Glossary
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Aeration
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Presence of air-filled space in the soil profile. 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.
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Allophane
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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.
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Alluvium
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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.
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Anaerobic
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Absence of air. The condition that pertains when the soil is waterlogged.
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Argillic
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Soils with a subsurface horizon into which clay has moved. It has at least 5% more clay than the horizons above.
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Backswamp
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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.
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Base saturation (BS)
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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.
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Bedrock
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The solid rock that underlies soil or other loose material.
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Bulk density
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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.
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Cation exchange capacity (CEC)
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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.
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Clay
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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.
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Colluvium
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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.
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Deep
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Soil that is 90 cm or more deep to gravel or bedrock is described as deep.
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Depth (phase)
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Soils are assigned to one of five depth phases according to their depth above gravel or bedrock:
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0 Deep: > 100 cm
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1 Moderately Deep: 45-100 cm
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2 Shallow: 20-45 cm
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3 Stony: 10-20 cm
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4 Very Stony: < 10 cm
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Downlands
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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.
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Drainage
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See Profile drainage
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Erosion
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The wearing away of the lands surface by running water, wind, ice, or other agents.
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Fans
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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.
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Field Capacity
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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.
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Floodplain
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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.
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Floodplain bench
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A long narrow relatively level or gently inclined strip or segment of a floodplain, sufficiently elevated not to be within the flooding zone.
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Fragipan
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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.
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Interfluve
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The raised area between two adjacent streams flowing in the same direction.
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Landform
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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.
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Leaching
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The removal of dissolved materials from the soil by water.
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Levee
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Any naturally produced low ridge, but usually built of sand and silt by a stream on its floodplain.
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Loess
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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.
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Lowland
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Low-lying land, often near the coast, (usually <450 m asl).
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Metamorphic (rock)
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Rock whose nature has been transformed by natural geological processes, usually heat and pressure, from a pre-existing form.
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Micro-nutrients
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Chemical elements needed in only very small (micro) amounts for the growth of plants, e.g., boron, molybdenum.
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Mineral soil / horizons
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Mineral horizons containing < 18% organic carbon.
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Mottles, Mottled, Mottling
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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.
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Nutrient leaching
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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.
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Organic carbon
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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.
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Organic matter
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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.
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P-retention
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See Phosphate retention.
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Parent material
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The geological origin of the sediments or rocks from which the soil has formed.
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Ped
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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.
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Perched water table
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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.
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Permeability
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A measure of the rate at which water can flow through the soil. A soil type’s overall permeability is usually based on the horizon with the slowest permeability class, and the depth at which this layer occurs. Permeability classes and their equivalent flow rates (mm/h) are:
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Slow: < 4
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Moderate: 4-72
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High: > 72
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pH
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A measure of the acidity or alkalinity of a soil.
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Phase(s)
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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.
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Phosphate retention
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A measure of the ability of the soil to remove phosphorus from solution, holding onto it firmly, tending to make it unavailable to plants. In acid soils the phosphorus is retained by reactive aluminum and iron minerals. High P-retention values indicate that plants will give a lower response to the same amount of phosphate fertiliser than those plants on a soil with low P-retention.
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Plateau
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An extensive flat area elevated above the surrounding land.
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Polygon
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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.
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Potential rooting depth (PRD)
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The effective depth of soil that can be exploited by the rooting systems of most common crops, and that can provide a medium for root development, water and nutrient uptake.
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Profile available water (PAW)
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The amount of water that would be available to a grass/pasture cover within the soil profile to a depth of 1 m when the soil is at field capacity. PAW is expressed in units of millimetres of water where a PAW of 100 mm implies that 10% of the soil volume is water available to plants. PAW takes into account variations in soil horizons, and is calculated by percent area weighted averages using the proportions of the siblings where more than one soil sibling is mapped in a soil polygon.
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Profile drainage
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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.
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Profile readily available water (PRAW)
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The amount of water held in a soil that can be easily absorbed by plant roots.
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Rolling
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One of the five slope phases to which all soils were assigned (see Slope phase); land with a slope of 8–15°.
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Rooting barrier
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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.
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Rooting depth
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See Potential rooting depth
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Sand
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The coarsest of the three soil textural classes (sand, silt and clay); a soil particle between 0.06 and 2.0 mm in diameter.
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Shallow
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Soil that is 20–45 cm deep to gravel or bedrock. One of the five depth phases used in the description of soils.
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Short-term waterlogging
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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)
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Silt
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The intermediate soil textural class between sand and clay; a soil particle between 0.002 and 0.06 mm in diameter.
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Slope (phase)
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Soils may be assigned to one of five slope phases according to the slope angle of the landscape on which they are located:
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G = Gently undulating (0-3°)
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U = Undulating (4-7°)
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R = Rolling (8-15°)
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H = Hilly (16-25°)
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S = Steep (> 25°)
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Soil complex
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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.
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Soil horizon
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A distinct layer in a soil profile, with different physical, chemical and biological properties from adjacent layers.
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Soil organic matter
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The organic part of the soil. Includes plant and animal remains and products at various stages of decomposition.
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Soil profile
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A vertical section of a soil showing all its horizons to 90 cm depth.
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Soil structure
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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.
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Soil texture
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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.
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Steep
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One of the five slope phases to which all soils were assigned (see Slope phase); land with a slope of >25°.
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Stony
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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.
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Structural vulnerability
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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.
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Subsoil
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A general term for the lower horizons of a soil; usually B horizons and below.
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Terrace
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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.
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Topsoil
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A general term for the upper part of a soil; usually the A horizons.
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Undulating
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One of the five slope phases to which all soils were assigned (see Slope phase); land with a slope of 4–7°.
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Upland
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An extensive region of relatively high land, usually distant from the coast.
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Variant(s)
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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.
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Very stony
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Soil that is <10 cm deep to gravel or bedrock. One of the five depth phases used in the description of soils.
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Waterlogging
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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 whereanaerobic conditions are sustained for periods of greater than one week due to a high groundwater table or perched water table.
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Watertable
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At a depth below the surface, the ground is saturated with water. The upper surface of this zone of saturation is termed the watertable.
