|
Welcome to OzSoils
OzSoils Authors and Editors: Dr. Heiko Daniel and Dr. Peter Lockwood,
Soils GlossarySelect an item from the list above to see the definition. Hitting a letter on your keyboard will go straight to that point in the list. A HORIZONMineral horizons close to the soil surface which are darkened due to organic matter accumulation, and/or appear to have lost clay, iron or aluminium with resultant concentration of quartz or other resistant minerals. A1 HORIZONAn A horizon at the soil surface which is characterised by relatively high organic matter and biological activity and a dark colour compared to horizons below. A2 HORIZONAn A horizon which is paler than A1 horizon above and a B horizon or other soil material below. ACCELERATED EROSIONWater or wind erosion resulting from human activity which is more rapid than normal or geological rates. ACTINOMYCETESA group of filamentous bacteria, which may be abundant in soils. ADSORPTIONThe process whereby ions or molecules are attracted and held close to the surfaces of soil minerals. For example cations are adsorbed by clay minerals carrying negative charges. AEOLIANPertaining to sediments transported and deposited by wind. Parna, loess and sand dunes are examples. AEROBICAn aerobic soil environment is one where there is free oxygen (ie O2) present. The opposite of anaerobic. See also respiration. AGGREGATEA soil particle formed when primary soil particles (sand, silt and clay) are bonded together. Peds are examples of soil aggregates. Contrast with primary particle. AGGREGATE STABILITYThe ability of compound particles of soil to maintain integrity when a disruptive force is applied. AIR-DRYSoil samples are air-dry when they have been allowed to dry at room temperature to a stable water content. The remaining water can be removed by heat to produce oven-dry soil. AIR-FILLED POROSITYThe proportion of the total soil volume that is occupied by air. ALLOPHANEA noncrystalline aluminosilicate soil clay mineral. ALLUVIUMMaterial deposited by flowing water. ALPINE HUMUSA great soil group. An alpine humus is a strongly acid organic soil with a thick loamy A horizon overlying a coarser textured C horizon. They form on a variety of parent materials and are restricted to the highlands of south-eastern Australia. The Great Soil Group system is now superseded by the Australian Soil Classification. ALUMINOSILICATEMinerals containing aluminium, silicon and oxygen as the main elemental constituents.. AMMONIA VOLATILISATIONThe loss of ammonia, and hence nitrogen, from the soil as ammonia gas escapes to the atmosphere. AMORPHOUSAmorphous minerals lack a regular crystal structure. AMPHIBOLESA group of primary aluminosilicate minerals with a crystal structure comprising double chains of linked silica tetrahedra. Hornblende is a common example. ANAEROBICAn anaerobic soil environment lacks free (molecular) oxygen (eg a waterlogged soil). The opposite of aerobic. See also respiration. ANGULAR BLOCKYAngular blocky peds are square or block like with relatively sharp and well defined edges and vertices ANION EXCHANGE CAPACITYThe number of moles of charge of anions that are adsorbed exchangeably per unit mass of soil. ANTHROPOSOLA soil order in the Australian Soil Classification. Anthroposols are soils resulting from human activities APEDALSoil structure is apedal if it lacks peds. Apedal structures may be massive or single grain. Contrast with pedal. AQUIFERA porous layer within the soil or underlying in which water is stored. ARABLELand which can be cultivated for the production of crops. ASPECTThe aspect of a slope is the direction you are facing if you are looking down the slope. ATTERBERG LIMITSThe soil plastic and liquid limits. Used in soil engineering to characterise how soil consistence varies with water content. AUSTRALIAN SOIL CLASSIFICATIONDeveloped by R.F. Isbell and introduced in 1996, the Australian Soil Classification is a hierarchical scheme which uses field and laboratory data to classify soil profiles via a key. The levels in the hierarchy are: order, suborder, great group, subgroup and family. The soil is assigned a name which suggests some of the properties of the soil. There are 14 soil orders: Anthroposols, Calcarosols, Chromosols, Dermosols, Ferrosols, Hydrosols, Kandosols, Kurosols, Organosols, Podosols, Rudosols, Sodosols, Tenosols, and Vertosols. B HORIZONA horizon which shows signs of illuviation (accumulation of clay, iron, aluminium, or organic matter), or differs from the A horizons above or horizons below in structure, consistence or by being more strongly coloured. BACTERIAA large group of procaryotic microorganisms which are abundant in soil and play a major role in nutrient and organic matter cycling. BASE SATURATIONThe proportion of the cation exchange capacity that is occupied by the exchangeable bases, expressed as a percentage. BED LOADLarge, heavy particles that are moved by water (eg in streams) by creep. BIOTITEA dark coloured mica common in some granites. BIOTURBATIONA form of pedoturbation (soil mixing) caused by living organisms (eg burrowing by worms, ants, termites, etc, or tree throw caused by the roots of falling trees). BLACK EARTHA great soil group which is dark coloured, has a uniform clay texture profile, cracks upon drying and contains calcium carbonate at depth. The Great Soil Group system is now superseded by the Australian Soil Classification. BROWN CLAYA member of the grey, brown and red clays great soil group. Deep, uniform cracking clay soils widespread in the alluvial plains of inland Australia. The Great Soil Group system is now superseded by the Australian Soil Classification. BUFFER CAPACITYA measure of the ability of the soil to resist change in pH when acid or alkali is added. May also be applied to