iii iiiii nun ill inn mil inn mil inn inn iiiii mi mi 
EXTENSION COUKSE 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 
