A
A-horizon
The A-horizon is often called topsoil; it is the top layer of mineral soil, immediately above the B-horizon or subsoil. Commonly at 0–100 millimetres depth in the south-western agricultural region of Western Australia, the Ahorizon contains more soil organic matter, nutrients and soil organisms than the B-horizon and is usually visually distinguishable by its darker colour. In some soil profiles, the A-horizon can be further divided into A1, A2 and A3 horizons. A layer of decomposing organic residues accumulated on top of the A-horizon is known as the O-horizon.
Accessibility of soil water
The accessibility of plant available water refers to the water potential of the plant available water. If the water potential is close to zero, water is readily available to plants. If the water potential is close to permanent wilting point (-1.5 megapascals) plants are so challenged by the lack of water that they die. When the soil solution is more saline, the water potential is lower (more negative), reducing the accessibility of soil water to plants.
Acellular
Organisms comprising a single cell, or lacking intact cells.
Adsorb
Adsorption describes the process by which an atom or molecule transfers from a gas, liquid or dissolved solid phase and adheres to the outer surface of another substance as a result of surface forces.
Adsorbed
A substance that is adhered to the outer surface of another substance as a result of surface forces is termed to be adsorbed.
Adsorbent
The substance onto which another substance can adsorb (get attached to the external surface).
Adventitious roots
Roots that grow from somewhere other than a root, such as a stem or leaf.
Aeolian accessions
Wind-blown deposits
Aeolian dust
Wind-blown dust.
Aerenchyma
Spongy plant tissue with air-filled spaces that allow for gas exchange, typical in aquatic plants.
Aerobic
Aerobic organisms require oxygen.
Agar plate
An agar plate is a Petri dish that contains a growth medium solidified with agar, used to grow microorganisms.
Aggregates
Soil aggregates are made up of sand, silt and clay particles bound together to form the soil matrix.
Agricultural Production Systems Simulator (APSIM)
Agricultural Production Systems Simulator (APSIM) is an internationally recognised model that simulates biophysical processes in agricultural systems. It predicts crop production in relation to selected climate, plant, soil and management factors over time. Foundation members are CSIRO, the State of Queensland and the University of Queensland. AgResearch Ltd., New Zealand, the University of Southern Queensland and Iowa State University followed as members.
Air-filled porosity
Volumetric content of air in soil.
Alcohol
Alcohols are characterised by one or more hydroxyl (OH) groups attached to a carbon atom in place of a hydrogen atom. Sterol and phytanol are organic alcohols.
Algae
A large, diverse group of eukaryotic organisms that photosynthesise. Algae can be unicellular or multicellular and occur in both terrestrial and marine environments, including both freshwater and saltwater environments. Algae require sunlight, nutrients and water for survival. Excess nutrients cause algae to undergo exponential growth and can lead to algal ‘blooms’ capable of depleting oxygen to a point where the environment can be deadly to other organisms. Algae lack true roots, stems, leaves and vascular system and can range in size from microscopic, single cells to fronds of giant kelp up to 60 m in length.
Alkanes
Alkanes consist of chains of carbon (C) atoms with hydrogen (H) atoms bonded to each carbon atom. These carbon chains vary in length. Phytane consists of 20 carbon atoms and a total 42 bonded hydrogen atoms.
Allelochemicals
Allelochemicals are secondary metabolites produced by plants, animals and microorganisms that are released from one organism to stimulate, regulate or inhibit another organism – affecting growth, physiology, behaviour or survival.
Aluminosilicate
Basic building blocks of clay particles consisting of oxides of silicon and aluminum.
Amino acids
Amino acids are organic compounds that combine to form proteins. Amino acids and proteins are the building blocks of life and are made up of carbon, hydrogen, oxygen and nitrogen – and for some, other elements found in the side chains.
Amoeba
Amoeba is a term used to describe single-celled eukaryotic organisms that can change shape and move around in a crawling fashion by extending their pseudopodia (‘false feet’). Genetic comparison indicates that organisms grouped as amoeba are not necessarily closely related.
Amphiphitic
Of, relating to, or being a compound consisting of molecules with a polar water-soluble group attached to a water-insoluble hydrocarbon chain.
Anaerobic
Without oxygen.
Anhydrobiosis
Anhydrobiosis refers to the ability of some organisms to survive in a suspended animation state in the event of severe water deficit, during which their metabolism comes to a stop – but is able to be reversed when rehydrated.
Anion
An anion is a negatively charged ion, e.g. nitrate NO3-.
Anthesis
Anthesis refers to the flowering period of a plant. Different plant species flower at different times of the day and for varying lengths of time. For example, in wheat flowering first begins in the central part of the spike, progressing downward and upward over 3 to 5 days with flowers usually open in the early morning and remain open for only a short time. The growth stage over which anthesis occurs in cereal plants is often described using the Zadoks (Z) growth scale as ‘anthesis’ and, in wheat, commences with the first emergence of anthers, usually a few anthers at the middle of the ear (Z60) through to anthesis of the whole ear (Z69).
Apedal soil
Soil that contains no obvious structural units (i.e. aggregates) . Apedal soil is also known as ‘massive’.
Apparent electrical conductivity (ECa)
The electrical conductivity of soil as measured by electomagnetic induction.
Aquifer
An aquifer is an underground body of water held within permeable rock or unconsolidated gravel or sand that allows water to percolate, or ‘flow’, through it.
Archaea
Archaea are a domain (the highest level of division in classification of organisms) of single-celled microorganisms with no cell nucleus (prokaryote). Archaea were previously classified as bacteria and are often morphologically similar (though archaea can be flat or square shaped) but exhibit genetic and metabolic differences that make them unique from bacteria. Archaea can be found in a broad range of habitats including the human body (e.g. methanogens) and are particularly numerous in the oceans. Archaea are a major part of Earth’s life and may play roles in both the carbon cycle and the nitrogen cycle.
Autotrophic
An autotroph is an organism that can produce its own food using light, water, carbon dioxide, or other chemicals without needing a living source of carbon.
Autumn feed gap
The autumn feed gap is the period over the autumn months where pasture or stubble feed for livestock can be insufficient.
B
B-horizon
The B-horizon or subsoil is immediately below the A-horizon or topsoil. The Bhorizon is visually distinct from the A-horizon, containing more clay and less organic matter. The B-horizon may consist of a number of layers. Some overseas methods of soil classifications indicate the top layer of the B-horizon as the E-horizon, or eluvial horizon, where leaching of minerals has occurred.
Bacteria
Bacteria are unicellular (one-celled) organisms, which have cell walls but lack organelles and an organised nucleus. Soil bacteria are approximately 1–3 micrometres in size (far less than a hair’s width) and vary in shape from spheres to rods to spirals. In a handful of soil there can be several billion bacteria. Bacteria are capable of surviving in most habitats including soil and water. Molecular approaches are rapidly enabling us to gain knowledge of previously unidentified species, but many bacteria are still unable to be cultured and most have not been characterised. Bacteria live in either a symbiotic or parasitic relationship with plants and animals. Bacteria are vital in many stages of the nutrient cycle by recycling nutrients.
Bacteroids
Can refer to either a differentiated symbiotic form of the nitrogen-fixing bacteria (Rhizobia); or a genus of Gram-negative, rod-shaped anaerobic bacteria.
Banks
Surface water control structures generally installed on hillsides to prevent the overland flow of excess rain onto lower-lying land.
Base cation
The base cations (positively charged) are calcium, magnesium, potassium and sodium ions. They are the most numerous on the cation exchange complex (except in acidic soils in which aluminium and hydrogen ions predominate) and are alkalinising when they react with water.
Bedrock
Bedrock is unweathered, solid rock. In some places it can be at the surface, for example, at a granite outcrop. Usually it is covered by weathered rock, sediments and soil.
Biochemical transformations
A biochemical transformation refers to a reaction regulated by enzymes, where one molecule transforms to a different molecule inside a cell.
Biocycling
A biocycle describes the flow of energy and essential substances among species; and between biotic and abiotic environments. An example of this is the cycling and interchange of phosphorous through various biological pools in the soil.
Biodiversity
The variety and variability among organisms. Biological diversity, which is often shortened to biodiversity can be considered within species (genetic diversity), between species (organism diversity) and at the ecosystem scale (ecological diversity).
Biogeochemical
Biogeochemical cycles occur where essential elements of living matter are transferred between living systems. Contraction that refers to the biological, geological and chemical aspects of each cycle.
