Acid. A solution containing free hydrogen ions (H+) or a chemical that will give off hydrogen ions into solution.

Acidic soil. A soil with a pH below 7. The lower the pH, the more acidic the soil.

Aggregates. The structures, crumbs, or clumps formed when soil minerals and organic matter are bound together with the help of organic molecules, plant roots, fungi, and clays.

Alkaline soil. A soil with a pH above 7, containing more base than acid.

Allelopathic effect. Suppression of the germination or growth of one plant by another. The chemicals responsible for this effect are produced during the growth of a plant or during its decomposition.

Ammonium. A form of nitrogen (NH4+) that is available to plants and is produced in the early stage of organic matter decomposition.

Anion. A negatively charged element or molecule such as chloride (Cl) or nitrate (NO3).

Aquifer. A source of groundwater below the land surface.

Available nutrient. The form of a nutrient that a plant is able to use. Nutrients are commonly found in the soil in forms that the plant can’t use (such as organic forms of nitrogen) and must be converted into forms that the plant is able to take into its roots and use (such as the nitrate form of nitrogen).

Ball test. A simple field test to determine soil readiness for tillage. A handful of soil is squeezed into a ball. If the soil holds together, it is in the plastic state and too wet for tillage or field traffic. If it crumbles, it is in the friable state.

Base. Something that will neutralize an acid, such as hydroxide or limestone.

Beds. Small hilled-up, or raised, zones where crops (usually vegetables) are planted. They provide betterdrained and warmer soil conditions. They are similar to ridges but generally broader, and they are usually shaped after conventional tillage has occurred.

Buffering. Slowing or inhibiting changes. For example, buffering can slow pH changes by neutralizing acids or bases. A substance that can buffer a solution is also called a buffer.

Bulk density. The mass of dry soil per unit volume; an indicator of the density and compactness of the soil.

Calcareous soil. A soil in which finely divided lime is naturally distributed; it usually has a pH between 7 and slightly more than 8.

Cation. A positively charged ion such as calcium (Ca++) or ammonium (NH4+).

Cation exchange capacity (CEC). The amount of negative charge that exists on humus and clays, allowing them to hold on to positively charged chemicals (cations).

Chelate. A molecule that uses more than one bond to attach strongly to certain elements such as iron (Fe++) and zinc (Zn++). These elements may later be released from the chelate and used by plants.

C:N ratio. The amount of carbon divided by the amount of nitrogen in a residue or soil. A high ratio results in low rates of decomposition and can also result in a temporary decrease in nitrogen nutrition for plants, as microorganisms use much of the available nitrogen.

Coarse-textured soil. Soil dominated by large mineral particles (the size of grains of sand); may also include gravels. Used to be called “light soil.”

Colloid. A very small particle with a high surface area that can stay in a water suspension for a very long time. The colloids in soils—the clay and humus molecules—are usually found in larger aggregates and not as individual particles. These colloids are responsible for many of the chemical and physical properties of soils, including cation exchange capacity, chelation of micronutrients, and development of aggregates.

Compost. Organic material that has been well decomposed by organisms under conditions of good aeration and high temperature, often used as a soil amendment.

Controlled traffic. The restriction of field equipment to limited travel or access lanes in order to reduce compaction on the rest of the field.

Conventional tillage. Preparation of soil for planting by using a moldboard plow followed by disking or harrowing. It usually breaks down aggregates, buries most crop residues and manures, and leaves the soil smooth.

Coulter. A fluted or rippled disk mounted on the front of a planter to cut surface crop residues and perform minimal soil loosening prior to seed placement. Multiple coulters are used on zone-till planters to provide a wider band of loosened soil.

Cover crop. A crop grown to protect the soil from erosion during the time of the year when it would otherwise be bare. Sometimes called a green manure crop.

Crumb. A soft, porous, more or less round soil aggregate. Generally indicative of good soil tilth.

Crust. A thin, dense layer at the soil surface that becomes hard upon drying.

Deep tillage. Tillage that loosens the soil at a greater depth (usually more than 8 inches) than regular tillage.

Denitrification. The process by which soil organisms convert dissolved nitrate to gaseous nitrogen under anaerobic (low-oxygen) conditions. This occurs when soils become saturated and results in losses of nitrous oxide (a potent greenhouse gas) and dinitrogen (N2, an inert gas).

Disk. An implement for harrowing, or breaking up, the soil. It is commonly used following a moldboard plow but is also used by itself to break down aggregates, help mix fertilizers and manures with the soil, and smooth the soil surface.

Drainage. The loss of soil water by percolation down through the soil as a result of the gravitational force. Also: Removal of excess soil water through the use of channels, ditches, soil shaping, or subsurface drain pipes.

Elements. Components of all matter. Seventeen elements are essential for plant growth; elements such as carbon, oxygen, and nitrogen combine to form larger molecules.

Erosion. The wearing away of soil by runoff water (water erosion), wind shear (wind erosion), or tillage (tillage erosion).

Evaporation. The loss of water from the soil surface as vapor.

Evapotranspiration. The combined processes of evaporation and transpiration.

