130 AGRICULTURE ON THE PACIFIC SLOPE 



Such drains are particularly useful in clay or adobe lands, 

 which are likely to remain wet longest. But the tile drains 

 are useful not only to remove excess of water, but also to 

 deepen the loose layer of soil and subsoil, so that roots can 

 penetrate to greater depths and so find more moisture and 

 plant food. Such a deep, loose layer down to the depth of 

 the drains (three to five feet) also insures the better soaking- 

 in of rain water, thus diminishing the run-off and surface 

 washing and increasing the moisture supply in the land. It 

 permits us to plow the land long before it would otherwise 

 have been possible. 



' ,2 3 v -M- 3 v~ i 



I'iG. 69. Diagram showing the effect of underdrains in loosening more and 

 more soil year after year. 



The cause of this loosening is that whenever a clay soil 

 is wetted it swells, and then shrinks in drying. When this 

 happens slowly, the surface of the ground rises and sinks. 

 But over the underdrains the change is so quick that the soil 

 clods are torn into small pieces or grains, which then let the 

 water, air, and roots pass like so much sand. Gradually 

 the effect extends sideways, until the whole space between 

 the drains is loosened. This is shown in figure 69. 



How water gets into the underdrains. People sometimes 

 ask how water gets into the drains through the close joints 

 between the tiles. This can easily be seen by looking into the 

 ends of drains during a wet time. We can then see the water 

 spurting in at the joints. The danger is not that the water 

 will not get into the drains, but that sand and earth may, and 

 so clog the pipes, compelling us to dig them up and clean them. 



