254 



THE PRINCIPLES OF SOIL MANAGEMENT 



to place the drain on the boundary between the two. 

 If the clay is below, the water will percolate along its 

 surface through the sand and enter the tile. On the 

 other hand, if the clay is underlain by sand, it is easier 

 for the water to percolate downward into the coarse- 

 texture stratum, and through this into the tile, entering 

 from below. 



(b) Frequency. There are two general systems of 

 arranging drains: (1) The gridion or regular system. 



Fig. 84. Two systems of arranging tile drains. Compare the amount of 

 double draining in each system, due to junctions. Note the relative lengths 

 of tile required for the same area under each system. 



(2) The natural or irregular system. In the first, the 

 drains are arranged at definite regular intervals apart, 

 this interval depending chiefly on the texture of the soil. 

 This is necessary where the surface is very uniform and 

 the soil very homogenous. It may be applied to a slope 

 as well as to level land. In clay soil the interval must be 

 less than in coarse-textured soil. This is because there 

 is a drainage gradient between the drains. In fine- 