Fertigation. The application of soluble fertilizers through an irrigation system, which allows for nutrient spoonfeeding of plants.

Field capacity. The water content of a soil following drainage by gravity.

Fine-textured soil. Soil dominated by small mineral particles (silt and clay). Sometimes called “heavy soil.”

Friable soil. Soil that crumbles when force is applied. A soil generally goes from the plastic to the friable state when it dries.

Frost tillage. Tillage performed when a shallow (2–4inch) frozen layer exists at the soil surface.

Full-field (full-width) tillage. Tillage that results in loosening soil over the entire width of the tillage pass— for example, moldboard plowing, chisel tillage, and disking.

Green manure. A crop grown for the main purpose of building up or maintaining soil organic matter; sometimes called a cover crop.

Groundwater. Water contained below the ground surface, typically in the pore spaces of underground geologic deposits.

Heavy soil. Nowadays usually called “fine-textured soil,” it contains a lot of clay and is usually more difficult to work than coarse-textured soil. It normally drains slowly following rain.

Humus. The well-decomposed part of the soil organic matter. It has a high cation exchange capacity.

Infiltration. The process of water entering the soil at the surface.

Inorganic chemicals. Chemicals that are not made from chains or rings of carbon atoms—for example, soil clay minerals, nitrate, and calcium.

Irrigation. The application of water to soil to provide better moisture conditions for crop growth. Flood and furrow irrigation practices pond the soil with water for a limited time and allow it to infiltrate. Micro-irrigation,including drip, trickle, and microsprinkler irrigation,refers to a set of practices that apply localized irrigation water at low rates through small tubes and emitters and are generally water conserving. Supplemental irrigation refers to a practice used in humid regions where rainfall provides most crop water needs and irrigation is primarily used to maintain adequate soil moisture levels during limited drought periods. Deficit irrigation refers to a water-conserving practice whereby water supply is reduced below maximum levels and mild crop stress is allowed, with minimal effects on yield.

Landslide. The instantaneous downward fall of large soil volumes as a result of gravity. Landslides may occur on steep slopes when they become supersaturated with water.

Least-limiting water range. See Optimum water range.

Legume. Plants—including beans, peas, clovers, and alfalfa—that form a symbiotic relationship with nitrogenfixing bacteria living in their roots. These bacteria help to supply the plants with an available source of nitrogen.

Lignin. A substance found in woody tissue and in the stems of plants that is difficult for soil organisms to decompose.

Lime or limestone. A mineral consisting of calcium carbonate (CaCO3) that can neutralize acids and is commonly applied to acid soils.

Loess soil. Soil formed from windblown deposits of silty and fine-sand-size minerals; they are easily eroded by wind and water.

Micronutrient. An element, such as zinc, iron, copper, boron, or manganese, that is needed by plants in only small amounts.

Microorganisms. Very small and simple organisms such as bacteria and fungi.

Mineralization. The process by which soil organisms change organic elements into the “mineral” or inorganic form as they decompose organic matter; for example, organic forms of nitrogen are converted to nitrate.

Moldboard plow. A commonly used plow that completely turns over the soil and incorporates any surface residues, manures, or fertilizers deeper into the soil.

Mole drainage. A practice used on heavy clay soils whereby water is removed through subsurface channels 2–3 feet deep. This practice does not involve pipes; the channels are generated with the use of a bullet-type plow. Channels generally need to be rebuilt every four to six years.

Monoculture. Production of the same crop in the same field year after year.

Mulch. Organic materials like straw and wood chips that are applied to soil as a surface cover; generally also includes cover crop material left on the surface and heavy amounts of crop residues left at the soil surface after harvest.

Mycorrhizal relationship. The mutually beneficial relationship that develops between plant roots of most crops and fungi. The fungi help plants obtain water and phosphorus by acting like an extension of the root system and in return receive energy-containing chemical nutrients from the plant.

Nitrate (NO3). The form of nitrogen that is most readily available to plants and is normally found in the greatest abundance in agricultural soils.

Nitrification. The process by which soil microorganisms convert ammonium into nitrate.

Nitrogen fixation. The conversion of atmospheric nitrogen by bacteria to a form that plants can use. A small number of bacteria, including the rhizobia living in the roots of legumes, are able to make this conversion.

Nitrogen immobilization. The transformation of available forms of nitrogen, such as nitrate and ammonium, into organic forms that are not readily available to plants.

No-till. A system of planting crops without tilling the soil with a plow, disk, chisel, or other tillage implement.

Optimum water range. The range of soil water content in which plants do not experience stress from drought, high soil strength, or lack of aeration.

Organic chemicals. Chemicals that contain chains or rings of carbon connected to one another. Most of the chemicals in plants, animals, microorganisms, and soil organic matter are organic.

Oxidation. The combining of a chemical such as carbon with oxygen, usually resulting in the release of energy.

Penetrometer. A device that measures soil resistance to penetration, which indicates the degree of compaction; it has a cone-tipped metal shaft that is slowly pushed into the soil while the resistance force is measured.

