THE MOISTURE IN THE SOIL 



61 



hard upon the surface that the water of rainfall never can pass 

 down. By placing the drains shallow, the soil is rendered mellow 

 and porous, water passes down readily, the level of free water is 

 raised, and the surplus is removed. 



766. The distance apart at which drains should be placed is 

 variable, but 30 feet is usually considered most advisable. The 



Fig. 13. Properly made open ditch. 



level of the free water tends to rise higher at a point midway 

 between drains, as shown in Fig. 8. If the drains are too far 

 apart, this tendency may be greater than the tendency to move 

 toward the drain. In soils through which the water moves some- 

 what readily, the drains may be farther removed than in close, 

 impervious soils. 



78a. In the spring, on undrained soils, free water remains 

 for a considerable time near the surface ; consequently the plant 



Fig. 14. Sides too steep. 



roots cannot penetrate deeply into the soil. When the drought 

 comes the surface is first affected, and the plants suffer at once. 

 It is a well-known fact that tap-rooted plants are admirably 

 fitted to withstand dry weather. Their feeders are deep in the 

 soil. It is this condition which is obtained to a certain extent by 

 under-drainage. The soil above the drain is made porous, the 

 water which cannot be held by capillarity is quickly removed, the 

 air penetrates, the soil becomes warm and congenial. Thus 