other chemical species. For example the P buffer capacity of a soil is the degree to which it resists change in soil solution P concentration when soluble P is added or removed. BULK DENSITYThe mass of oven-dry soil divided by the bulk soil volume at sampling. Contrast with particle density. C HORIZONLayers below the solum (A and B horizons) which have usually been partly weathered, but are little affected by soil forming processes such as pedoturbation. Often the structure of the underlying parent rock is still visible. C/N RATIOThe ratio by mass of carbon to nitrogen in organic matter. CALCAROSOLA soil order in the Australian Soil Classification. Calcarosols are characterised by pedogenic carbonate material through much or all of the solum. CAPILLARY FORCESForces of attraction between water and the soil surface which help retain water in soil micropores. CARBOHYDRATEA group of organic compounds composed of carbon, hydrogen and oxygen. Examples are sugars like glucose and sucrose, and polysaccharides like starch and cellulose. CARBOXYLThe organic functional group -COOH. Along with phenol groups, carboxyl groups are responsible for the acidity and cation exchange capacity of humus. CATENASee toposequence. CATION EXCHANGE CAPACITYThe amount of cations that a soil can adsorb exchangeably, expressed as moles of charge per unit soil mass. Equals the amount of negative charge possessed by the soil colloids. CECSee cation exchange capacity. CELLULOSEA carbohydrate which is abundant in plants and plant residues in soil. CHELATEA chemical compound in which a molecule (in soils usually an organic acid) wraps around and bonds to a polyvalent metal cation (eg ferric or aluminium ions). Chelation can increase the solubility of ions like aluminium and iron in soils. CHOCOLATE SOILA great soil group. A brown, well aggregated soil commonly formed in situ from basalt in an upper slope position. The Great Soil Group system is now superseded by the Australian Soil Classification. CHROMAThe intensity or saturation of a soil colour. See also Munsell colour. CHROMOSOLA soil order in the Australian Soil Classification. Chromosols have a strong texture contrast between the A and B horizons, and the B horizons are neither sodic nor strongly acidic. CHRONOSEQUENCEA series of soils for which all the factors of soil formation apart from time are approximately constant. CLAYIn the International system (used in Australia), clay is defined as soil particles less than 0.002 mm in diameter. CLAY MINERALSSecondary minerals which commonly occur in the clay fraction of soils and include layer silicate minerals such as kaolinite, montmorillonite and illite, as well as non-silicate minerals such as hematite, goethite and gibbsite. 1:1 clay minerals are silicate clay minerals with one tetrahedral layer to one octahedral layer (eg kaolinite). 2:1 clay minerals are silicate clay minerals with two tetrahedral layers to one octahedral layer (eg illite and montmorillonite). CLODA soil aggregate produced by artificial means such as by ploughing or digging. In contrast peds are naturally occurring soil aggregates. COHERENCEThe degree to which soil material is bound together. Soil is said to be coherent if more than two thirds of the material (whether in peds or not) is united at a given soil water content and in the absence of an applied force. For example, a massive clay is coherent, but beach sand lacks any coherence. Synonymous with cohesiveness. COHESIVENESSSee coherence. COLLOIDMaterial composed of particles less than 0.002 mm in diameter. COLLUVIUMMaterial that has crept downslope under the force of gravity and has accumulated at the foot of the hill. COLOURSee Munsell colour. COLUMNARA type of soil structure where peds have a vertical axis that is much longer than the horizontal axis and have domed tops. Compare with prismatic structure. COMPACTIONThe process of compression of soil into a smaller volume. Bulk density is a measure of the result of a compaction process. As a soil becomes more compacted, its bulk density increases. CONSERVATION FARMINGFarming that emphasises conservation of soil and water resources. Includes practices such as minimising tillage and retaining crop residues. CONSISTENCEField characterisation of the behaviour of soil when it is subject to stress. Consistence varies with water content, and includes assessment of strength, stickiness and plasticity. See also Atterberg limits. CONTOUR BANKAn erosion control measure where soil has been pushed up into a bank nearly on the contour, so that runoff is caught and redirected to a non-erodible slope such as a grassed waterway. CRACKING CLAYSA general term (not part of any soil classification scheme) to describe uniform clay soils which shrink to form a pattern of deep cracks upon drying. CRUSTA surface crust is a thin hard layer formed at the soil surface as a result of structural collapse associated with raindrop impact. CUTANA thin layer on the outside of a ped which is different to the soil material within the ped. Also called a clay skin. Cutans often have a polished appearance and are of a different colour to the interior of the ped. In clay B horizons they may be produced by clay illuviation. CYCLIC SALTSalts derived from the sea that are carried inland as aerosols and deposited in the landscape. DEEP DRAINAGEDrainage of water through the soil profile below the root zone. DEFLOCCULATIONSee dispersion. DENITRIFICATIONThe reduction of nitrate in soil by microorganisms to produce nitrous oxide (N2O) and dinitrogen (N2) gases. DERMOSOLA soil order in the Australian Soil Classification. Dermosols have pedal B horizons but lack strong texture contrast between the A and B horizons. DESORPTIONRelease of an adsorbed ion or molecule from a surface. The opposite of