K

K-strategy- Ecological strategy where organisms depend on physiological adaptations to environmental resources. K strategists are usually stable and permanent members of the community.

karyogamy- Fusion in a cell of haploid (N) nuclei to form a diploid (2N).

Koch's Postulates- Set of laws formulated by Robert Koch to prove that an organism is the causal agent of disease.

Krebs cycle- See tricarboxylic acid cycle.

L

lag phase- Period after inoculation of fresh growth medium during which population numbers do not increase.

lamella (plural, lamellae)- (i) A thin layer, platelike arrangement or membrane. (ii) Layers of protoplasmic membranes within the chloroplast that contain the photosynthetic pigments.

latin binomial- See binomial nomenclature.

leaching- (i) Removal of valuable metals from ores by microbial action. (ii) The removal of materials in solution from the soil.

lectins- Plant proteins with a high affinity for specific sugar residues.

leghemoglobin- Iron-containing, red pigment(s) produced in root nodules during the symbiotic association between rhizobia and leguminous plants. The pigment is similar but not identical to mammalian hemoglobin.

lichen- Fungus and an alga or a cyanobacterium living in symbiotic association.

ligand- Molecule, ion, or group bound to the central atom in a chelate or a coordination compound.

lime, agricultural -Soil amendment containing calcium carbonate, magnesium carbonate or other materials to neutralize soil acidity and furnish calcium or magnesium or both for plant growth.

lipopolysaccharide (LPS)- Complex lipid structure containing unusual sugars and fatty acids found in many Gram-negative bacteria.

lithotroph- Organism that uses an inorganic substrate such as ammonia or hydrogen as an electron donor in energy metabolism. There are two types of lithotrophs: chemolithotroph and photolithotroph.

litter- Surface layer of the forest floor consisting of freshly fallen leaves, needles, twigs, stems, bark, and fruits.

lophotrichous- Having a tuft of polar flagella.

luminescence- Production of light.

luxury uptake- The absorption by plants of nutrients in excess of their need for growth. Luxury contents accumulated during early growth may be used for later growth.

lysis- Rupture of a cell, resulting in loss of cell contents.

lysogeny- An association where a prokaryote contains a prophage and the virus genome is replicated in synchrony with the host chromosome.

lysosome- Cell organelle containing digestive enzymes.

M

macrofauna- Soil animals that are > 1000 micrometers in length (e.g., vertebrates, earthworms, and large arthropods).

macromolecule- Large molecule formed from the connection of a number of small molecules.

macronutrient- A substance required in large amounts for growth, usually attaining a concentration of > 500 mg kg-1 in mature plants. Usually refers to N, P, K, Ca, Mg, and S.

macropore- Larger soil pores, generally having a minimum diameter between 30 and 100 micrometers, from which water drains readily by gravity.

manure- Excreta of animals, with or without an admixture of bedding or litter, fresh or at various stages of decomposition or composting. In some countries the term may denote any fertilizer material.

mass flow (nutrient)- Movement of solutes associated with net movement of water.

matric potential- Portion of the total soil water potential due to the attractive forces between water and soil solids as represented through adsorption and capillarity.

medium (plural, media)- Any liquid or solid material prepared for the growth, maintenance, or storage of microorganisms.

meiosis- In eukaryotes, reduction division, the process by which the change from diploid to haploid occurs.

mesofauna- Soil animals between 200 to1000 micrometers in length, including nematodes, oligochaete worms, smaller insect larvae, and small arthropods.

mesophile- Organism whose optimum temperature for growth falls in an intermediate range of approximately 15 to 40¼C.

messenger RNA (mRNA)- RNA molecule transcribed from DNA, which contains the information to direct the synthesis of a particular protein.

metabolism- All biochemical reactions in a cell, both anabolic and catabolic.

methanogenic bacterium (methanogen)- Methane-producing prokaryote; member of the Archaea.

methanogenesis- Biological production of methane.

methanotroph- Organism capable of oxidizing methane.

microaerophile- Organism that requires a low concentration of oxygen for growth. Sometimes indicates an organism that will carry out its metabolic activities under aerobic conditions but will grow much better under anaerobic conditions.

