Common meanings for soil related terms.

A Horizon

Surface mineral horizon is usually darker in colour than the lower horizons due to organic matter accumulation. The A Horizon can be broken down into three distinct layers (A1- surface soil with some organic matter content; A2- usually paler in colour and less organic matter; A3- transitional horizon between the A and B horizons).

Acidity

Soils with a pH below 7 are acid. Growth and yield of most plants is unaffected until the pH drops below about 5.

Acidification

A process whereby soils become acidic over time caused by changing soil chemistry, leaching or oxidation.

Aerobic

Soils in which free oxygen is abundant such as well drained soils with good structure. Plant roots and soil organisms require oxygen to survive and grow.

Aluminium

Aluminium (Al) is a component of clay, and becomes soluble in acid soils with a pH less than 5. Below a pH of 4.5, it becomes toxic and may severely restrict plant growth or cause plant death.

Anaerobic

Soils deficient in free oxygen. This condition generally occurs in poorly drained or waterlogged soils, resulting in a bluey-grey mottled colour in soil.

Anion

Anions are negatively charged particles (ions) such as chloride, sulphate and nitrate.

Available nutrients

Available nutrients are present in soil solution, or as exchangeable ions attached to clay and organic matter, and are readily plant available.

Available water

Plant available water (PAW) is used to describe how much available water can be stored in a particular soil that can be extracted by a plant. Clay loams tend to store the most available water, and coarse textured sandy soil the least.

B Horizon

Subsoil horizon consisting of one or more mineral layers. It differs to the A Horizon in clay, iron, aluminium or organic matter concentrations; structure and/or consistence; and colour.

Biodynamic

An agricultural system that introduces specific additional requirements to an organic system. These are based on the application of preparations and subsequent management.

  

Biomass

Biomass of organic material.

Buffering capacity

The soils ability to resist change in pH. Soils with a high clay and organic matter content have a higher buffering capacity.

Bulk density 

The weight of a known volume of oven-dried soil Mg/m³ (equivalent to g/cm³).

C Horizon

Layers below the AB profile. Lacks pedological development, includes rock and sediments that are generally weak in strength

Calcareous

A soil with sufficient calcium carbonate to cause effervescence (fizzing) after the application of a few drops of hydrochloric acid.

Calcium

Calcium (Ca) is an important plant nutrient as well as being important for soil structure. It is strongly attracted to clay and causes flocculation (clumping together) of wet clays, which is important for good soil structure. Calcium works against the dispersion that occurs when a soil is high in sodium.

Calcrete

A hard layer where cemented carbonate accumulation has occurred.

Cation exchange capacity (CEC)

The measure of the capacity of a soil to store nutrient cations. Most soil tests measure calcium (Ca), magnesium (Mg), sodium (Na) and potassium (K), but a small component of the CEC in acid soils includes hydrogen (H), aluminium (Al) and manganese (Mn). It is a property of clays and organic matter.

Cation

Cations are positively charged particles. Cation exchange is used as a measure of the soil's ability to store nutrients.

Clay

Clay is the smallest of the particles that make up soil (less than 0.002 mm in diameter).

Clay minerals

Three broad groups of clay minerals are important in soils: kaolins, illites and smectites. Mixtures of clay minerals within a soil are very common.

Compaction

Increase in soil density resulting from tillage, stock trampling and/or vehicular traffic. A loss of pore space associated with compaction can lead to lower soil permeability and poorer soil aeration.

Crusting

Soil crusts are hard surface layers up to 1 cm thick, with very fine or no shrinkage cracks. They occur mainly on bare soil when soil aggregates are broken down into particles by the force of raindrops, or by rain and wind erosion. Soil particles fill the pore spaces between remaining aggregates and seal the soil surface.

Dispersion

Dispersion occurs when soil clay minerals fragment into sand, silt and clay components. Soils are usually high in exchangeable sodium, or low in organic matter and as a result are structurally unstable and disperse in water.

Drainage

Ability of water to permeate a soil.

Duplex soil

A duplex soil is one that has a sand, silt or loam topsoil over a clay subsoil. These soils may have restricted drainage, or lateral drainage at the top of the subsoil because of this texture change.

Earthworms

Earthworm casts cement the soil particles together in water-stable aggregates. Earthworm casts on the soil surface rebuild topsoil and under favourable conditions can bring up about 50 t/ha annually (enough to form a 5 mm layer). Earthworms form biopores in soil, enhancing water infiltration and drainage.

