturbance (e.g. tillage, deep ripping, subsoiling, recontouring)
AN	Animal disturbance (e.g. pugging, rooting, tunnelling, excretion)
CL	Complete vegetation clearance by humans (e.g. clearfelling, harvest)
CO	Selective vegetation clearance by humans (e.g. tree removal or weed clearance)
WI	Wind-induced treefall
SL	Natural erosion event – material lost from site
SD	Natural deposition event – material added to site
FC	Fire – controlled burn
FW	Fire – uncontrolled burn
FL	Flooding – freshwater
FS	Flooding – saline water (e.g. storm surge event)
PO	Pollution event (e.g. petrol spill)
NO	No recent events
UK	Unknown

Table 10.5: Generalised time since recent site-affecting events

Code	Name
D	Within the past day
W	Within the past week
M	Within the past month
S	Within the past 6 months
Y	Within the past year

10.5 Surface water

Information about surface water can contribute to drainage assessments. If surface water is present at the site, record its presence and estimate the area covered using the WT code in Section 10.3. Optionally, append type and degree of persistence using the codes in Table 10.6 and Table 10.7.

For example: use WT 20% (R, T) for a paddock experiencing limited surface ponding from a recent cyclone at the time of observation.

Table 10.6: Surface water character

Code	Name	Description
M	Saline surface water	Marine, estuarine, or inland-saline waters flowing over the surface
F	Fresh surface water	Palustrine, lacustrine or riverine waters flowing over the surface
C	Cold groundwater	Waters rising to the surface from cold springs or an elevated natural water-table
G	Hot groundwater	Waters rising to the surface from geothermal sources (temperature 30°C or more)
S	Snow	Snow falling on or near the site and accumulating on the surface
R	Rainfall	Rain falling on or near the site and accumulating on the surface
A	Anthropic	Water from a man-made discharge location or backing up behind a dam
N	None	Surface free from ponds or puddles
Source: adapted from IUSS Working Group WRB (2022) and Johnson and Gerbeaux (2004).

Table 10.7: Surface water persistence

Code	Name	Description
P	Permanent	Surface water persists year-round
S	Seasonal	Surface water is present during wet seasons
T	Temporary	Surface water is present due to a specific event
Source: adapted from IUSS Working Group WRB (2022).

10.6 Surface cracking

Soil surface cracking can occur for a number of reasons, including the following:

• The soil parent material includes shrink–swell clays, in an environment that allows desiccation.
• Sediment was originally deposited underwater (e.g. during a flood) and cracks on drying out.
• Freeze–thaw cycles, mass movement, or tectonic activity has caused surface deformation.

Surface cracks can have a strong influence on infiltration behaviour. Surface cracks are not strongly related to soil structure, generally being much larger than the spaces between peds and often extending through more than one horizon. However, cracks are more likely to form and re-form along existing planes of weakness.

Within the site, record surface crack presence/persistence using the codes in Table 10.8. If present, record spatial arrangement using the codes in Table 10.9, as well as median width, depth, and spacing between cracks in centimetres. Note that infilled cracks should be described as per Section 11.5.

For example: use RT P 0.5 cm, 10 cm, 35 cm for surface cracking patterns common to Vertic Melanic Soils.

Table 10.8: Crack persistence

Code	Name	Description
NO	None	No surface cracks
RT	Reversible cracks	Cracks that open and close with changing soil moisture
IT	Irreversible cracks	Cracks that persist year-round
Source: (IUSS Working Group WRB 2022).

Table 10.9: Crack spatial arrangement

Code	Name	Description
P	Polygonal	Surface cracks forming closed shapes on the land surface
L	Linear	Surface cracks running roughly parallel to one another
I	Irregular	Surface cracks with no discernible pattern
Source: (IUSS Working Group WRB 2022).

Recent weather data are useful for interpreting surface crack information (Section 5.2.2).

10.7 Microrelief

‘Microrelief’ is a term covering small-scale, regular or semi-regular repeating patterns of surface deformation. Microrelief occurs within a landform, so its components are smaller than the landform itself – often too small to be reliably sensed remotely.

Consider the following example of how microrelief fits into a particular landscape context. In a hill-country landscape a north-facing slope landform is used for sheep grazing. Over time, stock movement has created small, step-like terraces on the slope surface. The terraces, known as terracettes, are a form of biotic microrelief. They are 20–30 cm high and spaced 1–2 m apart. While the terracettes do not alter the overall form of the slope, they influence local surface water movement and erosion.

