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acid rock

acid rock  

An igneous rock that contains more than 60% silica by weight, most of which is as silicate materials, with the excess of about 10% as free quartz. Acid rock is low in magnesium. Acid rocks tend to be ...
acid soil

acid soil  

Soil with a pH under 7, such as podzols and brown earths. Acidity in a soil may be due to the leaching out of cations when precipitation exceeds evapotranspiration. Other factors include the nature ...
Acrisols

Acrisols  

A reference soil group in the soil classification scheme used by the FAO. Acrisols are acid soils with an argic B horizon (see argic horizon) having a cation-exchange capacity of less than 24 ...
aggregate

aggregate  

A group of soil particles held together by electrostatic forces, polysaccharide gums, and cementation by carbonates and iron oxides.
agric horizon

agric horizon  

A mineral-soil diagnostic horizon, formed from an accumulation of clay, silt, and humus, which has moved down from an overlying, cultivated soil layer. It is a soil horizon created by agricultural ...
Agriculture and Agricultural Land

Agriculture and Agricultural Land  

The use of land for the production of food, fiber, and fuel is the principal driver of land use and land cover change and, therefore, a key forcing function of ...
Agriculture and Climate

Agriculture and Climate  

The basic industry that produces natural fiber and food for human consumption began between 8,000 and 10,000 years ago in the region now known as the Middle East. Hunters and ...
Albeluvisols

Albeluvisols  

A reference soil group in the soil classification scheme used by the FAO. Albeluvisols have an argic B horizon (see argic horizon) with an irregular upper boundary (a condition sometimes known as ...
Alisols

Alisols  

A reference soil group in the soil classification scheme used by the FAO. Alisols have an argic B horizon (see argic horizon) with a cation-exchange capacity of more than 24 cmolc/kg clay and a base ...
andic horizon

andic horizon  

A soil horizon formed by the moderate weathering of pyroclastic materials. The name is from the Japanese an meaning dark, and do meaning soil.
Andosols

Andosols  

Soils that have developed from volcanic material and have either a vitric horizon more than 30 cm below the surface, or that have weathered volcanic rocks within 25 cm of the surface. Andosols are a ...
Anthrosols

Anthrosols  

Soils that have been strongly affected by human activities such as ploughing, irrigation, or the addition of manure. Anthrosols are a reference soil group in the FAO soil classification.
argic horizon

argic horizon  

(argillic horizon)A subsurface soil horizon, at least one-tenth the thickness of the overlying horizon, that is identified by the illuvial (see illuviation) accumulation of silicate clays. The amount ...
argic horizon

argic horizon  

In the soil classification used by the FAO, a soil horizon showing signs of clay enrichment.
basic rock

basic rock  

Any of a number of igneous rocks that are rich in the heavy compounds of iron, magnesium, and calcium. They tend to be darker than acidic rocks and many of them are weathered by chemical weathering ...
bedrock

bedrock  

A general term for the solid, unweathered rock that lies beneath soils or deposits and is often exposed at the ground surface. See also outcrop.
biogeochemical cycle

biogeochemical cycle  

The cyclical movement of elements between living organisms (the biotic phase) and their nonliving (abiotic) surroundings (e.g. rocks, water, air). Examples of biogeochemical cycles are the carbon ...
broadcasting

broadcasting  

The casting of seed by hand from a basket, alternately to the right and to the left, an ancient method of sowing which lasted in some districts into the 20th century.
brown podzolic soil

brown podzolic soil  

A freely draining, leached soil profile that has developed acid surface horizons, a mor surface humus, and a clearly visible enrichment of translocated iron oxide in the middle, or B, horizon. The ...
calcic horizon

calcic horizon  

A mineral soil horizon with evidence of secondary calcium carbonate deposition which is more than 15 cm thick, with a calcium carbonate content of more than 15 per cent by weight, and with 5 per cent ...

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