Aeration (gas exchange)—The process of replacing air in the soil with air from the atmosphere. In well-aerated soil, air in the soil is similar in composition to air above the soil. Poorly aerated soils contain a higher content of carbon dioxide and a lower content of oxygen than the atmosphere above the soil. Aeration is important because plant roots and aerobic soil organisms consume oxygen and release carbon dioxide during respiration.
Aggregation—The process whereby primary soil particles (sand, silt, clay) are bound together into large particles by physical and chemical forces and substances derived from root exudates and microbial activity. Soil aggregates are the building blocks of soil structure.
Bulk Density—The weight of dry soil of a given volume. Bulk density is determined by the texture of the soil, which is inherent, and by soil structure and the amount of soil pore space, which can be changed by management. Compaction increases bulk density by reducing soil pore space.
Carbon Cycle—The sequence of transformations in which carbon dioxide is converted to organic forms by plants through photosynthesis, organic carbon is recycled through a series of living organisms, and carbon is ultimately returned to its original state (gaseous CO2) through organic matter decomposition and biological respiration. Living organisms use carbon compounds as energy sources (respiration) and as building blocks for biological molecules essential for their bodies and life functions. The carbon cycle is also important because plant nutrients follow carbon through the organic phases of this cycle, so the carbon cycle overlaps and interacts with many nutrient cycles.
Cation Exchange Capacity (CEC)—The surfaces of clay minerals and organic matter have negative electrical charges that attract positively charged elements or molecules (cations), which are exchangeable with other cations in the soil solution. The net negative charge of a given weight of soil is equivalent to the cation exchange capacity. CEC is especially important for the essential plant nutrients potassium, calcium, and magnesium. When held in exchangeable form on particle surfaces, these nutrients are protected from leaching and are a reserve nutrient supply that can replenish ions taken up by plant roots. Another important function of CEC is the exchange of hydrogen and aluminum cations between particle surfaces and the soil solution, which helps buffer soil pH (i.e., pH remains more stable because of the ability to exchange acidic cations).
Compaction—An increase in bulk density and soil strength and a decrease in soil porosity by the application of mechanical forces to the soil. Wheel traffic, the action of tillage implements, and similar physical forces crush soil aggregates and push soil particles closer together, especially under wet soil conditions. Compacted soils or soil layers restrict root growth, water movement, and air exchange.
Crust—A dense, cemented layer at the soil surface, usually only a few tenths of an inch thick, that is very hard when dry and inhibits water infiltration and seedling emergence. Crusting is most common on fine-textured soils (high in silt and clay) and is often caused by the impact of raindrops on soil that has been extensively tilled to produce a very fine seedbed.
Drainage—Loss of water from soils, either by percolation through the soil or by surface flow across the soil. Adequate drainage is necessary for good soil aeration, but soils that drain too well have low soil water content and may dry too quickly for good crop growth without irrigation.
Erosion—The detachment and movement of soil by water, wind, ice, or gravity. Loss of topsoil by erosion is probably the most common cause of soil-quality degradation, so management practices to control erosion are critical to sustaining soil quality.
Humus—Well-decomposed organic compounds in soil that are very resistant to further degradation. Humus has high water-holding and cation-exchange capacity, binds soil particles, and improves soil structure.
Infiltration—The entry of water into the soil profile through openings in the soil surface. Infiltration capacity, or rate, determines how much water runs off and how much soaks in during rainfall, although after soil pores become filled, the permeability of soil layers below the surface may control water entry.
Leaching—The movement of soluble materials from one soil zone to another by water movement in the soil profile. Excessive leaching of agrochemicals like nitrate fertilizer and herbicides can be an environmental problem.
Mulch—Any material such as straw, wood chips, sawdust, leaves, plastic film, loose soil, etc., that is spread or formed upon the soil surface to protect the soil and/or plant roots from the effects of raindrops (crusting), wind or flowing water (erosion), temperature extremes (cold or hot), evaporation, etc. Mulches are also used primarily to control weeds, conserve soil moisture, or alter soil temperature.
Nutrient—Chemical elements or compounds that are essential raw materials for the growth and development of plants and/or soil organisms. Nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, boron, chlorine, copper, iron, manganese, molybdenum, and zinc are absorbed by roots from the soil and are necessary for completion of the normal life cycle of all plants.
Nutrient Cycle—The sequence of biochemical changes undergone by an essential plant nutrient where it is taken up by plant roots or soil microbes, used by a series of living organisms, and transformed to its original state upon the death and decomposition of the organism (at which time the cycle can start again). Chemical and physical changes (such as ammonia volatilization) are also parts of some nutrient cycles.
Permeability—The ease with which gases, liquids, or plant roots penetrate or pass through the soil or a layer of soil. Permeability is determined by soil texture, structure, and porosity.
pH—A measure of soil acidity or alkalinity; the hydrogen ion activity of a solution in equilibrium with the solid phase of a soil.