Electrical conductivity (EC)

EC is a measure of the concentration of ions in soil solution (as measured by the conduction of electricity through water, or a soil water extract). The value can reflect the amount of soluble salts in a soil extract - therefore providing an indication of soil salinity. Saline soils are defined as those with an EC of greater than 1.5 dS/m for a 1:5 soil water extract. EC units are microSiemens per centimetre (μS/cm). These units relate directly to deciSiemens per metre (dS/m) and parts per million (ppm) of total dissolved solids in mg/L. For example, 1 dS/m = 1000 EC (μS/cm) = 640 ppm.

Evaporation

Evaporation removes water from the soil by heating it and vaporising it into the air. Water loss by evaporation is more rapid on wet soils with evaporation slowing as soils dries.

Exchangeable sodium percentage (ESP)

The ESP is the amount of exchangeable sodium in a soil expressed as a percentage of the total exchangeable ions in the soil. An ESP of more than 5 defines a sodic soil.

Fertiliser

Fertiliser is a generic term for a product that adds nutrients to the soil. Products that are not registered cannot be sold as fertilisers. This is to make sure that all fertilisers conform to minimum nutrient requirements. Both inorganic (manufactured) and organic fertilisers exist.

Field capacity

Field capacity is a measure of the amount of water present in a soil when the macropores are empty and the micropores are full—usually 24 to 48 hours after rain or irrigation (applies only to freely draining soils). The amount of water the soil can hold at field capacity depends on the size of the pores. Sandy soils, for instance, hold very little water at field capacity because most of their pores are large. Clay and organic soils have a much higher field capacity water content because they contain many micropores.

Flocculation

Flocculation refers to the clumping together of particles in solution, and is the opposite of dispersion. In soils, the presence of calcium tends to flocculate clays, maintaining structure, while sodium tends to disperse clays in solution. Gypsum and lime, both calcium minerals, are used to achieve flocculation in some sodic soils.

Gleying

Gleying' is a term used for the reduction of iron that removes the reddish and yellowish iron minerals and reveals the greyish colour of the clays. Gleying is a sign of severe waterlogging, and gleyed soils often have a bluish or greenish tinge.

Gypsum

Gypsum (calcium sulfate) can occur naturally, or may be produced as a by-product of phosphate fertiliser production (phosphogypsum) or hydrofluoric acid production (calcium sulfate anhydrite). Gypsum that occurs naturally is of variable quality and contains impurities.

Hardsetting

Hardsetting refers to a soil in which the surface horizon sets hard when dry. Hardsetting soils are high in fine sand and/or silt and/or an easily dispersed swelling clay such as illite. They are also low in organic matter. Cultivation can make hardsetting worse, especially if the soil is too dry, because it encourages the breakdown of organic matter and destroys soil structure.

Humus

The relatively stable organic material after it has been weathered

Hydraulic conductivity

Hydraulic conductivity is a measure of the movement of water through the soil profile. It determines where the water goes after infiltration, and how quickly. The hydraulic conductivity depends largely on the soil texture and structure; it reflects the size, arrangement and number of pores.

Illite

Illites are clay minerals or clay micas. They are the dominant clay minerals in most of the red brown earths, solonised brown soils, calcareous red earths, and terra rossa soils of southern Australia. They are generally very stable. Illites vary in their ability to swell, and are generally more prone to dispersion than kaolins and smectites.

Infiltration

Infiltration refers to the movement of water into the soil profile. Many clay soils have a very high infiltration rate when dry because they have large cracks. In non-cracking soils, old root paths and biopores such as ant or worm holes are important for infiltration.

Ion

Ions are electrically charged particles made up of atoms or molecules. Cations are positively charged ions. Anions are negatively charged ions.

Irrigation

Flood and furrow irrigation are the most common methods of irrigation. In both cases, free water is supplied to the surface of dry soil.

Kaolin

Kaolin is the most weathered of the clay minerals, and has virtually no swelling potential. Soils dominated by kaolin clay occur mainly in areas of high temperature and rainfall. These soils are generally acidic due to the weathering process. Kaolin clay soils are frequently strongly structured, with good infiltration and drainage.

Landforming

Landforming is an earthmoving process whereby the natural topography is changed to suit a particular purpose, such as surface irrigation. When you are choosing an area to develop, consider the topography and how much landforming will be required. Carefully examine soils, paying attention to the depth and structure of the topsoil, and the structure of the subsoil. Avoid soils where the topsoil is shallow (less than 10 cm) or highly variable over the area, or areas where the topsoil and/or subsoil structure is poor.The most important factor that determines the benefit or harm done by working the soil is the moisture content - avoid working wet soils. In most soils, structure is poorer with depth, and the topsoil contains most of the nutrients.

