S

saline soil- Soil containing sufficient soluble salt to adversely affect the growth of most crop plants.

sand- Soil particle between 0.05 and 2.0 mm in diameter.

sanitization- Elimination of pathogenic or deleterious organisms, insect larvae, intestinal parasites, and weed seeds.

saprophyte- Nonparasitic nutritional mechanism by which an organism obtains its food exclusively from the degradation of nonliving organic material.

sclerotium- Modified fungal hyphae that form a compact, hard vegetative resting structure with a thick pigmented outer rind.

secondary metabolite- Product of intermediary metabolism released from a cell, such as an antibiotic.

selective medium- Medium that allows the growth of certain types of microorganisms in preference to others. For example, an antibiotic-containing medium allows the growth of only those microorganisms resistant to the antibiotic.

septum (plural, septa)- Crosswall (partition) dividing a parent cell into two daughter cells during binary fission or occurring between adjacent cells in hyphae.

serial dilution- Series of stepwise dilutions (usually in sterile water) performed to reduce the populations of microorganisms in a sample to manageable numbers.

serology- Study of antigen-antibody reactions in vitro.

sheath- Tubular structure formed around a chain of cells or around a bundle of filaments.

siderophore-Nonporphyrin metabolite secreted by certain microorganisms that forms a highly stable coordination compound (chelate) with iron; a high-affinity iron-binding compound. There are two major types: catecholates and hydroxamates.

silt- Soil particle with a diameter between 0.002 and0.05 mm.

site- (i) In ecology, area described or defined by its biotic, climatic, and soil conditions as related to its capacity to produce vegetation. (ii) Area sufficiently uniform in biotic, climatic, and soil conditions to produce a particular climax vegetation.

site-directed mutagenesis- Insertion of a different nucleotide at a specific site in a molecule using recombinant DNA methodology.

16S rRNA-Large polynucleotide (about 1,500 bases) that functions as a part of the small subunit of the ribosome of prokaryotes and from whose sequence evolutionary information can be obtained; the eukaryotic counterpart is 18s rRNA.

slime layer- Diffuse layer of polysaccharide exterior to the cell wall in some bacteria.

slime mold- Nonphototrophic eukaryotic microorganism lacking cell walls, which aggregate to form fruiting structures (cellular slime molds) or simply masses of protoplasm (acellular slime molds).

soil-(i) Unconsolidated mineral or material on the immediate surface of the earth that serves as a natural medium for the growth of land plants. (ii) Unconsolidated mineral or organic matter on the surface of the earth that has been subjected to and influenced by genetic and environmental factors of: parent material, climate (including water and temperature effects), macroorganisms and microorganisms, and topography, all acting over a period of time and producing a product--soil--that differs from the material from which it is derived in many physical, chemical, biological, and morphological properties, and characteristics.

soil aggregate- Unit of soil structure generally < 10 mm in diameter and formed by natural forces and substances derived from root exudates and microbial products which cement smaller particles into larger units.

soil atmosphere- Gases occupying the pore space in soil. Generally characterized as having a greater percentage of carbon dioxide and a lesser percentage of oxygen than the overlying air.

soil biochemistry-Branch of soil science concerned with enzymes and the reactions, activities, and products of soil microorganisms.

soil classification-See classification.

soil extract- Solution separated from a soil suspension or from a soil by filtration, centrifugation, suction or pressure.

soil health- See soil quality.

soil horizon- Layer of soil or soil material approximately parallel to the land surface and differing from adjacent genetically related layers in physical, chemical, and biological properties or characteristics such as color, structure, texture, consistency, kinds and number of organisms present, and degree of acidity or alkalinity.

soil microbiology- Branch of soil science concerned with soil-inhabiting microorganisms and their functions and activities.

soil organic matter (SOM)- Organic fraction of the soil exclusive of undecayed plant and animal residues. Often synonymous with humus.

soil population- (i) All the organisms living in the soil, including plants and animals. (ii) Members of the same taxa.

soil pore- That part of the bulk volume of soil not occupied by soil particles. Soil pores have also been referred to as interstices or voids.

soil quality- Continued capacity of soil to function as a vital living system to sustain biological productivity, maintain the quality of the environment, and promote plant, animal, and human health.

soil salinity- Amount of soluble salts in a soil. The conventional measure of soil salinity is the electrical conductivity of a saturation extract.

soil science- Science dealing with soils as a natural resource on the surface of the earth including soil formation, classification and mapping, and physical, chemical, biological, and fertility properties of soils per se; and these properties in relation to their use and management.

