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Glossary
Glossary
Apoenzyme: the portion of an enzyme (which contains a non-protein component) that is a protein. Arbuscules: specialized fungal structures formed within the cortical cells of roots by many mycorrhizal fungi. Cation exchange capacity: the potential of a soil (or other solid material) to adsorb cations, often expressed in milligram equivalents per 100 g of soil. Chelate: a chemical complex in which two or more atoms of an organic compound (acting as a ligand) bind to a single metal ion, thus forming a ring structure. Coenzyme: a molecule that acts with an enzyme in catalysing a biochemical reaction, usually by accepting or donating a particular chemical group. Complex: a compound in which a metallic atom or ion is bound by the donation of a lone pair of electrons. Concentrate feed: a feed that is high in energy and/or protein and low in fibre. Critical concentration: the concentration of a nutrient element in a plant tissue that occurs when the supply of the nutrient is just adequate, or slightly less than adequate, for maximum growth. Dry deposition: the transfer of material from the atmosphere to land (or sea) surfaces by the processes of gravitational settling, turbulent transport, molecular diffusion and impaction. Endogenous: from within the body. Erythrocyte: red blood cell. Forage (species): plants of which the foliage is used for feeding to ruminant animals. Goitre: enlargement of the thyroid gland (in the neck), often caused by a deficiency of I and/or the presence of goitrogens in the diet. Homeostasis (in animals): (i) the state of equilibrium in the chemical composition and functions of the body and body fluids; and (ii) the processes through which such equilibria are maintained.
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Hypomagnesaemia: a disorder in animals due to a low concentration of Mg in the blood plasma; it occurs particularly in recently calved cows, and less frequently in ewes. Inositol: a cyclic sugar alcohol, C6H6(OH)6. Iodophor: a compound comprising iodine linked to a surface-active compound, which has both antimicrobial and detergent properties. Isomorphous replacement: replacement of one atom by another of similar size in the lattice of a mineral, especially a clay mineral, without disrupting the lattice structure. Kaolinite: a clay mineral with a 1 : 1 crystal lattice, i.e. single Si tetrahedral sheets alternating with single Al octahedral sheets. Leaching: the removal of soluble constituents (usually from soil), by water moving downwards. Ley: a sown grass or grass–clover sward intended to remain in situ for between 1 and 5 years, often alternating with a similar period under arable cultivation. Ligand: a molecule, atom or ion that acts as the electron donor in one or more coordination bonds. If the ligand is organic and a heterocyclic ring is formed with a metal ion, the product is a chelate. Macropore (in soil): a pore or crack of sufficiently large diameter for water to flow readily, often formed by alternate wetting and drying of the soil or through the decay of roots or the action of soil fauna. Mass flow: the movement of nutrient ions in the flow of water in soils, often caused by the plant uptake of water. Mineralization (in soils): the processes by which the organic forms of elements in plant, animal and microbial residues and in humified organic matter are converted into inorganic or ‘mineral’ forms. Molar: the concentration resulting from 1 mole (see below) dissolved in 1 l of solution. Mole: the amount of a substance that has a weight in grams numerically equal to the molecular weight of the substance. Mycorrhiza: an association between fungal hyphae and plant roots, with the hyphae extending into the root cortex and also into the soil for a distance of several centimetres or more. Occlusion: the adsorption, usually of a soluble ion, in a non-exchangeable form, e.g. in the interior of a mineral particle. Oxidation: in a chemical reaction, the loss of electrons, often by transfer to an oxidizing agent, which is thus reduced. Pasture: grassland grazed, on a long-term basis, by ruminant animals. Prosthetic group (of enzyme): a non-amino acid group (e.g. haem) linked to the protein component of the enzyme. Redox condition: the balance between chemical oxidation and reduction in a given situation. Reduction: in a chemical reaction, the addition of electrons, often by the addition of hydrogen; reduction is the opposite of oxidation.
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Residence time: the mean period for which an element or compound remains in a specified form. Rhizosphere: the zone of the soil immediately surrounding a plant root, where there is usually a large microbiological population and where soil properties are modified by the presence of the root. Ruminant (animals): herbivorous mammals, such as cattle and sheep, that have complex stomachs containing microorganisms that are able to break down the cellulose and hemicellulose in plant cell walls. Shale: a sedimentary rock formed largely from compacted clay deposits, with variable amounts of organic material. Siderophore (or phytosiderophore): a naturally occurring compound that will chelate with trivalent or divalent metal ions, especially Fe3+, thus rendering the ion(s) more readily available for uptake. Slurry: a mixture of dung and urine, with a variable amount of water, from housed livestock. Smectite: a group of clay minerals with a 2 : 1 lattice structure (i.e. two Si tetrahedral sheets on each side of a single Al octahedral sheet) and with isomorphous replacement, producing a negative charge. Montmorillonite is an example. Stocking rate (or density): the number of grazing animals per unit area of land. Stolon: a stem growing horizontally at or just below the soil surface, capable of rooting and sending up new shoots at the nodes. Symplasm (in plants): the continuum of cytoplasm that results from the cytoplasm in different cells being connected by plasmodesmata (see Fig. 3.2, p. 47). Wet deposition: the transfer to the land (or sea) surface of gaseous and particulate matter from the atmosphere by rain or snow.
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