252 



THE PRINCIPLES OF SOIL MANAGEMENT 



(a) Depth. The depth of 

 the drain must be such that 

 the water can find entrance 

 before it shall have caused 

 serious injury to the crop. 

 Since water percolates through 

 sand and gravel so much more 

 readily than through clay, 

 drains may be placed much 

 deeper in the former than in 

 the latter. In coarse-textured 

 soil, drains attain their full 

 efficiency almost at once; but 

 in clay, owing to its dense 

 character from long wetness, 

 there is a gradual increase in 

 efficiency through several 

 seasons, as the soil becomes 

 better granulated and ac- 

 quires other favorable struc- 

 tural properties. In sand, 

 water percolates rapidly into 

 the drain, but in clay this gen- 

 eral movement is greatly re- 

 duced and takes place largely 

 from the sides and top of the drain. In fact, a dense 

 clay soil holds its pores almost full of capillary water, 

 which is not subject to percolation. Under such condi- 

 tions, a large part of the injury comes from water stand- 

 ing on the surface. Here the under-drains must be placed 

 very near the surface, and function chiefly as surface 



Fig. 82. Laying tile in the 

 bottom of ditch by use of the tile 

 hook. Shows arrangement of tile 

 preparatory to rilling the ditch. 