adsorption. DIFFUSIONMovement of molecules or ions down a concentration gradient (ie from a place of high concentration to one of low concentration). DIRECT DRILLSowing seeds directly into the soil without prior cultivation. This term is mainly used in southern and western Australia. See also no-till and fallow. DISPERSIONThe process whereby the clay fraction of soil aggregates is broken down into primary particles. Dispersion in water can be recognised by an increase in the turbidity of the water as clay particles form a colloidal suspension. Synonymous with deflocculation. See also slaking and flocculation. DUPLEXA duplex soil is one which has a duplex texture profile, which means there is an abrupt and marked increase in clay content with depth. E HORIZONAn A2 horizon. Not used in Australia. ELUVIATIONThe soil forming process whereby soil material is moved out of a soil layer in suspension. ERODIBILITYThe susceptibility of soil to erosion EROSIONThe process whereby soil is removed by the action of water or wind. EROSIVITYThe strength of the tendency of water (rain or overland flow) to cause erosion. ESPThe ESP or exchangeable sodium percentage is the percentage of the effective cation exchange capacity of the soil occupied by sodium ions. EVAPORATIONLoss of water as vapour to the atmosphere directly from the soil. Contrast with evapotranspiration and transpiration. EVAPOTRANSPIRATIONThe sum of evaporation and transpiration. EXCHANGE ACIDITYThe sum of exchangeable aluminium and hydrogen ions in the soil. EXCHANGEABLE BASEAn exchangeable cation other than hydrogen or aluminium ions. The most abundant exchangeable bases in soils are sodium, potassium, calcium and magnesium ions. Note that exchangeable bases are not bases in the strict chemical sense (ie proton acceptors). However soils that have high (close to 100%) base saturation do tend to be alkaline or neutral, because of the absence of the truly acidic cations of aluminium and hydrogen. Contrast with exchange acidity. EXCHANGEABLE IONSCations or anions that are held by electrostatic attraction to an oppositely charged surface (eg clay particle) and which may be displaced by other ions. EXCHANGEABLE SODIUM PERCENTAGEThe ESP or exchangeable sodium percentage is the percentage of the effective cation exchange capacity of the soil occupied by sodium ions. EXTERNAL DRAINAGEDrainage of water away from a site by runoff rather than through the soil profile. Contrast with internal drainage. FABRICThe appearance of the surface of soil material. In the field a hand lens is often used for close inspection. Where few, if any, peds are present the fabric is described as sandy (like beach sand) or earthy (a matt appearance due to the presence of fine pores). If peds are evident, the fabric is described as smooth-ped (peds dense and smooth-faced) or rough-ped (peds have relatively porous surfaces and are rough-faced). FACTORS OF SOIL FORMATIONThe environmental factors which define the nature of the soil formed at a particular place in the landscape. Five are usually identified: parent material, climate, topography, biota and time. FACTUAL KEYSee Northcote Key. FALLOWA period in the cropping cycle where the land is kept free of growing plants. This allows soil water and mineralised nutrients to accumulate. Weeds need to be controlled during the fallow, and this is achieved by cultivation or herbicides. The plant residues from the previous crop have traditionally been removed by burning (a bare fallow), but are increasingly being retained. Retained residues may be left untouched (no-till fallow), incorporated into the soil by tillage (stubble incorporation), or chopped and left on the soil surface (stubble mulching). FELDSPARThe most abundant group of primary aluminosilicate minerals, being found in all the common igneous rocks. Plagioclase feldspars contain Ca and Na, and alkali feldspars contain K and Na. FERROSOLA soil order in the Australian Soil Classification. Ferrosols lack strong texture contrast between the A and B horizons, and have B horizons which are high in free iron oxide. FIELD CAPACITYThe amount of water retained in the soil after it has been saturated and allowed to drain freely for 1 to 2 days. Often approximated by the soil water content at a matric potential of -10 kPa. FIELD TEXTURETexture as determined by a field method, such as the ribbon test. Is influenced by factors in addition to particle size distribution. FINE EARTHThe fraction of soil particle sizes with diameters less than 2 mm (ie clay + silt + sand). Soil chemical and physical analyses are usually carried out on the fine earth fraction after the gravel (> 2 mm)has been removed by sieving. FLOCCULATIONThe bonding together of soil colloids (clay particles) that are suspended in water to form compound particles (microaggregates). When a colloidal suspension flocculates gravity causes the particles to rapidly settle out. Flocculation is the opposite of deflocculation or dispersion. FLUXThe amount of a quantity transported per unit time. For example in soil hydrology the infiltration flux is the rate at which water enters the soil. In nutrient cycling nutrient flux is the rate at which a nutrient moves from one pool to another. FRIABLEEasily crumbled. For example a loam with a high organic matter level would be friable when moist. FULVIC ACIDComplex organic molecules that form part of soil humus and are soluble in water. FUNGAL HYPHAEThe individual filaments which make up the body of a fungus. GEOLOGICAL EROSIONErosion occurring at natural rates, and hence a natural soil and landscape forming process. Contrast with accelerated erosion. GIBBSITEA white sesquioxide soil clay mineral with formula Al(OH)3. GILGAIA form of soil surface microrelief comprising hummocks and/or hollows. Associated