Biopores
Biopores are fractures, channels or holes created in the soil by living organisms including plant roots, fungi and both macro and microfauna. Biopores support the exchange of water and gases, movement of soil biota and provide a pathway for root growth.
Biosurfactants
Biosurfactant usually refers to surfactants (biological wetting agents) of microbial origin.
Biota
The biota is all the living organisms within a region or habitat. For example the biota of agricultural soil includes plants; algae; fungi; soil fauna such as termites, worms, mites and nematodes; and microorganisms.
Biotite
A distinct dark-coloured mineral that falls under the mica group, recognised for its layered composition and glossy sheen on cleavage surfaces. It contains magnesium and potassium within its sheet-like structure, setting it apart from muscovite within the same group.
Boron-toxicity
Consequences to plant metabolism and growth, of high concentrations of boron in soil.
Buffering capacity
Buffering capacity in regards to acidity, is the ability of the soil to resist a change in pH. Finely-textured clay soils often have greater buffering capacities than coarse-textured sandy soils.
Bulk density
Bulk density is a measure of the weight of dry soil in a given volume. The soil volume is the combined volume of solids and pores, which may contain air and/or water. Bulk density is usually measured by pushing a bulk density ring (cylinder) of known volume into the soil and excavating around it to carefully remove the ring and soil. The soil is then dried and weighed. The bulk density of the soil (g/cm3) is equal to the dry weight of soil (g) divided by the soil volume (volume of the ring) (cm3). The critical value of bulk density of restricting root growth varies with soil type, but in general, bulk densities greater than 1.6 g/cm3 tend to restrict root growth. Sandy soil usually has higher bulk density (1.3–1.7 g/cm3) than fine silt and clay soil (1.1–1.6 g/cm3) because sand has larger, but fewer pore spaces. In clay soil with good soil structure, there is a greater amount of pore space because the particles are very small and many small pore spaces fit between them. Soil rich in organic matter (e.g. peaty soil) can have bulk density less than 0.5 g/cm3.
Bund
Embankment used to control the flow of water.
C
C4 plants
C4 plants fix CO2 into a molecule containing four carbon atoms before initiating the Calvin-Benson cycle of photosynthesis. C4 plants includes maize, sugarcane, sorghum and tropical grasses.
Capillarity action
Water movement in fine pores due to surface water tension.
Carbon fertiliser effect
Higher concentrations of atmospheric carbon dioxide can improve crop growth and yields by increasing the rate of photosynthesis. Together with a decrease in the amount of water lost through transpiration (associated with stomata not opening as much) water-use efficiency in crops can increase and higher yields result.
Cation
A cation is a positively charged ion, e.g. hydrogen H+, calcium Ca++.
Cation exchange capacity (CEC)
Cation exchange capacity is a measure of the ability of soil (or organic matter) to retain positively charged ions (cations) on particle surfaces, primarily influencing nutrient availability and exchange.
Cellular osmotica
Any substance that acts to supplement osmotic pressure in plant cells and hence retain water in the cell.
Chaff lining
Chaff lining involves the separation of the chaff and weed seed fraction from the straw residue at harvest, with chaff dropped into a narrow line behind the harvester via a chute attached to the main sieve. Alternatively the chaff is diverted into the dedicated wheel tracks/tramlines in a controlled traffic farming system using chaff decks. The chaff lines remain on the soil surface where weed seeds are left to rot, while the straw travels through the rotor to be chopped and spread.
Chemical cementation
Chemical cementation occurs when iron oxides, calcium carbonates or silica, for example, precipitate in the pore space between individual grains of soil under moist conditions. These precipitates permanently occupy the pore space, resulting in cementation of particles under drying conditions and the formation of hard layers in soil.
Chemical precipitation
Chemical precipitation occurs when, as a result of a chemical reaction, a solid is formed from substances that were dissolved in a liquid.
Clay platelets
Clay platelets are made up of silicon-oxygen tetrahedra and aluminum oxygen octahedra which combine to form a crystalline array. Put simply, clay platelets are made up of alumino silicate lattices bound together by electrostatic bonds.
Coleoptile
A pointed, protective sheath protecting the emerging shoot tip in a grass or cereal (monocotyledons).
Colwell-K
The sodium bicarbonate extraction for phosphorus can also be used to predict the plant available potassium in soil. There are other methods for extracting plant available potassium from soil. For soil testing, there is an increase in efficiency in the laboratory and a decrease in cost to the client if both phosphorus and potassium tests can be done on the same bicarbonate extraction.
Colwell-P
The most common soil test for phosphorus in Australia uses a sodium bicarbonate solution to extract phosphorus. The test was developed by Dr Jeff Colwell of CSIRO, and is commonly called the Colwell-P test. There are other methods for testing soil phosphorus, including the DGT (diffuse gradient thin film) test that has shown promising results for predicting plant available phosphorus on soil with high phosphate buffering index (PBI).
Complexes
A complex is formed when atoms or molecules comprising different substances are bound together by chemical bonds.
Compression
Compression is the packing of soil particles from a vertical force.
Compressive strength
Compressive strength is the capacity of a material to resist being pushed together by a load.
Concentration gradient
The difference in the concentration of solutes in a solution from one area to another.
Consortia
Two or more individual or entities (in this case microbial species), coexisting for a common purpose.
Cortical cells
Outer ‘spindle shaped’ cells of the cortex in plants, lying between the epidermis(surface cells) and the vascular, or conducting, tissues of stems and roots. May contain carbohydrates or other substances such as resins, latex, essential oils, and tannins.
Crown roots
A root crown, is that part of a root system from which a stem arises and links the roots to shoots. Crown roots in cereals are postembryonic stem.
D
Decomposition
Decomposition is the process of breaking down organic matter into simpler forms.
Deep seeding points
Also known as deep working points or deep sowing points. Deep tillage points differ from normal seeding points and discs by providing tillage below the depth of seed placement. They are commonly a narrow blade, or knife point, 130 to 230 millimetres in length. The seeding mechanism that follows the deep seeding point allows for seed placement at a depth appropriate for the species being sown.
Deflocculation
Deflocculation is a chemical process.Deflocculation is the breakdown of soil aggregates into the individual soil particles, often occurring in the presence of high levels of exchangeable sodium. Is a synonym of Dispersion.
Degradation of waterways
The degradation of waterways in or near agricultural regions can occur through water run-off containing sediment and nutrients entering the waterways, particularly during high rainfall or storm events. Sediment physically abrades the plant and animal life in the stream and, when excessive, can clog the streambed, making it an unsuitable habitat for many invertebrates and ultimately impacting fish and waterbirds. Altered water flow and leachates also impact on waterway quality, as does clearing of vegetation in the catchment and riparian zones. Excess nutrients can result in algal blooms (eutrophication), leading to low oxygen conditions and making the stream unsuitable for many plant and animal species. Sedimentation, salinisation, chemical contaminants and eutrophication result in reduced species diversity and abundance in waterways and can have an impact far downstream from the source.
Denitrification
Denitrification is the microbial process of reducing nitrate (NO3-) and nitrite (NO2-) to gaseous forms of nitrogen, mainly nitrous oxide (N2O) and nitrogen gas (N2). The most important chemical difference between waterlogged and welldrained soil is that waterlogged soil moves into a reduced state. As soil becomes more reduced, the chemistry is dominated by compounds that do not contain oxygen. Soil bacteria play crucial roles in converting compounds and ions from oxidised to reduced states. Well-drained soil generally contains mineral nitrogen in the form of nitrate (NO3-). Denitrification occurs after soil becomes depleted of atmospheric oxygen. Denitrification is the anaerobic process in which soil bacteria convert nitrate to nitrogen gas in the following sequence: NO3− → NO2− → NO → N2O → N2.
Dicotyledons
Any member of the flowering plants that has a pair of leaves or two cotyledons in the embryo of the seed.
Differential global position system (dGPS)
A system for providing a very accurate position, by calculating the difference between the actual location of a fixed-position ground station and the satellite-located position of the station, and providing a correction signal to a mobile user, either directly from a ground station or via a satellite. Accuracy is typically better than 3 metres and can be better than 1 metre.
Diffuse double layer
The diffuse double layer consists of the cloud of cations (positively charged) that are in solution around negatively charged clay surfaces. They are weakly held by electrostatic forces. The concentration of cations is highest close to the clay surfaces but further away from the clay surfaces, cations are more diffuse.
