IRRIGATION AND DRAINAGE 153 



primary importance. In many sections of the United States 

 it is resorted to not only in the regions of heavy rainfall, but 

 also in localities where rainfall is very slight. By means of 

 drainage, vast areas which otherwise would have been second 

 rate or worthless have been transformed into land of high 

 producing power. The subject is, therefore, as worthy of 

 consideration in some respects as is irrigation with which it 

 is commonly associated. 



251. Why drainage is important. In many sections of 

 the United States, the rainfall is excessively heavy, with the 

 result that the soil is saturated with moisture much of the 

 time. Under these conditions, the decay of vegetation and 

 other chemical processes is not carried to completion and a 

 large number of injurious substances results. Among other 

 things, it is a very common phenomenon in wet soils for 

 acids of one sort or another to be formed. When this hap- 

 pens, the soil is said to be sour. A few plants like the cran- 

 berry thrive on this type of land, but most plants require a 

 soil where acid is absent. 



The first step in correcting this acid condition is to remove 

 the excess water from the soil. The next step is the applica- 

 tion of lime, which reacts chemically with the acid of the 

 soil, neutralizes it, and leaves the land in a non-acid condition. 



The soil is not dead and inert as is so commonly supposed, 

 but is teeming with myriads of forms of life. It is also a 

 laboratory in which many and very complex chemical 

 reactions are taking place continually. The best conditions 

 for the life of the soil are: (1) an optimum amount of water, 

 and (2) a plentiful supply of atmospheric oxygen. If the 

 land is too dry, the optimum of water is not present and 

 conditions are poor for bacterial and chemical activity. If, 

 on the other hand, there is too much water in the soil, then 

 the oxygen of the air is excluded and many of the bacterial 

 and chemical changes are shortened on that account. Irriga- 

 tion supplies water in such a way as to keep as near as possible 



