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O horizon

A surface horizon, or a subsurface horizon occurring at any depth if it has been buried, that consists of poorly aerated organic material. It is usually undecomposed or partially decomposed organic matter (litter such as leaves, needles, twigs, moss, and lichens) (WRB, 2006). Often referred as the histic horizon (from Greek histos, tissue).

Organic farming

Agricultural production which typically places a higher emphasis on environmental and wildlife protection and, with regard to livestock production, on measures that are supposedly animal welfare friendly. Organic production aims at more holistic production management systems for crops and livestock, emphasizing on-farm management practices over off-farm inputs. This involves avoiding, or largely reducing, the use of synthetic chemicals such as inorganic fertilizers, pesticides, medicinal products, replacing them, wherever possible, with cultural, biological and mechanical methods. Organic producers explicitly aim to develop an allegedly healthier, fertile soil by growing and rotating a mixture of crops and using clover to fix nitrogen from the atmosphere. The production of genetically-modified (GM) crops and their use in animal feed is banned. In the context of European Union (EU) statistics, farming is considered to be organic if it complies with Regulation 834/2007 of 28 June 2007 on organic production and labelling of organic products. The detailed rules for the implementation of this Regulation are laid down in Regulation 889/2008.

Organic fertilizers

Livestock manures, digestates, green manures, compost, sewage sludge, (agro)industrial organic waste.

Organic matter

Plant and animal residue in the soil in various stages of decomposition. Any material that is part of or originated from living organisms. Includes soil organic matter, plant residue, mulch, compost, and other materials.

Organic matter loss

Decline of organic matter content in one or more soil layers when the annual loss of organic matter (e.g. due to oxidation or erosion) is insufficiently compensated for by the annual gain of organic matter, resulting from crop residues, composts and manures.

Organic matter, active fraction

The highly dynamic or labile portion of soil organic matter that is readily available to soil organisms. May also include the living biomass. Particulate organic matter (POM) and light fraction (LF) are measurable indicators of the active fraction. POM particles are larger than other SOM and can be separated from soil by sieving. LF particles are lighter than other SOM and can be separated from soil by centrifugation.

Organic matter, stabilized organic matter

The pool of soil organic matter that is resistant to biological degradation because it is either physically or chemically inaccessible to microbial activity. These compounds are created through a combination of biological activity and chemical reactions in the soil. Humus is usually a synonym for stabilized organic matter, but is sometimes used to refer to all soil organic matter.

Organic soil

A soil in which the sum of the thicknesses of layers comprising organic soil materials is generally greater than the sum of the thicknesses of mineral layers.

Organic soil material

Consists of organic debris that accumulates at the surface under either wet or dry conditions and in which any mineral component present does not significantly affect the soil properties. Organic soil material must have organic carbon (organic matter) contents as follows: (1) if saturated with water for long periods (unless artificially drained), and excluding live roots, either: 18 % organic carbon (30 % organic matter) or more if the mineral fraction comprise 60 % or more clay; or 12 % organic carbon (20 % organic matter) or more if the mineral fraction has no clay; or a proportional lower limit of organic carbon content between 12 and 18 % if the clay content of the mineral fraction is between 0 and 60 %; or (2) if never saturated with water for more than a few days, 20 % or more organic carbon.

Overland flow

Excess water leaving a field horizontally across the soil surface because it cannot infiltrate into the soil, eventually ending up in a ditch or stream (= surface runoff).

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