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Soils Glossary A - L

Some of the words and terms used in this website may be new to you. Here is a glossary and definition of some common soil-related words and terms and their meaning:

A | B | C | D | E | F | G | H | I | J | K | L | M-Z

Acid rain
Fossil fuels can release chemicals such as sulphur when they are burnt (as petrol is in a car, or coal is in a power station). These chemicals can dissolve in atmospheric water and make rainfall unnaturally acidic. This can cause many environmental problems.

Aerated soil
An aerated soil is a soil with a good movement of air through the soil structure. The opposite is a wet waterlogged soil, where the soil pores are filled with water.

Aggregates' in the soil are just soil lumps of a range of differing sizes. The word is also often used to describe gravel used to build roads.

Alluvium is a deposit made by a river or running water. This leads to an 'alluvial deposit' forming in the water. Alluvial soils are rich agricultural lands. Glaciers may also deposit sedimentary material, see 'glacial till'.

Anaerobic soils
Anaerobic soils have very little oxygen present - for instance the wet, waterlogged, marshy soils in a bog. While anaerobic conditions are important for some processes, such as bacterial reduction of nitrate to nitrogen, these conditions can also produce hydrogen sulphide, methane and other undesirable substances.

A negatively charged ion. The most commonly found anions in soil waters include bicarbonates, sulphates, carbonates, chlorides and nitrates.

Arid conditions
Arid conditions are dry! Where rainfall is less than about 200 mm/year an area is 'semi-arid', rainfall below below this leads to arid conditions such as deserts.

Autotrophic (bacteria)
Autotrophic bacteria can take carbon in from the atmosphere through the fixation, reduction and incorporation of carbon dioxide.

Available soil water
This is the amount of water held in the soil root zone, within which the 'easily available soil water' is the amount of soil water easily available for plants for evapotranspiration; which can be extracted by the plants. Soil also contains a smaller fraction of water which is 'held' too tightly by the soil particles for plants to use - this makes up the 'total available soil water'. Expressed in mm.


A bacteria is a simple microscopic single-celled organism. There are many forms of bacteria which thive in soil. Bacteria play a crucial role in the nutrient cycles in soil, helping to break down organic matter.

Bearing capacity
This is effectively the weight a soil can withstand before severe damage occurs to the structure of the soil. Bearing capacity varies throughout the year, for instance a very heavy tractor that causes no damage on dry soils may cause a lot of damage to wetted soils.

Biodiversity describes the diverse number and type of organisms such as animals, plants and microorganisms living in habitats. Healthy soil contains a staggering biodiversity of organisms and is a powerhouse of the wider ecosystem.

A Biotope is a distinct geographical area having similar environmental conditions, such as soil, climate and relief.

Buffering capacity
Buffering capacity is the ability of the soil to reduce high alkalinity or acidity levels coming perhaps from pollution (e.g. acid rain). Chalky or limestone soils for instance are very alkaline and can neutralise acids more effectively than acid, peat soils.

Bulk density
The dry mass of soil per unit bulk volume of soil. Expressed as kg/m-3 or g/cm3.


Calcicoles are plants able to grow in very alkaline conditions (high pH).

The overall term used to describe the process of producing maps characterising geographical variation in some property of soil. Traditional maps are made on paper. However, increasingly maps are held and manipulated in computer form.

A positively charged ion. The most commonly found cations in soil waters include sodium, potassium, calcium and magnesium.

That mineral fraction of the soil with particles less than 0.002 mm diameter. Yes this is what you make pots from - but clay soil is rarely that pure - it is often mixed with particles of different textures.

Climate change
Most scientists and politicians now accept the climate of planet Earth is changing due to human activities such as the burning of fossil fuels. Changing climates are not new on Earth - but this time the changes are strongly influenced by our actions.

Compost is a mixture of decomposing vegetable matter and animal remains, as well as manure, etc., which breaks down into the 'organic layer'. Gardeners create compost heaps to hold grass clippings and vegetable peelings etc. which rot down to a rich organic material used for fertilising and conditioning the soil.

