Soils Glossary M - Z

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-L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

Macronutrients: These are nutrients which are needed in large quantities by plants - such as calcium, magnesium and potassium.
Manure: Manure is the well-rotted remains of dung, say from horses, which can be applied as a rich organic fertiliser to garden soil. Note, never add fresh dung directly to garden soils but only well-rotted remains!
Marl: Marl is a calcareous clay, comprising about 30 to 65 % calcium carbonate (CaCO₃). Marl is most often found in old fresh water basins, swamps, or as the sediment of shallow lakes.
Microfauna: Microfauna are the smallest of the soil 'fauna' or animals, and are less than 0.1 mm in size, so you need a microscope to see them. The two most important ones are the nematodes and the protozoa.
Microflora: Microflora are tiny microscopic 'flora' in soils, such as bacteria, fungi and viruses. Microflora are extremely numerous in soils with billions in just one gram of soil and many thousands of species also within a single gram.
Micronutrients: These are nutrients which plants need only in small (or 'trace') amounts - such as iron, zinc, copper and boron.
Microorganism: Microorganisms are tiny organisms visible only with a microscope and include bacteria, protozoa, algae, and fungi. Note that viruses are not truly living organisms, but are sometimes classed as microorganisms. The soil is literally teeming with microorganisms!
Mineralisation: Mineralisation is the conversion of 'bound' organic nitrogen into the mineral (ionic or inorganic) form required for plant uptake. The inorganic nitrogen is then converted in a process known as nitrification from ammonium salts to nitrites, and from nitrites to nitrates by nitrifying bacteria which obtain their energy from this oxidation process. Once free nitrate is formed, the rapid recycling process offers many options, the nitrate may be assimilated by plants, denitrified or leached.
Mineral: An inorganic component of soil deriving from rock material, including substances such as calcium, iron, zinc, iodine and fluorine. There are also 'trace elements' such as selenium, manganese, molybdenum, chromium, cobalt, silicon, vanadium and nickel.
Moraine: A moraine is a form of glacial till, a soil sediment deposited at the terminus of a melting glacier. The moraine is a single, large mound deposited, typically at the edge of a glacier, being a jumble of rocks and finely ground material carried by the glacier and deposited when the ice melted. The deposit is left by the melting of a glacier where soil had been 'ploughed' along by the preceding advance of the ice sheet. Moraines comprise unstratified soil of varying composition. See also 'glacial till'.
Mud: Mud!! It may be mud to you - but it's soil to us!! OK, soil + water = mud
Mycorrhizae: What a strange word! This is a special fungus that grows in root tips in little strands passing from just outside the root to just inside it. They are too small for us to see. Mycorrhizae can help plants take in nutrients.
Nitrification: Nitrification is the process in which the nitrogen cycle works. Ammonia is created by urea and decomposition. Ammonia is turned into nitrites by nitrosomonas bacteria. Nitrites are then converted to nitrates by nitrobacter. Nitrates are used as an agricultural fertiliser. Applied incorrectly, nitrates can pose a pollution risk.
Nutrients: Nutrients are 'minerals' needed by plants and animals function and to remain healthy. Iron, calcium, potassium, and sodium are minerals.
Organic farming: Organic farming is a means of producing food and fibre products with an absolute minimal input of agrochemicals such as pesticides, which may remain as residues in the food we then eat. Organic farming is a balance to the industrial food production techniques which have come to dominate western agicultural production.
Organic material: Organic matter, also called 'humus', forms from the decay of leaves, plants and other life.
Overland flow: When rain falls to the ground, some evaporates away again, or is taken in by plants, and some water flows down into the soil. Some, however, may flow off across the soil surface towards the river by overland flow.
Oxidation: Oxidation is the addition of oxygen, removal of hydrogen, or the removal of electrons from an element or compound. In the environment, organic matter is oxidized to more stable substances. Oxidation is the opposite of 'reduction'. Oxidation of organic matter is termed 'burning', and that of iron 'rusting'.
Pan: A pan is a well-defined layer forming in the soil. There are two common types; a 'plough pan' which builds up in-field just below plough depth, and an 'iron pan' which forms naturally by iron oxide accumulations deposited in acid gley soils. Pans can impede the passage of water through the soil which in an agricultural context can lead to problems if uncorrected.
