A horizon: Surface layer of a soil profile. This horizon has more organic matter and dense microbial population hence greatest biological activity than other layers or horizons, such as B horizon and C horizon. It is also referred to as the surface mineral horizon with decomposed organic matter. Most important horizon from crop nutrition and microbial activity point of view.
Absolute weed: These are the plants, which are undesirable regardless of time and place.
Absolute Zero: Considered to be the point at which theoretically no molecular activity exists or the temperature at which the volume of a perfect gas vanishes. The value is 0o Kelvin, -273.15o Celsius and 459.67o Fahrenheit.
Acetobacter: N-fixing bacteria found in the roots, stems and leaves of sugarcane with a potential N-fixing capacity of 200 kg N ha-1. It also solubilises insoluble phosphorus. It is recommended for sugarcane.
Acetylene reduction assay (ARA): The quantitative method for estimating biological nitroigen fixation through gas chromatography. The enzyme nitrogenase (N-ase), which is responsible for biological N fixation, can reduce the gas acetylene (C2H2) to ethylene (C2H4) as well as dinitrogen (N2) to ammonia (NH3).
Acid rains: Acid rain contains excessive concentration of acidic compounds, primarily NO3-, SO42- and H+, having pH 5-7, received where atmospheric pollution through industrial activity or vehicular exhaust is high.
Acid sulphate soil:
• Very acid soil (pH<4) in which sulphuric acid is formed by the oxidation of S-bearing pyrite minerals.
• Found primarily in coastal, deltaic and estuarine areas of the humid tropics. Sometimes also called cat clays.
Actinomycetes: A group of microorganisms, intermediate between bacteria and true fungi that usually produce a characteristic branched mycelium. These organisms are responsible for earthy smell of the compost.
Activated charcoal: Charcoal, which has been treated to remove impurities. Activation carried out by heating charcoal under partial aeration. Used in chemical analysis.
Additive series: In intercropping, introduction of another plant species without reducing the population of the first species from the optimum.
Adjuvant: Substance added to a formulated pesticide product to act as a wetting or spreading agent, sticker, penetrant, or emulsifier in order to enhance the physical characteristics of the product.
Adsali sugarcane: Sugarcane, which takes 18 months for harvesting, usually planted in June-July.
Advance time (irrigation): The time it takes the first water applied to a dry irrigation furrow to travel the length of the furrow.
Advection: Advection involves the transfer of heat energy by means of hiorizontal mass motions through a medium.
Aerial root: A root usually arising adventitiously from a stem. Epiphytes produce aerial roots which have specialized tissue called velamen for absorbing moisture from the air. Aerial roots may contain chlorophyll and photosynthesize.
Aeroponics: A technique in growing plants wherein the plants derive their nutrients and water from a mist of air and aqueous solution that comes in contact with the roots.
Aerosol: Particulate matter, solid or liquid, larger than a molecule but small enough to remain suspended in the atmosphere. Aerosols can also originate as a result of human activities and are often considered pollutants.
Agricultural lime: Material containing oxides, hydroxides and / or carbonates of Ca and / or Mg, used for neutralizing soil acidity.
Agrobiology: A phase of the study of agronomy dealing with the relationship of yield to the quantity of an added or available fertilizer element.
Agroecology: The study of the interrelationships of living organisms with each other and with their environment in an agricultural system. (or) The use of ecological concepts and principles to study, design, and manage agricultural systems.
Agrology: The study of applied phases of soil science and soil management.
Ahu rice: Early rice similar to “Aus”; grown in Assam, India.
Akiochi disease: Japanese word used to describe a disease-like condition of rice plants caused by H2S toxicity, deficiency of Si, Mg, bases in general and N and K at later stages of plant growth.
Albumionous seed: A seed that contains endosperm at the time of germination, which nourishes the growing seedling.
Aleurone layer: The peripheral layer of endosperm of the grain beneath the seed coat, which envelops the endosperm and contains oil and protein. (or) It is a layer of high-protein cells surrounding the storage cells of the endosperm. Its function is to secrete hydrolytic enzymes for digesting food reserves in the endosperm.
Alfisols:
• Soils with grey to brown surface horizons.
• Medium to high supply of bases and B horizons of illuvial clay accumulation.
• Formed mostly under forest or savanna vegetation in climates with slight to pronounced seasonal moisture deficit.
Alien weeds: When a weed is allowed to move from the place of its origin to a new area, and it establishes itself there, it becomes an introduced weed in its new environment. Such weeds are known as alien weeds or anthrophytes.
Alkaloids: Any of a group of organic bases occurring in plants and containing carbon, hydrogen, oxygen and nitrogen for example, Dhurin/HCN, coumarins, oxalate etc.
Allelo-inhibition: Allelochemicals inhibit more the growth of plants of species other than producer species.
Alley cropping: A farming system in which arable crops are grown in alleys formed by trees or shrubs, established mainly to hasten soilfertility restoration and enhance soil productivity.
Alley crops: when arable crops are grown in alleys formed by trees or shrubs, established mainly to hasten soil fertility restotration, enhance soil productivity and reduce soil erosion they are known as alley crops.
Such crops should have slight shade tolerance and should be nontrailing. e.g., sweet potato, blackgram, turmeric and ginger in between the rows of Eucalyptus, Subabool and Cassia.
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