with the swelling and shrinking that occurs in expansive soils as they go through wetting and drying cycles. Linear gilgai are long parallel mounds and depressions that run down a gentle hill slope (relief usually less than 300 mm, mounds usually separated by 5 to 8 m). Melonhole gilgai are irregular large depressions on level ground usually more than 3 m in diameter and more than 300 mm deep. GLEYDull grey or bluish-grey colours produced in soil by extended periods of waterlogging and the chemical reduction or removal of iron minerals. GOETHITEA sesquioxide soil clay mineral with formula FeOOH, and which is commonly responsible for yellow-brown colours in soils. GRADATIONAA gradational soil is one which has a gradational texture profile, which means that the texture gradually becomes heavier (more clayey) with depth. GRANULARA structure with spheroidal peds that do not fit into the faces of adjacent peds. GRASSED WATERWAYA strip of land running down a slope which is kept permanently covered with grass and used as a safe channel to transport water down the slope without causing erosion. Often used in conjunction with contour banks. GRAVELIn the International scheme (used in Australia), gravel is the fraction of soil particles with diameter greater than 2 mm. Contrast with fine earth. GRAVIMETRIC WATER CONTENTThe mass of water contained in a soil sample divided by the mass of oven-dry soil. Also termed mass water content. GRAVITATIONAL POTENTIALThe component of water potential that is caused by the position of the water in the earth#39;s gravitational field (ie its elevation). Gravitational potential is measured relative to some arbitrary reference height (eg the soil surface) and is positive above that point and negative below that point. See also water potential. GREAT SOIL GROUPOne of 43 Australian soil types described in "A Handbook of Australian Soils". The Great Soil Group system is now superseded by the Australian Soil Classification. GREEN MANUREA crop grown and then ploughed into the soil without harvesting in order to improve soil fertility. GREY CLAYA member of the grey, brown and red clays great soil group. Deep, uniform cracking clay soils widespread in the alluvial plains of inland Australia. The Great Soil Group system is now superseded by the Australian Soil Classification. GULLYAn erosion channel cut into the soil by a concentration of runoff. Gullies are deeper than rills and are not able to be ploughed out. See also rill and sheet erosion. GYPSUMThe mineral name for calcium sulfate dihydrate. HARDSETTINGThe soil degradation process whereby a surface soil horizon dries to a hard apedal mass. HEMATITEA sesquioxide mineral with formula Fe2O3, that is often found in the clay fraction of soils, and which is responsible for the red colours of soils. HORIZONA soil layer which is approximately parallel to the land surface and differs in properties from adjacent layers. HUEThe component of soil colour which indicates its dominant spectral wavelength (eg yellow or red). See also Munsell colour. HUMIC ACIDA component of soil humus which is insoluble in water but soluble in dilute alkali. HUMINA component of soil humus which is insoluble in water and also insoluble in dilute alkali. HUMUSThe relatively stable, dark coloured mixture of organic compounds which is produced as plant or animal residues decompose in the soil. HYDRAULIC CONDUCTIVITYA measure of the ability of the soil to conduct water. More precisely it is the flux of water at unit hydraulic potential gradient. The saturated hydraulic conductivity is the hydraulic conductivity when the soil is saturated. HYDRAULIC POTENTIALThe sum of the matric, pressure and gravitational components of water potential. Differences in hydraulic potential determine the movement of water within the soil profile. Movement of water into plant roots, however, is also influenced by differences in solute (osmotic) potential. See also water potential. HYDROSOLA soil order in the Australian Soil Classification. Hydrosols include a range of soils which are seasonally or permanently wet. IGNEOUSIgneous rocks are those formed as magma cools and solidifies. ILLITEA 2:1 clay mineral related in structure and composition to mica, and in which the layers are held together by unhydrated potassium ions. ILLUVIATIONThe soil forming process whereby soil material in suspension is transported into and accumulates in a soil layer. IMMOBILISATIONAlso spelled immobilization. The process whereby inorganic chemical forms of an element are converted to organic forms. For example, if a microorganism takes up nitrate and converts the N to protein, immobilisation of N has occurred. The opposite of mineralisation. INFILTRATIONMovement of water through the soil surface and into the soil. INTERNAL DRAINAGEDrainage of water through the soil profile. Contrast with external drainage. IRONSTONEHardened material often found in soil as concretions and which is rich in iron oxides, but also often containing substantial quantities of aluminium and manganese oxides. See also laterite. ISOMORPHOUS SUBSTITUTIONSubstitution of one ion in a crystal structure with another without altering the structure. In clay minerals isomorphous substitution of one cation with another of a lower charge gives the structure a permanent negative charge which contributes to its cation exchange capacity. KANDITEA group of 1:1 clay minerals clay. Kaolinite is a common example. KANDOSOLA soil order in the Australian Soil Classification. Kandosols have weakly pedal or massive B horizons, little texture contrast, and are not calcareous throughout. KAOLINITEA 1:1 clay mineral common in soils. KRASNOZEMA great soil group which is typically red, high in clay content, leached and acid. The Great Soil Group system