Lime

Lime (calcium carbonate) is commonly used on strongly acid soils to make them more alkaline. The carbonate in lime neutralises the acidity in soils and so raises the pH. Good quality lime contains 37% to 40% calcium. There are several liming materials available, so determine effective neutralising value prior to application - the most effective lime has a higher proportion of small particles. The finer the particles the faster they react with the soil.

Magnesium

Magnesium is an important part of chlorophyll, the green colouring material of plants. It is also vital for photosynthesis, the process that converts the sun's energy into food for the plant. Magnesite or Epsom salts can overcome magnesium deficiency. However, an excess of exchangeable magnesium in a sodic soil can worsen the dispersion caused by exchangeable sodium.

Micro-organisms

Micro-organisms (or microbes) include bacteria, actinomycetes, fungi, algae and protozoa. There are hundreds of millions of micro-organisms in a spoonful of soil, most of them of benefit to plants and soil. They are important for soil structure, as they form organic bonds that hold soil particles together. They also release carbon dioxide and energy as well as mineral nutrients that plants can use. They do this because they derive their energy by breaking down plant and animal material. When they die they decompose and release more nutrients for plant use. The type of organic matter present influences the microbial activity in the soil. Material high in lignins and tannins (from hardleafed plants such as eucalypts) takes much longer to break down than less fibrous plants such as most vegetables. Microbes are generally more active at higher temperatures, and require moist soil to function. Microbial activity is influenced by pH; it slows down in very acid or very alkaline soil. Finally, most microbes need oxygen, though some are anaerobic and thrive in waterlogged conditions.

Moisture content

Moisture content affects soil strength, aeration, temperature, consistency and infiltration. It is an important soil property to observe, especially when you are planning tillage operations. A simple and effective way to test soil moisture content for tillage is to mould some soil in your hand. See if it will form a ball, then try to form a ribbon, and finally try to roll it into a thin (3 mm diameter) rod.

Mulch

Mulch is a layer of material (usually organic) that sits on top of the soil. Some heavy clay soils are actually self-mulching in that they form a layer of loose granules on their surfaces that acts as a moisture and temperature mulch.

Mycorrhizae

Mycorrhizae are soil fungi that act as extensions to plant roots, helping roots take in more nutrients (in particular phosphorus) in exchange for carbohydrates. They also help improve soil structure by binding soil particles into aggregates.

Nitrogen

Nitrogen is a key element in plant growth. Plants take nitrogen from the soil during the growing season, and this nitrogen is harvested mainly as protein. Nitrogen has to be replaced after a crop is grown. Soils high in organic matter are generally higher in nitrogen too. Nitrogen needs to be converted into inorganic form, mostly nitrate, to be available to plants.

Organic matter

Soil organic matter consists of any living or dead plant and animal material. Organic matter is important for both the chemical and the physical properties of the soil. Organic matter makes bonds between soil particles, creating and maintaining soil structure. It also contains nutrients that are essential for plant growth. Organic matter eventually decomposes to humus, a dark crumbly material that cannot be broken down any further and is the basis of a soil's natural fertility because it allows the recycling of nutrients.

Permanent wilting point

As soil becomes drier, remaining water is held more tightly by the soil and is harder for the plant to extract. When the plant can no longer obtain water from the soil, it may wilt and cease growing. Temporary wilting occurs in hot, dry climates in the heat of the day because the plant cannot transmit water fast enough to replace that evaporated into the atmosphere. The wilting disappears when the temperature drops. If a plant does not recover when the temperature drops, the wilting is permanent and the soil is said to be at the permanent wilting point. In sandy soils, the permanent wilting point is sharply defined. In clays and loams it is much less sharply defined, and plants in these soils will show increasing signs of wilting over several days.

pH

The term pH stands for a measurement of the number of hydrogen cations in a solution. The 'H' stands for hydrogen, and the 'p' indicates a negative logarithmic scale. This means that a solution with a pH of 7 has 10 times more H⁺ cations than a solution of pH 8. The pH scale goes from 1 to 14, with 7 being neutral. A solution with a pH greater than 7 is alkaline, while a solution with a pH less than 7 is acid. The pH level is very important in the soil solution, because plant growth and yield are reduced in highly acid or highly alkaline soils. One reason is that some plant nutrients become unavailable at certain pH levels. Standard methods of measuring soil pH is with a suspension of 1 part air-dried soil (by weight) to 5 parts liquid (by volume). The recommended liquid is 0.01 M CaCl₂ (calcium chloride) though distilled water is sometimes used in place of calcium chloride. Soil tested in CaCl₂ solution registers about 0.5 to 0.8 units lower than the same soil tested in water.

