416 Irrigation and Drainage 



the great world problems of better homes and all 

 which these mean. 



Further than this, on account of the fact that a 

 large majority of swamp lands and lands which may 

 be improved by drainage are not massed together, but 

 are scattered broadly in small tracts, so related to 

 the higher and better -drained lands that these must 

 often be improved in order to work the others to 

 the best advantage, the principles of farm drainage 

 become a matter of great importance to a large pro- 

 portion of the rural population, and through good 

 roads to the people of cities as well. 



THE NECESSITY FOR DRAINAGE 



The first and most fundamental necessity for land 

 drainage, as has been pointed out in discussing 

 alkali soils, is the removal of the more soluble salts 

 formed by the decay of rock and organic matter, 

 because too strong a solution of salts in the soil water 

 is fatal to the growth of vegetation, and gives rise 

 to the alkali lands. So long as there is sufficient 

 leaching to hold the soluble salts down to small per- 

 centages, so that neither plasmolytic nor toxic effects 

 result, then the first imperative demand for thor- 

 ough drainage in all soils is met. 



The second imperative demand for drainage is to 

 prevent a stagnation of the soil water, which means, 

 to avoid the exhaustion of oxygen from the air in 

 the soil water and in the spaces not occupied, by 

 water, because an abundance of free oxygen in the 