is now superseded by the Australian Soil Classification. KUROSOLA soil order in the Australian Soil Classification. Kurosols have a strong texture contrast between the A and B horizons and the B horizons are strongly acid LAND CAPABILITYThe ability of land to sustainably support a given land use. LATERITEA zone of ironstone within the soil which may form a continuous mass or be composed of concretions. LAYER SILICATESThose aluminosilicate minerals which have crystal structures comprising octahedral and tetrahedral layers, yielding plate like crystals. The silicate clay minerals and micas are layer silicates. LEACHINGThe removal from the soil of materials dissolved in percolating water. Contrast with eluviation LENTICULARPeds which are lens shaped. LIGNINLignin is a complex polymer of phenolic compounds which gives strength to woody tissue. In soil lignin is quite resistant to decay, and partially altered lignin is a component of humus. LIQUID LIMITThe water content at which a soil passes from a plastic (retains its shape after being deformed) to a liquid state (flows). LITHOSOLA great soil group which includes soils that are shallow and stony. The Great Soil Group system is now superseded by the Australian Soil Classification. LOAMA mid range soil texture class containing approximately equal quantities of clay, silt, fine sand and coarse sand. LOESSSoil material predominantly of silt (with some clay) size that has been deposited after being transported by wind. Also see parna. MACRONUTRIENTSThe plant nutrients that are needed in the largest quantities: N, P, K, S, Ca and Mg. MACROPORESSoil pores greater than 0.5 mm in diameter. MASS WATER CONTENTSee gravimetric water content. MASSIVESoil structure is massive if no peds are evident (ie it is apedal) and the soil is coherent (soil primary particles are bonded together). MATRIC POTENTIALThe component of water potential that results from the adsorption of water molecules onto soil particle surfaces and the capillary attraction of soil pores for water. In unsaturated soil the matric potential is negative, reaching zero at saturation. Pressure potential and matric potential are sometimes lumped together, as pressure potential is zero in unsaturated soil, but positive in saturated soil. See also water potential. MICAA group of primary aluminosilicate minerals with a layered crystal structure similar to that of the 2:1 silicate clay minerals. Two common forms are biotite and muscovite. MICRONUTRIENTSPlant nutrients that are required in relatively small quantities (though they are still essential for plant growth). Micronutrients include: Cl, B, Zn, Fe, Mo, Mn, Cu and Ni. MICROPORESPores less than 0.5 mm in diameter. MINERALISATIONAlso spelled mineralization. The process whereby organic chemical forms of an element are converted to inorganic forms. For example, if, as humus is slowly decomposed, ammonium is released into the soil, then mineralisation of N has taken place. The opposite of immobilisation. MINERALSSoil minerals are the solid inorganic constituents of soil, each of which has defined chemical and crystalline properties. Primary minerals (eg quartz) were formed as igneous rocks crystallised and have been inherited chemically unaltered by the soil. Secondary minerals (eg clays) have been formed by the chemical weathering of primary minerals. MINIMUM TILLAGEPart of conservation farming practice in which the number of tillage operations is reduced to the minimum necessary for crop production. MONTMORILLONITEA member of the smectite group of 2:1 clay minerals. Sometimes montmorillonite is used synonymously with smectite. MOTTLESPatches in a soil horizon which have a different colour to the dominant colour for that horizon. MULCHMaterial which is spread over the surface of the soil, and which modifies conditions near the soil surface (eg by altering soil temperature, evaporation or erodibility). MUNSELL COLOURThe colour of soil described objectively in terms of hue, value and chroma by reference to the Munsell colour charts. MUSCOVITEA pale coloured mica common in some granites. NITRIFICATIONThe oxidation of ammonium to nitrate by microorganisms in soil. NITROGEN FIXATIONConversion of atmospheric dinitrogen gas (N2) to a chemical form than plants can use (eg ammonium or nitrate). NO-TILLA cropping system where there is no tillage, the stubble is untouched during the fallow, weeds are controlled with herbicide, and the next crop is sown directly in the standing stubble. In Australia this term is mainly used in northern New South Wales. See also direct drill and fallow. NORTHCOTE KEYA key developed by Keith Northcote in the 1960s and formerly used for classifying soils in Australia based on objective assessment of soil profiles. Soils were given a letter and number code (eg Dr4.21). Also termed the Factual Key. NUTRIENTIn general, a nutrient is a substance which contributes to the growth and functioning of an organism. In the context of soils, the term nutrient is often taken as synonymous with plant nutrient. The essential plant nutrients are those elements (apart from C, H and O) which are essential for plants to complete their life cycle. Currently the nutrients which are known to be essential to plants in general are: N, P, K, S, Ca, Mg, Cl, B, Zn, Fe, Mo, Mn, Cu and Ni. NUTRIENT CYCLEA nutrient cycle is the set of pools and fluxes that describe the translocation and transformation of a nutrient through an ecosystem. OCTAHEDRONAn eight sided volume with 6 vertices. In soil clay minerals an aluminium ion is often surrounded by 6 oxygen anions situated at the vertices of an octahedron. Linked octahedrons form the octahedral layers in clay mineral crystals. ORGANIC MATTERSoil organic matter comprises a wide range of organic carbon compounds which