soil series- Lowest category of U.S. system of soil taxonomy; a conceptualized class of soil bodies (polypedons) that have limits and ranges more restrictive than all higher taxa. The soil series serve as a major vehicle to transfer soil information and research knowledge from one soil area to another.

soil solution- Aqueous liquid phase of the soil and its solutes.

soil structure- Combination or arrangement of primary soil particles into secondary particles, units, or peds. The secondary units are characterized and classified on the basis of size, shape, and degree of distinctness into classes, types, and grades, respectively.

soil texture- Relative proportions of the various soil separates in a soil. The major textural classes are sand, silt, and clay.

soil water potential-(total)- Amount of work that must be done per unit quantity of pure water in order to transport reversibly and isothermically an infinitesimal quantity of water from a pool of pure water, at a specified elevation and at atmospheric pressure, to the soil water (at the point under consideration). Informally, the amount of energy that must be expended to extract water from soil. The total potential (of soil water) consists of the following: gravitational potential, matric potential, and osmotic potential.

solarization- Method to control pathogens and weeds where moistened soil in hot climates is covered with transparent polyethylene plastic sheets, thereby trapping incoming radiation.

Southern blot- Hybridization of single-stranded nucleic acid (DNA or RNA ) to DNA fragments immobilized on a filter.

spatial variability- Variation in soil properties (i) laterally across the landscape, at a given depth, or with a given horizon, or (ii) vertically downward through the soil.

specific activity- Amount of enzyme activity units per mass of protein. Often expressed as micromoles of product formed per unit time per milligram of protein. Also used in radiochemistry to express the radioactivity per mass of material (radioactive and nonradioactive).

species- In microbiology, a collection of closely related strains sufficiently different from all other strains to be recognized as a distinct unit.

specific epithet- Designation of a particular organism in the binomial nomenclature system. For example, coli is the specific epithet of Escherichia coli.

spermosphere- Area of increased microbial activity around a germinating seed.

spirillum (plural, spirilli)- (i) Bacterium with a spiral shape which is relatively rigid. (ii) Bacterium in the genus Spirillum.

sporangiospore- Spore formed within a sporangium by fungi in the phylum Zygomycota.

sporangium- Fungal structure which converts its cytoplasm into a variable number of sporangiospores; formed by fungi in the phylum Zygomycota.

spores- Specialized reproductive cell. Asexual spores germinate without uniting with other cells, whereas sexual spores of opposite mating types unite to form a zygote before germination occurs.

spread plate- Method for performing a plate count of microorganisms. A known amount of a serial dilution is spread over the surface of an agar plate. After growth the number of colony-forming units is counted.

stationary phase- Period during the growth cycle of a population in which growth rate equals the death rate.

sterilization- Rendering an object or substance free of viable microbes.

storage polysaccharide- Energy reserve deposited in the cell when there is an excess of carbon available. These are usually deposited as large granules in the cell. The most common example of a storage polysaccharide in plants is starch. Its counterpart in animal cells is glycogen.

strain- Population of cells all descended from a single pure isolate.

structure- See soil structure

structural polysaccharide- Polysaccharide that serves primarily as a structural element in cell walls and coats and intercellular spaces, and connective tissue where they give shape, elasticity, or rigidity to plant or animal tissues and protection and support to unicellular organisms. Cellulose is the predominant structural polysaccharide in plant cell walls and chitin is abundant in fungal cell walls and insect exoskeletons.

substrate- (i) Substance, base, or nutrient on which an organism grows. (ii) Compounds or substances that are acted upon by enzymes or catalysts and changed to other compounds in the chemical reaction.

substrate- level phosphorylation- Synthesis of high-energy phosphate bonds through reaction of inorganic phosphate with an activated (usually) organic substrate.

succession- Gradual process brought about by the change in the number of individuals of each species of a community and by the establishment of new species that gradually replace the original inhabitants.

sulfur cycle- Sequence of transformations undergone by sulfur where it is taken up by living organisms, transformed upon death and decomposition of the organism, and converted ultimately to its original state of oxidation.

surface area- Area of the solid particles in a given quantity of soil or porous medium.

surface soil- Uppermost part of the soil, ordinarily moved in tillage, or its equivalent in uncultivated soils ranging in depth from 7 to 20 cm. Frequently designated as the surface layer, the Ap layer, or the Ap horizon.

symbiosis- Living together in intimate association of two dissimilar organisms. The interactions between the organisms can be commensal or mutualistic.

synergism- Association between organisms that is mutually beneficial. Both populations are capable of surviving in their natural environment on their own although, when formed, the association offers mutual advantages.

syntrophy- Interaction of two or more populations that supply each other's nutritional needs.

systemic-Not localized in a particular place of the body; an infection disseminated widely through the body is said to be systemic.