Parent material: Soil parent material refers to the rocks which were weathered to form the soil in the first place. Usually the parent material is below the soil, but it can be distant if glaciers translocated the soils during the ice ages. Parent material is the focus of the study of geology.
Peat: Peat is a type of soil formed in water-logged conditions from incompletely decomposed plant material. Peat forms in wetlands or peatlands, also commonly called bogs, moors, mires, swamps and fens.
Pedology: The science of studying soils and their interaction with the wider environment.
Pedon: A three-dimensional body of soil with lateral dimensions large enough to include representative variations in the shape and relation of horizons and in soil composition. Usually 1 to 10m² in area depending on variability.
Perennial plants: Perennial plants like trees are 'hardy' and can continue their growth from year to year. The opposite is an annual plant that grows, flowers and dies in just one year, an 'annual'.
Permanent Wilting Point (PWP): The water content a soil reaches at the time when water extraction by plants has ceased as they wilt beyond recovery. PWP depends on the soil, the plant and the climate.
Pesticide: A pesticide is a chemical applied to plants (such as farm crops, or garden vegetables) to eradicate pests which would otherwise spoil the crop. Pesticides have permitted an increase in agricultural production and consistency of produce. However, applied incorrectly pesticides can pollute waterbodies and aquifers and even foodstuffs can retain a minute residue of pesticides.
pH: pH stands for Potenz Hydrogen ..... well now you know! It is measured from 1 (acid) through 7 (neutral) to 14 (alkaline), expressed on a logarithmic scale. Most soil is about pH 3 to 8. Note the case of pH, a lower case 'p' and an upper case 'H'.
Photosynthesis: Photosynthesis is the important process in plant cells in which the sun's energy is used to join carbon dioxide and water to make glucose (a type of sugar), the food of green plants.
Podsol: A Podsol, or Podzol, is a soil with a profile having distinctive horizons of a bleached lower topsoil, often having a cemented iron oxide 'pan' in the subsoil. Podsols are common in heathland and coniferous, or boreal, forests where there are temperate to cold moist climates.
Pores: A soil pore is the hole in-between particles of soil that becomes filled with air or water.
Precipitation: Precipitation means water reaching the ground from both rainfall, snow and hail.
Preferential flow: If the soil is cracked and fissured then the water can pass quicky down these 'preferential' pathways. Cracks can form in drying clay soils, or beside plant roots. When rain falls to the ground, some water evaporates away again, or is taken up by plants, and some water flows off the surface towards the river. Some, however, seeps down through the soil towards the water table. Normally this water soaks through the soil and this can be quite slow depending on the soil type.
Profile: The soil profile is a vertical column of soil, essentially three dimensional and being large enough in dimension to be used to characterise the soil condition at a particular place.
PSC: PSC or Particle Size Class. Soil particles are divided into classes according to their size. Soils differ by having different proportions of particles in each of these classes. In the UK, these classes are expressed as clay having particle sizes of less than 0.002 mm, silt being 0.002 ? 0.06 mm, fine sand being 0.06 ? 0.2 mm, medium sand being 0.2 ? 0.6 mm and course sand being 0.6 ? 2.0 mm. Other differing classification systems are also used, such as in the USA.
Reduction: Reduction is the addition of hydrogen, removal of oxygen, or the addition of electrons to an element or compound. Under anaerobic conditions (where there is no dissolved oxygen present) such as in 'gley' soils, sulphur compounds are reduced to odour-producing hydrogen sulphide (H₂S and other compounds. Reduction is the opposite of oxidation.
Respiration: Respiration involves the exchange of oxygen and carbon dioxide between an organism and its external environment.
Runoff: Runoff occurs as water falling as precipitation does not soak deep into the soil, but passes across the surface and near-surface towards the rivers.
Saline soil: Soil subject to salinisation, where the soil contains sufficient soluble salts to interfere with plant growth.
Salinisation: Sometimes underground water, pumped up for irrigation, can contain mineral salts. When the water is sprayed onto the land the water is used or evaporates, but the salt can remain. Over time soil salt levels can increase - and this can be toxic to the crop plants!
Sand: That mineral fraction of the soil with particles from 0.063-2.0 mm diameter
fine sand: 0.063-0.212 mm diameter
medium sand: 0.212-0.6 mm diameter
coarse sand: 0.6-2.0 mm diameter
Sediment: Sediment is a deposit of 'alluvium' laid down in water (such as lakes or the sea). Sediments can, over time, form rocks such as chalk and limestone.
