SOIL DRAINAGE EXPLAINED

The objectives of cultivation.

a) The main object in cultivation is to provide conditions which will enable crops to grow healthily and give a good yield. This will involve the establishment and maintenance of a good soil structure.

b) To provide conditions that will enable seeds to be sown and plants to be planted.

c) To enable fertilisers and manures to be incorporated, and the residue of crops and weeds to be disposed of.

Of primary importance is drainage. The vast majority of crops thrive only in a well-drained soil. A badly drained soil may be responsible for both difficulties and failures. Some idea of the condition of a soil’ s drainage can be obtained by a study of the natural herbage or of the condition and appearance of growing crops. Better still, an intelligent use of the soil auger will provide reliable evidence. Imperfect drainage is generally revealed by dark brown mottling of the soil at the point where drainage is impeded. Students must be familiar with the common methods of draining soils:-

i) Tile draining, ii) Mole draining, iii) Sub-soiling.

As its name implies, tile draining consists of laying drainage tiles underground. The sizes of the tiles and the depths at which they are laid will vary according to the requirements of the particular project under consideration. Fundamental principles of tile draining are:

a) The tiles are laid carefully so that they will not easily become dislodged.

b) They must be of adequate size to cope with the amount of water with which they may have to deal.

c) There must be an even flow towards the point of outlet which must first be determined before any pipes are laid.

d) Branch drains must be laid so that they flow easily into the main drains.

i) Steps must be taken to ensure that the drains are not covered with an impermeable substance such as clay which will prevent water from reaching the pipes. Generally this is achieved by covering the tiles with a clean gravel.

[n the garden, drainage systems are laid out by hand, beginning at the lowest point, and require considerable skill. Students should make themselves familiar with irainage tools. In the field, drains are laid mechanically, md the work is usually carried out by contractors who are specialists at the job. It should be noted that as an ilternative to the 4 inch (100 mm) clay land drains, plastic )iping is now being used.

The size of the pipes might be a nominal 3 (75 mm) diameter for the laterals and 4 (100 mm) for the main drains. The depth of laying must be below the level of cultivations, I.e. 18 (450 mm) down to 2 feet (600 mm) or more.

The spacing of the laterals depends on the soil type, the extent of the problem and the rate (speed) at which the water has to be removed from the area. It could vary from 2 metres to 10 metres apart depending on the situation.

Mole draining. A special implement draws through the soil a bullet-shaped piece of metal called the mole. This leaves behind it a channel through which water can pass. The system is suitable only for heavy soils. On light soils the channel soon collapses and becomes blocked. The system is sometimes used in conjunction with tile drainage.

Pulling the mole uphill is best and pulling over landdrains covered with porous backfill provides a positive connection between the pipe drains and the mole channel.

Sub-soiling. It is perhaps debatable whether sub-soiling should be included under the heading of draining. There are two distinct types of sub-soiling, one of which is expressly designed to improve drainage; this is deep sub-soiling, carried out to a depth just above any tile drainage system that may exist. Usually 18 to 20 (45 – 51 cm) is considered the maximum depth; the new N.I.A.E. (National Institute of Agricultural Engineering) Winged Sub-Soiler tines are of special value. The operation requires special equipment and a powerful tractor by means of which a heavy blade or tine is drawn through the soil at the required depth at intervals of about 3 feet (0.9 m) The object is to shatter the sub-soil thus improving the passage of water. This operation is most effective in the autumn when the sub-soil is dry, thus enabling the maximum shattering effect to be achieved.

The other method of sub-soiling consists of drawing a smaller tine through the soil, a few inches below that at which ploughing can be done. Sometimes it is done at the same time as ploughing, the tine being drawn aiong the bottom of the furrow. This will break up any pan caused by continually ploughing at the same depth. Both these operations are important and students should fully understand not only the object of the operations but also the method of carrying them out.

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