11  Soil surface

The surface at a site is described primarily in terms of its slope, aspect, and surface cover. If present, surface water and microtopography should also be recorded. Ongoing or recent erosion and deposition events may be briefly noted or described in more detail.

11.1 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 meters. Stand 10 m upslope from the point of observation and measure downhill. Record in units of degrees, with a maximum precision of 0.1 (e.g. 6°). Record ND if slope cannot be reliably determined. Record NR if no measurement was taken.

Precision needs 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. Slope classes may be applied to the recorded data; some options are presented in Section 23.1.

Record the equipment used to measure slope using Table 11.1.

Table 11.1: Slope measurement methods

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

Note 11.1: Extracting slope from DEMs

Slope data for a site 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 Section 3.1.3)
• Extract the median slope within a 20 m radius, not the single value directly under the recorded point
• Review and, if necessary, adjust slopes extracted from sites described on exposures. These may be overestimates (e.g. where a track cuts into a slope, the values upslope behind the site would better represent the landform). Sites located close to landform or landscape boundaries may also need review.

11.2 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 11.2). Use NR if no measurement was taken. Aspect classes may be applied to the recorded data; some options are presented in Section 23.2.

Record the equipment used to measure aspect using Table 11.2.

Table 11.2: Aspect measurement methods

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

Note 11.2: Defining ‘flat’ in soil survey

Truly flat land is extremely rare over distances of more than a few meters. The cutoff slope value for flatness is not always provided in soil description standards (e.g. Schoeneberger et al. (2012)) and is not consistent when it is, e.g.

• 0.58° (~1%) for level ground in Soil and Terrain (2023), p. 19
• 2.86° (5%) in Hurst et al. (2022), p. 14

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°, while survey-grade equipment (e.g. a tripod-mounted laser level) will have a 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 site-scale aspect measurement, a flatness threshold of 1° (1.75%) is acceptable. This equates to a ~35 cm fall over 20 m.

11.3 Surface cover

Surface cover helps visualise the site and its landform and provides some information on infiltration behaviour. Soil 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 following generalised surface cover components as a percentage composition of the site (Section 10.1). Consider the land cover from the surface to a height of 1.35 m, for consistency with Hurst et al. (2022).

Table 11.3: Surface cover types

Code	Name	Description
BE	Earth	Bare sediment or soil, < 2 mm
BR	Fragments	Small rock fragments, shells, or bones, 2 - 60 mm
BO	Rock	Bare rock outcrop or rock fragments >60 mm
WT	Water	Open water, ice or snow
PT	Peat	Plant material decomposing under persistently saturated conditions
LH	Litter	Plant material decomposing under intermittently wet to 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

GRAPHIC: Turn the above into a 3x3 grid with labels underneath conceptual diagrams

It will be unusual to encounter more than three components at any one site.

11.4 Surface water

Information about surface water presence can contribute to drainage assessments.

Where water is present above the landform surface, as well as noting its areal dominance above, note the type and degree of persistence using the codes in Table 11.4 and Table 11.5 below.

Table 11.4: Surface water character (adapted from IUSS Working Group WRB (2022))

Code	Name	Description
S	Seawater	Saline ocean or brackish tidal waters
I	Surface water	Waters flowing over the surface from further inland
G	Groundwater	Waters rising from adjacent springs or an elevated natural water table
R	Rainfall	Rain falling on or near the site and perching on the surface
A	Anthropic	Water from a manmade discharge location or backing up behind a dam

Table 11.5: Surface water persistence (adapted from IUSS Working Group WRB (2022))

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

e.g. R T 20% for a paddock experiencing limited surface ponding from a recent cyclone at the time of observation.

11.5 Surface cracking

Soil surface cracking can occur for a number of reasons:

• Soil parent material includes shrink-swell clays.
• Sediment was originally deposited underwater (e.g. during a flood) and cracks on drying out.
• Mass movement or earthquake has caused surface heave.

Within the landform, record surface crack presence/persistence using the codes in Table 11.6. If present, record spatial arrangement using the codes in Table 11.7. For detailed descriptions, report median width and spacing between cracks in cm.

e.g. RT P 0.5 cm, 15 cm for surface cracking patterns common to Vertic Melanic soils.

Table 11.6: Crack persistence (IUSS Working Group WRB 2022)

Code	Name	Description
NO	No surface cracks
RT	Reversible cracks	Cracks that open and close with changing soil moisture
IT	Irreversible cracks	Cracks that persist year-round

Table 11.7: Crack spatial arrangement (IUSS Working Group WRB 2022)

Code	Name	Description
P	Polygonal	Surface cracks forming closed shapes on the land surface
L	Linear	Surface cracks running roughly parallel to each other
I	Irregular	Surface cracks with no discernible pattern

Recent weather data is useful for interpreting surface crack information (Section 6.3.2).

11.6 Microrelief

Microrelief occurs within a landform, so its components are smaller than the landform itself - usually too small to be sensed remotely. Its presence helps signify particular soil types and influences many small-scale soil processes such as local water infiltration, accumulation of surface litter, and topsoil development.

