406 Irrigation and Drainage 



have completely used up the organic nitrogen, then it will be 

 only partly purified ; or if the flow over the field is made con- 

 tinuous, then the supply of oxygen in the soil becomes so 

 small that the germs are unable to carry forward the work, 

 and organic nitrogen passes through largely unchanged and 

 liable to become the food in drinking water of other but 

 dangerous forms. 



SOILS BEST SUITED TO SEWAGE IRRIGATION 



In humid climates, where the rainfall is both frequent 

 and abundant, the lighter loams and sandy soils are best 

 suited to this type of irrigation, because upon them there is less 

 danger of water -logging. It should be understood, however, 

 that from the agricultural standpoint sewage may be applied 

 to any soil, provided it is not used in too large quantities or too 

 continuously ; but as the sandy soils are usually more in need 

 of artificial fertilization, and at the same time likely to be 

 deficient in water, they are preeminently suited to this use, and 

 will usually be chosen by city authorities when they are avail- 

 able, but simply because a smaller number of acres will answer 

 the purpose and the cost of the plant be less. 



The agricultural value of sewage when properly applied to 

 land has been so thoroughly demonstrated under so many condi- 

 tions of soil and climate that there can no longer be any doubt as 

 to the desirability of its use if the expense of getting it to the 

 land were eliminated, and it would appear that lands enough in 

 the vicinity of most cities could profitably receive and use the 

 sewage if only it were led to them. 



DESIRABILITY OP WIDER AGRICULTURAL USE OF 

 SEWAGE IN IRRIGATION 



In countries like Italy, where there are extensive canal 

 systems largely used for irrigation, it would appear that sewage 

 disposal by irrigation should become the general practice, pro- 