Microrelief presence and character can help signify particular soil types, and also influence many small-scale soil processes such as local water infiltration, accumulation of surface litter, and topsoil development.

10.7.1 Natural microrelief

Natural microrelief emerges from a similar set of drivers to soil formation itself. Record the dominant microrelief driver and form using the codes in Table 10.10 and Table 10.11. In practice the type categories can overlap and interact, but a dominant force should usually be apparent. Optionally, record the average height and spacing of repeating microrelief elements in centimetres.

Where the microrelief pattern is larger than the profile being examined, note the location of the profile relative to the pattern (e.g. whether a core has been drilled in a dip).

Table 10.10: Drivers of microrelief

Code	Name	Description
N	None	Site surface is naturally smooth or level
G	Soil	Induced by shrink–swell clay activity (e.g. gilgai)
B	Biotic	Induced by plant or animal activity (e.g. forest dimples from treethrow)
C	Climatic	Induced by temperature extremes or wind and water (e.g. frost heave, sand ripples)
T	Terrain	Induced by mass movement effects on slopes
S	Substrate	Induced by changes to the substrate (e.g. small karst features)
A	Anthropogenic	Induced by deliberate human activity
Source: adapted from the Australian Soil and Land Survey Field Handbook and IUSS Working Group WRB (2022).

Table 10.11: Natural microrelief forms

Code	Name	Description
R	Ripples	Small elongate rises above a planar surface
H	Hummocks	Small rounded rises above a planar surface
D	Dips	Small hollows set into a planar surface or slope
U	Undulating	A pattern of alternating small hummocks and dips
T	Terracettes	Small shelves or steps on a larger slope

Figure 10.1: Reference for measuring microrelief width and height parameters on flat and sloping land.

10.7.2 Anthropogenic microrelief

Small-scale human alterations to the soil surface are common in agricultural and urban areas (both current and former). Microrelief can be both removed from and created by these activities.

If the alteration is relatively recent, a surface disturbance code (e.g. MC) from Table 10.4 can be used. Otherwise, record the dominant anthropogenic microrelief type using the codes in Table 10.12. The driver (Table 10.10) in all cases will be A. For detailed descriptions of anthropogenic microrelief, record the average height and spacing of repeating elements in centimetres.

Table 10.12: Types of anthropogenic microrelief

Code	Name	Description
C	Smoothed	Site surface has been smoothed, contoured, rock-picked, infilled or levelled, beyond normal tillage effects
B	Raised beds	Elevated areas have been created and maintained for specific crops; e.g. rows for vegetables or puke (mounds) for kūmara
H	Hump and hollow	Broad, alternating surface drains and ridges constructed across a surface, usually for pasture production
R	Terraces and tracks	Hillsides have been cut/filled into small terraces, or tracks cut in across the slope
D	Drained	Open drainage ditches have been cut into the surface
Source: adapted from the Australian Soil and Land Survey Field Handbook and IUSS Working Group WRB (2022).

10.8 Erosion and deposition

Erosion and deposition processes operate at the landscape and landform scale and have effects on the site and profile description. When documenting erosion and deposition across landforms and landscapes, the LUC erosion assessment framework should be applied (Lynn et al. 2009). At the site scale (radius up to ~10 m, see Section 9.1), the system below allows description of the localised nature and impact of these processes.

10.8.1 Erosion

Where there are active signs of erosion, its presence can be recorded using the profile disturbance code SL (see Section 10.4). If a more detailed assessment is desired, record using the type(s) given in Table 10.13. If erosion is present, also record severity and activity using Table 10.14 and Table 10.15. Only record erosion directly affecting the profile.

For example: use RI S PR for rills observed on a tilled paddock after a storm, or NO - - for an unaffected area.

