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A seed drill is a sowing device that positions seeds in the soil and then covers them. The seed drill sows the seeds at equal distances and proper depth. This ensures that the seeds also get covered with soil. This saves them from being eaten by birds. Before the introduction of the seed drill, the common practice was to plant seeds by hand. Besides being wasteful, planting was very imprecise and led to a poor distribution of seeds, leading to low productivity. Jethro Tull is widely thought of as having invented the seed drill, though earlier the Sumerians used a single-tube seed drill, and the Chinese had also used a multi-tube seed drill.〔 〕 The use of a seed drill can improve the ratio of crop yield (seeds harvested per seed planted) by as much as nine times. In short, the seed drill can be described as a modern agricultural implement used for sowing seeds. == Design == In older methods of planting, a field is initially prepared with a plough to a series of linear cuts known as ''furrows''. The field is then seeded by throwing the seeds over the field, a method known as ''manual broadcasting''. In this method there was not a right depth nor proper distance. Seeds that landed in the furrows had better protection from the elements, and natural erosion or manual raking would preferentially cover them while leaving some exposed. The result was a field planted roughly in rows, but having a large number of plants outside the furrow lanes. There are several downsides to this approach. The most obvious is that seeds that land outside the furrows will not have the growth shown by the plants sown in the furrow, since they are too shallow on the soil. Because of this, they are lost to the elements. Much of the seed remained on the surface where it never germinated or germinated prematurely, only to be killed by frost. On the surface, it was also vulnerable to being eaten by birds or carried away on the wind. Since the furrows represent only a portion of the field's area, and broadcasting distributes seeds fairly evenly, this results in considerable wastage of seeds. Less obvious are the effects of overseeding; all crops grow best at a certain density, which varies depending on the soil and weather conditions. Additional seeding above this limit will actually reduce crop yields, in spite of more plants being sown, as there will be competition among the plants for the minerals, water and the soil available. Another reason is that the mineral resources of the soil will also deplete at a much faster rate, thereby directly affecting the growth of the plants. The invention of the seed drill dramatically improved germination. The seed drill employed a series of runners spaced at the same distance as the ploughed furrows. These runners, or drills, opened the furrow to a uniform depth before the seed was dropped. Behind the drills were a series of presses, metal discs which cut down the sides of the trench into which the seeds had been planted, covering them over. This innovation permitted farmers to have precise control over the depth at which seeds were planted. This greater measure of control meant that fewer seeds germinated early or late, and that seeds were able to take optimum advantage of available soil moisture in a prepared seed bed. The result was that farmers were able to use less seed, and at the same time experience larger yields than under the broadcast methods. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「seed drill」の詳細全文を読む スポンサード リンク
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