120 



LAND DRAINAGE AND IRRIGATION 



broad, flat areas several lines of tile are necessary. Just how 

 far apart these drains should be placed to secure adequate drainage 

 depends largely on the character of the subsoil. In some very 

 heavy soils and in springy areas, it is necessary to lay them not 

 more than two rods apart, while in porous and open soils they may 

 be placed as far apart as eight to twelve rods. In ordinary 

 loams and silt loams, and in peat and muck, four rods is the 

 common distance. 



=20 RODS 



M, main; 



FIG. 54. Natural systems of tile drainage. Three systems in an 80-acre field. 

 L, lateral; S, sub-main. (Wisconsin Station.) 



How Tile Works. Water gets into tile drains through the joints 

 between the tile (Fig. 59). In case of porous tile, a little water 

 goes through the walls, but at least ninety-five per cent enters 

 through the joints. Care should be taken in laying tile not to 

 leave cracks between them so big as to permit soil to fall in, yet 

 not so close together as to exclude the water. In tight clays it is 

 well to " blind" the tile with black soil to facilitate the entrance of 

 water. In sand it is necessary to blind the tile with black soil 

 to keep the sand from entering. 



It is a mistake to think that thorough underdrainage is harmful 

 to crops, particularly in dry seasons. Tile cannot "draw" water 

 out of soil, nor can they drain a soil too thoroughly. It is impos- 