microaggregate- Clustering of clay packets stabilized by organic matter and precipitated inorganic materials.

microbial biomass- Total mass of microorganism alive in a given volume or mass of soil.

microbial population- Total number of living microorganisms in a given volume or mass of soil.

microbiology- Study of microorganisms.

microcosm- A community or other unit that is representative of a larger unity.

microenvironment- Immediate physical and chemical surroundings of a microorganism.

microfauna- Protozoa, nematodes and arthropods generally < 200 micrometers long.

microflora- Bacteria (including actinomycetes), fungi, algae, and viruses.

microhabitat- Clusters of microaggregates with associated water within which microbes function. May be composed of several microsites (e.g., aerobic and anaerobic).

micrometer- One-millionth of a meter, or 10-6 meter, the unit usually used for measuring microorganisms.

micronutrient- Chemical element necessary for growth found in small amounts, usually < 100 mg kg-1 in a plant. These elements consist of B, Cl, Cu, Fe, Mn, Mo, and Zn.

microorganism (microbe)- Living organism too small to be seen with the naked eye (< 0.1 mm); includes bacteria, fungi, protozoans, microscopic algae, and viruses.

micropore- Relatively small soil pore, generally found within structural aggregates and having a diameter < 30 micrometers.

microsite- Small volume of soil where biological or chemical processes differ from those of the soil as a whole, such as an anaerobic microsite of a soil aggregate or the surface of decaying organic residues.

mineralization- Conversion of an element from an organic form to an inorganic state as a result of microbial decomposition.

minor elements- See micronutrients.

mitochondrion (plural, mitochondria)- Eukaryotic organelle responsible for processes of respiration and oxidative phosphorylation.

mitosis- Highly ordered process by which the nucleus divides in eukaryotes.

mixotroph- Organism able to assimilate organic compounds as carbon sources while using inorganic compounds as electron donors. Compare with autotroph and heterotroph.

moisture-characteristic curve- See water-retention curve.

moisture content- See water content.

moisture-release curve- See water-retention curve.

mold- A filamentous fungus.

molecule- Result of two or more atoms combining by chemical bonding.

monoclonal antibody- Antibody produced from a single clone of cells. This antibody has uniform structure and specificity.

monokaryon- Fungal hypha in which compartments contain one nucleus.

most probable number (MPN)- Method for estimating microbial numbers in soil based on extinction dilutions.

motility- Movement of a cell under its own power.

mucigel- Gelatinous material at the surface of roots grown in normal nonsterile soil. It includes natural and modified plant mucilages, bacterial cells, and their metabolic products (e.g., capsules and slimes), and colloidal mineral and organic matter from the soil.

mucilage- Gelatinous secretions and exudates produced by plant roots and many microorganisms.

mulch- (i) Any material such as straw, sawdust, leaves, plastic film, and loose soil, that is spread upon the surface of the soil to protect the soil and plant roots from the effects of raindrops, soil crusting, freezing, or evaporation. (ii) To apply mulch to the soil surface.

municipal solid waste- Combined consumer and commercial waste generated within a defined geographic area

murein- See peptidoglycan.

mushroom- Large, sometimes edible, fruiting body produced by some fungi.

mutagen- Substance that causes the mutation of genes.

mutant- Organism, population, gene, or chromosome that differs from the corresponding wild type by one or more base pairs.

mutation- Heritable change in the base sequence of the DNA of an organism.

mutualism- Interaction between organisms where both organisms benefit from the association.

mycelium (plural, mycelia)- Mass of hyphae that form the vegetative body of many fungal organisms.

mycophage- see mycovirus.

mycophagous- Organisms that consume fungi, such as mycophagous nematodes.

mycoplasma- Group of bacteria without a cell walls that do not revert to walled forms. Phylogenetically related to clostridia.

mycorrhiza- Literally "fungus root." The symbiotic association between specific fungi with the fine roots of higher plants.

mycorrhizosphere- Unique microbial community that forms around a mycorrhiza.

mycovirus- Virus that infects fungi.