BONDS Chemical forces holding atoms together to form molecules.

BORDER IRRIGATION Irrigation in which the water flows over narrow strips that are nearly level and are separated by parallel, low-bordering banks or ridges.

BROAD-BASE TERRACE A low embankment, with such gentle slopes that it can be farmed, constructed across sloping soils approximately on the contour. Broad-base terraces are used on pervious soils to reduce runoff and soil erosion.

BROWN FOREST SOILS An intrazonal group of soils that have dark-brown surface horizons, relatively rich in humus, grading through lighter colored soil into the parent material. They are characterized by a slightly acid or neutral reaction and a moderately high amount of exchangeable calcium. They are commonly developed under deciduous forests from parent materials relatively rich in bases, especially calcium.

BROWN PODZOLIC SOILS A zonal group of soils with thin mats of partly decayed leaves over thin, grayish-brown mixed humus and mineral soil. They lie over yellow or yellowish-brown, acid B horizons, slightly richer in clay than the surface soils. These soils develop under deciduous or mixed deciduous and coniferous forests in cool-temperate humid regions, such as parts of New England, New York, and western Washington.

BROWN SOILS A zonal group of soils having a brown surface horizon that grades below into lighter colored soil. These soils have an accumulation of calcium carbonate at 1 to 3 feet. They develop under short grasses, bunchgrasses, and shrubs in a temperate to cool semiarid climate.

BUFFER, BUFFERING Substances in the soil that act chemically to resist changes in reaction or pH. The buffering action is due mainly to clay and very fine organic matter. Highly weathered tropical clays are less active buffers than most less weathered silicate clays. Thus with the same degree of acidity, or pH, more lime is required to neutralize (1) a clayey soil than a sandy soil, (2) a soil rich in organic matter than one low in organic matter, or (3) a sandy loam in Michigan, say, than a sandy loam in central Alabama.

BUFFER STRIPS Established strips of perennial grass or other erosion-resisting vegetation, usually on the contour in cultivated fields, to reduce runoff and erosion.

BULK DENSITY The mass or weight of oven-dry soil per unit bulk volume, including air space. This mass in relation to the weight of a unit volume of water, was formerly called "apparent density" or "volume weight."

C HORIZON The unconsolidated rock material in the lower part of the soil profile like that from which the upper horizons (or at least a part of the B horizon) have developed.

CALCAREOUS SOIL A soil containing calcium carbonate, or a soil alkaline in reaction because of the presence of calcium carbonate. A soil containing enough calcium carbonate to effervesce (fizz) when treated with dilute hydrochloric acid.

CALICHE A broad term for the more or less cemented deposits of calcium carbonate in many soils of warm-temperate areas, as in the Southwestern States. When it is very near the surface or exposed by erosion, the material hardens. (Caliche is also used for deposits of sodium nitrate in Chile and Peru.)

CAPILLARY POROSITY The volume of small pores within the soil that hold water against the force of gravity.

CAPILLARY WATER The water retained in the fine pores in soil by surface tension that moves as a result of capillary forces.

CARBOHYDRATES Compounds containing carbon, hydrogen, and oxygen. Usually the hydrogen and oxygen occur in the proportion of 2 to 1, such as in glucose (C₆H_l2O₆).

CARBON One of the commonest chemical elements, occurring in lampblack, coal, and coke in varying degrees of purity. Compounds of carbon are the chief constituents of living tissue.

CARBON DIOXIDE A colorless gas (CO₂) composed of carbon and oxygen and normally found in small amounts in the air. It is one of the end products of the burning (oxidation) of organic matter, or carbon-containing compounds.

CARBON-NITROGEN RATIO The ratio of the weight of organic carbon to the weight of total nitrogen in a soil or in an organic material.

CATALASE An enzyme capable of decomposing hydrogen peroxide into water and oxygen: ₂H₂O₂--→₂H₂O₂+O₂.

CATALYST A material that increases the rate of a chemical reaction.

CATENA A group of soils, within a specific soil zone, formed from similar parent materials but with unlike soil characteristics because of differences in relief or drainage.

CATION An ion carrying a positive charge of electricity. The common soil cations are calcium, magnesium, sodium, potassium, and hydrogen.

CATION EXCHANGE The exchange of cations held by the soil-adsorbing complex with other cations. Thus if a soil-absorbing complex is rich in sodium, treatment with calcium sulfate (gypsum) causes some calcium cations to exchange with some sodium cations.

CATION-EXCHANGE CAPACITY A measure of the total amount of exchangeable cations that can be held by the soil. It is expressed in terms of milliequivalents per 100 grams of soil at neutrality (pH 7) or at some other stated pH value. (Formerly called base-exchange capacity.)

