Plant species selection in Planterguide is based on stipulation of both the Ecological Region and the broad soil type or Soil Order. The Ecological Region is an indicator of climate – the major driver of ecosystem functioning. But soils fine tune, or in some cases dictate, land capability. These notes background the nature of soils and provide aids to their identification.

Soils are the interface between the Earth’s crust and the atmosphere. They derive from physical weathering of raw rock and sediment and associated biological activity. Soils are integrators of the environment and determine species composition and productivity potentials of vegetation. Soils develop or change over time.

They are the essential rooting medium for plant survival and growth - providing anchorage, reservoirs of water and nutrients, and buffering against extremes of climate. Plant survival and growth depend on both the above and below ground environment. Thus the soil condition must also be identified if correct selection and placement of plants is to occur in restoration projects. The climate may be right, but soils may be too wet, too dry, too infertile or too toxic for some plant species.

Mature soil profiles have distinctive layers or horizons that reflect these site conditions. These layers are broadly divided into topsoil (which includes a litter layer on top of an organic horizon - perceptibly darker than the deeper layers), subsoil (with paler upper horizon leached of nutrients and clay particles, and a deeper accumulation zone), and parent rock or sediment at the base, where there has been no visible weathering.

Soils may vary over short distances (at a finer scale than it is practicable to map), especially where there are sharp changes in land form, slope or drainage - such as edges of cliffs, coastal shores, riverbanks in otherwise dry environments, and urban areas or transport corridors where cut and fill earthworks have occurred. Such heterogeneity below ground is often recognised above ground by sequences of distinct bands or zones of vegetation.

To determine what type of soil you have (major groups are called Soil Orders), you must first dig a hole (or clean a cutting back at least 10 cm) down to about 75 cm below ground surface or to the water table. You inspect the dark organic topsoil and its thickness, depth to water table, the colour and texture of the subsoil, the presence of mottling or layering, the underlying rock or sediment type, the degree of colour change from one layer (horizon) to the next, and the landform or position of the site in the landscape. Note, cuttings are generally better drained (and show less mottling) than adjacent intact land.

Identifying soils accurately is a specialist’s job, but with a bit of practice you should be able to make an informed choice among the broad types. Consulting a regional or local soil map will assist the task. If you look at all the soil types mapped on the same basic landform (plains or hills) within a 50 km radius of your site, you should be restricted to no more than half a dozen possibilities at the Soil Order level. A soil legend or booklet, which can be purchased with many soil maps, will also explain the common patterns, e.g. those soil types specific to lower slopes, mid slopes or ridges of hills. However, mapped soils are only an approximation, reflecting the dominant soil of the locality. It is impracticable to map all the minor variations. So it is only where the land is very uniform that maps can be definitive at fine scales. Thus, a combination of soil map, spade in the field, and soil key will give the best results.

Correlations between the old Genetic soil group names (of most older maps) and the Soil Orders (NZ Soil Classification), used in Planterguide, are given in the table below.

Reference: A.E.Hewitt 1992. New Zealand Soil Classification. NZ DSIR Land Resources Scientific Report No. 19.

The following Soil Key is based on the observable features in a vertical soil profile as outlined above. The first part divides the Soil Orders into five broad groups (A-E). Follow the instructions or the most likely choice. Then check with the descriptions (under xxxxx). When you are satisfied with the identification (and there may be more than one type in your restoration site), you can then apply these names to the Planterguide search engine where it allows you to generate a plant list for a soil type within each Ecological Region. If there is still uncertainty about the soils, try species lists generated from alternative soil types and then check the ecology of individual species to see if they fit.

A
Artificial or severely modified soils – that are ‘cut’ (topsoil and some of subsoil removed) or ‘filled’ (often with buried foreign material or artefacts) – go to section A below.

B   
Continuously wet in winter and spring (water slow to drain away after rain; squelchy underfoot) – not only flat land, but also hollows or seepages on hills and mountainsides. Includes areas artificially drained and not currently always wet, but reverts if drainage is blocked. Subsoil is grey with rust coloured mottles – go to section B below.

C   
Thin topsoil directly overlies rock at depths of <30cm, and/or any subsoil present consists of fresh or unweathered material more or less the same colour as the underlying parent rock or sediment; not continuously wet; occurs in areas of active disturbance on flood plains, screes, dunes, estuaries, eroded slopes, and rocky areas where raw rocks, stones or sands are exposed or are at depths shallower than 30cm – go to section C below.

D
Deeply weathered soils (yellow, brown or reddish brown clay, >1m thickness to parent material, that is hard and dense when dry and firm and sticky when wet, and if stones are present they are soft enough to cut with a knife or spade) on very old surfaces in uplands and hills (but not steeplands) north of Coromandel and Waikato, and also coastal hills of Marlborough Sounds, Nelson and Wellington not subject to past glaciation – go to section D below.

E
All others: free-draining, mature, ‘normal’ or ‘zonal’ soils, >30 cm to rock, or subsoils distinct from parent rock or sediment, and with well-developed horizons (layers) and weathered substrate down to maximum depth of 1m – go to section E below.

Section A (drastically human modified soils)

Anthropic
A1 If semi-toxic, strongly compacted, or over landfill with methane gas being generated, see specialised list of plants.

A2 If neither toxic nor over landfill, go to B, C, D or E below for equivalent soil types for restoration purposes. If topsoils have been scraped then remedial treatment of fertiliser and mulch may overcome some of the deficiencies.