were originally part of living organisms. ORGANIC SOILA soil with a high proportion of organic matter throughout the solum. ORGANOSOLA soil order in the Australian Soil Classification. Organosols are dominated by organic materials. OSMOTIC POTENTIALSee solute potential. OVEN-DRYSoil samples are oven-dry when they have been allowed to dry in an oven at 105 C to a stable water content. Usually this takes about 24 hours. PARENT MATERIALThe material from which the soil is formed. PARNAAeolian deposits of calcareous clay which are common in the Riverina. PARTICLE DENSITYThe density of the particles within the soil, often assumed to be 2.65 grams per cubic centimetre. Contrast with bulk density. PEATAn organic soil formed where organic matter decomposition has been impeded by permanent waterlogging. PEDPeds are aggregates that occur naturally in the soil profile. They are separated one from another in the soil profile by planes of weakness which may be indicated by cracks or cutans. Peds are different to clods, which are formed artificially (eg by tillage). PEDALSoil structure is pedal if peds are present. Contrast with apedal. PEDOGENESISSoil formation. Hence pedogenic means formed within the soil profile. PEDOLOGYThe study of soils in relation to their genesis, classification and distribution over the landscape. PEDOTURBATIONA soil forming process in which soil material is mixed within the soil profile. Examples of pedoturbation are the shrinking and swelling of cracking clays leading to gilgai, or bioturbation due to burrowing animals. PENETRABILITYThe ease with which a soil can be penetrated. See also penetrometer. PENETROMETERA device consisting of a steel rod with a conical tip that is used to measure soil strength. PERCOLATIONThe downward movement of water though the soil under the influence of gravity. PERMANENT CHARGEThat component of the electrical charge possessed by soil colloids which does not vary with environmental conditions. Permanent charge is related to isomorphous substitution in the clay crystal lattice. Contrast with variable charge. PERMANENT WILTING POINTThe water content of soils at which plants will not regain turgor even if the soil water content is raised. Often taken as the lower limit at which plants can use soil water and approximated by the soil water content at a matric potential of -1500 kPa. PHASE RELATIONSThe relationship between the solid, liquid and gaseous components of soil. PHOSPHORUS FIXATIONThe process by which phosphate ions become adsorbed so strongly by soil colloids that they are no longer exchangeable. PLANT AVAILABLE WATERSoil water that plants are able to take up and transpire. Excludes soil water held in large pores that drain rapidly and soil water held in pores so fine that the plant roots cannot extract it. Numerically plant available water is equated to the difference in water content between field capacity and permanent wilting point. PLASTIC LIMITThe water content at which a soil passes from a solid to a plastic state. Operationally defined as the water content at which soil can be rolled into 3 mm diameter rods without the rods breaking. Tillage when the soil is at or wetter than the plastic limit is likely to result in compaction or smearing. The plasticity index is the numerical difference between the liquid limit and the plastic limit. See also Atterberg limits. PLATYA type of soil structure where the horizontal dimensions of peds exceed the vertical dimension. PODOSOLA soil order in the Australian Soil Classification. Podosols have B horizons which characterised by accumulations of organic matter, iron and/or aluminium. PODZOLA great soil group characterised by a uniform sandy profile where a B horizon has been formed by the illuviation of humus and sesquioxides. Found in sand dunes in humid areas. The Great Soil Group system is now superseded by the Australian Soil Classification. PODZOLICThere are several podzolic great soil groups, which are duplex or gradational soils, mostly with A2 horizons, and acid to neutral pH. The Great Soil Group system is now superseded by the Australian Soil Classification. POLYHEDRALA type of structure with approximately equidimensional peds that have more than six faces and fit one into another in the soil profile. PONDINGAccumulation of free water on the soil surface. POOLIn nutrient cycling a pool is a defined form or location in which a nutrient element occurs within the ecosystem. For example, in the soil-plant N cycle, pools could include soil inorganic N, soil organic N and plant N. PORESThat part of the soil not occupied by solid particles. Synonymous with voids. Soil pores are filled by air or water. The pore size distribution is the relative proportions of pores of different sizes in the soil. POROSITYThe volume of soil pores (water + air filled) divided by the total soil volume. POTENTIAL GRADIENTThe rate at which a potential varies with distance. For many quantities their flux is proportional to a potential gradient. For example plant water uptake from the soil is proportional to the water potential gradient. PRECIPITATIONPrecipitation has two different meanings in the context of soils. Precipitation refers to material falling to the earth's surface from the atmosphere, and in particular means the sum of rain and snow. Chemically, however, precipitation means the formation an insoluble substance by the chemical reaction of substances in solution. PRESSURE POTENTIALThe component of water potential that is caused by the hydrostatic pressure of overlying water. Pressure potential is zero in unsaturated soil. Below a water table pressure potential is positive and increases with depth. Pressure potential is sometimes lumped with matric potential. See also water potential. PRIMARY PARTICLEThe individual