Diffusion-controlled
A diffusion-controlled process is one where the rate of chemical reaction of two substances is limited by the time it takes for the substances to move (diffuse) together in a solution. Diffusion is inherently a slow movement.
Dispersion
Dispersion is a chemical process. Dispersion is the complete separation of soil aggregates into sand, silt and clay fractions. A term often used interchangeably with Deflocculation.
Dissolution
The process of dissolving.
Domain
Evolutionary trees show the relationships between organisms based upon common ancestry. The three-domain system divides cellular life forms into Archaea, Bacteria, and Eukaryota domains. This taxonomy uses these groupings: Domain—Kingdom—Phylum—Class— Order—Family—Genus—Species.
Drains
Structures used to remove excess water from low-lying land, thereby decreasing waterlogging and improving crop growth.
Dryland salinity
Dryland salinity results from rising water tables bringing stored salt to the surface leading to the accumulation of high levels of salts near the soil surface. In the original, pre-European landscape, the native vegetation used nearly all rainfall, the water-table was deep and salt was stored tens of meters deep in the soil profile. Salt in the soil profile was therefore not a problem. After the arrival of Europeans, much of this native vegetation was cleared for the growth of annual crops and pastures. This new shallow-rooted annual vegetation had reduced water use compared to the original deep-rooted perennial vegetation, leading to a rise in the water table, and the mobilisation of salt to the soil surface. Dryland salinity causes reduced pasture and crop growth and yield, and most commonly occurs in low-lying areas. Dryland salinity can result in progressively increasing bare patches and visible salt crust on the soil surface.
DualEM
DualEM is a brand name of company manufacturing electromagnetic induction instruments. It is widely used in ground based EM surveys where the instrument is towed behind a vehicle on a sled. The units have multiple coil separations that allow EM measurements to be made at differing depths simultaneously within the soil profile.
Duplex soil
Duplex soil has an abrupt change in texture (greater than 1.5 texture units) between the coarser A-horizon and the finer underlying B-horizon. Duplex soil is known as texture contrast soil.
Duricrust
A hard mineral crust formed at, or near, the surface of soil, varying in thickness (centimetres to metres) and formed through deposition of soluble minerals associated with the movement of groundwater.
E
Earth’s Critical Zone
The critical zone is the thin veneer of our planet from the top of the tree canopy to the bottom of the aquifers, upon which humanity is utterly dependent for life support.
Ecosystem
The ecosystem is the living and non-living components of an environment functioning as an interconnected system.
Ecosystem services
Ecosystem services is an overarching term for the wide range of benefits that are freely gained from the natural environment (i.e. soil, plants and animals, air and water). Examples of these benefits include food production, climate regulation, cultural use, decomposition of wastes, recycling of nutrients, cycling of water and pollination of plants.
Electrical conductivity (EC)
The electrical conductivity of soil describes the degree to which the soil solution can conduct electricity and in Western Australia is usually expressed in deciSiemens per metre (dS/m). It is used to give an indication of the salinity of the soil. Soil with more salt conducts electricity to a greater degree. Electrical conductivity in Western Australia is commonly measured using an electrode in the extractant from a mixture of 1 part soil to 5 parts deionised water and is represented by the abbreviation EC1:5. A measurement often used for scientific research is the electrical conductivity of the saturation extract (represented by the abbreviation ECe) in which the electrical conductivity is measured using an electrode in an extract from soil which has been mixed to a saturated paste with deionised water.
Electro chemical stability index
Electro chemical stability index = electrical conductivity ÷ exchangeable sodium percentage The index was proposed to predict dispersion in soil that has varying levels of sodicity and salinity.
Electrolyte
A substance that conducts an electrical current when dissolved in water. This term is often used as a synonym for a salt.
Electrolyte effect
The electrolyte effect is the impact of high salt concentrations in causing flocculation in dispersed soil. The degree of flocculation that occurs depends on the type of salt and the concentration with which the soil is equilibrated.
Electromagnetic induction
Electromagnetic induction sensors measure the electrical conductivity (EC) of the soil over varying depths and are generally towed behind a vehicle; a growing number of agricultural consultants offer these services. The soil’s texture, water and salt content are the primary factors that affect the readings and, with well calibrated models for the site, can be used to map changes in these properties. These surveys are often used to identify soil zones and potential sub-soil constraints (such as soil compaction or salinity).
Electrostatic bonds
Bonds formed by the attraction of oppositely charged ions (i.e. ionic and hydrogen bonds).
Elemental sulfur
Elemental sulfur is pure sulfur; its chemical symbol is S. It is mined from natural deposits or recovered as a by-product from oil and gas production. Elemental sulfur applied as a fertiliser needs to be oxidised to sulfate (SO42-) by bacteria in the soil before it can be taken up by plant roots. This process contributes to soil acidification. Sulfur fertiliser applied as sulfates do not require oxidation and are less acidifying than elemental sulfur.
EM38
An EM38 is an easy-to-use geophysical surveying instrument that reflects net signal arising from soil electrical conductivity, soil moisture and clay content. ‘EM’ refers to electromagnetic and ‘38’ refers to a specific configuration of coils used in the instrument. It is this design principle of inductive electromagnetics that determines how sensitive and deep the signal would travel. The EM38 senses up to 1.5 metres depth. Other configurations in use are EM31 and EM34, sensing as deep as 6 metres and 60 metres respectively.
Endemic
Endemic species are plants and animals that exist only in one geographical location. Species can be endemic to large or small areas of the world – continents, state, island or other defined zone. Indigenous animals may be native but can also be found elsewhere.
Endoparasite
A parasite that lives within the internal organs or tissues of its host.
Endophyte
An endophyte is an organism (often bacteria or fungus) that lives, for at least part of its life cycle, within plant cells, without causing apparent disease. Endophytes have been found in all species of plants. Some are thought to enhance plant growth or nutrient acquisition, while others are considered to improve the plant’s tolerance to stresses such as drought, or increase plant resistance to insects, pathogens and herbivores. The related term endosymbiosis is from the Greek language and helps to explain the meaning of the word: within (endon), together (syn) and living (biosis).
Enzymes
Enzymes are molecules (typically proteins) that act as a catalyst and regulate the rate of nearly all chemical reactions taking place within cells – normally speeding them up.
EPSP
5-enolpyruvylshikimate-3-phosphate synthase, a key enzyme in the shikimic acid pathway. Glyphosate (Group M herbicides) are inhibitors of EPSP, which is required for plant growth.
Eukaryota
Biological domain including any single-celled or multicellular organisms, with cells which each contain a distinct membrane-bound nucleus containing genetic material. Eukaryotes include plants and animals which are multicellular, protoctists (e.g. protozoa, amoeba, slime molds, some algae) and fungi which are usually unicellular.
Evapotranspiration
Water use: sum of plant transpiration and soil evaporation.
Exchangeable sodium percentage
The exchangeable sodium percentage in soil is determined by the percentage of exchangeable sodium ions (Na+) relative to the total exchangeable cations—calcium (Ca2+), magnesium (Mg2+), potassium (K+), sodium and aluminium (Al3+).
F
False break
A false break is a small and discrete autumn rainfall event which is sufficient to germinate the seed, which is then followed by an extended dry period. This results in there being insufficient moisture reserves in the soil for the germinated plant to survive through the following dry period. A false break can result in germinated seedling death, either before or just after emergence. In some cases, the seeds imbibe enough moisture to start germination, but there isn’t enough and the seed starts to rot. False breaks are a particular problem for regenerating pasture as a large proportion of the pasture seed bank can fail to establish, leaving limited seed remaining to establish when more effective and persistent rain arrives later in the season.
Fauna
Soil fauna includes representatives of many groups of terrestrial and aquatic animals. Protozoans, rotifers and nematodes inhabit soil water films. Small arthropods including mites and collembola occupy pore spaces between soil particles not filled with water. Larger invertebrates (mesofauna), such as, earthworms, ants and termites tunnel into soil. All use the soil as their habitat. Soil fauna is divided into microfauna (10–200 μm) including protozoa and nematodes, mesofauna (0.2–2 mm, the same size as a sand grain) including mites, collembolan and tardigrades and macrofauna (>2 mm) including ants, termites, earthworms and beetles.
Feldspar
Any of a group of aluminosilicate minerals that contain calcium, sodium, or potassium.