Section B (wet, poorly drained soils)

B1 Organic   
Black-brown peat at or close to surface (>30cm thick);
Infertile usually acid

B2 Gley
Peat <30 cm thick;
Subsoil bluish grey or grey (with rust coloured mottles);

Freshwater habitats associated with streams, steep slopes, foodplains, fans or dunes

Peaty Gley, Recent Gley, Perch-gley Pallic Soils

Freshwater habitats on terraces and undulating to hilly slopes

Perch-gley Podzols, Perch-gley Ultic soils

Saline habitats

Saline Recent Gley

Section C (young, well-drained soils)

C1    Rock exposures on cliffs, ledges, or outcrops, with fresh unweathered very shallow subsoil (total soil <30 cm thick) more or less the same colour as the underlying parent rock or sediment:

Rendzic Melanic

Parent material limestone or other white or cream coloured limey rock (that fizzes in reaction to a drop of battery acid);
Topsoil is black & nutty:
Regional endemic plant species; see rock/scree garden list.

Rocky Raw

Any parent material (other than limestone or other white or cream coloured limey rock);
Topsoil absent or <5cm thick;
See rock/scree garden list.

Rocky Recent

Any parent material (other than limestone or other white or cream coloured limey rock);
Topsoil is >5cm thick;
See rock/scree garden list.

C2    Rock is deeper than 30cm or is not present

Raw

Topsoil absent or <5cm thick;
Low N-fertility, high base & P fertility;
The dominant materials to depth of 50cm are:

Very stony (many stones >2mm),
Sandy (dunes) (gritty with visible grains) - cf Brown Soils if rainfall >1000 mm,
Loamy or silty (mixed, or can’t be moulded without cracks when moist) - cf Pallic Soils if rainfall >800 mm,
Clayey or silty clay (including loess) (may be moulded without cracks when moist - cf Pallic Soils if rainfall >750 mm;

See rock/scree garden list or Semiarid Soils for coarse textures or rainfall <500 mm.

Recent

Topsoil is >5cm thick
Fertile, well drained;
The dominant materials to depth of 50cm are:

Very stony (many stones >2mm),
Sandy (dunes) (gritty with visible grains) – cf Brown Soils if rainfall >1000 mm,
Loamy or silty (mixed, or can’t be moulded without cracks when moist) - cf Pallic Soils if rainfall >750 mm,
Clayey or silty clay (including loess) (may be moulded without cracks when moist) – cf Pallic Soils if rainfall >700 mm;
See rock/scree garden list for coarse textures and rainfall <500 mm.

Section D (ancient soils on old stable landscapes; usually poorly drained)

D1    Pale upper subsoil passing to yellow or yellow brown. Parent rocks are light or mid grey coloured greywacke, schist, granite or rhyolite ash.

Ultic
Subsoil clayey (or sandy on stable Northland dunes);
Subsoil colour pale grey, passing to yellow or yellow brown;
Acid, infertile, impermeable

D2    Brown to reddish brown subsoils. Parent rocks are dark coloured basaltic or andesitic volcanic rocks.

Oxidic
Subsoil clayey;   
Subsoil colour reddish brown to grey brown;
Only fine subsoil structure
Infertile, limited root depth [because of firmness], upper layers well-drained.

Granular
Subsoil clayey;
Subsoil colour reddish brown to brown;
Fine and coarse subsoil structure;
Slowly permeable, infertile.

Section E (moderately mature, well-drained, “normal” soils)

E1    Rainfall <500mm per year. Soil is usually very dry in spring through to late autumn; fertile.

Semiarid
Subsoil grey-brown;
White patches of lime in many subsoils that fizz in reaction to strong acid; [Note automotive battery acid may be used with care by using a clean twig to transfer a small drop of acid to the soil sample];
Confined to the lowlands of Central Otago and Mid Waitaki Basin.

Melanic
Topsoil black & nutty;
Parent material dark volcanic rock.

E2    Rainfall 500-800 mm in the South Island, or 700-1000 mm in the North Island. Soil is usually droughty during summer.

Pallic
Subsoil pale yellow to straw coloured;
North Island, rainfall <1000 mm;
South Island, rainfall 500–800 mm;
In silty to loamy soils;
Moderately fertile, low permeability, droughty summers, moist winters.

Brown
Subsoil yellow-brown;
North Island, rainfall >1000 mm;
South Island, rainfall > 800 mm;
In sandy or stony to loamy soils;
Moderate to low fertility.

Melanic
Topsoil black & nutty;
Parent material

limestone or other limey rock
or dark volcanic rock;

Fertile.

E3    Rainfall >1000mm per year, soil moist in summer in most years,
but not continuously wet.

Pumice
Parent material is in pumice, scoria or sandy volcanic ash;
Rainfall >1200 mm;
Confined to Central NI or Auckland;
Deep rooting medium, well drained, infertile.

Allophanic
Parent material is volcanic ash;
Rainfall >1200 mm;
Subsoil fine, friable, light, uniform, with greasy texture and brown colour;
Well drained, moderate fertility.
       
Melanic
Topsoil black & nutty;
Parent material

limestone or other limey rock
or dark volcanic rock;

Fertile.

Brown
Subsoil yellow-brown;
Moderate fertility, not poorly drained.

Podzols
White to grey layer below topsoil passing with depth to darker material;
Rainfall >1400 mm;
Infertile, acid, poor rooting depth.