IRRIGATION IN HUMID CLIMATES 415 



tity of sewage, are, as yet, located under a considerable 

 rainfall. The materials dissolved or suspended in irrigation 

 water are often, as shown in Chapter V, of high value as 

 plant-food. Of all known waters, however, sewage water 

 is usually of the highest value hi crop-growth, since its 

 chief constituent, human waste, approaches in composition 

 the more valuable portions of plants. 



It has been roughly calculated that each person, living 

 in a city, wastes annually eight pounds of nitrogen, three 

 pounds of potassium, two pounds of phosphorus, not 

 counting the organic matter of which these three funda- 

 mentally important elements are parts. When these 

 quantities are multiplied by the millions residing in many 

 of the large cities, the sewage which passes into the rivers 

 and oceans, rises to tremendous value. The large cities 

 cause the largest single losses, but the smaller cities of the 

 country are now installing sewage systems, and all should 

 give some attention to the conservation of sewage waste. 

 Sewage can best be put to use by its application in irri- 

 gation. 



249. The use of sewage. In many countries, sewage 

 water is used for plant-production. The most famous 

 example is that of Craigentinny meadows, receiving sewage 

 from Edinburgh. According to Storer and King, one 

 hundred years ago, when sewage irrigation began on these 

 fields, they were originally a waste. With the aid of sewage 

 irrigation they have produced continuously since that time 

 large crops of grass, with a profit far above that of the 

 best fields of the country. Similarly, near Milan hi Italy, 

 sewage is let into great canals that lead to great meadows. 

 These have produced remarkably, as a result of the appli- 

 cation of the heavily fertilized water. China, and the 

 Orient generally, are perhaps the greatest examples of the 



