Presence of air-filled space in the soil profile. Roots need to be able to breathe. Inadequate aeration has adverse effects on root function and can result in suppression of yield and crop failure. Aeration limitation is defined as the proportion of the year in which anaerobic conditions may prevail in the soil.
A non-crystalline soil mineral; an oxide of silicon and aluminium with a high water content, variable-charge surfaces, and a very high surface area.
Material that has been deposited by water action. Alluvial deposits are described as fine where they are dominated by particles less than 2 mm in diameter, and gravelly when particles are greater than 2 mm.
Absence of air. The condition that pertains when the soil is waterlogged.
Soils with a subsurface horizon into which clay has moved. It has at least 5% more clay than the horizons above.
A natural swampy depression with poor drainage developed in the floodbasin behind the river levees on a floodplain, in which layers of silt and clay are deposited.
Base saturation (BS)
A general measure of soil fertility and leaching. Base saturation is calculated as the sum of the exchangeable cations of calcium, magnesium, potassium, and sodium, expressed as a percentage of the total number of sites available for cation exchange (CEC). A low to very low BS indicates strong leaching. The BS of a soil can be raised by adding lime, or by adding fertilisers containing cations in a readily available form.
The solid rock that underlies soil or other loose material.
Bulk density
The mass of oven-dry soil contained in a sample of known volume. Soils with low bulk density generally have fewer problems of root penetration and water permeability than soils with high bulk density.
Bypass flow
An assessment of the potential for water infiltrating at the soil surface to bypass the soil matrix as it drains through the soil. Soils with coarse structure (>10-mm diameter) have a high susceptibility to bypass flow. Bypass flow allows nutrients and bacteria to bypass the soil material on their way from the soil surface to the groundwater below. Some soils, such as Granular soils, are more prone to bypass flow than others.
Cation exchange capacity (CEC)
The total number of sites in a soil available for cation exchange; a measure of the ability of the soil to retain added nutrients such as calcium, magnesium, and potassium. Nearly all the cation exchange sites are on the surfaces of clay particles or organic matter. Consequently, soils with large amounts of clay or organic matter usually have a higher CEC than soils with low amounts of clay or organic matter.
The finest of the three texture classes of soil, the others being sand and silt; soil material consisting of particles <0.002 mm in diameter. Clay is responsible for the swelling and shrinking properties of soil and the stickiness or slipperiness of moist soil, different clay minerals having different characteristics in this regard; plant nutrient elements (cations) attach themselves to the clay particles in the soil. Organic matter works with clay particles to hold and supply nutrients to plants.
Rock fragments and soil material, which have accumulated on slopes as a result of gravity. Colluvial deposits may also be described as fine or gravelly.
Dairy effluent (FDE) risk category
Five soil classes (A to E) based on the DairyNZ guide to determining soil risk for farm dairy effluent application.
Soil that is 90 cm or more deep to gravel or bedrock is described as deep.
Depth Class
Otherwise known as ‘diggability’, this is the soil depth to the layer that makes digging difficult. This could be a very stony layer, a soft or hard rock surface, shattered rock or other firm material. Depth class is classified as very shallow (<20 cm), shallow (20–40 cm), moderate (40–90 cm) and deep (>90 cm).
Depth (phase)
Soils are assigned to one of five depth phases according to their depth above gravel or bedrock:
  • 0 Deep: > 100 cm
  • 1 Moderately Deep: 45-100 cm
  • 2 Shallow: 20-45 cm
  • 3 Stony: 10-20 cm
  • 4 Very Stony: < 10 cm
Depth to hard/soft rock
Shallow rock will make soils prone to drought when dry, and may cause the soils to be poorly drained, therefore leading to pugging when the soils are wet.
Depth to slowly permeable horizon
The depth of the soil from the surface down to the slowly permeable layer. If a slowly draining layer of soil is near the surface, the soil will be more susceptible to pugging and surface water ponding, and will not be able to hold much water, making it droughty. If irrigated it would require small, frequent applications.
Downlands are extensive areas of gently to strongly rolling land often with a deep mantle of windblown loess. Downlands may be underlain by terrace gravels or bedrock, but the loess is often the dominant soil parent material.