Shrub: A 'shrub' is a bush which has grown up to about 1.5 metres high. How tall are you?
Silt: That mineral fraction of the soil with particles from 0.002-0.063 mm diameter
fine silt: 0.002-0.006 mm diameter
medium silt: 0.006-0.02 mm diameter
coarse silt: 0.02-0.063 mm diameter
Smectites: Smectites are mineral particles in clay soils. Smectites are disc-like in shape and can slide across one-another. 'Smectitic' clays are associated with shrinking and swelling that can cause foundation subsidence to buildings.
Sodic soil: A soil with sufficient exchangeable sodium (alkali) to interfere with plant growth and cause dispersion and swelling of clay minerals.
Soil: Soil is a combination of four constituents: mineral material (sand, silt, clay and rock particles), organic material, air and water. Soil is made from the breaking down of rocks and organic matter by physical, chemical and biological weathering processes.
Soil acidity: Most soils are of a pH from about 5.5 to 8, still a pretty big range - but pH 3 (see pH below) is really acid! Remember the smaller the pH number the more acid it is
Soil mechanics: The study of how soil behaves when it is built on and great pressures build up on it. Does the soil compress, slip away or is it stable? How does the weather affect the soil?
Soil minerals: These refer to the trace elements found in soil - also called nutrients.
Soil sealing: Soil sealing is the covering of the soil by a built surface, such as a road, pavement or building. Every year Britain loses the equivalent area of a medium-sized city to soil sealing under tarmac and concrete.
Specific Heat Capacity: A substance's Specific Heat Capacity (or SHC) refers to the amount of heat which is required in order to increase its temperature. Measured in 'joules per kilogram kelvin', or J/(kg·K), it is specifically the amount of heat energy in joules needed to increase the temperature of one kilogram of the substance by one kelvin.
Stomata: A tiny pore opening in the epidermis of a plant. Stomata are present in large numbers in leaves where water vapour exchange takes place (transpiration). Stomata can be opened and closed by the plant. See 'transpiration'.
Structure: Soil structure is the 'architecture' of soil - how it is constructed and made up. The structure is the aggregation of soil particles into units separated from each other by surfaces of weakness. An individual natural soil aggregate is called a ped, in contrast to a clod caused by disturbance, or a concretion caused by cementation.
Substrate: An inclusive term for the soil used when describing, for instance, the portion in which plant roots exist.
Symbols: Scientists use symbols to refer to these. sand is written as 'S', clay as 'C' and silt as 'Z'. Organic material is written as 'O'.
Taiga: 'Taiga' is a Russian word for the boreal conifer forest areas stretching from Eurasia to Northern America in areas of previous glaciation and commonly permafrost - or frozen soil.
Temperate climates: Temperate regions are those not experiencing very hot or very cold conditions for long periods of the year. Britain has a 'temperate' climate.
Texture: The description of the balance in the soil between the constituents including sand, silt and clay as well as organic matter. With experience, texture can be established accurately by touch.
Topography: The physical shape of the landscape - hills, valleys, ridges etc... just as you learned in geography lessons!
Topsoil: Topsoil is the surface layer of soil containing partly decomposed organic debris, and which is usually high in nutrients, containing many seeds, and is rich in fungal mycorrhizae. Topsoil is usually a dark colour due to the 'organic matter' present. In arable land, 'topsoil' refers to the soil down to plough depth.
Transpiration: The process whereby plants lose water by evaporation of liquid water at the surface of the stomatal cells, the water vapour diffusing out through the leaf via the stomatal openings. See 'Stomata', See 'Transpiration'.
Water holding capacity: Can the soil hold lots of water like a sponge? If so it has a large 'water holding capacity'. Soil organic matter increases the water holding capacity. Pure running sand has a low water holding capacity.
Waterlogged soil: A waterlogged soil is wet, with lots of water in the pores of the soil structure. The opposite is a dry aerated soil.
Weathering: The process by which materials are broken down into smaller parts and ultimately their constituents. An example is 'freeze thaw' expansion and cracking. There are both physical and chemical weathering processes.
Weedkiller: A weedkiller is a type of 'herbicide' used to eradicate certain types of unwanted plants or vegetation. Weedkillers can be specific to certain types of plants (a 'selective weedkiller').
Windblow: 'Windblow' is the proper term used to describe one or more trees blown over in the wind.