Field capacity
Field capacity is the maximum amount of water a soil can hold after excess water has drained. Field capacity is commonly referred to as the drained upper limit of the soil.
Fine lime
Fine lime is a general term used to describe agricultural lime with the majority of lime particles less than 150 micron diameter, and typically less than 50 microns.
Flocculate
Flocculation of clay particles that are dispersed in the soil water is the process whereby the particles become grouped together in clumps. Dispersed clay can flocculate when the chemical conditions of the soil water change, for example, in response to the addition of gypsum or some other electrolyte to the soil, or when the pH falls to neutral or acidic levels.
Fungal hyphae
Hyphae are the tubular branching structures produced by multicellular fungi that form a filamentous mat or ‘mycelium’. Hyphae are capable of penetrating the plant root tissue, absorbing nutrients from plants and/or transferring minerals and nutrients from soil into the plant.
Fungi
Fungi include microorganisms such as yeasts and moulds, as well as the more familiar mushrooms. Fungi secrete digestive enzymes into their environment to gain nutrients and do not photosynthesise. Often microscopic in size they are the primary decomposers in ecological systems. A huge diversity of fungi exists with varied ecologies, life cycle strategies, and morphology. An arbuscular mycorrhiza fungi is a type of mycorrhiza often referred to in an agricultural context in which the fungus (AM fungi, or AMF) penetrates the cortical cells of the roots of a vascular plant.
G
Genes
A gene is the basic physical and functional unit of heredity and carries the information that determine your traits. Genes are made up of DNA and are found on chromosomes inside cells.
Gibbsite
An aluminum hydroxide mineral of the oxides and hydroxides group, with structural formula [Al(OH)3].
Gradational soil
A gradational soil has a gradual increase in texture (becomes more clayey) down the soil profile.
Graminaceous
Graminaceous plants are herbaceous or woody plants with hollow jointed stems and long narrow leaves, for example rice, wheat, maize and other cereal crops.
Groundwater
Groundwater is water that is located below the surface of the Earth in aquifers.
Groundwater recharge
Deep drainage or deep percolation of water which then enters an aquifer through the unsaturated zone following natural rainfall events, or where reclaimed water is directed to the subsurface.
Gypsum
Gypsum is a mined product comprised predominantly of calcium sulfate. In agriculture gypsum is predominantly used to ameliorate sodic soil.
H
Halophytic
Halophytic plants have physiology that enables survival and growth in saline soil.
Harvest index
Harvest index is a measure of crop yield that takes into account the amount of biomass produced for the grain yielded and gives an indication of the efficiency of the crop in producing grain – the proportion of the above ground biomass that is grain. It is calculated as the weight of grain harvested divided by the total dry weight of above ground biomass including grain.
Heterogeneity
Heterogeneous refers to a structure with dissimilar components or elements, appearing irregular or inconsistent.
Heterogeneous
Heterogeneous refers to something that has a diverse or inconsistent composition, or shows spatial variance in its measure, for example, a paddock with multiple soil types; such heterogeneity would need to be accounted for in soil sampling.
Heterotrophic
A heterotroph is an organism that cannot produce its own food, instead it obtains nutrition from organic substances. For heterotrophic soil biota, nutrition is mainly obtained from plant or animal litter. Heterotrophs are consumers, not producers, and include all animals and fungi and some bacteria. This contrasts to autotrophic organisms (primary producers), which make organic compounds using carbon from simple substances such as carbon dioxide (CO2) and obtain energy from light or inorganic chemical reactions.
Higher plants
A large group of plants that have vascular tissues (with veins) to distribute resources through the plant.
Homogeneous
Homogeneous refers to something (in this case a soil, or soil properties) that has a similar or uniform composition, with little spatial variance.
Horizon/Soil horizon
Soil horizons are the noticeably different zones that make up a soil profile. For example, the A-horizon, or topsoil, is visually distinguishable from the subsoil B-horizon by its darker colour resulting from a greater concentration of soil organic matter.
Hostile subsoil
Hostile subsoil has physical or chemical characteristics that limit or decrease the ability of crop or pasture roots to grow and access water and nutrients deeper in the soil profile.
Humic or fulvic acid
Humic and Fulvic acids are natural organic compounds that are the main components of the organic fraction of soil.
Hydraulic conductivity
Hydraulic conductivity is a measure of how easily water can pass through soil: high values indicate a permeable soil through which water can pass easily; low values indicate that the soil is less permeable. Hydraulic conductivity is calculated as the ratio of the volume fluid that moves through the soil to the hydraulic gradient.
Hydrophilic
Literally meaning water loving, this refers to the attraction and hence bonding (close association) between a compound and water molecule.
Hydrophobic
Literally meaning ‘water fearing’ or ‘water hating’ and describes a water repelling behaviour; a hydrophobic substance repels water. In water repellent soil, water does not bind with or wet the surface of hydrophobic materials, decreasing infiltration or entry of water into the subsoil.
Hypoxic
Condition of oxygen deficiency.
Hysteresis
The difference in the relationship between the water content of the soil and the corresponding water potential during drying/drainage and wetting processes.
I
Illite
Illite is a clay mineral group. Illite has a moderate cation exchange capacity, is non expandable and a 2:1 silicon to aluminium structure. Presence in Western Australian soil is variable and can be at moderate levels in some soil.
Infiltration
Infiltration is the process by which water moves into and through the soil.
Ingress
The action or capacity of entering. An example being the movement of organisms from the soil into the hydrogel.
Inoculant
Microbial inoculants or bioinoculants are agricultural products that use particular species (e.g. Rhizobium for legume inoculation), or general communities of organisms to promote plant health or combat soil pathogens. Many of the soil microorganisms used as inoculants form a symbiotic relationship with the target plant species where both microbe and plant benefit from the mutual relationship/interaction.
Inoculum
Inoculum generally refers to a small amount of material containing bacteria, viruses, or other microorganisms used to start a culture for inoculation to initiate infection.
Inorganic nutrients
Nutrients can be available in both organic and inorganic forms. Inorganic nutrients in the context of a soil refer to the mineral elements, such as boron, originating from soil or rock; or the inorganic fertilisers such as phosphorus, sulphur and nitrogen. Separating organic and inorganic nutrients can be difficult as many are continually cycling from one form to another. Organic nutrients are bound with carbon and must be decomposed before the nutrients they contain are released.
Ion
Ions are atoms, or a group of atoms, that have an electrical charge (positive or negative). This electrical charge is caused by the loss or gain of one or more electrons.
Ion toxicity
Ion toxicity occurs where ions (e.g. Na+, Cl-, B3+) accumulate in plant tissue in excessive concentrations so as to inhibit crop growth.
Ionic strength
Concentration of ions in solution.
Isotopes
Atoms with the same number of protons but different numbers of neutrons are called isotopes. They share almost the same chemical properties, but differ in mass and therefore in physical properties. There are stable isotopes, which do not emit radiation, and there are unstable isotopes, which do emit radiation. The latter are called radioisotopes.
K
Kaolin
Kaolin is a specific form of the kaolinite group of clays.
Kaolinite
Kaolinite is a group of clay minerals, also known as Kandite. The presence of these clays indicate significant weathering and leaching and are predominant in Western Australian soil. They have low cation exchange capacity, no interlayer cations and are non expandable. Kaolinite has a 1:1 silicon to aluminium structure.
L
Labile carbon
Labile carbon is the biologically available carbon that results from the addition of fresh residues, including plant roots and living organisms, or from plant roots as exudates and is the major food source for soil microorganisms.
Leaching of nitrate
Leaching of nitrate usually occurs in association with rainfall when nitrate in solution moves to a greater depth in the soil profile and away from the root zone, preventing uptake by plant roots.
Leaching; Leaching potential
Leaching is the loss of water-soluble substances from the root zone by the action of water percolating deeper through the soil profile. The leaching potential of soil is the likelihood for leaching to occur.
Leaf blades
The expanded thin and green part of the leaf which performs photosynthesis. It is also known as the lamina.
Ley
The period when arable land is covered with grass or herbage for hay or grazing.
Liming
Liming of agricultural soil is the application of agricultural lime (usually limesand, crushed limestone or dolomitic limestone) to soil to raise the pH and ameliorate soil acidity.
Lipids
A lipid is an organic molecule that is produced by a living organism (biomolecule) that is soluble in non-polar solvents (the molecules lack charges). Three main types of lipids are triacylglycerols (also known as triglycerides), phospholipids and sterols.
