EXTENSION COUESE IN SOILS. 35 



movement of water in silt loam, clay loam, and clay soils is essen- 

 tially all through granulation spaces, as very little takes place be- 

 tween the finest soil grains. In such soils tile must not be placed 

 much below the level to which granulation extends. This usually 

 means about 3 to 3J feet. Occasionally it is necessary for short 

 distances to place tile much deeper in order to keep the necessary 

 grade. 



Grade. — The difference of levels between the outlet and the highest 

 point of the drainage system divided by the distance between these 

 positions naturally establishes the maximum grade possible. Where 

 the fall is slight the minimum grade permissible for effective drainage 

 depends largely upon the length of the drain and the size of the tile. 

 Water will flow more rapidly in large tile having a given gradient 

 than in small tile. Lateral or branch tile having a diameter of 3 or 

 4 inches may be laid with as little fall as 1 inch to 100 feet for several 

 hundred feet in length, provided the soil is of a clayey nature. If 

 laid in fine sandy soils, so that there is danger of the sand finding 

 entrance to the tile through joints, the grade must be not less than 

 3 or 4 inches per 100 feet, in order that the current in the tile 

 may be sufficient to keep it clean. Soil may often be kept from 

 passing into the tile by placing straw or other similar material over 

 the joints before covering the tile. 



System (Ref. No. 8, pp. 38-43). — By ''drainage system" is meant 

 the arrangement of the lines of tile which are to collect the surplus 

 waters from any piece of land. There are several of these systems. 

 The one which should be used in any case will depend upon the shape, 

 the size, and the surface topography of the area to be drained. In 

 many instances two or more of these systems may be effectively 

 combined. 



Distance apart of laterals. — When wet lands are in the form of 

 narrow runs or sloughs, tile ditches should be dug as nearly as prac- 

 ticable along the courses in which the water naturally runs, although 

 it is frequently necessary to straighten these considerably. When 

 broader areas are being drained, so that laterals or side branches of 

 tile are necessary, the distance between these will be determined by 

 the degree of drainage to be secured and by the character of the soil. 

 In the case of fine-textured clay soils it is necessary to put tile drains 

 as close as 2 rods apart in order to secure the thorough drainage neces- 

 sary for garden or truck crops, though laterals placed 4 rods apart 

 should, give sufficient drainage for practically all staple crops. In wet 

 sandy soils the laterals may be placed farther apart, though, as 

 mentioned above, they must be of sufficient size to remove the water 

 freely. 



Size of tile (Ref. No. 8, p. 82). — In determining the sizes of lateral 

 and of main tile to use under different conditions, certain principles 