Phosphorus

Phosphorus is a nutrient that is vital for seedlings and young plants. Many Australian soils are naturally low in phosphorus in forms that agricultural plants can use. Some exceptions are the grey clays and black earths of northern New South Wales and the red brown earths and sands of south-western New South Wales. Phosphorus is a relatively immobile element and thus very little leaches through the soil, with the possible exception of sandy soils. It is mainly lost from the soil by erosion of soil particles containing phosphorus. For this reason, controlling soil erosion is the main method of minimising the loss of soil phosphorus to waterways in cultivated areas.

Plant Available water (PAW)

Plants are able to use only a part of the water held in a soil. Some water is held so tightly in the soil pores by surface tension that plant roots cannot extract it. By contrast, when a soil is saturated most plants suffer from lack of oxygen. Available water is the water in the soil between the field capacity and the permanent wilting point, and is thus available for plant use. Plant-available water capacity is the term used to describe how much available water can be stored in a particular soil. Soils of intermediate texture, the clay loams, tend to have the highest levels of available water. Sandy soils have little available water, while clay soils have high field capacity levels but also a high permanent wilting points.

Plastic limit

This is the moisture content at which clay behaves like plasticine, because the clay plates slip smoothly across one another with very little force. The plastic limit applies only to clay soils and varies with the amount of clay in the soil, but it can easily be determined by hand. At the higher plastic limit the soil is very weak and is easily remoulded and damaged by external pressure (e.g. machinery, stock), resulting in smearing and compaction. At the lower plastic limit the soil is firm and shatters on cultivation.

Potassium

Potassium is important for plant growth and disease resistance. Some potassium, held on clay minerals and organic matter, is easily available to plant roots. Potassium occurring naturally in some soil minerals is released only slowly for plant use. Potassium is low or deficient in some sandy soils, and deficiency can occur on soils used for intensive grazing, intensive cropping and haymaking. The most common potassium fertilisers are potassium chloride (muriate of potash) and sulfate of potash. Care needs to be taken with potassium chloride, as chloride levels can build up, causing salinity problems.

Salinity

Salinity refers to the concentration of all salts in the soil solution. Although the most common salt in most Australian soils is sodium chloride (common table salt), any readily soluble salt (including fertilisers) may contribute to soil salinity. Salts are present as ions when dissolved in the pore water. If the soil solution is saline, plants may have difficulty taking up both water and nutrients. Soil salinity is measured in soil tests as electrical conductivity. The more electrical current that is conducted in a soil solution, the higher the salt content of the solution.

Sand

Sand refers to particles between 0.02 and 2 mm in diameter. Fine sand is between 0.02 and 0.2 mm and coarse sand is between 0.2 and 2 mm in diameter. Sand comprises mainly quartz particles that will not break down further. Since sand has no electrical charge it does not hold water or nutrients, and does not contribute to soil fertility. However, the large pores between the relatively large particles are important for the movement of air, water and plant roots.

Self-mulching

Some heavy clay soils form a layer of loose granules at the surface when they dry. Soils with this behaviour are called 'self-mulching'. Although water is lost from this and the adjacent layer by evaporation, the self-mulched layer acts as a moisture and temperature mulch. The remaining moisture in the profile is then conserved better than in clay soils that do not self-mulch.

Shrinking and swelling

In some clay minerals, such as smectites, water can enter between the clay particles, causing swelling. As these clays dry, the water evaporates from between the clay particles and the soils shrink, in some cases leaving quite large cracks. Most soils shrink and swell on drying and wetting to some extent, but this is most marked in soils containing smectite clays. Those soils that do shrink and swell have the capacity to repair any structural damage (for example, compaction and smearing) over the space of a few wetting and drying cycles.

Silt

Silt is the particle size fraction in soils that lies between sand and clay (that is, between 0.002 and 0.02 mm in diameter). Silty soils are prone to hardsetting and crusting.

Smearing

Smearing is the realignment of clay particles from a random to a parallel orientation, producing a hard, shiny surface on drying. Smearing results from horizontal shear forces, produced by, for example, a spinning tractor wheel or a tine moving through the soil. Smearing occurs when a soil is wetter than the plastic limit.