Long-chain fatty acids
Fatty acids consist of chains of carbon atoms with oxygen (O) + hydroxyl (OH) bonded at one end creating a hydrophilic end to an otherwise hydrophobic molecule. Esters are derived from fatty acids.
Lower plastic limit
Lower plastic limit the is the water content at which a thread of fine-grained soil starts to crumble when rolled to a diameter of 3 millimetres.
M
Megapascal
A megapascal (MPa) = 1 000 000 pascal units, which is the SI* unit for pressure. Primarily used for higher range measurement, MPa is used to describe pressure ranges and ratings of hydraulic systems. * SI refers to ‘The International System of Units’, which is a standard recognised world-wide.
Metabolism
Metabolism refers to the chemical processes within a living organism needed to maintain life, including converting food and/or fuel to energy for cellular functions, the building blocks for cellular growth and the elimination of wastes.
Metabolomics
Metabolomics is the large-scale study of small molecules, commonly known as metabolites, within cells, biofluids, tissues or organisms. Collectively, these small molecules and their interactions within a biological system are known as the metabolome.
Mica
A general term for a group of minerals with a layered structure, known as complex hydrous aluminosilicates. These minerals are widespread in igneous and metamorphic rocks, playing a role in their formation.
Microbial biomass
Microbial biomass (bacteria, fungi and archaea) is a measure of the living component of soil organic matter. The microbial biomass includes all soil organisms smaller than 5000 μm3 in volume.
Microorganisms
Microscopic organisms, such as bacteria, archaea, eukaryotes and fungi, exist either in a single-celled form or in a colony of cells and are smaller than can be seen with the naked eye. Microorganisms are commonly called microbes. Microbial is the term used when referring to microscopic organisms.
Mineral/mineralogical
The mineral component of soil comprises all solid forms that are not living or dead organic matter, liquid or gaseous.
Mineralisation
Mineralisation is the release of inorganic chemical compounds (nutrients) into the soil as a result of decomposition of soil organic matter by microorganisms. Mineralised nutrients are more-readily taken up by plants than nutrients that are part of organic compounds comprising soil organic matter.
Mineralogy
Study of the crystalline structure of clay minerals.
Minimum tillage
Minimum tillage is an umbrella term for all ‘less tillage’ systems. It is also known as ‘conservation tillage’. Zero-tillage – disc seeding without soil throw (but note that some discs do throw soil). No-tillage (or no-till) – sowing without prior cultivation while disturbing less than 20% of the topsoil. Direct drilling – one pass seeding with a full-cut or greater than 20% topsoil disturbance. Reduced tillage – one pass or full soil disturbance prior to seeding.
Monocotyledons
A flowering plant group whose seed typically contains only one embryonic leaf or cotyledon. Flowering plants that are not dicotyledons are designated as monocotyledons.
Montmorillonite
Montmorillonite is a group of clay minerals also known as the Smectite group. Derived from magnesium-rich parent rocks with a 2:1 silicon to aluminium structure. Is highly expandable and has a high cation exchange capacity. Minerals within the group include smectite, pyrophyllite, talc, vermiculite, sauconute, saponite, nontronite.
Mucilage
Mucilage is a high molecular weight, insoluble, polysaccharide-rich material that acts as a lubricant for the growing root tip as it forces its way past abrasive sand, silt and clay particles. Mucilage also provides protection for the root from desiccation, assists in nutrient acquisition and helps to bind soil particles together to form soil aggregates.
Multiple tillage
Two or more tillages before seeding. Also known as ‘intensive tillage’ and ‘conventional tillage’. The term ‘conventional’ may be misleading for the south-western agricultural region of Western Australia because the current convention for most broardacre agriculture in the region is minimum tillage.
Muscovite mica
A hydrated sheet silicate mineral containing aluminum and potassium, sometimes known as potash mica or white mica.
Mycorrhizal symbiosis
Most plants form mycorrhizal associations with plant roots. The arbuscular mycorrhizal association forms when fungal hyphae penetrate the root cortex. The fungus is then able to acquire carbon from the infected plant and in return nutrients absorbed by the hyphae that extend out into the soil are supplied to the host plant. The mycorrhizal association is most beneficial for uptake of phosphate and other nutrients supplied to the roots by diffusion since the network of fungal hyphae extend the volume of soil from which the plant can acquire nutrients.
N
NDVI (normalised difference vegetation index)
Normalised difference vegetation index, or NDVI is used to provide an indication of plant heath, biomass and ground cover via remote sensing and can be used as a mapping layer for prescription mapping for precision agriculture. Optical sensors (usually from aircraft or satellites) are used to measure the intensity of near-infrared and red light wavelengths. Near-infrared light is strongly reflected by vegetation and red light is strongly absorbed by healthy vegetation, while unhealthy or dying plants reflect more red light and less infrared light. NDVI compares the intensity of near-infrared light and red light to give an indication of biomass, cover and plant health. NDVI values range from -1 (no vegetation) to 1 (highest density of green leaves).
Nematicide
A nematicide is a chemical pesticide used to kill nematodes. Though used primarily in agriculture to target plant parasites, it will also kill beneficial nematodes.
Nematode
Nematodes are non-segmented worms that are present in a wide variety of habitats. They are the most abundant group of multicellular organisms on Earth. Many are free-living, such as those that live in the soil, while others are parasitic and must attach themselves to a host, such as a plant or insect, to survive and reproduce. Nematodes constitute a large and diverse group of variable and adaptable organisms. They occur virtually everywhere, from hot springs, to ocean depths, and Polar Regions. They are found in large numbers in soil, fresh or salt water, in humus and decaying plant material, and in moist tissues in most groups of animals. Only 10% of 30 000 nematode species described are plant parasites. Fifty percent are marine, 25% free-living, such as those that live in soil, and 15% are parasites of animals (including humans) and must attach themselves to a host to survive and reproduce. Nematodes are non-segmented and translucent when in their vermiform (worm shaped) life stages, so are virtually impossible to see in soil with the naked eye. Some species develop into cysts or become like swollen globes that make them visible at 0.4–1 mm length. All nematodes hatch from eggs.
Net negative charge
The net negative charge rates dispersion based on exchangeable cations (Ca2+, Mg2+, K+, Na+) and their relative dispersive and flocculating effects on clays.
Net nitrogen mineralisation
Net mineralisation is the difference between gross mineralisation (supply) and microbial immobilisation (consumption).
Net primary productivity
Refers to the conversion of atmospheric carbon dioxide via photosynthesis to organic mass (biomass)—primarily by plants.
Nitrogen fixation
Nitrogen fixation is a biochemical process performed by specialist microorganisms (e.g. Rhizobium) where dinitrogen (N2) gas in the air is converted into ammonia (NH3) or related nitrogenous compounds, which can be used by many organisms.
Northern Agricultural Region
The Northern Agricultural Region (NAR) covers 7.5 million hectares of WA and extends from the Murchison River in the north to Moore River in the south and out to eastern grain growing areas from Kalannie north. Broadacre agriculture is the predominant industry, dominated by the production of cereal, oilseed and pulse grains. Sheep and cattle are typically found in the higher rainfall areas in the Western third of the region. The region’s climate varies from a mild Mediterranean in the coastal areas to semi-arid climate further inland. Annual median rainfall varies from 350–565 millimetres. The soil in this region ranges from deep sand, sandy earth and sandy gravel in the Western two-thirds of the region with heavier-textured loamy earth and loamy duplex soil along the eastern areas and around he townsite of Northampton. Low natural nutrition, topsoil water repellence as well subsoil acidity are the major soil constraints in the region. Evidence of a drying climate in the region is strong, with a clear trend of reduced rainfall over the past few decades, particularly in the autumn period corresponding with the typical break-ofseason rains for the crop growing season.
Nucleus
The nucleus is the largest organelle inside most cells separated by the nuclear membrane and found only in eukaryotic organisms. The nucleus contains the genetic material (DNA) of organisms and controls and regulates activity of the cell.
O
Optical sensors
Optical sensors measure the reflectance of light across the visible and nonvisible spectrum and are the most common sensor type mounted on aircraft or satellites to map aspects of the land surface and atmosphere, with vehicle mounted sensors also increasingly being used. Different sensors will detect different parts of the spectrum and can be used to measure crop and soil properties. For example, the red and near infra-red parts of the spectrum are often used to detect crop health or biomass characteristics (for example, the NDVI) while the mid-infrared is often used to examine properties of the soil. Tools have been developed using this technology to look at specific crop issues (for example, nitrogen status or weed detection), while maps of biomass and crop health over time can be used to identify consistently underperforming patches within paddocks that may be caused by an underlying soil issue.
