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 dich ho em bai nay theo chuyen de nong nghiep nhe

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Tổng số bài gửi : 1
Tổng số điểm : 13030
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Tâm trạng : Tôi vui
Ngày tham gia : 19/10/2009

Bài gửiTiêu đề: dich ho em bai nay theo chuyen de nong nghiep nhe   Mon Oct 19, 2009 12:07 pm

One meaning of drainage is the natural ability of soil to allow a downward movement of water . The ease with which water can pass through a soil depends on the proportion in it coarse and fine particle such as sand and clay . The finer the particles become , the more slowly the water percolates , or passes through the soil . So heavy soils such as clay are more impermeable than light soils .
When there is too much water in the soil , some of it must be drained off . This is the other meaning of drainage : the removal of excess water from the soil by ditching or tiling . This is done in order to maintain a correct balance of air and water in the soil . Good drainage makes a soil easier to work . It also helps to increase the feeding area of the soil for the root of plants . Another advantage is that a well-drained soil will have enough air aerobic bacteria to break down humus and so provide food for the plant .
Ditching is one of the most important techniques for draining land . Ditching can be cut at certain intervals between the crops . These will remove surface water . They should be wide and straight , with sloping sides , and they should be regularly cleaned . Another important technique is tile drainage . Porous drainage tiles may be laid in or on the land and these will help to draw off the surplus water . The distance between the drains will depend on the level of the land , the permeability of the soil , and the amount is used where water accumulates underground . A tunnel is bored about 3 inches in diameter through the earth at a depth of about 2 feet .
Where and when water is in short supply , irrigation is needed to make up the deficit . we should distinguish between the collection of water and its application . There are two main sources of irrigation water : surface water and ground water . The former may be obtained from rivers , lakes or reservoirs , and the later is provided underground water deposits . Irrigation from rivers is mainly along canals from dams which have been built across the rivers . The water collects behind the dam during the wet season . And it is applied in the fields later during the dry season . Subterranean water is obtained by digging or drilling a well . In either case it is necessary to lift the water before it can be used for irrigation .
The amount of water which is required for irrigation depends on a number of factors . It depends , firstly , on the type of soil , and the deficit in the soil . By this we mean the amount of water which is needed to bring the soil to full capacity . It also depends on the type of crop , the stage of growth of the crop and the amount which is will use at that particular time . The irrigation requirement of a crop is not the same throughout its growing period . Most plants require larger quantities of water during the later stages than in the earlier stages . Sugar can need heavier irrigation or more frequent irrigation from about the sixth or seventh month onwards . In the same way , grain crops require their maximum irrigation during the time earheads are forming .
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