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Soil and Water Quality: An Agenda for Agriculture GLOSSARY acidification a chemical process by which the soil or water environment becomes more acidic; occurs naturally over time in humid regions and is also caused by the application of acid-forming, nitrogenous fertilizers. Soils become acidic when bases (Ca+2, Mg+2, K+, Na+) are leached out and replaced by H+, resulting in pH levels less than 7.0. Highly acidic soils may limit plant growth due to insufficient calcium and magnesium, toxic levels of exchangeable aluminum (in soils with pH <5.5), decreasing availability of nutrients, and changing decomposition rates of soil organic matter and organic residues. advection a method of nutrient transport through water flows. aerobic characterized by the presence of oxygen; pertains to organisms (such as some bacteria) that require oxygen to maintain life processes (such as composting or biological treatment processes). Carbon dioxide is a product of aerobic processes. allelopathy the suppression of growth of one species of plant or microorganism by another due to the release of toxic substances by the organism. alluvial deposits materials such as clay, silt, sand, gravel, and mud that have been eroded, transported, and deposited by running water: alluvion; alluvium. anaerobic characterized by the absence of oxygen; pertains to organisms (such as some bacteria) that do not require oxygen to maintain life processes (such as digestion of sewage sludge or manure and decomposition). Denitrification occurs under anaerobic conditions.
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Soil and Water Quality: An Agenda for Agriculture carbonation a process of chemical weathering of minerals that contain soda, lime, potash, or basic oxides contributing to an increase in the dissolved mineral load in the soil solution and in waters; the conversion to a carbonate—a salt or ester compound—as a reaction to some corrosive agent or dissolved CO2. denitrification the bacterial reduction of nitrate or nitrite to gaseous molecular nitrogen (N2), and nitrogen oxides (NOxs), such as NO2, N2O, and NO; the process by which nitrogen is returned from the soil environment to the atmosphere; occurs under anaerobic conditions and results in loss of available nitrogen from surface soils of wet riparian forests and vegetative filter strips. dissolution a chemical weathering mechanism that involves the dissolving or breaking up of a material such as salt or minerals in water. The deposition of salts in soil solution and water affects properties such as electrical conductivity (EC) and total dissolved solids (TDS). ephemeral erosion erosion of the soil by running water that creates small channels, called rills, that cannot be completely filled in by tillage. Ephemeral rills will collect water during heavy rains and increase the severity of erosion. erosion the loosening, transportation, and wearing away of the land surface by running water, wind, ice, or other geological agents; the single most important process of soil degradation resulting in loss of soil productivity and increased water pollution from sediment and agricultural chemicals. eutrophication the process by which a body of water becomes rich in nutrients, characterized by high concentrations of phosphorus (P) and nitrogen (N), frequently shallow depths, and seasonal oxygen deficiency in the deeper areas; occurs naturally and by human activity, usually in the form of industrial or municipal wastewater or agricultural runoff. Eutrophication causes algal blooms, fish kills, and other water quality problems. evapotranspiration the loss of pure water from the soil as a result of both soil surface evaporation (the process by which liquid is changed to a vapor or gas) and plant transpiration (the photosynthetic and physiological process by which plants release water vapor into the air). fertility (soil) the quality of a soil that enables it to provide nutrients in adequate amounts and proper balance for plant growth when
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Soil and Water Quality: An Agenda for Agriculture other growth factors such as light, moisture, and temperature are favorable. Two main methods for ensuring fertility are biological nutrient cycling and the addition of chemical fertilizers. greenhouse effect warming of the earth's surface partly caused by concentration of gases such as water vapor (H2O), carbon dioxide (CO2), methane (CH4), and chlorofluorocarbons (CFCs) in the earth's atmosphere. The concentration of gases acts as a cover—absorbing longer, infrared waves and trapping heat in the atmosphere. Soil management can have a significant impact on greenhouse gas concentrations inasmuch as soils can be sources or sinks of greenhouse gases. gully erosion severe erosion of the soil by running water that creates deep channels: gully; gullies. halophytes a plant or microorganism adapted to high-saline soil environments. Examples include greasewood (Sarcobatus), and salt-grass (Distichlis ). Halophytes can be irrigated with reused drainage water that is higher in salinities. humus the fraction of organic matter in the soil that is relatively resistant to further breakdown and decomposition; contains complex and more stable (less reactive or labile) organic matter including decaying plant and animal tissue. hydrolysis a process that dissolves minerals and breaks down chemical compounds by reaction with hydrogen and oxygen; a method of pesticide degradation and a primary source of salt deposits. labile carbon that fraction of organic matter in soils that is most readily decomposable by soil microorganisms. The amount of labile carbon in the soil has an important effect on soil biological activities such as mineralization of nutrients, generation of soil structure, and specific enzyme activities. laterization the desiccation and hardening of exposed plinthitic materials. laterite refers to a zonal group of red soils, rare in the United States, developed in hot, humid climates characterized by intense weathering, chemical change, and residues of aluminum and iron oxides. ligand the molecule, ion, or group bound to the central atom in a chelate (cyclical structure with a central metallic ion) or coordination compound forming a complex. Trace elements such as copper (Cu) and molybdenate (MoO4-2) that tend to form complexes with ligands have greater mobilities than those that are not complexed.