See Profile drainage
Drainage Class
Indicates how wet a soil is likely to be under high rainfall conditions. Well-drained soils allow water to drain through the profile in all periods of the year. Poorly drained soils either have a water table close to the surface or a compact subsurface layer that limits the rate that water can drain through the soil. Imperfectly drained soils are in an intermediate condition between well-drained and poorly drained. A poorly drained soil is susceptible to pugging and it will lose nitrogen to the atmosphere. A deep, poorly drained soil will cope better in a drought. Well-drained soils can sometimes be droughty, but are less likely to pug. Provided they are not prone to bypass flow, poorly drained soils will not leach to the same extent as well-drained soils.
Drought vulnerability (if not irrigated)
Determined by the amount of water a soil will hold within the potential rooting zone.
Dry bulk density topsoil
The density of the soil measured as the weight of dry soil within a given volume. Different soils have higher or lower volumes of pores within them. The porosity of soil affects the bulk density of the soil. Soil with low bulk density is easy to aerate, so biological activity will be higher, the soil will have good drainage and it will be easier to cultivate. When bare ground is exposed, soil with low bulk density can be prone to wind or sheet erosion. Plants find it more difficult to penetrate into soil with higher bulk densities, and there will be less aeration. High dry bulk density near the surface will also make soil droughty. Examples of different dry bulk densities: a peat soil will have a very low bulk density of level of 0.03–0.4 tonnes/m3, an Allophanic soil will have a low bulk density level of less than 0.9 tonnes/m3, whereas Pallic Soils are typically more dense, about 1.2–1.4 tonnes/m3.
Dry bulk density subsoil
As above but below 20 cm depth.
Erodibility of soil material
A measure of the likelihood of soil particles detaching from the soil surface as water flows over it. Factors involved in determining the erodibility of a soil include phosphate retention, clay content and organic matter levels.
The wearing away of the lands surface by running water, wind, ice, or other agents.
See Soil family
Gently sloping, fan-shaped masses of material formed along the margins of hills and mountain ranges by the streams that drain their slopes. A fan commonly occurs where there is a marked decrease in gradient, for example where a stream meets the gentler floodplain or river terrace. Fan gravels are generally sub-angular in shape, while those of river terraces and floodplains are more rounded.
Field Capacity
The total amount of water that a soil contains after water has drained away by gravity over a period of two days after it has been saturated by rainfall.
Relatively smooth land adjacent to a river or stream channel; built of alluvium deposited by that river or stream, which, in the absence of flood protection works, may still be subjected to flooding.
Floodplain bench
A long narrow relatively level or gently inclined strip or segment of a floodplain, sufficiently elevated not to be within the flooding zone.
A natural sub-surface horizon with high bulk density, and sufficiently compacted to provide a barrier to root penetration. Fragipans are slowly or very slowly permeable to water. The presence of a fragipan frequently gives rise to impeded drainage and perched water tables.
Functional horizons
A functional horizon is a soil layer defined by stoniness, texture, structure size and soil density. The soil profile is described by a set of soil functional horizons (or layers parallel to the soil surface). The horizons are listed from the soil surface at the top, to the base of the soil profile at the bottom.
Hydrological soil group
Four soil classes (A to D) that classify soils according to their vulnerability to runoff. Class A soils have rapid permeability and rainfall and irrigation is readily transmitted through the entire soil profile and Class D soils can only infiltrate a small amount of water at the soil surface before water begins to pond or run off.
The raised area between two adjacent streams flowing in the same direction.
Ease of irrigation. This includes factors such as the slope of the land and intensity of management.
The characteristic shape of the earth’s surface on which a soil type is developed. Landform types include floodplains, terraces, fans, downlands, moraines, hill and steep lands.
The removal of dissolved materials from the soil by water.
Any naturally produced low ridge, but usually built of sand and silt by a stream on its floodplain.
A blanket deposit of windblown silt-sized material. Although loess is being deposited continually, extensive deposits occurred mostly during the ice ages, when glaciers were producing large quantities of ground-up rock dust.
Low-lying land, often near the coast, (usually <450 m asl).
Metamorphic (rock)
Rock whose nature has been transformed by natural geological processes, usually heat and pressure, from a pre-existing form.
