CHAPTER XII 



DRAINAGE, IRRIGATION, AND DRY FARMING 

 I. DRAINAGE 



148. Effects of drainage. Soils in which water stands near 

 the surface for a considerable time during the growing season 

 are benefited by drainage. Such soils are usually either so level 

 that water does not flow off the surface or so fine-grained that 

 water cannot flow out readily after heavy rains. The standing 

 water shuts out the air and furnishes a condition that is not 

 favorable to the growth of roots (Fig. 50). It also hinders the 

 bacteriological processes necessary to a productive soil. It makes 

 the soil cold and delays its preparation for a crop. 



Tile laid beneath the surface remove the excess water and 

 bring in air, while at the same time they admit air to the lower 

 areas of the soil through the openings of the tile themselves. 

 Water falling as rain upon the soil displaces the air, and as it 

 drains out, fresh air comes in. Probably the most important natu- 

 ral cause of soil aeration is the diffusion of gases, and the most 

 important means by which the farmer can further soil aeration 

 are cultivation, addition of vegetable matter, and tile drainage. 



149. Kinds of drainage. There are two general methods of 

 draining land surface drainage and tile drainage. Surface 

 drains are used as a cheap means of ridding the land of the 

 surface water. Tile drains are pipes laid at a depth of three 

 or four feet to remove the standing water from the soil. The 



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