CELLULOSE The principal constituent of the cell walls of higher plants. It is made up of glucose molecules arranged in long chains and has the chemical formula (C₆H₁₀O₅)x. The long molecules give it a fibrous nature. Cotton fibers are almost pure cellulose. Paper is mainly cellulose separated by chemical processes from wood or other plant remains.

CHELATES A type of chemical compound in which a metallic atom is firmly combined with a molecule by means of multiple chemical bonds. The term refers to the claw of a crab illustrative of the way in which the atom is held.

CHERNOZEM SOILS A zonal group of soils having deep, dark to nearly black surface horizons and rich in organic matter, which grades into lighter colored soil below. At 1.5 to 4 feet, these soils have layers of accumulated calcium carbonate. They develop under tall and mixed grasses in a temperate to cool subhumid climate.

CHERT A structureless form of silica, closely related to flint, which breaks into angular fragments. Soils developed from impure limestones containing fragments of chert and having abundant quantities of these fragments in the soil mass are called cherty soils.

CHESTNUT SOILS A zonal group of soils with dark-brown surface horizons, which grade into lighter colored horizons beneath. They have layers of accumulated calcium carbon. ate at 1 to 4 feet. They are developed under mixed tall and short grasses in a temperate to cool and subhumid to semiarid climate. Chestnut soils occur in regions a little more moist than those having Brown soils and a little drier than those having Chernozem soils.

CHISEL A tillage machine with one or more soil-penetrating points that can be drawn through the soil to loosen the subsoil, usually to a depth of 12 to 18 inches.

CHITIN A nitrogen containing polysaccharide found in the outer part of insects.

CHLOROPHYLL The constituent responsible for the green color of plants. Chlorophyll is important in photosynthesis in plants, the process by which sugar is manufactured.

CHLOROPLASTS Small bodies in cells of plants in which the green pigment chlorophyll is concentrated.

CHLOROSIS A condition in plants resulting from the failure of chlorophyll (the green coloring matter) to develop, usually because of deficiency of an essential nutrient. Leaves of chlorotic plants range from light green through yellow to almost white.

CLAY As a soil separate, the mineral soil particles less than 0.002 mm. in diameter. As a soil textural class, soil material that contains 40 percent or more of clay, less than 45 percent of sand, and less than 40 percent of silt.

CLAY LOAM Soil material that contains 27 to 40 percent of clay and 20 to 45 percent of sand.

CLAY MINERAL Naturally occurring inorganic crystalline material in soils or other earthy deposits of clay size particles less than 0.002 mm. in diameter.

CLAYPAN A compact, slowly permeable soil horizon rich in clay and separated more or less abruptly from the overlying soil. Clay-pans are commonly hard when dry and plastic or stiff when wet.

CLOD A mass of soil produced by plowing or digging, which usually slakes easily with repeated wetting and drying, in contrast to a ped, which is a natural soil aggregate.

COLLOID, SOIL Colloid refers to organic or inorganic matter having very small particle size and a correspondingly large surface area per unit of mass. Most colloidal particles are too small to be seen with the ordinary compound microscope. Soil colloids do not go into true solution as sugar or salt do, but they may be dispersed into a relatively stable suspension and thus be carried in moving water. By treatment with salts and other chemicals, colloids may be flocculated, or aggregated, into small crumbs or granules that settle out of water. (Such small crumbs of aggregated colloids can be moved by rapidly moving water or air just as other particles can be.) Many mineral soil colloids are really tiny crystals and the minerals can be identified with X-rays and in other ways.

COLLUVIUM Mixed deposits of soil material and rock fragments near the base of rather steep slopes. The deposits have accumulated through soil creep, slides, and local wash.

COMPANION CROP A crop grown with an- other crop, usually a small grain with which alfalfa, clover, or other forage crops are sown. (Formerly such small grain crops were known as nurse crops, but because the small grain does not "nurse" the other crop this older term is being abandoned.)

COMPLEX, SOIL An intimate mixture of tiny areas of different kinds of soil that are too small to be shown separately on a publishable soil map. The whole group of soils must be shown together as a mapping unit and described as a pattern of soils.

COMPOST A mass of rotted organic matter made from waste plant residues. Inorganic fertilizers, especially nitrogen, and a little soil usually are added to it. The organic residues usually are piled in layers, to which the fertilizers are added. The layers are separated by thin layers of soil. The whole pile is kept moist and allowed to decompose. The pile is usually turned once or twice. The principal purpose in making compost is to permit the organic materials to become crumbly and to reduce the carbon-nitrogen ratio of the material. Compost is sometimes called artificial or synthetic manure.