Relation of Soils to Moisture and Air 51 



Drainage and manently is known as the water-table. 

 Mechanical ^j^^^.^ ^j^.^ ^ater-table is within a foot or 

 Improvement 



of Soils two of the surface any addition from rain- 



fall soon floods the land and destroys crops. 

 Hence the importance of drainage for lands of this char- 

 acter. Open ditches will to some extent lower this water- 

 table and carry off the rainfall. But open ditches to be 

 effective in the drainage of the land need to be quite close 

 together and thus are seriously in the way of cultivation. 

 Their banks also become the nurseries of weeds and 

 bushes and need constant clearing. They occupy too 

 much of the land. Hence the importance of taking the 

 water away in underdrains. Underdrains are not pri- 

 marily for carrying off the rainfall rapidly but for lowering 

 the permanent water-table in the soil, thus making room 

 for the air and enabling the soil the more rapidly to take 

 up the water that falls in rain. The air, too, warms the 

 soil and lightens its mechanical condition. To accompHsh 

 this the drains must of course be placed in the soil deeper 

 than the natural water-table, and the nearer the Hnes of 

 drains are to each other the more effectually they will 

 lower the water-table, since each drain will draw water 

 only from a certain distance each side of it, and if too far 

 apart the intervening water-table will be very Httle lowered. 

 The best material for underdrains is tile 

 Materials and baked Uke bricks. These are made in short 

 Methods for j j^^ ^^^ ^^ ^^^..^^^ diameters. Many 

 Drammg ° i i • i 



Lands shapes of tiles have been made, but practical 



tile drainers have settled down on the round 

 tubular tiles as best. Various other materials have been 

 used and answer very well for a time where tiles may be 