Organic acids
A type of organic compound that typically has acidic properties. A common group of organic acids are called carboxyl acids, which are generally known as weak acids.
Organic compounds
Organic compounds consist of chains (branched or unbranched) or rings of carbon (C) atoms with predominantly hydrogen (H) or hydrogen + oxygen (OH) bonded to the carbon atoms. The number of carbon atoms in an organic compound can vary from one to many. Organic compounds with carbon atoms numbering 16–36 have been implicated as the main causative agents of soil water repellence. The classes of organic compounds responsible for water repellence include long-chain fatty acids, esters, alkanes, phytanols, phytanes and sterols.
Organic matter
Organic matter is matter composed of organic compounds from any living or decomposing plant, animal or microorganism and/or their waste. Carbon is a primary component of organic matter.
Organic molecules
Organic molecules are usually composed of carbon atoms in rings or chains of varying length, with other atoms of elements, such as hydrogen, oxygen and nitrogen. They form the cells found in living organisms and chemical reactions that facilitate life.
Osmotic stress
Water deficit caused by salinity; decrease in availability of water to plant cells due to salinity.
P
Parent material
Primary material that the soil has formed from.
Particle size distribution
Soil particles are classified into sand, silt, and clay. Sand ranges in size from 0.05 to 2.0 millimetres diameter, silt between 0.002 and 0.05 millimetres and clay < 0.02 millimetres diameter.
Ped
Peds are aggregates that are separable from each other by lines of weakness. Peds may be less than 2 millimetres to greater than 500 millimetres.
Persistent organic pollutants
Organic compounds such as pesticides that are difficult to degrade and thus persist in the environment for a long time. They may also accumulate in the food chain and become more toxic.
Petioles
The stalk containing xylem and phloem conducing vessels that attaches the leaf blade to the stem.
pH
A measure of acidity or alkalinity, measured as the negative decadal logarithm of the hydrogen ions; see soil pH.
pHca
It is standard practice in Western Australia for the pH of acidic soil, with pH below 6, to be measured in a weak calcium chloride solution (1 part soil to 5 parts 0.01 molar calcium chloride). In alkaline soil with a pH above 8, the standard practice is to measure pH in the extract of 1 part soil to 5 parts water. Soil pH measured in calcium chloride is designated pHCa and gives a lower value than pH measured in water (pHW).
Phloem
The living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis (known as photosynthates), in particular the sugar sucrose, to all other parts of the plant.
pHw
Soil pH measured using water as the solvent is designated pHW. In alkaline soil with a pH above 8, it is standard practice in Western Australia to measure pH in the extract of 1 part soil to 5 parts water. It is standard practice to measure the pH of acidic soil, with pH below 6, in a weak calcium chloride solution (1 part soil to 5 parts 0.01 molar calcium chloride) and is designated pHCa. Soil pH measured in calcium chloride gives a lower value than pH measured in water.
Physicochemical
Physicochemical describes a process or attribute that has both physical and chemical properties. For example, chemical properties, such as, nutrients, salinity and water repellency, and physical properties, such as, porosity, structure and water availability, in a soil environment.
Phytanol
An organic alcohol.
Plant available water
Plant available water is usually considered to be the difference between the drained upper limit (field capacity) and the amount of water in the soil at the plant lower limit (wilting point), where the plant can no longer extract water from the soil. This is usually measured over 100 centimetres or the maximum rooting depth.
Plant rooting depth
The depth to which plant roots grow; see ‘rooting zone’.
Polymer
A polymer is a carbon-based material consisting of very large molecules that are made up of many repeating smaller subunits.
Potential yield
The water-limited potential of a crop in a particular environment is the potential yield where the only factor limiting production is the availability of water (agronomy, soil conditions and other climatic factors are optimal).
Precipitation
The process whereby a solid is formed (and thus removed from a solution) via a chemical reaction between substances in the solution.
PREDICTA® B
PREDICTA® B is an Australian DNA-based soil testing service that helps grain and pulse producers identify which soilborne disease and/or nematode pest pose a significant risk to their crops before seeding so that steps can be taken to minimise risk of yield loss. The amount of pathogen DNA detected is used to: 1. determine the soilborne disease and/or nematode pest risks category (not detected, low, medium, high risk) 2. help inform crop and variety decisions 3. guide management to minimise crop losses. The presence of the pathogen DNA identifies the potential risk of the disease/ root lesion nematode with the expression/severity determined by the crop/ varieties (host) planted and environmental conditions.
Premature haying-off
Premature haying-off is the early cessation of grain filling in crops and pastures and occurs when plants mature prematurely due to a lack of available water after flowering. Premature haying-off in crops is most often associated with vigorous early plant growth, which when the available water supply is used up is unable to support grain fill, resulting in low yield and shrivelled (pinched) grain.
Prilled lime
Prilled lime is manufactured from fine lime particles, which are formed into prills (or granules), typically with the addition of a binding agent.
Primer
A primer is a short nucleic acid sequence that provides a starting point for DNA synthesis.
Primer plant
‘Primer plant’ plant species have mechanisms that enable them to grow through compacted (or other) soil constraints, allowing them to grow on ‘hostile’ soils and provide biopores for following crops.
Prokaryota
A prokaryote is a simple cellular organism that lacks an envelope-enclosed nucleus, and is characterised by a cell membrane and cell wall. Prokaryota consist of two kingdoms that inhabit virtually all known habitats: archaebacterial and eubacteria.
Protonation/deprotonation
The addition of a hydrogen ion (proton) to a substance is termed protonation and conversely the removal of a hydrogen ion is deprotonation.
Protozoa
Microscopic, single celled animals in the eukaryote domain that vary widely in size and shape. Protozoa are heterotrophic and derive their intake of nutrition from organic matter. They occur in a diverse range of habitats including soil and both fresh and salt water, with some surviving in extreme environments. Most are free-living, though some are parasitic and can cause disease.
Q
Quartz
One of the most common constituents of the Earth’s crust, abundant in sandstones, granite, and soils. It is made up of silica (SiO2) having a superior ability to conduct heat.
R
Radicle
The part of a seed embryo that develops into a root.
Radiometrics
Gamma radiometrics sensors measure the strength of gamma rays produced by the natural radioactive decay of potassium, thorium and uranium, which is linked to the mineralogy, past weathering and particle size of the soil. The key use of radiometric survey data is to identify soil zones, and is often measured at the same time as EM38 as the two methods are complimentary. Radiometric sensors are either vehicle or airborne mounted and, as with EM surveys, the data needs to be calibrated to local site conditions.
Real time kinematics (RTK)
The highest level of positioning offered by a GPS system, +/– 2 cm. This system requires a base station (on a tripod or building), with a dGPS receiver and radio transmitter, to get a very local correction signal, accurate to a few centimetres. The base station can transmit to multiple vehicles up to five or six miles away depending on the terrain.
Recalcitrant
Difficult to break down. Recalcitrant materials can resist biodegradation associated with environment or chemical factors (or both) and some are more resistant than others.
Recombinant DNA
Exchange of genetic material between different organisms which results in offspring with combinations of traits that differ from those found in either parent. i.e. these traits could not occur naturally through mating or traditional breeding.
Reductants
A chemical species that ‘donates’ an electron to an electron recipient.
Redundancy
A characteristic of ecosystems where different (or multiple) species can perform the same functional role, so that changes in species diversity do not affect ecosystem function. Also referred to as ‘functional redundancy’.
Relative yield
Relative yield is usually expressed as a percentage change in grain yield of a treatment relative to a constant. The constant can be the control treatment, paddock mean or calculated rainfall limited potential yield.
Residence time
Period of time that a gas molecule stays in the atmosphere; or a measure of the average time a molecule of water spends in a reservoir. Residence time refers to the total time spent inside a known volume control, prior to it exiting that same volume.
Resilience
Ability of an ecosystem to adapt to, maintain or recover function following a single, or multiple stresses.
Resilient soil
Soil resilience refers to the ability of soil to resist change, or recover to a similar state or function, following a disturbance or natural stress.
Rhizobacteria
Rhizobacteria are root-associated bacteria that form symbiotic relationships with many plants. Plant growth-promoting rhizobacteria are a group of freeliving rhizobacteria that benefit plant growth. Bacteria of diverse genera have been identified as plant growth-promoting rhizobacteria of which Bacillus and Pseudomonas species are predominant.