11.6.1 Natural Microrelief

Natural microrelief emerges from slope processes, biotic activity and climate effects.

Record the dominant microrelief type using the codes in Table 11.8. For detailed descriptions, record the average height and spacing of repeating microrelief elements in cm.

Table 11.8: Types of microrelief (adapted from Soil and Terrain (2023) and IUSS Working Group WRB (2022))

Code	Name	Description
N	None	Landform surface is naturally smooth or level
G	Gilgai	Induced by shrink-swell clay activity
S	Swamp	Hummocky landforms common in wetland areas, derived by biotic activity
K	Karst	Induced by subsurface collapse in limestone-dominated landscapes
A	Animal	Trampling or pugging induced by larger animals standing on low-slope ground (cows, pigs, ungulates)
T	Terracettes	Small terraces on moderate to steep slopes, generally induced by larger animals navigating the terrain
F	Frost	Induced by freeze-thaw activity

11.6.2 Anthropogenic microrelief

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

Record the dominant anthropogenic microrelief type using the codes in Table 11.9. For detailed descriptions of induced microrelief, record the average height and spacing of repeating elements in cm.

Table 11.9: Types of Anthropogenic microrelief (adapted from Soil and Terrain (2023) and IUSS Working Group WRB (2022))

Code	Name	Description
N	None	Landform surface is not significantly altered by humans
C	Smoothed	Landform 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. raised rows or puke for kūmara
R	Terraces and tracks	Hillsides have been cut/filled into terraces, or tracks cut in across the slope
D	Drained	Open drainage ditches have been installed

11.7 Erosion

Where signs of erosion exist at a site, its presence can simply be noted using the site disturbance code SL (see Section 10.3).

If a more detailed assessment is desired, note the type(s) per Table 11.10. If erosion is present, note also severity and activity using Table 11.11 and Table 11.12. Only record erosion directly affecting the site.

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

Table 11.10: 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 >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 and the formation of narrow tunnels through which soluble or spheroidal soil material is removed.
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 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
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
Wind-driven
WI	Wind	Detachment, transport, and deposition of loose materials by wind action, especially in dust storms and in arid or semi-arid regions or where a protective mat of vegetation has been removed. Erosion forms are characteristically ovate in plan, and concave in contour and profile.
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 11.11: Erosion severity (reproduced from IUSS Working Group WRB (2022))

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

Table 11.12: Erosion activity (adapted from IUSS Working Group WRB (2022))

Code	Name	Description
PR	Active	Active at present
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

11.8 Deposition

Where signs of deposition exist at a site, its presence can simply be noted using the site disturbance code SD (see Section 10.3).

If a more detailed assessment is desired, note the type(s) per Table 11.13. If deposition is present, note also severity and activity using Table 11.14 and Table 11.15. Only record deposition directly affecting the site.

For example, FL M FR for a recent flood that has dropped sediment on a low terrace and partly smothered grass, or NO - - for an unaffected site. Optionally, the generalised texture class of the material can be reported using the codes in Table 16.3.

Table 11.13: Types of deposition (partly adapted from Soil and Terrain (2023))

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
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
Wind-driven
VR	Volcanic eruption	Airfall deposition from pyroclastic eruptions
SA	Sand movement	Wind-driven deposition of sand from beaches, banks, or remobilised dunes
SI	Silt movement	Wind-driven deposition of silt and finer material 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

Table 11.14: 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 < 10cm thick, original ecological functions partly destroyed
V	Severe	Cover complete and > 10cm thick, original ecological functions largely destroyed
E	Extreme	Cover complete and permanent, original ecological functions fully destroyed

Table 11.15: Deposition activity

Code	Name	Description
FR	Fresh	Active within the past year
RE	Recent	Active within the past 10 years
HI	Historical	Active within the past 100 years
UK	Unknown	Activity history cannot be reliably determined

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

Hurst JB, Allen RB, Fergus AJ 2022. The recce method for describing new zealand vegetation - a manual. [Internet]. Lincoln, Canterbury, New Zealand. https://doi.org/10.7931/p48h-zb65

IUSS Working Group WRB 2022. World reference base for soil resources: International soil classification system for naming soils and creating legends for soil maps. 4th ed. Vienna, Austria: International Union of Soil Scientists.

OECD 2001. Environmental indicators for agriculture: Methods and results volume 3. [Internet]. [place unknown]: OECD. https://doi.org/10.1787/9789264188556-en

Schoeneberger PJ, Wysocki DA, Benham EC, Soil Survey Staff 2012. Field book for describing and sampling soils, version 3.0. [Internet]. Lincoln, NE, USA: Natural Resources Conservation Service, National Soil Survey Center. https://www.nrcs.usda.gov/sites/default/files/2022-09/field-book.pdf

Soil NC on, Terrain 2023. Changes to the australian soil and land survey field handbook in the draft 4th edition.

Turner AK, Schuster RL 1996. Landslides: Investigation and mitigation. Landslides [Internet]. Washington, D. C. https://onlinepubs.trb.org/Onlinepubs/sr/sr247/sr247.pdf