Table 10.13: Types of erosion

Code	Name	Description
Water-driven
SH	Sheet	Thin layers of surface material are gradually removed more or less evenly from an extensive area of sloping land
RI	Rill	Formation of predominantly steep-sided erosion channel(s) ≤0.5 m deep.
GU	Gully	Formation of predominantly steep-sided erosion channel(s) >0.5 m deep, not capable of being safely crossed by a wheeled tractor or obliterated by ploughing or deep cultivation
TN	Tunnel	Erosion by percolating water in a layer of subsoil, resulting in caving in and the formation of tunnels through which soil material is removed. Includes the linear dips and sinkhole features created by tunnel collapse
CH	Channel	Erosion by water flowing in stream and river channels, including stream bank erosion
Gravity-driven
FA	Fall	A very rapid downward movement of a mass of rock or earth that travels mostly through the air by free fall, leaping, bounding or rolling, with little or no interaction between one moving unit and another (e.g., rock-fall, debris fall)
TO	Topple	Large blocks of earth material or rocks falling forward off a very steep to precipitous slope
SL	Slide	Lateral displacement of earth materials moving as a block along a deep-seated slip face, either rotational (‘slump’) or translational
SP	Spread	Rapid displacement of wet, plastic materials that liquefy, often bringing along intact blocks or beds
FW	Flow	Moderate to rapid movement of earth materials that behave as a viscous fluid (earthflow or mudslide)
CR	Creep	Slow, gradual movement of earth materials down a slope
Wind-driven
WI	Wind	Detachment, transport, and deposition of loose materials by wind action, especially where a vegetation cover is not present.
Other
UK	Unknown	Mode of erosion cannot be clearly determined
NR	Not recorded	Mode of erosion not recorded
NO	No erosion	No evidence of erosion

Table 10.14: Erosion severity

Code	Name	Description
S	Slight	Some evidence of damage to surface layers; original ecological functions largely intact
M	Moderate	Clear evidence of removal of surface layers; original ecological functions partly destroyed
V	Severe	Surface layers completely removed and subsurface layers exposed; original ecological functions largely destroyed
E	Extreme	Substantial removal of deeper subsurface layers; original ecological functions fully destroyed
Source: IUSS Working Group WRB (2022).

Table 10.15: Erosion/deposition activity timescales

Code	Name	Description
PR	Active	Active at present
FR	Fresh	Active within the past year
RE	Recent	Active within the past 100 years
HI	Historical	Active in historical times (>100 years ago)
UK	Unknown	Activity history cannot be reliably determined
Source: adapted from IUSS Working Group WRB (2022).

10.8.2 Deposition

Where there are signs of deposition, its presence can be recorded using the profile disturbance code SD (see Section 10.4). If a more detailed assessment is desired, record using the type(s) given in Table 10.16. If deposition is present, also record severity and activity using Table 10.17 and Table 10.15. Only record deposition directly affecting the profile.

For example: use FL M FR for a recent flood that has dropped sediment on a low terrace and partly smothered grass, or NO - - for an unaffected area.

Table 10.16: Types of deposition

Code	Name	Description
Water-driven
FL	Flood	Deposition driven by inundation events along streams
SS	Surge	Deposition driven by storm surge
TI	Till	Glacial deposition
Gravity-driven
FA	Fall	Rapid, freefall collapse of earth material on precipitous slopes
TO	Topple	Large blocks of earth material falling forward off very steep to precipitous slope
SL	Slide	Lateral displacement of earth materials moving as a block along a deep-seated slip face, either rotational (‘slump’) or translational
SP	Spread	Rapid displacement of wet, plastic materials that liquefy, often bringing along intact blocks or beds
FW	Flow	Slow to rapid movement of earth materials that behave as a viscous fluid (earthflow or mudslide)
CR	Creep	Slow, gradual movement of earth materials down a slope
Wind-driven
VR	Volcanic eruption	Deposition from pyroclastic fall or flow materials (tephra) from explosive volcanic eruptions
SA	Sand movement	Wind-driven deposition of sand (e.g., from beaches, banks, or remobilised dunes)
SI	Silt movement	Wind-driven deposition of silt and finer material (e.g., from banks or bare earth)
Other
UK	Unknown	Mode of deposition cannot be clearly determined
NR	Not recorded	Mode of deposition not recorded
NO	No deposition	No evidence of deposition
Source: partly adapted from the *Australian Soil and Land Survey FIeld Handbook.

Table 10.17: Deposition severity

Code	Name	Description
S	Slight	Patchy or thin cover over previous surface layers; original ecological functions largely intact
M	Moderate	Cover mostly complete but <10 cm thick; original ecological functions partly destroyed
V	Severe	Cover complete and >10 cm thick; original ecological functions largely destroyed
E	Extreme	Cover complete and permanent; original ecological functions fully destroyed

More detailed assessments of local erosion and deposition are possible using the characterisation principles in Turner and Schuster (1996), but they are considered beyond the scope of a soil profile description.

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