TERRACING AND DRAINING 79 



penetrate farther, and in drained soils therefore plant roots 

 are deeper than in soil that has ordinarily been saturated. 

 When the upper soil dries, the shallow-rooted plants in 

 undrained lands are no longer able to obtain moisture ; 

 but the deep-rooted plants in drained soil, being nearer the 

 ever moist subsoil, are uninjured. 



Other benefits from drainage. Drainage makes soils 

 more crumbly and less inclined to be cloddy. It increases 

 in the soil the number of helpful germs, or tiny living 

 plants, that change vegetable matter into available plant- 

 food. This it does by supplying an abundance of air, with- 

 out which they cannot live. Moreover, drainage makes the 

 land ready for plowing earlier. Plants start to grow earlier 

 on drained than on wet soil, for drainage warms the soil 

 by drawing off a part of the water that would otherwise 

 evaporate, and which, in evaporating, would cool the soil. 



Two classes of drains. The usual drain is an open ditch. 

 Another kind is the covered or underdrain. A field in 

 which there are underdrains shows no sign of them, for they 

 are two to four feet below the surface and completely 

 covered over. One of their advantages over open ditches 

 is that crops can be grown above the drains. 



Underdrains. These are usually made of tiles, which are 

 hollow tubes of burnt clay one foot long, laid end to end. 

 The water runs into them at the joints, which do not fit 

 together tightly, and trickles in through the porous walls 

 (Figs. 52, 53). Sometimes underdrains are made of four 

 narrow planks nailed together like a long box, with 

 numerous holes for water to enter. In other cases they 

 are made of three large poles in a triangular pile, and 



