SOIL IMPROVEMENT BY DRAINAGE. 67 



t-his country, with larger sizes for mains in most cases. 

 In the central Western States tile twelve, fourteen or 

 sixteen inches in diameter are not uncommon in main 

 drains on large farms, or where several farmers have 

 united in a system of drainage. 



Depth and Distance Apart of Drains — There 

 is a close relation between these two points. The 

 deeper the tile is laid, ordinarily, the greater may be 

 the distance between the drains. Shallow drains 

 cannot affect the land to any considerable distance, 

 and will not lower the line of saturation sufficiently 

 to give the best results. The tile may be injured by 

 frost. Little good comes from cutting deep drains in 

 very retentive soils. Frequent, shallow drains are 

 best for such. In open, porous soils, such as those of 

 the prairie States, drains laid three feet deep will ap- 

 preciably affect the soil for 100 feet or more on either 

 side. In some very compact English clay soil drains 

 are laid not more than 15 feet apart. 



Usually a deep laid drain will begin to flow sooner 

 when rain comes, after a period of drouth, will con- 

 tinue to flow longer, and will carry off somewhat more 

 water in a given time than a shallow drain. The 

 reason these statements are true will be apparent if 

 the results of pouring water into a barrel filled with 

 earth, and with holes bored into its sides at different 

 depths, be thought of. 



The cost of digging ditches rapidly increases with 

 their depth. There is no advantage, and some disad- 

 vantages, from lowering the line of saturated soil far 

 below the surface. In practice three feet is a desir- 

 able depth for tile di-ains in ordinary soils. The 

 nature of the soil and other conditions vary so much 