mineral particles of which soil is composed, and which the soil can be separated into in the laboratory by dispersion techniques. Primary particles are those which comprise the clay, silt, sand and gravel fractions of the soil. Contrast with aggregate. PRISMATICA type of soil structure where peds have a vertical axis that is much longer than the horizontal axis and have angular edges. Compare with columnar structure. QUARTZA primary mineral, chemically silicon dioxide, which is abundant in all acid igneous rocks, as well as in many sedimentary and metamorphic rocks. It is resistant to weathering and is the main component of the sand fraction of most soils. R HORIZONBedrock beneath the soil profile. RED CLAYA member of the grey, brown and red clays great soil group. Deep, uniform cracking clay soils widespread in the alluvial plains of inland Australia. The Great Soil Group system is now superseded by the Australian Soil Classification. RED EARTHA great soil group characterised by a red, uniform to gradational, weakly pedal profile. The Great Soil Group system is now superseded by the Australian Soil Classification. RED-BROWN EARTHA great soil group characterised by red-brown gradational to duplex profiles, sometimes with an A2 horizon, alkaline at depth with calcium carbonate concretions. The Great Soil Group system is now superseded by the Australian Soil Classification. REGOLITHThe layers of unconsolidated material at the earth#39;s surface, comprising the solum and any underlying weathered rock. RESIDUEThe organic remains of plants or animals. RESPIRATIONThe biochemical processes by which organisms obtain energy by oxidising sugars to carbon dioxide and water. In aerobic respiration molecular oxygen (O2) is used as the oxidising agent. In the absence of oxygen (eg in waterlogged soils), other oxidising agents can be used, including nitrate and ferric ions. This form of respiration is termed anaerobic respiration. RHIZOSPHEREThe zone in the soil that is adjacent (eg within 1 mm) to the surface of a root. RHYODACITEA fine grained acid igneous rock. RILLA small erosion channel cut into the soil by a concentration of runoff. Contrast with gully. See also sheet erosion. RUDOSOLA soil order in the Australian Soil Classification. Rudosols are soils which have negligible pedologic organisation (ie negligible evidence of the effects of soil forming processes like horizon formation, colour differentiation or formation of peds). Very young soils might be Rudosols. RUNOFFWater that flows over the soil surface and away from a site. Contrast with runon. RUNONWater that flows over the soil surface and onto a site. Contrast with runoff. SALINISATIONThe process by which soil becomes saline. Saline soils are those containing high enough concentrations of soluble salts to adversely affect the growth of most plants. Primary salinisation is a natural soil forming process, but secondary salinisation is salinisation induced by human activity. SALTATIONIn wind or water erosion it is the transport of particles over the soil surface in a series of short jumps. SANDThe International particle size classification scheme (used in Australia) defines sand as particles sizes between 2 mm and 0.02 mm in diameter. Coarse sand is from 2 to 0.2 mm, and fine sand is from 0.2 to 0.02 mm. SAPROLITESoft weathered rock. SATURATIONSoil is at saturation when all the pores are filled with water. SELF-MULCHINGA surface soil condition where a loose mass of fine peds forms a natural mulch. SESQUIOXIDESA group of soil minerals comprising the oxides and hydroxides of iron and aluminium. SHEET EROSIONA form of water erosion in which a thin uniform layer is removed from the soil surface. See also rill and gully. SILICATESMinerals containing silicon and oxygen as the main elemental constituents. SILICEOUS SANDA great soil group characterised by a sandy profile where there has been little profile development apart from the possible accumulation of surface organic matter. The Great Soil Group system is now superseded by the Australian Soil Classification. SILTIn the International system (used in Australia), silt is defined as the particle size range between 0.002 mm and 0.02 mm in diameter. SLAKINGThe breaking down of aggregates to smaller aggregates due to internal stresses that accompany wetting. Contrast with dispersion. SLICKENSIDESPolished surfaces of B horizon peds that occurs in cracking clays as they shrink and swell during drying and wetting cycles. SMECTITEGroup of 2:1 clay minerals characterised by high surface areas and cation exchange capacities, and variable interlayer spacing. Montmorillonite is a smectite commonly found in soils. SODICSodic soils are defined in Australia as those having an ESP (exchangeable sodium percentage) equal to or greater than 6%. SODOSOLA soil order in the Australian Soil Classification. Sodosols have a strong texture contrast between the A and B horizons, and the B horizons are sodic and not strongly acid. SOIL COLOURSee Munsell colour. SOIL PROFILEA vertical section through the soil. SOIL REACTIONAn old term meaning soil pH. SOIL SOLUTIONSoil water and substances dissolved in it (ie solutes). SOIL SURVEYThe systematic examination, description, classification, and mapping of soils in an area. SOLODICA great soil group characterised by a duplex profile with a bleached A2 horizon and a sodic B horizon which is alkaline at depth. The Great Soil Group system is now superseded by the Australian Soil Classification. SOLUM(Plural: sola). The A and B horizons of the soil profile. SOLUTE POTENTIALAlso termed osmotic potential. That component of the soil water potential which is caused by the osmotic effect of solutes in soil water. The solute potential is always negative, becoming more so as the salt concentration