Porosity—The non-solid portion of a volume of soil, consisting of air- or water-filled pore space. Soil porosity, both the total amount of pore space and the distribution of pore sizes, controls soil water content, air movement, and water movement. The rates of air exchange and water movement depend on both the volume and continuity of pore spaces within the soil.
Raised Bed—A slightly dome-shaped or flat-topped ridge of soil. Raised beds are generally a series of parallel ridges formed by cultivation with shallow furrows in between. Bed size depends on the crop grown and the number of plant rows per bed but is usually in the range of four- to eight-inches high and two- to four-feet wide, with one to three rows per bed. Advantages of raised beds include improved soil drainage, earlier soil warming, and easier picking of some crops. But beds also dry out more quickly and make irrigation a more critical requirement for many crops.
Residue—Leaves, stems, stalks, stubble, and other plant parts that are left on the soil surface after the harvested portion of a crop is removed. Crop residue protects the soil surface, improves water infiltration, and reduces crusting, erosion, and evaporation. But large amounts of residue also can keep soil excessively cool and wet early in the growing season.
Respiration Rate—The rate of carbon dioxide release (or oxygen consumption) by biological respiration. Soil respiration rate is a measure of the size and activity of the overall population of soil organisms. Soil microbes generally make the largest contribution to soil respiration, although measurements in the field can include significant contributions from larger organisms and plant roots. Soil temperature, moisture, aeration, and food supply all have major effects on biological activity, and therefore respiration rate, so these factors must be taken into account when interpreting and comparing respiration measurements.
Soil Organic Matter (SOM)—The organic fraction of the soil. The broadest definition includes undecayed plant or animal residues, living soil organisms, and plant roots as categories of soil organic matter. Many definitions restrict the term to humus and organic materials that are at least partially decomposed, and whose origin (plant stems, leaves, animal remains) is no longer recognizable. Organic matter that bacteria, fungi, earthworms, and other soil organisms can decompose is a critical component of the soil food web, which is important in nutrient cycling and all other soil biological processes.
Soil Strength (penetration resistance, cone index)—The hardness, or resistance to a physical force, of a soil layer, zone, or specific point in the soil. Soil strength is often defined in terms of a measurement of resistance to penetration with an instrument such as a cone penetrometer. Measurements may be difficult to interpret, because they are strongly affected by differences in soil water content. Penetration resistance is also determined by soil texture and many properties related to soil aggregation and structure. Soil zones with very high soil strength, such as clay pans, plow pans, and other types of hardpans, restrict root growth, water movement, and air exchange.
Soil Structure—The combination or arrangement of primary soil particles (sand, silt, clay) into larger, aggregated particles with pore spaces between them. Soil with "good" structure is about 50% solids and 50% pore space, with the pore space evenly distributed between large, air-filled pores and smaller, water-holding pores.
Soil Texture—The relative proportions of primary soil particles (sand, silt, clay) in a soil, which determine a soil's "textural class" (loamy sand, loam, silty clay, etc.). Texture is not a measure of soil quality, because it is an intrinsic soil property that cannot be changed. But texture is an important soil characteristic that affects most soil management decisions.
Tilth—The physical condition of soil, generally described in terms of its ease of tillage (workability), fitness as a seedbed, and its resistance to seedling emergence and root penetration. Soil tilth is closely related to soil structure, but structure is usually described in physical terms and tilth is usually evaluated in relation to a crop response or soil management operation.
Topsoil (surface soil, plow layer, Ap horizon)—The uppermost part of the soil, ranging in depth from three to 10 inches, and generally containing higher amounts of organic matter and plant nutrients than the soil below. Topsoil is often functionally defined as the soil layer moved in primary tillage or the equivalent depth in uncultivated soils. This definition is rather arbitrary in deep, untilled soils with no clear indication of where topsoil ends and the next soil layer begins.
Water (hydrologic) Cycle—The fate of water from the time it leaves the atmosphere as precipitation until the water has been returned to the atmosphere by evaporation or plant transpiration. Soil plays a very important role in the water cycle, because a substantial portion of the precipitation reaching the earth falls on soil. The condition or quality of the soil determines such things as how much water runs off to rivers or lakes, how much enters the soil and can be taken up by plants or evaporated, and the rate and the amount of water that moves through the soil to groundwater.
Water-Holding Capacity (field capacity)—The amount of water soil can hold against the downward force of gravity. Soil texture, structure, porosity, and organic-matter content determine soil water-holding capacity. Some soil water is held too tightly to be taken up by plant roots, either in thin films on particle surfaces or in very small soil pores. Plant-available water-holding capacity is the portion of the total amount of water a soil can hold that can be taken up by plant roots. Available water in a soil can vary by crop, because roots of some plants can absorb water at lower soil water contents (held more strongly by the soil) than other types of plants.