Rhizobia
Rhizobia are a group of diazotrophic bacteria found in the soil, or applied as an inoculant to seed. After infecting plant legume (Fabaceae or Leguminosae) roots, they produce nodules where they fix nitrogen gas (N2) from the atmosphere turning it into a more readily useful form of nitrogen.
Rhizosphere
The rhizosphere is the narrow zone of soil surrounding a plant root where the biology, chemistry, physics and hydrology of the soil are influenced by the root. The rhizosphere represents an organic carbon and nutrient rich ‘hotspot’ for bacteria and other microorganisms to live.
Root exudates
Roots exudates provide carbon and nutrients to rhizosphere organisms including a variety of sugars, amino acids, organic acids, vitamins and larger molecules. The composition and volume of root exudate compounds released is influenced by a number of factors including plant species and age, climatic conditions and the biological, chemical, physical and hydrological properties of the surrounding soil.
Rooting zone
The typical depth to which plant roots grow. The rooting depth is influenced by factors such as plant species and the interaction between soil type, presence or absence of soil constraints and seasonal conditions.
S
Saprophyte
Plant, fungi or microorganism that lives on dead or decaying organic matter. Can also use the term ‘saprophytic’: organisms that break down and obtain nourishment dead or decaying organic matter.
Sedimentary
Sedimentary rock is made by sediments/particles deposited by water, ice or wind, for example sandstone or limestone.
Sedimentation
Sedimentation is the process of sediment accumulating, resulting from either settling of particles that were suspended in water or deposited after wind transport.
Self-repair
Soil with a high percentage of smectite (a 2:1 clay) has the capacity to swell with increases in moisture and shrink as moisture content decreases. This shrink–swell cycle produces cracking throughout the profile, which can reduce soil strength and improve structure.
Seminal roots
Seminal (seed/embryonic) roots initiated after germination dominate early root growth in monocots; the nodal (crown) roots appear a few weeks after germination.
Shear strength
Shear strength, is indicative of the stress or force that a soil can sustain before changing its state and is influenced by:
- Soil composition (mineralogy, grain size and shape, grain size distribution,
porosity) - Soil state (i.e. loose, dense, soft, etc.)
- Soil structure: matrix and distribution of particles within the soil mass, and
soil features such as strata, layers, cracks, voids, cementation, etc. - Loading conditions (i.e. drained or undrained; weight and frequency of
loading, whether static or dynamic, and temporal history.
Shearing
Shearing is the deformation of soil from a horizontal force.
Shells of hydration
Clusters of water molecules that surround and are electrostatically attracted to ions (such as Na+). They have the effect of increasing the size of the hydrated ion.
Slaking
Slaking is a physical process. It is the disintegration or collapse of dry soil aggregates into smaller aggregates upon wetting. It can occur in combination with dispersion though one factor, either slaking or dispersion, will usually be the dominant process.
Smearing
Smearing is the realigning of soil particles from random to parallel.
Smectite
Smectite is group of clay minerals also known as Montmorillonite group. Derived from magnesium-rich parent rocks with a 2:1 silicon to aluminium structure. Is highly expandable and has a high cation exchange capacity. Other minerals within the group include montmorillonite, pyrophyllite, talc, vermiculite, sauconute, saponite, nontronite.
Sodic soil
A sodic soil is defined as a soil with an exchangeable sodium of greater than 6% of the cation exchange capacity.
Sodicity
Sodicity refers to the amount of sodium present in soil and is measured as the amount of sodium ions (Na+) present relative to other cations in the soil – the exchangeable sodium percentage (ESP). Soil with an exchangeable sodium percentage greater than 6 is considered to be ‘sodic soil’.
Sodium absorption ratio
The sodium adsorption ratio is a measure of the concentration of sodium ions (Na+) relative to calcium (Ca2+) and magnesium (Mg2+) ions, generally in the saturated extract of a soil. It is the ratio of the sodium concentration divided by the square root of one-half of the calcium plus magnesium concentration where all concentrations are in millimoles per litre.
Soil acidity
Soil with pH below 7 is acidic. Acidity is considered a soil constraint to plant growth when the top 100 millimetres of soil is less than pHCa 5.5 or the subsurface soil below 100 millimetres is less than pHCa 4.8.
Soil aggregate
A soil aggregate is a group of organic and inorganic soil particles held together, usually by fungal hyphae or ‘glues’ produced by soil organisms.
Soil alkalinity
Soil with pH above 7 is alkaline. Alkalinity is considered a soil constraint to plant growth above pHW 8. Soil pH between 5.5 and 7.5 is generally considered best for plant growth.
Soil biota
The biota is all the living organisms within a region or habitat. For example soil biota includes all organisms that spend a significant portion of their life cycle within a soil profile, or at the soil-litter interface. This includes: algae; fungi; soil fauna (such as, termites, worms, mites and nematodes); and microorganisms.
Soil colloid
Microscopic particles of clay, organic matter and hydroxyoxides (e.g. Al and Fe) that form part of the soil solid phase.
Soil organic carbon
Soil organic carbon is the carbon component of soil organic matter; it is not the same as soil organic matter, which also includes other elements such as hydrogen, oxygen and nitrogen. Soil organic carbon includes carbon in fresh plant residues in the particulate fraction, as well as more degraded and inert carbon in the humus and resistant fractions. The organic carbon content in soil is often used to calculate soil organic matter using a conversion factor of 1.72 (i.e. 58 % of the soil organic matter is composed of carbon) – while this factor can vary it provides a reasonable estimate.
Soil organic carbon stock
Soil organic carbon stock is the amount of organic carbon in the depth of soil sampled, expressed as tonnes of carbon per hectare. This takes into account differences in soil bulk density.
Soil organic matter
The organic component in soil; plant and animal residues in various stages of decomposition, plant roots and living organisms able to pass through a 2-mm sieve that exist in various forms including soluble fractions.
Soil ped
Soil peds are aggregates that are separable from each other by lines of weakness. Peds may be less than 2 millimetres to greater than 500 millimetres.
Soil pores
Soil pores are the space between soil particles which, besides the organic and inorganic solids present in a soil, can contain either water or air.
Soil porosity
Soil porosity describes the fraction of total soil volume that is taken up by the pore space. Pore spaces facilitate the availability and movement of air and water within the soil.
Soil respiration
Key ecosystem process where carbon is released from soil as carbon dioxide (CO2). Soil respiration is a measure of CO2 resulting from soil organic matter decomposition via organisms such as animals, bacteria and fungi. The process can be aerobic or anaerobic and is often used as a measure of biological activity.
Soil solution
The soil solution is the liquid component of the soil matrix. It contains dissolved minerals, gases and organic matter.
Soil strength
Soil strength refers to the reaction of the soil to applied force and is used to estimate the degree of soil compaction.
Soil test calibration curves
Soil test calibration curves are the main approach used to assess soil nutrient supply in relation to plant demand in Australia. Soil test calibration curves are developed using nutrient response trials, by relating the yield response to fertiliser to a measurement of soil nutrient availability at each trial for a database of trials. Soil nutrient availability is usually measured using laboratory-based methods, where a sample of soil is agitated in a solution to extract plant available nutrients. The soil test calibration curve is created by plotting the yield response to fertiliser against the measured plant-available nutrient concentration and fitting a curve to that data.
Soil volume
Total soil volume refers to the combined volume of space taken up by solids (i.e. mineral soil particles and organic matter) and pore spaces. Bulk density is the mass of soil (dry weight) within that same volume.
Soluble
A substance that is able to be dissolved.
Solute
Solute is the chemical term for a substance that is dissolved in another substance (the solvent) to form a solution. In the solution, the solute takes on the characteristics of the solvent. For example, in a solution of salt and water, salt is the solute.
Solution
A liquid in which a substance (the solute) is dissolved in another (the solvent). In the solution, the solute takes on the characteristics of the solvent.
Sorption
Sorption is used to describe the joining of two substances where it is not important to specify whether the process is adsorption (the process by which an atom or molecule transfers from a gas, liquid or dissolved solid phase and adheres to the outer surface of another substance as a result of surface forces) or absorption (the process by which one substance is incorporated into another involving a change of state—solid, liquid or gas).