Chemical elements needed in only very small (micro) amounts for the growth of plants, e.g., boron, molybdenum.
Mineral soil / horizons
Mineral horizons containing < 18% organic carbon.
Mottles, Mottled, Mottling
Spots or blotches of colour different from the predominant soil colour. Often mottles are rusty red in colour, indicating the presence of iron oxides. Mottles indicate that there are periods of restricted profile drainage. The severity of the restriction to profile drainage is indicated by the abundance and depth at which mottles and gleying occur.
N leaching vulnerability
The susceptibility of the soil to nitrogen leaching. See also Nutrient leaching
Nutrient leaching
The process whereby excess nutrients not held by the clay and organic matter complex of the soil leach beyond the root zone and contaminate groundwater.
Organic carbon
The amount of carbon stored in the soil — it is expressed as a percentage by weight (g C/kg soil) and is closely related to the amount of organic matter in the soil. There are two discrete pools in which soil carbon is stored: the soil’s microbial biomass and easily-decomposed plant residues that are cycled rapidly and in which carbon may reside for as little as a few weeks, and; the pool in which carbon is more tightly held by physical encapsulation within soil aggregates (singular masses of coherent soil particles, or peds) or chemical complexing—here, carbon may reside in the soil for tens of thousands of years.
Organic matter
The organic as distinct from mineral component of the soil, including plant and animal residues at various stages of decomposition and the cells and tissues of soil organisms; the lifeblood of productive soil, it plays a crucial role in improving soil structure, storing and recycling nutrients and modifying soil pH.
P leaching vulnerability
The susceptibility of the soil to phosphorus leaching. See also Nutrient leaching
See Phosphate retention.
Parent material
The geological origin of the sediments or rocks from which the soil has formed.
Parent material origin
Describes the origin of the soil material (rock fragments and fine earth) to 100 cm depth or to a rock contact. Classes for parent material origin reflect the mode of origin and emplacement of parent material. For example the fine earth may be derived from in situ weathering of rock, or formed in alluvium or volcanic ash. Up to two classes can be entered where the soil material is derived from more than one source (e.g. loess over alluvium).
A unit of soil structure; an aggregate of soil particles formed by natural processes, as distinct from a clod, which is formed artificially by humans.
Perched water table
A zone in the soil where, due to an impermeable layer such as a fragipan, water is unable to percolate downwards and the water table is said to be ‘perched’ above the impermeable layer of soil.
The ability of a soil to drain (regardless of external influences on water tables, e.g. being low-lying in the landscape). Permeability is affected by texture, structure and soil density. A soil with moderate permeability is better for soil productivity, drainage, nutrient retention and effluent adsorption than a soil with slow permeability. Permeability profile is often expressed as two values such as ‘moderate over slow’. This is where a layer of moderate-draining soil sits above a layer of slow-draining soil. Permeability of a profile under extended rainfall conditions is governed by the permeability of the layer that is most slowly permeable. Permeability classes and their equivalent flow rates (mm/h) are:
  • Slow: < 4
  • Moderate: 4-72
  • High: > 72
Permeability of slowest horizon
The permeability of the slowest permeable layer of the soil. As well as being expressed as ‘slow’, ‘moderate’ or ‘rapid’, this is also expressed as the movement of water in millimetres per hour. For example, if soil has a slowly permeable layer (less than 4 mm/h) then ponding can occur if effluent is applied at a greater rate, especially if that slowly permeable layer is close to the surface.
A measure of the acidity or alkalinity of a soil.
Minor variations in the soil type. While these variations are expressed on the soil map, they are not considered to have a significant difference in overall soil properties sufficient to justify a new soil series.
Phosphate retention
A measure of the soil’s ability to adsorb P onto soil particles. In most soils there is sufficient P retention to retain added P in the topsoil and thus minimise leaching of P. Some soils, particularly those with high amounts of allophanic material in them (volcanic soils), bind P so tightly that a significant proportion of the added P is not available to plants. Phosphorus accumulates in these soils over time.
An extensive flat area elevated above the surrounding land.
A soil mapping unit; a defined area of soil type distinguished on the soil maps by a black line border, coloured fill and identified by soil symbols.