Perennial forage crops. Crops such as grasses, legumes, and grass-legume mixtures that form a complete soil cover (sod) and are grown for pasture or to make hay and haylage for animal feed.

pH. A way of expressing the acid status, or hydrogen ion (H+) concentration, of a soil or a solution on a scale on which 7 is neutral, less than 7 is acidic, and greater than 7 is basic.

Photosynthesis. The process by which green plants capture the energy of sunlight and use carbon dioxide from the atmosphere to make molecules needed for growth and development.

Plastic. The state of a soil that molds easily when force is applied. Compare to Friable.

Plastic limit. The water content of soil at the transition from the plastic to the friable state; the upper limit of soil moisture at which tillage and field traffic do not result in excessive compaction damage.

Polyculture. The growth of more than one crop in a field at the same time.

PSNT. The pre-sidedress nitrate test is a soil test for nitrogen availability in which the soil is sampled to a depth of 1 foot during the early crop growth.

Raised beds. Crops grown in rows that are raised from the inter-row areas to provide better drainage and aeration and deeper topsoil. Raised beds are wider than ridges but aim to achieve the same benefits.

Recycled wastewater. Water derived from the treatment of municipal wastewater and used for crop irrigation.

Respiration. The biological process that allows living things to use the energy stored in organic chemicals. In this process, carbon dioxide is released as energy is made available to do all sorts of work.

Restricted tillage. Tillage that includes only limited and localized soil disturbance in bands where plant rows are to be established—for example, no-till, zone-till, strip-till, and ridge-till systems. Compare with Fullfield tillage.

Rhizobia bacteria. Bacteria that live in the roots of legumes and have a mutually beneficial relationship with the plant. These bacteria fix nitrogen, providing it to the plant in an available form, and in return receive energy-rich molecules that the plant produces.

Ridge tillage. Planting crops on top of small ridges (usually 2–4 inches in height), which are generally re-formed annually with a special cultivator.

Rotation effect. The crop-yield benefit from rotations, which includes better nutrient availability, fewer pest problems, and better soil structure.

Runoff. Water lost by flow over the soil surface.

Saline soil. Soil that contains excess free salts, usually sodium and calcium chlorides.

Saturated soil. Soil whose pores are filled with water, resulting in a virtual absence of soil air. 

Silage. A feed produced when chopped-up corn plants or wilted hay is put into airtight storage facilities (silos) and partially fermented by bacteria. The acidity produced by the fermentation and the lack of oxygen help preserve the quality of the feed during storage.

Slurry (manure). Manure that is between solid and liquid; it flows slowly and has the consistency of a very thick soup.

Sod crops. Grasses or legumes such as timothy and white clover that tend to grow very close together and form a dense cover over the entire soil surface.

Sodic soil. Soil containing excess amounts of sodium. If it is not also saline, clay particles disperse, and the soil structure may be poor.

Soil structure. The physical condition of the soil, which depends on the number of pores, the arrangement of soil solids into aggregates, and the degree of compaction.

Strip cropping. Growing two or more crops in alternating strips, usually along the contour or perpendicular to the prevailing wind direction.

Surface water. Water at the land surface, including streams, ponds, lakes, estuaries, seas, and oceans.

TDR (time-domain deflectometry). Method for assessing water contents of soils by measuring the medium’s dielectric properties (its ability to conduct electromagnetic waves). Typically involves metal rods that are inserted into soil.

Texture. A soil’s sand, silt, and clay content. “Coarsetextured” means that a soil has a high sand content, while “fine-textured” means that a soil has a high clay content.

Thermophilic bacteria. Bacteria that live and work best under high temperatures, around 110°–140°F. They are responsible for the most intense stage of decomposition that occurs during composting.

Tile drainage. Removal of excess soil water through pipes buried in the soil, typically 3–4 feet deep. Traditionally, the pipes were made of clay tile, but they are now corrugated flexible PVC pipes with perforations.

Tillage. The mechanical manipulation of soil, generally for the purpose of loosening the soil, creating a seedbed, controlling weeds, or incorporating amendments. Primary tillage (moldboard plowing, chiseling) is a more rigorous practice, primarily for loosening soil and incorporating amendments. Secondary tillage (disking, harrowing) is a less rigorous practice, following primary tillage, that creates a seedbed containing fine aggregates.

Tillage erosion. The downslope movement of soil caused by the action of tillage implements.

Tilth. The physical condition, or structure, of the soil as it influences plant growth. A soil with good tilth is very porous and allows rainfall to infiltrate easily, permits roots to grow without obstruction, and is easy to work.

Transpiration. The loss of water from the soil through plant uptake and evaporation from leaf surfaces.

Wilting point. The point at which a soil contains only water that is too tightly held to be available to plants.

Yield monitor. A computerized data acquisition system on a crop harvester—typically, a grain combine—that records and provides maps of crop yield in fields on the go.

Zone tillage. A restricted tillage system that establishes a narrow (4–6-inch) band of loosened soil with surface residues removed. This is accomplished using multiple coulters and row cleaners as attachments on a planter. It may include a separate “zone-building” practice that provides deep, narrow ripping without significant surface disturbance. It is a modification of no tillage, generally better adapted to cold and wet soils.

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