South-western agricultural region
The south-western agricultural area of Western Australia covers about 25 million hectares. About 18 million hectares is used for agriculture, including broadacre cropping and pasture, irrigated cropping and pasture, and horticulture. Broadacre grain cropping is the dominant land use in this region and is primarily conducted between the 750 and 275 millimetre rainfall isohyets. This area is known as the wheatbelt and covers about 17 million hectares.
Spatial and temporal variability
Variability in the context of soil properties refers to measures that differ across locations (spatial) or with time (temporal). Spatial variability can be assessed on a horizontal plane, for example differences in values (or the range) that may be measured across a paddock or as the vertical change with increasing depth in a soil profile. Temporal variability largely refers to differences in measures (changes) within or between seasons at the same site.
Stability index
The stability index is the sum of scores for exchangeable sodium percentage, soil organic matter, exchangeable calcium:magnesium ratio and electrical conductivity of the saturated soil extract.
Sterol
An organic alcohol.
Stoichiometric
A stoichiometric process is one where the chemical interaction between two substances is predictable and quantifiable, meaning that the amount of the product from the interaction can be calculated.
Summer cropping
In Western Australia, summer cropping is the use of crops and pastures outside of the normal winter cropping period to: use excess moisture, take up nutrients in the soil that would otherwise leach, provide ground cover, control weeds, and supply nitrogen to subsequent crops.
Supernatant
The water lying above the compacted soil pellet after a soil: water mix is centrifuged.
Symbiosis
A mutually beneficial association between two organisms living in close physical proximity.
Synctia
A syncytium or symplasm (plural syncytia) is a multinucleate cell which can result from multiple cell fusions of uninuclear cells (i.e., cells with a single nucleus).
T
Tensile strength
Tensile strength is the maximum stress that can be applied to material before it breaks. Measured as force per unit area.
Terminal restriction fragment length polymorphism
Terminal restriction fragment length polymorphism (TRFLP or sometimes TRFLP) is a molecular biology technique for profiling of microbial communities based on the position of a restriction site closest to a labelled end of an amplified gene.
Tortuosity
Tortuosity is a property of a porous material describing the ratio of actual flow path length to the straight distance commonly used to describe diffusion and fluid flow in porous materials such as soil. A tortuous path describes a curve with many twists and turns.
Total organic carbon
Same as soil organic carbon: Soil organic carbon is the carbon component of soil organic matter; it is not the same as soil organic matter, which also includes other elements such as hydrogen, oxygen and nitrogen. Soil organic carbon includes carbon in fresh plant residues in the particulate fraction, as well as more degraded and inert carbon in the humus and resistant fractions. The organic carbon content in soil is often used to calculate soil organic matter using a conversion factor of 1.72 (i.e. 58 % of the soil organic matter is composed of carbon) – while this factor can vary it provides a reasonable estimate.
Trafficability
A vehicle’s ability to travel on certain types of terrain or ground.
Transient bonding
Transient bonding is the mechanism of binding micro-aggregates into larger components mostly via organic compounds, thus reducing pore space. However, this is not permanent and, therefore, a more manageable soil constraint.
Transient salinity
The salinity of dispersive (alkaline, sodic) soil; the salts are not associated with a shallow water-table, but are derived from rain and aeolian dust.
Translocate
Move from one place to another by displacement, dislocation or change of location. Agricultural examples could be the transport of solutes within an organism or across a membrane; or movement of a segment of a chromosome to a new position on the same or different chromosome.
Transpiration
A plant’s loss of water at the surface, mainly through the stomata of leaves.
Transporters
Molecules, frequently proteins, specialised for transporting water, food, wastes and other materials between cells within plants.
Triticale
Triticale is a hybrid of wheat (Triticum) and rye (Secale).
Turnover time
The decomposition rate; the time taken for organic matter to move into and through the various soil organic matter fractions.
V
Valency
The valency of an atom is determined by the number of electrons in its outer electron ring. Valence electrons can be lost, gained or shared. An outer electron ring with eight electrons is full (stable); it has no valence electrons and hence no valency. The valency is the number of electrons needed to be gained or lost to complete the outer electron ring. If the outer ring has 1–4 electrons, it can loose electrons and has a positive valency. If the outer ring has 5, 6 or 7 electrons, it can gain electrons and has a negative valency. For example, sodium has one electron in its outer electron ring and therefore its valency is 1 (indicated as Na+). Chlorine on the other hand has seven electrons in its outer ring and its valency is -1 (indicated as Cl-).
Van der Waals
In the context of soil chemistry, Van Der Waal forces explain the attraction and repulsion between ions found in the diffuse double layer between clay plates. In this context the Van Der Waal forces are weak electrostatic bonds that exist between negatively charged clay surfaces and ions in solution.
Vascular tissue
Vascular tissue is a complex conducting tissue, formed from multiple cell types found in vascular plants. The xylem and phloem constitute the primary components of vascular tissue and transport water and nutrients in solution internally.
Vector
Vector is a DNA molecule used as a vehicle to artificially carry foreign genetic material into another cell. Vectors for gene delivery can be either recombinant viruses or synthetic vectors.
Vermiculite
A naturally occurring mineral that belongs to the phyllosilicate group. It has a layered structure and is characterised by its ability to expand significantly when heated, due to the presence of water molecules within its structure.
Virion
A virion refers to an entire virus particle, consisting of an outer protein coat (capsid) and an inner core of RNA or DNA. The virion is the infectious form of a virus as it exists outside the host cell.
Volatilisation
Volatilisation is the process of a substance changing from a solid or liquid state to a gaseous state.
W
Water-limited potential
The water-limited potential in a particular environment is the grain yield or pasture dry weight that could be achieved for a particular soil type when soil conditions (e.g. nutrient supply, diseases, pests, pH, compaction) are not limiting plant growth.
Waterlogged
Condition of soil saturation.
Waxy organic compounds
Waxy materials or substances result from the decomposition of organic matter, and are implicated in causing water repellence in soils.
Weathering
Weathering is the process of breaking down rocks, sediments and minerals via biological, chemical and physical processes such as, hydration, oxidation, extreme temperatures, salt spallation or the action of substances produced by organisms.
Western Australian sandplain
Western Australian sandplains are broad areas of relatively flat land with a variety of soil types that are sandy textured (loamy sand to sandy loam) in the upper part. Depending on the location of the sandplain (South Coast, Wheatbelt or Mid West), the soil can vary in colour and the subsoil textures can range from clay or loam to sands which extend to greater than a metre in depth. Soil textures down the profile are commonly gradational, but can also be duplex (clay subsoil often at <800 mm) and can contain variable amounts of ironstone gravels.
Y
Yield information
Yield information, generated by grain flow sensors fitted to the header, is an important and often under-utilised tool for identifying potential soil quality issues within a paddock. Analysis of multiple years of yield information can be used to identify areas that consistently underperform and may be caused by a soil constraint. The pattern in lower yielding areas may also help distinguish different factors, such as the distinctive pattern of soil compaction in past wheel tracks.
Z
Zadoks growth scale
The Zadoks (or Decimal) growth scale is an internationally recognised standard for describing the growth stage of cereal plants. Using the scale, growers can assess crop development stage and make informed decisions in regards to effective fertiliser management and/or herbicide application (or other sensitive management decisions).
The code is based around ten principal growth stages:
- 0 Germination
- 1 Seedling growth
- 2 Tillering
- 3 Stem elongation
- 4 Booting
- 5 Ear emergence
- 6 Anthesis (flowering)
- 7 Milk development
- 8 Dough development
- 9 Ripening
These principal growth stages are each divided into 10 secondary stages, for example: Z00 – Dry seed, Z01 – Start of water absorption, Z03 – Seed fully swollen. More than one growth stage can occur simultaneously, for example, a plant could be scored Z13 (three fully unfolded leaves) and Z21 (one tiller – branch arising from the base of the main stem).
Zone of Ancient Drainage
The Zone of Ancient Drainage is an area that encompasses a large part of the eastern portion of the central and northern grainbelt in Western Australia. It consists of ancient drainage lines that drain into chains of salt lakes on valley floors. These are the remnants of an ancient paleo-drainage system that flows only in very wet years. The landscape consists of an extensive undulating plain characterised by a gently undulating plateau, long gentle sideslopes and broad valley floors which can be from 2–10 kilometres wide. The soil is predominantly deep sands and sandy gravels on the catchment divides, sandy duplex soils on the valley slopes and loamy earths and shallow duplex soils on the valley floors.