increases. See also water potential. SPECIFIC SURFACE AREASurface area per unit volume or mass STRENGTHSoil strength is a measure of the resistance of soil to externally applied forces. STRIP CROPPINGA strategy to reduce water or wind erosion in which different crops are grown in strips across the slope, in such a way that no adjacent strips are bare at any one time. STRUCTURESoil structure is the three dimensional arrangement of primary particles in the soil profile. Primary particles are often bonded together to form peds or natural aggregates. In the field, soil structure is characterised by describing the shape (type), size and distinctness (grade) of the peds. SUBANGULAR BLOCKYSubangular blocky peds are square or block like with rounded edges. SURFACE SEALINGSealing of the soil surface during rain, resulting from breakdown of aggregates. May lead to the formation of a crust when the soil dries. SURFACE TENSIONThe force per unit length necessary to pull apart a liquid surface. SUSPENSIONA fluid containing particles which are small enough to settle out only slowly, but are not dissolved. TENOSOLA soil order in the Australian Soil Classification. Tenosols include a wide range of soils which have only weak pedologic organisation (ie negligible evidence of the effects of soil forming processes like horizon formation, colour differentiation or formation of peds) apart from the formation of A horizons. Contrast with Rudosols. TENSIOMETERA suction device that is used to measure the soil matric potential in the field. TETRAHEDRONA volume with four sides and four vertices. In soil clay minerals a silicon (+4) cation is often surrounded by 6 oxygen anions situated at the vertices of a tetrahedron. Linked tetrahedrons form the tetrahedral layers in silicate clay mineral crystals. TEXTURESoil texture is the relative proportions of the sand, silt and clay fractions in the soil. These fractions can be determined quantitatively in the laboratory, and the soil texture triangle is then used to assign a texture grade (eg sandy loam). In the field the ribbon test is used to determine the texture grade. Field texture is not identical to that determined in the laboratory, because factors apart from particle size distribution can influence the field method. TILLAGEAny process which disturbs the upper part of the soil profile, mostly for the purpose of aiding crop production. Tillage is used to kill weeds, incorporate plant residues, decrease soil strength, decrease aggregate size, increase porosity, level the soil surface, etc. TOPOSEQUENCEA series of soils for which all the factors of soil formation apart from topography are approximately constant. For example a sequence of soils formed from the same parent material down a hill slope usually forms a toposequence. Toposequence can be taken as synonymous with the older term catena. TRACE ELEMENTSElements present in small quantities. Sometimes used synonymously with micronutrients. TRANSPIRATIONLoss of water as vapour to the atmosphere that has been taken up from the soil by plants. Contrast with evaporation and evapotranspiration. TURGOR PRESSUREPressure exerted on the cell walls by water contained within the cell. Turgor pressure prevents plant tissues from wilting and drooping. UNIFORMA uniform soil is one which has a uniform texture profile, which means that the texture remains approximately the same at all depths. UNIVERSAL SOIL LOSS EQUATIONAn equation developed in the USA to predict soil loss by water erosion in that country. Modifications have been made to allow the USLE to be used successfully in parts of Australia. UREAThe dominant form of N in urine and a widely used N fertiliser. USLESee universal soil loss equation. VALUEIn the measurement of soil colour, the value indicates how light or pale the colour is (ie white has a high colour value, dark grey has a low value). See also Munsell colour. VARIABLE CHARGEElectrical charge present on soil colloids that varies with the soil chemical environment, particularly with pH. Contrast with permanent charge. VERTOSOLSA soil order in the Australian Soil Classification. Vertosols are clay soils which exhibit shrink-swell characteristics, crack strongly when dry and have lenticular peds and/or slickensides at depth. VOID RATIOThe volume of soil pores divided by the volume of solid matter. VOIDSSee pores. VOLATILISETransform from liquid or solid state to gaseous state. See also ammonia volatilisation. VOLUMETRIC WATER CONTENTVolume of soil water per unit bulk volume of soil. WATER HOLDING CAPACITYThe amount of water held by soil at field capacity. WATER POTENTIALThe soil water potential is the potential energy per unit quantity of soil water. Its value in understanding the behaviour of soil and plant water is based on the fact that water can only move spontaneously from a place of high potential to one of low potential, and that the rate of water movement is proportional to the potential gradient (See also hydraulic potential). Soil water potential can be split into a number of component potentials including matric potential, pressure potential, gravitational potential and solute potential. WATER TABLEThe upper surface of free water in the soil. Below the water table the soil is saturated. WATERLOGGEDSaturated with water. WEATHERINGThose chemical and physical processes which by which minerals are physically altered and chemically decomposed near the earth's surface by air and water. WILTING POINTThe water content or soil water tension at which plants lose turgor and wilt. See also permanent wilting point. YELLOW EARTHA great soil group characterised by a yellow to yellow-brown, uniform to gradational, weakly pedal profile. The Great Soil Group system is now superseded by the Australian Soil Classification. |