Coniferous (or evergreen) trees, such as Larch, are ones who retain their leaves all year long. Their leaves are designed to withstand adverse weather conditions such as cold or drought. The opposite is a 'deciduous' tree.


Deciduous (or broad-leaved) trees are ones whose leaves fall off, or are shed seasonally to avoid adverse weather conditions such as cold or drought. The opposite is an 'evergreen' tree.

Denitrification is the mechanism that allows the return of nitrogen to the atmosphere thus completing the cycle: atmosphere-fixation in the soil-mineralisation and nitrification for use by plants-denitrification and return to the atmosphere. Denitrification is the reduction of nitrate to gaseous nitrogen, enabling it to pass from the soil to the atmosphere again.

Deposition is the natural process of deposits being laid down which ultimately become soils. An example of a deposited soil is 'alluvium' which is sediment laid down in water. The opposite is 'erosion'.

Desertification is the change in arid and semi-arid regions from formerly productive land to desert. This is often the cause of erosion, particularly by wind, and climate changes exacerbated by poor land management (such as deforestation).


Effective rainfall (or efficient rainfall)
The rainfall useful for meeting plant water requirements. This does not include water percolating down to aquifers, or surface runoff of water.

Effective rooting depth
The soil depth from which a fully grown plant extracts most of the water needed for transpiration.

Erosion is the wearing away of land or soil through one or more processes. The main causes of erosion include the actions of water (rills, inter-rill, gully, snowmelt and river and lake bank erosion), wind (dessication and wind-blow), translocation (tillage, land levelling, harvesting of root crops, trampling and burrowing animals) and geological (internal subterranean erosion by groundwater, coastal erosion and landslides). Erosion can also be increased by poor land management such as overgrazing, deforestation or inappropriate use of mechanisation (e.g. ploughing down a hill slope).

Eutrophication describes the process where a waterbody, such as a lake or a soil solution, becomes loaded with dissolved nutrients. This can be natural, but is often due to pollution. Algal blooms can remove oxygen in the water, harming fishlife.

This is the rate of water loss from liquid to vapour (gaseous) state from an open water, wet soil or plant surface, usually expressed in mm day-1.

The process whereby water is passed from liquid to vapour (gaseous) state through transpiration from vegetation, and evaporation from soil and plant surfaces. See transpiration.

Evergreen (or coniferous) trees are ones who retain their leaves all year long. Their leaves are designed to withstand adverse weather conditions such as cold or drought. The opposite is a 'deciduous' tree.


Fauna and Flora
Cousins to the Adams family ... Oops wrong definition! Fauna are the organisms that live in and near the soil, from ants, snails and slugs to fungal mycelia, mycorrhizae and algae. Flora are the plants living in soil.

Substances added to the soil (in granules or liquid form) containing one or more recognized plant nutrients, such as nitrogen, potassium and phosphorus (called an 'NPK' mixture). Fertilisers, which can be either organic or inorganic in nature are really plant foods, and are added to the soil to improve the quality or quantity of plant growth.

Field capacity
Field capacity describes soil when it is completely wetted (excepting free drainage), and where there is plenty of water for plant roots. This occurs after ample irrigation or rainfall, when the rate of downward movement of water has substantially decreased, usually 1-3 days after rain or irrigation. It is expressed as a mass or volume fraction of soil water or a depth of water per metre of soil or mm m-1.

Nitrogen fixation is the conversion of atmospheric nitrogen into organic nitrogen compounds which are then available to green plants; a process that can be carried out only by certain strains of soil bacteria (e.g. Rhizobium, Clostridium and Azotobacter).

Fossil fuels
Fossil fuels are the petrified (that means old not scared!!) remains of ancient plant and animal matter. Organic matter was compressed and heated in the eath's crust over geological time to form oil and coal and natural gas.

A fungicide is a chemical 'pesticide' designed to eradicate fungi that can grow on and spol plants such as crops and fruit.


Solid material from which most soil derives, characterised by the horizon symbol 'R' for rock. Soil may lie on top of the geology it came from - or maybe not if ancient glaciers pushed the soil around like a primaeval bulldozer! The lower horizon may also be denoted 'C' where there are soft deposits such as alluvium or loess. Geology is also a scientific field concerning the study of rock.

