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Disc harrow
A disc harrow is a farm implement that is used to cultivate the soil where crops are to be planted. It is also used to chop up unwanted weeds or crop remainders. It consists of many carbon steel and sometimes the longer lasting boron discs, which have many varying concavities and disc blade sizes and spacing (the choices of the later being determined by the final result required in a given soil type) and which are arranged into two sections ("offset disc harrow") or four sections ("tandem disc harrow"). When viewed from above, the four sections would appear to form an "X" which has been flattened to be wider than it is tall. The discs are also offset so that they are not parallel with the overall direction of the implement. This arrangement ensures that the discs will repeatedly slice any ground to which they are applied, in order to optimize the result. The concavity of the discs as well as their offset angle causes them to loosen and lift the soil that they cut.
==History==
Before invention of the modern tractor, disc harrows typically consisted of two sections, which were horse-drawn and had no hydraulic power. These harrows were often adjustable so that the discs could be changed from their offset position. Straightening the discs allowed for transport without ripping up the ground; also, they were not as difficult to pull. Overuse of disc harrows (called "disc plows") in the High Plains of the United States in the early 20th century may have contributed to the "Dust Bowl".
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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