Potential rooting depth (PRD)
The depth of soil that a plant can exploit water from. In soil with a deep potential rooting depth, some grass roots will penetrate to over 100 cm deep. When plants extract water and nutrients from deep in the soil they are more resistant to drought and the soil is less prone to leaching.
Profile available water (PAW)
The amount of water potentially available to plant growth that can be stored in the soil to 100 cm depth. PAW takes into account variations in soil horizons and is expressed in units of millimetres of water, i.e. in the same way as rainfall. A PAW of 100 mm implies that 10% of the soil volume is water available to plants. Low PAW is <60 mm, moderate is between 60 and150 mm, and high is ≥150 mm.
Plants can only extract water where roots can grow. Thus where a root barrier occurs within 100 cm, the PAW reported will be the PAW to the root barrier. It is important to recognise that PAW is a potential value and not all the water is equally available. For example, as the soil dries out the water becomes more difficult to extract. As a general ‘rule of thumb’, plant growth will begin to slow down when 50% of PAW has been extracted. There are some crops that have shallow rooting depth, e.g. potatoes usually only root to a depth of 60 cm. In this case the PAW to 60 cm depth should be used.
Top 60 cm available water As above but to a depth of 60 cm.
Top 30 cm available water As above but to a depth of 30 cm, available to the widest range of crops, including shallow-rooting grasses and crops.
Note that the Soil Moisture – Profile Available Water map shows the average PAW in the polygon (i.e. area-weighted by the sibling proportions).
Profile drainage
Profile drainage provides an indication of how long a soil, or part of a soil, is saturated with water, and how quickly it can rid itself of excess water.
Profile readily available water (PRAW)
The amount of water held in a soil that can be easily absorbed by plant roots.
Pugging vulnerability
The risk of the topsoil becoming deformed by heavy animals when the soil is wet.
Rock class of stones/rocks
Describes the rock class of underlying rock (if rock occurs within 100 cm) or of stones occurring within the soil profile within a depth of 0–100 cm. Twenty-two rock classes have been identified to record rock type.
Rock origin of fine earth
Describes the rock class from which the fine soil material (< 2 mm) within a depth of 100 cm has been derived.
One of the five slope phases to which all soils were assigned (see Slope phase); land with a slope of 8–15°.
Rooting barrier
Root barriers control the depth of soil that is available for roots to extract water and nutrients, and to anchor the plant. Tightly packed, very stony layers, sandy very stony layers, rock, hard pans, compact soil layers, or wetness in the form of high, prolonged, water table can all form barriers to root penetration.
Rooting depth
See Potential rooting depth
Runoff potential
The susceptibility for nutrients (particularly P), and bacteria, bound to soil particles to run off the soil surface into waterways.
The coarsest of the three soil textural classes (sand, silt and clay); a soil particle between 0.06 and 2.0 mm in diameter.
Soil that is 20–45 cm deep to gravel or bedrock.  One of the five depth phases used in the description of soils.
Short-term waterlogging
The risk of short periods (up to one week) of anaerobic conditions in the soil after heavy rain, which can have marked effects on plant growth and yield. (See also Waterlogging)
The intermediate soil textural class between sand and clay; a soil particle between 0.002 and 0.06 mm in diameter.
Slope (phase)
Soils may be assigned to one of five slope phases according to the slope angle of the landscape on which they are located:
  • G = Gently undulating (0-3°)
  • U = Undulating (4-7°)
  • R = Rolling (8-15°)
  • H = Hilly (16-25°)
  • S = Steep (> 25°)
Soil classification
The New Zealand soil classification, which describes the characteristics, qualities and limitations of different soils. More information can be found here.
Soil complex
A soil polygon on the soil map that includes more than one soil type within its boundary. S-map is designed to provide more realistic information on the range of soil-individuals expected within map units and allows for up to five individuals to be shown on a map unit. This means that S-map contains more information and conveys soil complexity in a more realistic way.
Soil family
Families of mineral soils are identified by four sets of criteria describing soil materials occurring within 100 cm of the soil surface. The criteria are: soil-profile-material class, rock class, texture class and the permeability class of the slowest horizon. Soil families are given a geographic name and are also identified with a 4- or 5-character abbreviated name. The family name is suffixed with an italicised f to distinguish the family from the old soil series name. For example Templeton family is written as Templetonf and is abbreviated as Temp.
