SOIL WATER 139 



at the top to accommodate the workman, and the earth in the bottom of 

 the trenches is more difficult to remove. Where the soil is free from 

 stones and hardpan, trenches are frequently excavated to the depth of 

 three feet, and the tiles placed ready for filling the trenches, at a cost of 

 thirty cents per linear rod. Below the depth of three feet and up to five 

 feet, excavating under similar conditions will cost about one cent per 

 inch per rod. 



Depth and Frequency of Drains. — The depth at which to place the 

 tile drains will be determined by the class of crops to be grown and the 

 character of the subsoil. Three feet in depth is considered ample for 

 most farm crops, but for orchards, alfalfa and especially deep-rooted 

 crops, a depth of four feet is preferred. There are many localities, how- 

 ever, where the impervious character of the subsoil is such that tiles can 

 be placed only twenty-four or thirty inches deep, and permit the water 

 to enter. Even under these conditions, tile drainage is generally advisable. 



The distance between lines of drain will depend chiefly on the char- 

 acter of the soil, with special reference to its permeability to water. A 

 soil and subsoil that is sandy or loamy in character will frequently be 

 satisfactorily drained with lines of tile 200 to 300 feet apart. On the 

 other hand, a dense clay will sometimes necessitate the lines of drains 

 being placed at intervals of not more than 30 to 40 feet. This, of course, 

 makes underdrainage much more expensive than in the former case. 

 The deeper the tile is placed the farther the lines may be apart. 



Where land to be drained is uniformly wet, the gridiron or regular 

 system is to be preferred. The irregular system will answer the purpose 

 for the drainage of wet spots or sloughs. The main lines should follow 

 approximately the natural depressions or water courses, while the laterals 

 may run up and down the slopes. Rather long parallel lines are more 

 economical than short ones with numerous branches. 



Grades, Silt Basins and Junctions. — All lines of underdrainage should 

 be laid with uniform grades. If the topography of the land necessitates 

 a change in the grade, in which the grade in the lower portion of the line 

 is less than in the upper portion, a silt basin should be placed at the point 

 where the change of grade takes place. When the reverse is true, a silt 

 basin is not necessary. Where laterals enter a main or sub-main which 

 has a lesser fall than the laterals, silt basins should also be installed. 

 Laterals should enter the main above the center of the pipe, rather than 

 below it. All junctions should be made at an angle of about forty-five 

 degrees up-stream. A fall of one foot in one hundred feet is considered 

 a heavy grade. A fall of one inch in one hundred feet will give good 

 results, although more fall than this is better. In the level prairie sections 

 of the country hundreds of miles of tile are laid with a grade of only one- 

 half inch in one hundred feet, and where great care is exercised in laying 

 the tile, difficulty has seldom been encountered. 



On level land a fair grade may be obtained by gradually lessening 