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Soil and Water Quality: An Agenda for Agriculture loess an essentially unconsolidated, unstratified, calcareous silt transported and deposited by wind; usually homogeneous, permeable, and buff to grey in color. organic carbon the total amount of carbon held in the organic matter in the soil; chemical compounds based on carbon chains or rings that also contain hydrogen with or without oxygen, nitrogen, or other elements. Cultivation causes marked reductions in total organic carbon content in the soil; but it can be replaced by crop residues, manures, or other sources of organic matter added to soils. osmotic potential the work per unit quantity of pure water that has to be done to prevent the transport or flow of a solvent across a membrane that separates (1) solutions of differing concentrations or (2) pure solvent from solute (as in pure water and salt water): osmotic pressure; osmotic gradient. Salt water in the root zone decreases the osmotic potential of the soil solution relative to the osmotic potential of the root, thereby reducing the amount of fresh water available to plants. oxidation-reduction a simultaneous chemical reaction wherein an oxidation reactant (reducing agent) gains a positive charge (loses an electron), while a reduction reactant (oxidizing agent) loses a positive charge (gains an electron); a process of chemical weathering that is a source of salt deposits in water: redox reactions. pedon the smallest volume of soil capable of representing all the horizons of a soil profile, including soil horizon shapes and relations. The three-dimensional profile is usually hexagonal horizontally ranging from 1 to 10 m2 with depths to the lower limit of the genetic soil horizons. pedotransfer functions functions that relate different soil attributes and properties to one another (for example, predicting changes in soil's organic matter on the basis of the amount of crop residue added). pH a measurement scale indicating acidity and alkalinity in which values less than 7 are acidic, the value of 7 is neutral, and values from 7 to 14 are basic (alkaline). pH of a soil environment affects productivity by influencing solutbilities of heavy metals and minerals, mobilities of anionic trace elements, and activities of microorganisms. photolysis chemical decomposition or dissociation by the action of radiant energy (i.e., light); a method of chemical degradation of
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Soil and Water Quality: An Agenda for Agriculture pesticides with a short (on the order of hours and days) reaction rate. plinthitic materials earthy material of clay and quartz high in iron oxides, aluminum hydroxides, and some silica; poor in humus, highly weathered, and usually red in color. runoff rainfall excess that is not absorbed by the soil. rill erosion erosion of soil by running water carving out visible channels that are small enough to be filled in by tillage. salinity the concentration of dissolved salt in water traditionally referring to major anions and cations (Na, Ca, Mg, K, Cl, SO4, HCO 3, CO3, NO3) found in irrigation water, but now also including toxic trace elements. Salinity in soils has deleterious effects on physical soil condition and is frequently accompanied by waterlogging, which results in poor aeration of the root zone. Salinity reduces crop yields, affects germination, seedling and vegetative growth, and has adverse effects on water quality. Salinity is measured using electroconductivity (µS/cm) or total dissolved solids (mg/L). salinization the process by which salts accumulate in soil. Chemical weathering of minerals in soils and rocks—dissolution, hydrolysis, carbonation, acidification, and oxidation-reduction—is the primary cause of dissolved mineral load (salinity). Secondary causes of salinity include evaporation, release and dissolution of fossil salts, atmospheric deposition, seawater intrusion, human-induced irrigation, and salt seeps. seep an area, generally small, where a fluid (water, soil, gas, saline water) contained in the ground percolates slowly to the surface and forms a pool. Saline seeps, where salty groundwater moves to the surface, are a source of salinization of soils. sheet erosion erosion of the soil by running water that removes soil in thin uniform sheets. slaking the breakdown of soil aggregates as a result of exposure to air or water. Slaking can destroy pore interstices, reducing the hydraulic conductivity of a soil, leading to reduced permeability, poor crop establishment, inadequate water intake rates, and increased runoff and erosion. sodicity of, relating to, or containing sodium (Na). Soils are classified as sodic when Na is the prevalent cation in saline soils. Sodic soils may limit plant growth by having toxic concentrations of exchangeable Na that keep soil dispersed and maintain poor soil structure.
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Soil and Water Quality: An Agenda for Agriculture The sodium absorption rate (SAR) is a comparison of the concentration of sodium ions (Na+) to that of calcium ions (Ca+2) and magnesium ions (Mg+2) and is considered with the electroconductivity measurement of salinity when assessing the potential effects of water quality on soil water penetration. soil horizon an approximately horizontal layer of soil differing from adjacent layers in physical, chemical, or biological properties such as color, structure, texture, consistence, kinds and numbers of organisms present, or degree of acidity or alkalinity. solum the upper horizons of the soil profile in which the natural processes of soil formation take place; true soil. The solum is where most plant roots grow. tailwater irrigation water that reaches the lower end of a field. Tailwater ratio is an indication of irrigation water application systems; tailwater return systems increase efficiency of surface water irrigation systems. turgor the normal state of turgidity and tension in plant cells caused by water pressure when plant cells are full of water. This pressure keeps stems upright and leaves expanded to receive sunlight. turgor pressure the pressure developed as the result of fluid in a turgid (swollen) plant cell caused by the osmotic diffusion toward the inside of a cell. Saline soil environments can affect the necessary plant cell turgor pressure by disrupting the osmotic balance. vadose zone the unsaturated zone of the soil above the permanent groundwater level. The vadose zone is of concern in considering the potential contamination, transport, degradation, and mobilization of nutrients, pesticides, salts, and trace elements. water-holding capacity the capability of soils to store and release available water to plants. Estimation of water-holding capacities are made using regression models from parameters such as particle size distributions, organic matter content, and bulk density.
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