Smectite

Smectites are the clay minerals that shrink and swell on drying and wetting. They are an important component of black earths and other cracking clays. A soil that is dominated by smectites is a relatively young soil—as it weathers with time, the smectites will break down into the other, less reactive clay minerals.

Sodicity

Sodicity refers to the amount of exchangeable sodium cations (Na) in the soil, expressed as the exchangeable sodium percentage (ESP). Soils with an ESP greater than 6 are regarded as sodic. Sodic soils swell strongly and may also disperse on wetting ('spewy' soils). Sodic soils are generally poorly structured. Crusting is often a result of sodicity. A soil that is saline, but not sodic, will have fine crumbly aggregates, while a soil that is only sodic is cloddy or crusted. If sodicity remains the same, but salinity increases, a soil's behaviour may move across the line from dispersion to flocculation. On the other hand, if the salinity remains the same and the sodicity increases, a previously flocculated soil may disperse.

Sodium

Sodium is a cation with a single positive charge. For this reason, the bonds between clay minerals with a high proportion of exchangeable sodium cations are weak. A high level of exchangeable sodium causes the dispersion of individual clay particles in solution. Although all ions in solution contribute to salinity, sodium is usually present in significant quantities in saline soils.

Soil organisms

Earthworms, termites, ants and other soil organisms are very important for soil structure, especially in soils that do not crack. Soil animals are also important for incorporating organic matter and breaking it down so that plants can use the nutrients. Overcultivation and loss of organic matter will discourage soil organisms, leaving the soil hard and lifeless.

Soil profile

Soil profile refers to the sequence of horizons (layers) down to, and including, the parent material. The A horizon is the topsoil and the B horizon is the subsoil. Observing the soil profile can help you to manage your soil by giving you a good idea of its advantages and limitations.

Soil solution

The water that is in the soil pores, or that coats the pores as the soil dries out, is always a solution of cations and anions. Since plants take up nutrients in solution, the types and proportions of these ions determine the ease with which a plant can take up nutrients. Salinity, alkalinity and acidity are all a result of imbalances in the soil solution.

Strength

For successful plant growth, the soil must not be too weak (the wind can blow plants over, lift seedlings from the ground, or cause erosion), or too strong (seedlings cannot emerge, roots will be unable to grow vertically through a hardpan, or may grow flattened in narrow cavities). The moisture content is the most important modifier of soil strength, although texture, structure, organic matter and clay type are also important.

Structure

Soil structure refers to the arrangement of the sand, silt and clay particles and organic matter to form aggregates, as well as the arrangement of pores within and between aggregates. It has an important effect on the distribution of water and air in the soil, and is therefore important for the ease with which plant roots can exploit the soil for water and nutrients. Texture, moisture content, organic matter, clay type, cultivation and even the root systems of crops can affect soil structure.

Texture

Texture is a measure of the proportion of sand, silt and clay and organic matter in a soil. Organic matter and the clay minerals provide the bonds that hold a soil together. In clay soils the clay minerals do most of the bonding. However, as the proportion of clay in a soil decreases, organic matter becomes increasingly more important.

To assess texture:

• Take a small handful of soil and remove any gravel, stones, leaves or twigs. (Gravel should not be ignored, however, as any of the above can be modified by the prefix 'gravelly', for example, gravelly clay loam.)
• Break up aggregates if possible.
• Adding a small amount of water at a time, knead the soil in your hand to make a small ball. Make sure that the soil is wet right through and there are no lumps.
• Stop adding water as soon as the ball begins to stick to your hand. Knead for another 30 seconds.

You will find it very difficult to mould a sandy soil, and quite easy to mould a clay soil. A gritty feel indicates sand. A silky feel indicates silt, and a plastic, sticky feel indicates clay. Press the ball between your thumb and forefinger to form a ribbon. The longer the ribbon, the more clay is in the soil. The table on the following page allows you to class the texture of your soil based upon 'feel'.

Water

Soil water is a very important component of the soil, because it affects aeration and soil strength. It also controls soil temperature, and affects evaporation rates. Plants need water for growth and to obtain water through their roots. The roots are in soil (except for hydroponics), so the relationship between soil and water is very important for farmers.

Unavailable water

After a soil has reached the permanent wilting point it will continue to lose water until it becomes air dry. The water between the permanent wilting point and an air-dry state is generally unavailable to plants.

Waterlogging

Waterlogging refers to a soil that is saturated with water—that is, all the pores are full of water and contain no air.