Germination is the first stage in the development of a plant from its seed. Seeds lie 'dormant' in the soil waiting for the right conditions (related to sustained favourable temperature and moisture levels).

Geographical Information System. A term for a computer-based system for storing, accessing and representing maps and other data in electronic form. A GIS is a powerful tool for the contemporary environmental scientist.

Glacial till
Glacial till is sediment ground up beneath a glacier and deposited as 'sub-glacial till', or glacial till deposited at the 'terminus' as a 'terminal moraine'. Till is a jumble of rocks and finely ground material ground up by the movement of the glacier over the rock. Till deposited in this way leaves an unstratified soil of varying composition. See also 'moraine'

Gley soils
Gley (or gleyed) soils are soils developed under conditions of poor drainage, resulting in reduction of iron and other elements and also in a typical grey/blue soil colouring. There are two main types of gley soil:
- surface water gleys where water saturating the soil comes from surface drainage; and
- ground water gleys where saturation is due to fluctuating groundwater levels. A third type, unripened gley soil forms in brackish flooding conditions (tidal creeks).

Global warming
Global warming is the slow warming up of the Earth's atmosphere, caused by the 'Greenhouse Effect'. Rising temperatures lead to melting polar ice in turn leading to rising sea-levels. Global warming will have a big impact on the soil resource world-wide.

Greenhouse effect
The greenhouse effect describes the effect greenhouse gases have in the atmosphere. These gases allow short wavelength solar radiation to reach the Earth but keep in the radiation of longer wavelength from the earth. This causes warming of the atmosphere. Water vapour, carbon dioxide, methane and nitric oxide are the main greenhouse gases. Soil can be a major source of carbon.


A herbicide is a type of chemical applied to eradicate certain types of unwanted vegetation, such as weeds (see also pesticide).

One of the layers that form in the soil profile as a result of soil-forming processes such as weathering. A horizon is often highly visible and distinct. In garden soils and where there has been much soil movement these deeper layers may be lost or harder to observe - equally, where lots of organic matter and compost has been added, garden soils will have a strong organic rich horizon.

Organic matter, also called 'humus', forms from the decay of leaves, plants and other life.


Immobilisation is the process by which available inorganic forms of nitrogen are returned to the organic form and in so doing become unavailable again. Immobilised nitrogen is reasonably secure in the soil in this form and can continue to be recycled within the soil from organic-mineral-organic states again via the mineralisation, nitrification immobilisation stages.

A chemical 'pesticide' designed to eradicate insect pests on plants such as crops and fruit.

Infiltration is the movement of water from the surface down into the soil before moving down to the aquifers, or out to rivers. A portion of soil water may also be lost via the process of evapotranspiration.

Infiltration rate
The infiltration rate is the speed at which water can pass into the soil. You can imagine that with wet clay, this is slower than with dry sand.

Infrastructure means the 'built' world all around us - the roads, railways, bridges, houses, office blocks etc., even your school! That's 'infrastructure' too!

Crops need lots of water to grow. Where the soil is too dry, farmers can water their fields by irrigation - like a sprinkler in the garden but on a massive scale.


Leaching is the process where soluble materials (including nutrients and salts) in the soil are washed down the soil profile by water.

Lime and liming
Farmers add lime to improve their soils and to 'buffer' acidity. The production of lime was a major industry in the UK at the turn of the last century where limestone blocks were roasted in lime kilns to produce the powdery white lime which was then spread on fields. Modern 'fertilisers' also contain lime.

What a great word! But it's really dangerous in fact! This means that the soil becomes like a free-flowing mud - very bad news if you live at the bottom of the hill!

The Lithosphere is the outer rock shell of the planet, extending down several kilometres into the earth from the surface - the rocks and sediments well below the surface of the land and oceans.

Loam is a soil which contains clay, silt and sand as well as a mixture of organic material. Keen gardeners love loam - it's the best soil for potting plants and growing seeds in the greenhouse. Farmers like loam too as they are ideal for agricultural crops!