Soil horizon
A distinct layer in a soil profile, with different physical, chemical and biological properties from adjacent layers.
Soil organic matter
The organic part of the soil. Includes plant and animal remains and products at various stages of decomposition.
Soil profile
A vertical section of a soil showing all its horizons to 90 cm depth.
Soil profile material
Soils are assigned to one of 10 ‘soil-profile-material’ classes depending on the presence or absence, within specified depths, of distinctive types of mineral substrates with 100 cm depth (e.g. gravels, bedrock).
Soil structure
The way in which soil particles are aggregated into soil peds. Structure is described by ped size (fine, medium, or coarse), shape, and how strongly they are formed (weakly, moderately or strongly). The presence of peds is important because spaces are left between and within them. These spaces are necessary for root growth and the movement of water and air within the soil.
Soil texture
Soil texture is used to describe the particle distribution of those particles in a mass of soil that are less than 2 mm in diameter. Particles coarser than 2 mm are described as gravel and are not regarded as a textural component. Soil texture is usually described as a class determined from a standard texture triangle based on the relative proportions of sand, silt and clay.
One of the five slope phases to which all soils were assigned (see Slope phase); land with a slope of >25°.
Soil that is 10–20 cm deep to gravel or bedrock is described as stony; one of the five depth phases used in the description of soils.
Structural vulnerability
Untimely cultivation, grazing and forestry operations when soils are wet give rise to a rapid and marked loss of soil structure and a reduction in soil permeability and aeration owing to the compaction, deformation and consolidation of the topsoil and upper subsoil. Some soils are more at risk than others due to their poor resistance and resilience to compacting forces, poor drainage, limited water holding capacity, or higher rainfall.
A general term for the lower horizons of a soil; usually B horizons and below.
Terraces are near level, narrow plains bordering rivers of various ages and heights formed when river floodplains are abandoned during periods of land uplift, river incision or falling sea level. Each major river has an associated sequence of terraces, and these are described as Old, Intermediate, or Young according to their position and relative elevation in that sequence.
Texture Profile
There are four soil texture types with different qualities and limitations. They are sandy (<8%clay and <40% silt) clayey (>35% clay) and silty or loamy. Loamy soils are similar to silty soils but have less than 40% silt.
A general term for the upper part of a soil; usually the A horizons.
Topsoil clay range
An indication of the clayey texture of the topsoil. The amount of clay and sand in the topsoil have effects on susceptibility to erosion and compaction and affect land management practices such as cultivation.
Topsoil stoniness
The volume of stones in the topsoil. The majority of the life-supporting capacity of the soil is in the topsoil, i.e. the top 20 cm. Stones in the topsoil limit root density, its ability to store nutrients and water, and make the ground more difficult to work.
One of the five slope phases to which all soils were assigned (see Slope phase); land with a slope of 4–7°.
An extensive region of relatively high land, usually distant from the coast.
Soil variants are described where there is a difference in soil properties that is sufficient to justify separation into a new soil. The soil is recorded as a variant because it does not occupy a significant area in its own right (usually less than 200 ha) or occurs in a complex that cannot be separated from the main soil type at the scale of mapping used.
Very stony
Soil that is <10 cm deep to gravel or bedrock. One of the five depth phases used in the description of soils.
Periods of anaerobic conditions in the soil after heavy rain or flooding, which can have marked effects on plant growth and yield. Short-term waterlogging is where anaerobic conditions may occur after heavy rainfall for periods of up to one week Waterlogging is where anaerobic conditions are sustained for periods of greater than one week due to a high groundwater table or perched water table.
Waterlogging vulnerability
The vulnerability of the soil to being saturated, with little or no air in the soil pores for some or all of the year. Waterlogging is dependent on texture, structure, and permeability. Waterlogging limits the range of crops that can be grown, increases the risk of pugging, N and P losses, and ponding of effluent at the soil surface.
At a depth below the surface, the ground is saturated with water. The upper surface of this zone of saturation is termed the watertable.