164 HORTICULTURE FOR SCHOOLS 



the optimum condition as regards moisture. Drainage 

 removes excess water and thus allows the necessary atmos- 

 pheric oxygen to enter. 



252. Drainage of swamp lands. There are large areas 

 in the United States where the topographical conditions are 

 such that water collects year after year, forming swamps. 

 These areas are commonly very rich in natural soil fertility, 

 for plant-food has accumulated here for centuries and very 

 frequently organic matter has been added in large quantities. 

 The drainage of these lands is an engineering problem which 

 offers in many cases great inducements to the capitalist or the 

 state; financial returns are more or less certain once drain- 

 age has been accomplished. On that account, large areas 

 have been reclaimed and are now under cultivation. At 

 the present time there are still many thousands of acres of 

 such lands, especially in the South. When this task is com- 

 pleted, a two-fold good will have been accomplished: a 

 menace to health in the vicinity of these swamps will have 

 been removed, and large areas of land will have been put to 

 productive use. 



253. Drainage in irrigated regions. In many of the 

 irrigated sections of the United States, drainage is coming to 

 be quite as important as in regions of too abundant rainfall. 

 It has been found that irrigation water, applied year after 

 year, very frequently soaks into the soil in such large quan- 

 tities that the water-table below the surface rises until it 

 becomes a menace to the growing crops. This is especially 

 true if the more deep-rooted plants like fruit-trees are grown. 

 There are two ways of overcoming this difficulty. One is by 

 application of less water. This can frequently be brought 

 about through more intensive conservation of the moisture 

 already in the soil. When, in spite of this, the moisture 

 becomes excessive, the only practical remedy is to drain off 

 the excess through underground tiles. 



254. The necessity for irrigation and drainage. The 



